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.