Radiator under window sill

[JohnW2]

One further (I hesitate to dare say 'final'!) question ...

In that bay, I have 1,960 mm horizonal space ('between walls') available, so an 1,800 mm rad (with valves) ought to just about fit. However, the TRV will then be 'tucked into the corner', very close to the reveal of the bay,and probably beneath the sill (about 600 mm above TRV). In that situation, will the TRV 'work' satisfactorily?

 

[Agile]

TRVs only operate on the generality of the temperature around them.

In that case, with 100mm gap at the top, the heat should be able to get out well at the top and so the temperature around the valve will be pretty much that of the lower part of the room.

So I do not forsee any significant problem.

 

[JohnW2]

TRVs only operate on the generality of the temperature around them.

Thanks. That was my slight concern - it felt as if the TRV might be trapped within its own little 'microclimate', very close to the rad, in that corner!

In that case, with 100mm gap at the top, the heat should be able to get out well at the top and so the temperature around the valve will be pretty much that of the lower part of the room. So I do not forsee any significant problem.

Thanks for the reassurance!

I have to say that I've always wondered how/why TRVs seem to work as well as they do, because they are sensing the temp so close to the radiator. If one puts one's hand in the vicinity of the TRV (i.e. very close to the rad), it feels much hotter than 'the room' in general. However, I presme that's an unnecessary concern, since they do seem to work reasonably well

Kind Regards, John

 

[scw24489]

Having calculated a heat loss for room, adjust for dT of system which will be average rad temp ie flow+return/2 minus room temp, this obviously varies subject to the type of heat source and the level of efficiency at which you want the design to operate (lower is ALWAYS better).

The dT50 rating is a hangover from pre-condensing boilers that needed high return temps to prevent condensing and then corrosion at boiler return.
Since condensing boilers came in typical design would be ( (75+55)/2 )-21 = 44. correction factor for a dT50 rating would be 0.846
Current practice would be ((55-35)/2)-21 = 24 correction factor 0.383.

The new flow temps are to make rad systems heat pump ready but make rads impractically large in most domestic premises so you will find many ignore the regs!!
The correction factor falls off a little faster than Pro-rata and can be looked up on internet lists or calculated (actual dT/spec sheet dT)^1.3

Having corrected for spec sheet to operating dT you would apply a correction factor for the method of water connection - in domestic work connections are invariably Bottom Opposite Ends or BOE which meean rad derated by 0.943

Any significant cill or shelf over rad would usually attract a derate factor of 0.95 and a rad cabinet 0.7

The final adjustment would be applied on basis of operation - if the heating is going to be regularly turned off with energy saving controls and then required to bring a room back to temperature in a relatively short period then an output uplift would be required somewhere from 10-20% for resonably well insulated property reheating in half hour or so.

Rads are traditionally sited under a window because glazed areas will usually have significantly greater heat loss than walls and the cold air against the glass will create a downdraught that can then be counteracted by the rad and the resultant turbulent mixing will lessen the convection current effects already discussed in this thread.
More uncharitably the space under the window gives a rad size for those that can't manage the above process

Trv sensor are best mounted horizontally where possible for exactly the reasons you have mentioned but as they are rarely calibrated in actual temperatures any offset caused by local 'microclimate' can just be adjusted for with an appropriate setting.

 

[JohnW2]

Thanks for your interest and your detailed and very interesting explanations. I've re-arranged the quotes of your message a bit to facilitate this reply! ...

The dT50 rating is a hangover from pre-condensing boilers that needed high return temps to prevent condensing and then corrosion at boiler return.
Since condensing boilers came in typical design would be ( (75+55)/2 )-21 = 44. correction factor for a dT50 rating would be 0.846
Current practice would be ((55-35)/2)-21 = 24 correction factor 0.383.
.... The correction factor falls off a little faster than Pro-rata and can be looked up on internet lists or calculated (actual dT/spec sheet dT)^1.3

Fair enough, and thanks for answering what was going to be my next question (how to determine that 'correction factor').

The examples you quote assume a temp loss in the rad (flow-return) of 20 C, for differing flow temps. Why is that,and is it necessarily the case in practice? In fact, one presumably can change dT by altering flow rate (hence altering temp drop in the rad) without changing flow temp, can't one?

* Having calculated a heat loss for room, adjust for dT of system which will be average rad temp ie flow+return/2 minus room temp, this obviously varies subject to the type of heat source and the level of efficiency at which you want the design to operate (lower is ALWAYS better).
* Having corrected for spec sheet to operating dT you would apply a correction factor for the method of water connection - in domestic work connections are invariably Bottom Opposite Ends or BOE which meean rad derated by 0.943
* Any significant cill or shelf over rad would usually attract a derate factor of 0.95 and a rad cabinet 0.7
* The final adjustment would be applied on basis of operation - if the heating is going to be regularly turned off with energy saving controls and then required to bring a room back to temperature in a relatively short period then an output uplift would be required somewhere from 10-20% for resonably well insulated property reheating in half hour or so.

Having estimated heat lost as accurately as one can, and then applied all of those adjustments is all very well (and sounds like an attempt at 'pretty good science') but then seems to be completely overshadowed/undermined by ....

Having calculated a heat loss for room, adjust for dT of system .... <for> the type of heat source and the level of efficiency at which you want the design to operate
AND
The new flow temps are to make rad systems heat pump ready but make rads impractically large in most domestic premises so you will find many ignore the regs!!

It therefore seems that (in domestic settings) having attempted to undertake a series of fairly accurate calculations and adjustments thereto, one then applies an essentially arbitrary (and potentially massive) 'alteration' dependent upon what flow temp/dT one 'wants' and what is 'practical'. You refer to the 'many who ignore the regs' somewhat pejoratively, but if to do otherwise would be impractical (or just unaffordable), then it would be impractical, and therefore would/could not be done.

So this is where a science seems to turn into more of an art - with the bottom line that, regardless of all the calculations and adjustments (and 'regs'), the rads which are installed will often simply be 'the rads that will fit' (i.e. 'practical'). Is that not the case?

.... if the heating is going to be regularly turned off with energy saving controls and then required to bring a room back to temperature in a relatively short period then an output uplift would be required somewhere from 10-20% for resonably well insulated property reheating in half hour or so.

It's not really something which concerns me very much, but the 'warming up' process needs somewhat more sophisticated modelling, and probably happens faster than some would imagine - because,if one starts heating a room from cold, the temp difference across walls/floors/ceilings etc. (and hence heat loss) is initially pretty (or very) small. Our usual heat loss calculations refer to the steady state after 'warming up', such that 'heat loss' of a room is a constant. However,during the warming up phase, heat loss starts low and then gradually increases as the room warms up.

Rads are traditionally sited under a window because glazed areas will usually have significantly greater heat loss than walls and the cold air against the glass will create a downdraught that can then be counteracted by the rad and the resultant turbulent mixing will lessen the convection current effects already discussed in this thread. ....

Indeed, and ...

More uncharitably the space under the window gives a rad size for those that can't manage the above process

Not really 'uncharitable', since it's an undeniable truth that such is what many have done (and probably still do). As I've said,it's certainly what I've done in the (fairly distant) past,and it always seems to have resulted in a system which worked reasonably well, if not necessarily very efficient! Furthermore, as above, it seems that this is probably essentially what very often still happens in domestic settings - since the science/calculations are likely to call for impractically large radiators , such that what gets installed is "what will fit"

Trv sensor are best mounted horizontally where possible for exactly the reasons you have mentioned ...

I can see that would help a bit to address the issue I mentioned, but I have very rarely seen it done, and cannot recall ever having seen any TRVs for sale that would enable that to be done directly - since, whether 'straight', 'angled' or 'corner', the sensor head seems to invariably be at right angles to the spigot that goes into the rad. If one wanted a horizontal sensor, it therefore looks as if one would have to improvise with a 'straight' TRV and a M/F 1/2" elbow - which would be a little messy (and a little'wide') ... or am I missing something?

... but as they are rarely calibrated in actual temperatures any offset caused by local 'microclimate' can just be adjusted for with an appropriate setting.

You speak as if there is just a fixed 'offset' between the temp seen by the sensor and the temp of the room, but is it really that simple? My 'concern' was that, for example, when a radiator is first turned on in a very cold room, the temp 'seen by' the sensor (which is 'up close to the radiator' will presumably initially be a lot higher than the actual temp of the room air in general (as a result of radiation, if not also some convection) but that difference between sensor temp and general room-air-temp will gradually decrease as the room warms up?. However, as I've said, they seem to work fairly well, so my concerns are presumably largely unwarranted!

Kind Regards, John

 

[scw24489]

The examples you quote assume a temp loss in the rad (flow-return) of 20 C, for differing flow temps. Why is that,and is it necessarily the case in practice? In fact, one presumably can change dT by altering flow rate (hence altering temp drop in the rad) without changing flow temp, can't one?

The dT across the system will relate to the type of heat generator you are using. I have assumed (maybe incorrectly) that 20 years after madation you are using a condensing boiler (gas or oil) The controls of which will be attempting to maintain dT20 because that is what the manufacturer will have designed for when sizing all the components.
When small bore wet CH was developed in the 50's by the NCB design temps would typically be 180F flow with a dT20F drop (82C/11C drop), 180F being a sufficient margin of safety for the thermostat tech of the day to prevent boiling and 20F drop being realistic for the single pipe circulation systems and the return temperature being high enough to prevent condensation corrosion of cast iron heat exchangers without more sophisticated back end protection.
With the introduction and subsequent mandation of condensing boilers as energy efficiency moved up the agenda these typical temperatures were altered. but with a nod back to the old paradigm so as to keep existing pipework and emitter systems viable.
75/55 flow return became the norm. 75 not being so very much less than the 80 that the existing rad systems were designed for but spreading the dT so as to achieve a return temperature low enough to gain some latent heat through condensation of flue gases (full condensation does not happen until return temps are iro 30C) Additionally dT20 results in a halving of water flow rate (cf dT11), as you allude to, and therefore a reduction in pump energy requirements (CH pump circulators being a significant fraction of home energy use).
Does it happen in practise? Well it can and it should but...... Balancing a system is a tedious and time consuming task, made VERY much more difficult by 'sizing rads by window cill' rather than heat loss. The introduction of modern electronics to circulators (Grundfos Alpha3 etc) and pressure independent control valves in domestic rad valve form are making the job easier, as they remove some element of interaction between the adjustment of different elements.
The establishment of correct flow and dT is ESSENTIAL to achieving full efficiency and performance from a given design and the common lack is what is chiefly responsible for the performance gap between design/regulatory and reality, where rules of thumb plus a bit for luck, cos that was what my old gaffer said etc etc are more common.

Heat pumps (that the government would like us all to move to) are a totally different kettle of fish to boilers. Thermodynamically they operate in a completely different envelope to fossil fuel boilers and this means the systems that accompany them need to be specced with a completely different starting point. Heat pumps operate well in a very narrow band of conditions typically with a dT5C and commensurately higher flow rates (larger pipe work often needed)
Additionally in order to maintain an acceptable efficiency (COP) flow temperatures need to be substantially lower resulting in a derating of existing rads by two to three times.
Short cycling is not great for any heat generator but is disastrous for a heat pump where it will decimate the efficiency achieved.
Heat pumps do are not a good fit with highly zoned systems, it is readily possible to achieve a situation where turning off/down the heat in one zone with resultant increased heat loss from heated area into unheated causes the HP controls to raise flow temp into heated zone with a resultant drop in COP that means you use more energy to heat less rooms - crazy but true!!
These are SOME of the issues, that mean HP's are rarely a drop in replacement for a boiler, the cost and disruption of the consequential changes to an existing wet system are one of the reasons that HP's are not gaining the traction that might be desirable.

It therefore seems that (in domestic settings) having attempted to undertake a series of fairly accurate calculations and adjustments thereto, one then applies an essentially arbitrary (and potentially massive) 'alteration' dependent upon what flow temp/dT one 'wants' and what is 'practical'. You refer to the 'many who ignore the regs' somewhat pejoratively, but if to do otherwise would be impractical (or just unaffordable), then it would be impractical, and therefore would/could not be done.

While you are not wrong with regard to the result, this just further illustrates my points above that you cannot specify/design a single element (say a replacement rad) in isolation, you need to understand the design rational behind the existing equipment and specc the new equipment on the same basis. Where this cannot be practically achieved (as is often the case with change to HP) you have to start again from scratch - enter cost and disruption, prohibitive to most.
Situation exacerbated by the high chance that the tradesperson you engage will be unaware of any of these issues (or the many others that might be in play).

It's not really something which concerns me very much, but the 'warming up' process needs somewhat more sophisticated modelling,

Absolutely. Something that might be done for a commercial property where the savings might have some chance of offsetting the design costs, in domestic work you are reliant on a best guess which requires both the installer and the client to be aware of the factor.

cannot recall ever having seen any TRVs for sale that would enable that to be done directly - since, whether 'straight', 'angled' or 'corner', the sensor head seems to invariably be at right angles to the spigot that goes into the rad. If one wanted a ho

Most angle TRVs on sale these days have 15mm compression fittings on both ends which enable the sensor to be mounted vertical or horizontal just by rotating the body to the appropriate orientation.
TBOE is the most efficient connection pattern and then TRV head can be both above and the side of the rad, however not generally domestically acceptable. TRV's are such a crude control that the issues of offset, as discussed, are not real. TRV's are really just temperature limiters that are used to compensate for poor (over)sizing of rads and and their response and repeatability is, in any case, very dependant on cost/quality.

 

[JohnW2]

Many thanks again. I already knew much of what you say, but I'm pretty confused about what, in practice, is (realistically/'practically') can actually be done, and is done, in domestic properties.

The dT across the system will relate to the type of heat generator you are using. I have assumed (maybe incorrectly) that 20 years after madation you are using a condensing boiler (gas or oil) The controls of which will be attempting to maintain dT20 because that is what the manufacturer will have designed for when sizing all the components.

I am attempting to design the entire system from scratch (there is currently no heating system) so, yes, it will inevitably have a condensing (gas) boiler.

..... With the introduction and subsequent mandation of condensing boilers as energy efficiency moved up the agenda these typical temperatures were altered. but with a nod back to the old paradigm so as to keep existing pipework and emitter systems viable. 75/55 flow return became the norm. ..... spreading the dT so as to achieve a return temperature low enough to gain some latent heat through condensation of flue gases (full condensation does not happen until return temps are iro 30C) Additionally dT20 results in a halving of water flow rate (cf dT11), as you allude to, and therefore a reduction in pump energy requirements (CH pump circulators being a significant fraction of home energy use).
Heat pumps (that the government would like us all to move to) are a totally different kettle of fish to boilers. .... higher flow rates (larger pipe work often needed)
Additionally in order to maintain an acceptable efficiency (COP) flow temperatures need to be substantially lower resulting in a derating of existing rads by two to three times.

It is the real-word ('realistic') implementation of this (in domestic installations) (i..e. 'what is actually done ') that is getting me confused. Much of what is written ('today') seems to say or imply that with (inevitably condensing) gas boilers, dT50 (with ~75° flow temp) is the 'standard'/norm in the UK - e.g., this extract from a Stelrad article ...

Delta 50 is the UK standard for all domestic gas boilers. It allows professionals, end-users and consumers like yourself to make fair and reasonable comparisons and judgements when buying radiators — for example, comparing hydronic radiators and towel rail radiators output across a mix of product types.
If you’re looking into newer, more renewable heating systems, you can also purchase radiators with a lower-level output. For example, Delta t30 and Delta t40 work well for lower water temperature systems.

Needless to say, I understand the need for low temp, hence low dT, for heat-pump etc. systems but that is certainly not something I want to design for. However, I also understand that, as you have said, a lower return temp, (implying a lower dT) is also required for efficient condensing.

However, I think we have more-or-less agreed that the radiator sizes required for a dT30, let alone dT20 or lower, are usually going to be totally impractical in domestic properties (T33 rads, which might be a theoretical solution, when available, are 'impractically' expensive!) .

So what "is actually being done" in systems using (condensing) gas boilers in domestic properties? - just stick with the 70/75flow temp (and ~dT50), and 'ignore' all the efficiency-related theory/considerations (and maybe 'regs'), or what? ... and what on earth is being (or is going to be) done about people who want to run their heating system off a HP?

While you are not wrong with regard to the result, this just further illustrates my points above that you cannot specify/design a single element (say a replacement rad) in isolation, you need to understand the design rational behind the existing equipment and specc the new equipment on the same basis.

As above, I'm attempting to design an entire system, not just a 'replacement rad'- so, yes, I'm 'starting from scratch'- hence everything 'on the same basis'.

Situation exacerbated by the high chance that the tradesperson you engage will be unaware of any of these issues (or the many others that might be in play).

Quite. Why do you think I'm trying to get to grips with this exercise myself

Absolutely. Something that might be done for a commercial property where the savings might have some chance of offsetting the design costs, in domestic work you are reliant on a best guess which requires both the installer and the client to be aware of the factor.

I doubt whether (m)any would bother in relation to a domestic property but one wouldn't have to 'guess'. As I said, one can model the warm up period as well as the steady state - it's just a bit more complicated (and, of course, just like the steady-state calculations is definitely imperfect because of the need to estimate heat transfer through the various structural elements of the building).

Most angle TRVs on sale these days have 15mm compression fittings on both ends which enable the sensor to be mounted vertical or horizontal just by rotating the body to the appropriate orientation.

Right. Thanks. I didn't realise that (haven't had reason to look at what's available for ages!) - so that is 'what I was missing' ... so, since it's actually easy to implement (provided there is enough space beside the radiator),would you recommend always having the sensor head horizontal?

TBOE is the most efficient connection pattern and then TRV head can be both above and the side of the rad, however not generally domestically acceptable.

Indeed. To be frank, I don''t think I can recall ever having seen a valve 'at the top' of a domestic radiator (and it certaily would not be very 'pretty'!).

Kind Regards, John

 

[scw24489]

Hi John,

Most mainstream manufacturers still publish their output tables as dT50 for continuity and because it is a BS standard, some, particularly those who may be aiming at the renewables market, will also publish lower dT tables for the convenience of the informed customer. It is up to the system designer to understand the basis of the specs to which equipment is sold and to apply 'corrective' factors as appropriate.

The reality of what is actually done - it depends entirely on the level of skills, knowledge and drivers of those involved, there is no effective mechanism for enforcing regulations.
The unfortunate truth is that many of the trades persons who find a career in the domestic heating industry are from the pool of school leavers who are less academically able and who learn best by example from their colleagues, the result being that outdated practises are handed on for extended periods. Additionally if one is not academic, being presented with a whole bunch of calculations is daunting at best.
The result is failure to apply best practice more often than not, much of what we have discussed here will be of little interest to many trades and certainly not applied. There is NO policing/enforcement so the status quo endures,
So to answer your question bluntly - proper design is routinely not done because the installer is unwilling, unaware, unable to undertake it. This is a totally different thing to being good at the physical act of installing and is a completely different subject to gas safety!

I would agree with you that triple panel rads are not a domestically acceptable solution, there are others, UFH is great but only really financially sensible at (re)build, fan assisted rads are now available in domestic formats and can almost double the output of a rad, skirting heating can sometimes be a solution and can help alleviate those pesky convection drafts.
All of the solutions are more expensive than the mass produced steel panel rad and this is a major part of the barrier to installation of low temp systems, a fight which the government seems to expect the installer community to take up for them!

If you are attempting to size your new system then one strategy you might adopt to achieve realistic financial and physical size whilst ensuring best performance/efficiency is:

Make your heat loss calculations for steady state losses on 'design day conditions' ie the coldest conditions you intend the system to cope with. (typical industry standards are -3 to -5 depending on region)
Determine the radiators that will supply those heat requirements based on the dT50 tables.
Establish which room is most physically constrained size wise and decide what is the largest affordable radiator that will fit in that room eg. 120% larger than the dT50 tables would lead you to choose.
Reselect ALL the rads to be 120% larger than the original selection.
Establish, using the dT correction factors, what average rad temperature will give the 'design day output' and the design boiler flow will be this plus 10C.

Hopefully this will give some useful reduction in boiler flow temperature, the lower the better in terms of boiler efficiency.
Also it will give you a margin for heating startup if you elect to operate the heating discontinuously eg. programming it off while out to work

Some asides to consider - design day temperatures do not need to be the lowest temp that you would ever expect to see weatherwise, the coldest days are but a few a year and on those days it may be more practical to supplement the CH with secondary sources (gas fire, wood stove, electric heater) and/or extra clothing, doing so will give you a CH system that will operate that much more efficiently the other 98% of the year.

Having fitted the system that will do the job for the acceptable worst conditions of the year, it is worth considering that almost the entirety of the year will require significantly less output than a 'design day'
The flow temperature you operate the boiler at is not fixed in stone just because of the figures used at design.
During warmer spring and autumn days the boiler can be operated at lower flow temperatures and still achieve the required heat output from the rads to make good the lower heat losses from the building.
The simplest way is to manually adjust the boiler stat based on the weather forecast or just a seasonal expectation very easy particularly on a combo where you do not have to consider a minimum boiler flow temp to achieve DHW.

On stored hot water systems or where you wish to track this method of operation tightly to the actual weather many manufacturers offer 'Weather Compensating' controls where the flow temperature of the boiler is automatically controlled based on an exterior temperature sensor.
Such controls are not expensive and can save 10-15% of energy.

As regards the TRV sensor - personally I would choose larger rad/lower flow temp over the minimal improvement in TRV sensor accuracy afforded by horizontal mounting, if there is a real problem with an overheating sensor, TRVs with remote capillary connected sensors are available.

Best of luck with your endeavors!

 

[Harry Bloomfield]

Hopefully this will give some useful reduction in boiler flow temperature, the lower the better in terms of boiler efficiency.
Also it will give you a margin for heating startup if you elect to operate the heating discontinuously eg. programming it off while out to work

Which was exactly what I pointed out, in my post #25

 

[JohnW2]

Hi there. Many thanks for your interest and very helpful post (which confirms much of that which I've been coming to believe!)

Most mainstream manufacturers still publish their output tables as dT50 for continuity and because it is a BS standard...

Indeed, so I've discovered!

.... some, particularly those who may be aiming at the renewables market, will also publish lower dT tables for the convenience of the informed customer.

I take your word for that, but I have to say that I don't think I've come across such tables, so they are not particularly 'accessible'

... It is up to the system designer to understand the basis of the specs to which equipment is sold and to apply 'corrective' factors as appropriate. .... The reality of what is actually done - it depends entirely on the level of skills, knowledge and drivers of those involved, there is no effective mechanism for enforcing regulations.
The unfortunate truth is that many of the trades persons who find a career in the domestic heating industry are from the pool of school leavers who are less academically able and who learn best by example from their colleagues, the result being that outdated practises are handed on for extended periods. Additionally if one is not academic, being presented with a whole bunch of calculations is daunting at best. The result is failure to apply best practice more often than not, much of what we have discussed here will be of little interest to many trades and certainly not applied. There is NO policing/enforcement so the status quo endures, So to answer your question bluntly - proper design is routinely not done because the installer is unwilling, unaware, unable to undertake it. This is a totally different thing to being good at the physical act of installing and is a completely different subject to gas safety!

I'm sure that, unfortunately, that is all very probably true, and I suppose that it is a little ironic that I, definitely not a relevant 'trades person' is the one who is at least wanting/trying to 'do things properly'!

However, as has been discussed, what you say may be a little harsh, since even the trades person who is ';willing, aware and able' to undertake 'proper design' (there must be at l;east some ) is likely to end up faced with the inescapable truth that, at least with basic steel panel rads, the radiator sizes which would be needed to achieve their design criteria (e.g. at dT30) will usually be impractical/unrealistic in domestic situations.

I would agree with you that triple panel rads are not a domestically acceptable solution, there are others, UFH is great but only really financially sensible at (re)build, fan assisted rads are now available in domestic formats and can almost double the output of a rad, skirting heating can sometimes be a solution and can help alleviate those pesky convection drafts. All of the solutions are more expensive than the mass produced steel panel rad and this is a major part of the barrier to installation of low temp systems, a fight which the government seems to expect the installer community to take up for them!

Indeed. I did consider fan-assisted radiators, but rather dismissed the idea. As I've said, I'm not at all sure what those with heat pumps can, or can be expected, to do!

If you are attempting to size your new system then one strategy you might adopt to achieve realistic financial and physical size whilst ensuring best performance/efficiency is: .... Make your heat loss calculations for steady state losses on 'design day conditions' ie the coldest conditions you intend the system to cope with. (typical industry standards are -3 to -5 depending on region)

I'd have to look back to check, but I think my calcs were based on 23 or 24 °C across outside walls - so down to -3 or -4 with an indoor temp of 20 °C.

Determine the radiators that will supply those heat requirements based on the dT50 tables.
Establish which room is most physically constr.ained size wise and decide what is the largest affordable radiator that will fit in that room eg. 120% larger than the dT50 tables would lead you to choose.
Reselect ALL the rads to be 120% larger than the original selection.

When you say "120% larger" do you mean that literally (e.g. a 1,000 W dT50 would be 'corrected' to 2,200 W), or do you mean 20% larger (1,000 W corrected to 1,200 W)? My present plan has been to add on about 25% to the dT50-based figures (i.e. a 1,000 W calculated requirement would be taken to be 1,250 W in terms of dT50 figures)

Establish, using the dT correction factors, what average rad temperature will give the 'design day output' and the design boiler flow will be this plus 10C.
Hopefully this will give some useful reduction in boiler flow temperature, the lower the better in terms of boiler efficiency.
Also it will give you a margin for heating startup if you elect to operate the heating discontinuously eg. programming it off while out to work
Some asides to consider - design day temperatures do not need to be the lowest temp that you would ever expect to see weatherwise, the coldest days are but a few a year and on those days it may be more practical to supplement the CH with secondary sources (gas fire, wood stove, electric heater) and/or extra clothing, doing so will give you a CH system that will operate that much more efficiently the other 98% of the year.
Having fitted the system that will do the job for the acceptable worst conditions of the year, it is worth considering that almost the entirety of the year will require significantly less output than a 'design day'
The flow temperature you operate the boiler at is not fixed in stone just because of the figures used at design. During warmer spring and autumn days the boiler can be operated at lower flow temperatures and still achieve the required heat output from the rads to make good the lower heat losses from the building.
The simplest way is to manually adjust the boiler stat based on the weather forecast or just a seasonal expectation very easy particularly on a combo where you do not have to consider a minimum boiler flow temp to achieve DHW.

That really all corresponds with my thinking and 'plans'.

On stored hot water systems or where you wish to track this method of operation tightly to the actual weather many manufacturers offer 'Weather Compensating' controls where the flow temperature of the boiler is automatically controlled based on an exterior temperature sensor.
Such controls are not expensive and can save 10-15% of energy.

Indeed, but II'm not sure that I want to get as 'new fangled as that

As regards the TRV sensor - personally I would choose larger rad/lower flow temp over the minimal improvement in TRV sensor accuracy afforded by horizontal mounting, if there is a real problem with an overheating sensor, TRVs with remote capillary connected sensors are available.

Fair enough.

Best of luck with your endeavors!

Many thanks - as I've said, I am trying to 'do it properly', even though it looks as if I'll be somewhat thawted by practicalities and realities!

Do you think that my estimate of about 14,000 W of (dt50) rad output is credible for the property I have described!"

Kind Regards, John

 

[Mister Banks]

This is a useful resource for converted radiator outputs based on different delta t.

https://www.radiatorsdirect.co.uk/delta-t-conversion#:~:text=If

 

[JohnW2]

This is a useful resource for converted radiator outputs based on different delta t. ...

Thanks - but has been pointed out, one doesn't really need a 'calculator' or tables, since the 'correction factor' dT can be simply calculated as:

(dT1/dT2)^1.3

 

[scw24489]

I'm sure that, unfortunately, that is all very probably true, and I suppose that it is a little ironic that I, definitely not a relevant 'trades person' is the one who is at least wanting/trying to 'do things properly'!

However, as has been discussed, what you say may be a little harsh, since even the trades person who is ';willing, aware and able' to undertake 'proper design' (there must be at l;east some ) is likely to end up faced with the inescapable truth that, at least with basic steel panel rads, the radiator sizes which would be needed to achieve their design criteria (e.g. at dT30) will usually be impractical/unrealistic in domestic situations.

It is often the way that a PROPERLY informed 'DIYer' can spend significantly more time on such matters than is economically feasible for a trade and so achieve a more refined result.
I don't mean to be harsh to my industry colleagues, there are certainly some out there who can and will apply good design practice, and there are organisations like Heatgeeks that are pushing better practical knowledge, the other side of the coin is that under distress purchase conditions when most systems are renewed customers have little time patience or money to address these issues and such installers as could implement proper design are being priced out by less scrupulous trades who are happy to bang any old gear in.

Indeed. I did consider fan-assisted radiators, but rather dismissed the idea. As I've said, I'm not at all sure what those with heat pumps can, or can be expected, to do!

They are expected to spend a few tens of thousands on consequential improvements - most can't or won't and therefore the lack of demand follows which begets a lack of competent installers and the merry go round continues!!

When you say "120% larger" do you mean that literally (e.g. a 1,000 W dT50 would be 'corrected' to 2,200 W), or do you mean 20% larger (1,000 W corrected to 1,200 W)? My present plan has been to add on about 25% to the dT50-based figures (i.e. a 1,000 W calculated requirement would be taken to be 1,250 W in terms of dT50 figures)

The latter, so you are already along the right lines, remember to keep the balance you need to apply the uplift to all rads.

Indeed, but II'm not sure that I want to get as 'new fangled as that

Sure, get the basics right first - such controls can be easily added later should you ever wish to delve that far

Many thanks - as I've said, I am trying to 'do it properly', even though it looks as if I'll be somewhat thawted by practicalities and realities!
Do you think that my estimate of about 14,000 W of (dt50) rad output is credible for the property I have described!"

Don't think of it as thwarted, just doing the best job with the resources available to you!

I'm not sure you have fully described the property, single glazed came up but 14kW on design day is in right ballpark for an older property of 3 or 4 bedrooms but ballpark and right can be a wide gap and oversizing a boiler from say 12kw to 15kw can have far reaching effect on operating efficiency during the shoulder seasons.
Getting those initial heat loss calcs as accurate as possible is time consuming but is the foundation upon which all the rest sits, if you're interested, PM me and I can send you a copy of my basic heat loss spreadsheet that structures the calcs and will give you dT50 rad sizes for different boiler design flow temps.

Kind Regards, Simon

 

[JohnW2]

It is often the way that a PROPERLY informed 'DIYer' can spend significantly more time on such matters than is economically feasible for a trade and so achieve a more refined result. I don't mean to be harsh to my industry colleagues, there are certainly some out there who can and will apply good design practice ....

I'm sure there are a good few such trades people. Much as this is often not considered or acknowledged (the general feeling often being that DIYers are inevitably 'inferior' to professionals/trades people), "time is money" is much more relevant to the professionals than it is to most DIYers - such that (given adequate knowledge and skill) the latter can often afford to spend much more time and effort on a task.

As I've said, I'm not at all sure what those with heat pumps can, or can be expected, to do!

They are expected to spend a few tens of thousands on consequential improvements - most can't or won't and therefore the lack of demand follows which begets a lack of competent installers and the merry go round continues!!

Maybe I'm too pessimistic, but that sounds rather like a way of saying that it just "isn't going to happen" to any appreciable extent, isn't it?

The latter, so you are already along the right lines, remember to keep the balance you need to apply the uplift to all rads.

Thanks. Yep, my plan is to add about 25% to the estimated required heat output for all rooms before determining what rad can provide that (at dt50).

Don't think of it as thwarted, just doing the best job with the resources available to you!

Maybe. However, if, having expended appreciable time and effort on a 'proper design' one finds that the resulting requirements of rads (of whatever type) is totally impractical and/or unaffordable in a domestic setting, then that unfortunately sounds pretty much like 'thwarted'; to me

Put another way, if, instead of 'expending all that time and effort', I had simply guessed what rads to install (or based that decision on window widths etc., or just on floor area) I think I wouldhave ended up with something very similar to what, in practice, I'm probably going to have to do after undertaking all the attempts at 'proper design'!

I'm not sure you have fully described the property, single glazed came up but 14kW on design day is in right ballpark for an older property of 3 or 4 bedrooms but ballpark and right can be a wide gap ...

Sorry, I get a bit confused by the two parallel threads I have running (the other one being here ), and my description of the property seems to be in the other thread, where I wrote:

To remind you all, I am talking about a fairly large late Victorian 3-storey mid-terraced house, with a total floor area of about 110 m²). Other than for the pitched roof, it’s fairly poorly insulated, with solid 9” brick external walls, single-glazed wooden-framed windows and a suspended wooden ground floor. It is a 'narrow but deep' house - some 5m x 17m overall, hence with relatively little in the way of outside walls, and the two adjacent properties are occupied and heated.

Is there anything more that you would like to know? It originally had 5 bedrooms, but no bathrooms, but now has four bedrooms and two bathrooms. The rooms are very tall, varying from nearly 3m to ground floor to about 2.8m on top floor.

Getting those initial heat loss calcs as accurate as possible is time consuming but is the foundation upon which all the rest sits, if you're interested,

That's obviously what I have been trying to do.

PM me and I can send you a copy of my basic heat loss spreadsheet that structures the calcs and will give you dT50 rad sizes for different boiler design flow temps.

I would hope that it's fairly similar to the one I have created, but I would certainly be interested to see yours for comparison. I will PM you.

Kindest Regards, John

 

[JohnW2]

Hi again folks,

Having just about recovered from the festive season (living with multiple small grandchildren!), I'm close to the stage of having to make some final decisions about the sizing of the radiator being discussed in the OP (and all the others in the house) and, in relation to the one (under a window sill) we've been discussing,there seems to be a bit of a 'swings and roundabouts' situation.

I think the consensus was that I should probably go with a 500 mm x 1800 mm T22, and that's probably what I'll do. However, given that there is almost exactly 700 mm available below the sill, another possibility would be a 600mm x 1800 mm one. That would be 20% 'more radiator',, but with only about 50 mm space above and below it (rather than about 100mm top and bottom with a 500 mm one).

Do you think that the winner is the swing or the roundabout?

Kind Regards, John