Radiator under window sill

[Harry Bloomfield]

If you look at Eric's diagram of heat circulation it exactly represents what happens in our lounge, the radiator is behind our seating position, although we do not have a sofa tight up to the radiator. The heat goes up from the radiator, across the ceiling, down the far wall and then cool air at low level flows back towards the radiator. That cool air hits our legs when sitting on the sofa and feels like a cold draft even though there is no draft. When the radiator goes off because the room is up to temperature the cold draft ceases.

You can combat that issue, to a large extent, by using a slow turning ceiling fan, to push the heat down.

 

[JohnW2]

You can combat that issue, to a large extent, by using a slow turning ceiling fan, to push the heat down.

Indeed ...

... If one does experience the issue you describe, and perceives it as a problem, I imagine that it could probably be addressed with a ceiling fan or suchlike?

 

[Agile]

I've been doing a bit of experimenting with fan heaters. Admittedly, the current ambient temp is not that low (about 11-12 °C) but a 2kW fan hear is a lot more than enough to heat the room - and that's with none of the adjacent rooms heated.

Kind Regards, John

You should measure the room temp and then run the fan heater for an hour and measure the new temperature.

With -1 C outside you need to be able to raise the temperature by 22 C to give you 21 C inside.

But generally fit the largest rad that you can get to fit in the space so presumably a 500 high and double panel.

An 1800 x 500 double panel T21 will give about 2.5 kW but I would recommend a T22 double panel with double fins as that is about 3.2 kW and give a bit of extra output for very cold weather and increase the boiler efficiency a little. You only fit the radiator once.

I find the Kudox brand good for looks and reasonable prices and generally well available including from Toolstation.

 

[JohnW2]

You should measure the room temp and then run the fan heater for an hour and measure the new temperature.

Thanks. That's exactly what I am currently doing.

With -1 C outside you need to be able to raise the temperature by 22 C to give you 21 C inside.

Indeed. As I observed, my current fan heater experiments are being undertaken in the context of an outside temp of 11-12 °C, so it could be far worse.

However, extrapolation from the empirical observations is far from straightforward. With 11 °C outside and 21 °C inside,the heating has has tyo raise the temp by 10 degrees. If it were, say +1 °C outside, it would have to raise the temp by double that (i.e. 20 °C). However, the amount of power required to do that will not be double. Certainly in the house in question, no room has more than one outside wall, and that one outside wall is often quite small, with all other walls (and also floors and ceilings in some cases) having fairly modest temperature differences (if any) across them. Dramatic increase in temp difference across that one small wall therefore dose not necessarily have a massive effect on the overall heat loss from the room.

But generally fit the largest rad that you can get to fit in the space so presumably a 500 high and double panel.

Indeed.

An 1800 x 500 double panel T21 will give about 2.5 kW but I would recommend a T22 double panel with double fins as that is about 3.2 kW and give a bit of extra output for very cold weather and increase the boiler efficiency a little. You only fit the radiator once.

Again agreed. I would generally always use T22s, unless there were some good reason not to. The slightly deeper profile does not usually worry me.

I find the Kudox brand good for looks and reasonable prices and generally well available including from Toolstation.

Yes, I've used them in the past. Mind yiou, I'm not sure that there is much difference between any of the makes, in price or anything else. It's interesting that, I presume because of 'larger markets', some sizes of radiators are usually appreciably cheaper than one would expect by looking at other sizes - sometimes to the extent that such an (PPrently) 'preferred size' is appreciably cheaper than smaller ones!

Kind Regards, John

 

[Agile]

I do not think it is in any way due to manufacturing costs.

I have always thought that a size like 900 x 600 is very commonly used and therefore if they make that bit cheaper they might get all 10 rads bought from them just because one or two common sizes are better value.

 

[JohnW2]

I have always thought that a size like 900 x 600 is very commonly used and therefore if they make that bit cheaper they might get all 10 rads bought from them just because one or two common sizes are better value.

Yes, you could be right. In fact, from what I've seen it's the 1000 x 600 ones that most commonly seem unexpectedly cheap- often appreciably cheaper than 900 x 600 ones.

 

[Agile]

Of course the 900 x 600 are virtually 3 feet x 2 feet and probably a "metric replacement exact size" to avoid repiping and to preserve a similar heat output.

 

[JohnW2]

Of course the 900 x 600 are virtually 3 feet x 2 feet and probably a "metric replacement exact size" to avoid repiping and to preserve a similar heat output.

That would make some sense - but looking at, say, Wickes, at £105, a 900 x 600 is much more expensive than a 1000 x 600 (£75)

 

[JohnW2]

..... I've been doing a bit of experimenting with fan heaters. Admittedly, the current ambient temp is not that low (about 11-12 °C) but a 2kW fan hear is a lot more than enough to heat the room - and that's with none of the adjacent rooms heated.

A more realistic/relevant experiment today, given the much lower outdoor temp. Although it took quite a while to 'warm up' a 2 kW fan heater has been keeping the temp of the room at about 22 °C all day today, with the outside temp hovering around 0 °C - and that's without any heating at all on the other side of the two internal walls (there's additionally one external wall and one party wall, the adjacent property being occupied and heated).

On that basis, I would imagine that I don't need a rad much more than 2,000 W - such that a 500 x 1800 mm T22, or even T21, would probably be adequate?

Kind Regards, John

 

[Harry Bloomfield]

On that basis, I would imagine that I don't need a rad much more than 2,000 W - such that a 500 x 1800 mm T22, or even T21, would probably be adequate?

The more over-sized the radiator, the cooler it can be run. The cooler it is run, the cooler the return to the boiler temperature. The cooler the return temperature, the more able the boiler to condense, and the more economically it can be run..

 

[JohnW2]

The more over-sized the radiator, the cooler it can be run. The cooler it is run, the cooler the return to the boiler temperature. The cooler the return temperature, the more able the boiler to condense, and the more economically it can be run..

I need to think about that one, after which I'll probably have something to say! When you say "run the radiator cooler", are you referring vto the temp of the input ('flow') water?

As I said before (in this thread or a different one), unless plans change dramatically and unexpectedly, I will never be living in this house nor paying the energy bills - so that obviously has some influence of my decision-making, particularly if there is 'additional cost' on the other side of the equation

However, returning to the room I've been talking about a 500 x 1800 T22 is the largest single radiator that would fit - so if I decided that, for whatever reason, I needed more total heat output, I would be into the realm of 'multiple radiators', which would certainly have an impact on costs. Another option would be a single 500 x 1800 T33, but I think that would be pretty expensive.

 

[Harry Bloomfield]

I need to think about that one, after which I'll probably have something to say! When you say "run the radiator cooler", are you referring vto the temp of the input ('flow') water?

No, a TRV will restrict the flow, when the room is up to temperature, so the water contained in the radiator, will be cooler. The cooler it is in the radiator, the cooler the outflow, the cooler at the return to the boiler.

 

[JohnW2]

No, a TRV will restrict the flow, when the room is up to temperature, so the water contained in the radiator, will be cooler. The cooler it is in the radiator, the cooler the outflow, the cooler at the return to the boiler.

In that case, it would seem that you're talking about something rather different from what is usually said in relation to 'over-sizing' radiators - since the whole idea of what is usually said is to allow the flow temp to be reduced (from a historical delta T 50 down to delta T 30, or even 20), which obviously also reduces the return temp, improving/facilitating condensation.

However, with that approach, although it might be 'over-sizing' in terms of physical dimensions, it may not be in terms of heat output, since that obviously falls when one reduces flow temp. In other words, a physically larger radiator is used to facilitate a lower flow temp whilst giving as much heat output as a smaller one would with a higher flow temp.

One issue with that approach is presumably that, if flow temp is going to be reduced, the 'over-sizing' has to happen with all rads in the system - since, if not, those that have not been 'over-sized' will probably not produce enough heat (with the reduced flow temp).

... but that begs a question, to which I don't know the answer (but am about to try to find!). I am used to naively just 'accepting' the 'power outputs quoted for radiators of a particular size, without actually having any idea what flow temp (i,e. which 'delta T') those figures actually relate to.

 

[JohnW2]

... but that begs a question, to which I don't know the answer (but am about to try to find!). I am used to naively just 'accepting' the 'power outputs quoted for radiators of a particular size, without actually having any idea what flow temp (i,e. which 'delta T') those figures actually relate to.

...easier than I thought - I simply had never looked before!

The 'Homeline by Stelrad' from Wickes, the Kudox from Toolstation and the Flomasta from Screwfix are all quoting heat outputs ('Wattage') on the basis of Delta T50° - in other words, are assuming a flow temp of approximately 70 °C. If one reduced that flow temp to, say, 50 °C (delta T about 30°), the heat output would obviously decrease appreciably (I imagine probably roughly pro-rata to reduction in delta T)

When people here and elsewhere talked about over-sizing/'overkill'of radiator sizing, I think I was assuming thatr they were thinking of using a rad which, with the applicable flow temp, was capable of producing more heat than one actrually 'needed' (as determined by heat loss calcs etc.). However, if they are going to use the larger physical size of radiator to facilitate a reduction in flow temp, then they presumably could well 'be back to square one' in terms of heat output (at the reduced flow temp)?

 

[JohnW2]

Needless to say, now I'm a bit confused ...

It had always been my intention to go for radiators at least a reasonably bit larger than was suggested as being 'required' by some sort of 'consensus' of my own heat-loss calculations and various on-line 'calculators' - but it hadn't really occurred to me that, because of the nature of the 'wattages' being quoted by manufacturers and suppliers, I was thereby assuming a flow temp of around 70 °C.

I still intend to do that (as regards rad selection), but now have the 'dilemma' of deciding whether or not to appreciably reduce the flow temp. However, I suppose that's not really a problem since, once the rads have been selected and installed, all options regarding flow temp will remain.

Any thoughts/suggestions would be much appreciated!

(I'm still somewhat struggling with the actual 'size estimation' issue, per se, so will probably soon be resurrecting my recent thread about that!)

Kind Regards, John