Radiator advice sought

[Matt Beard]

I am currently planning my new central heating system and have a dilemma that I thought the group could help with?

The living room in this house has an internal bay for the window. To explain, the walls are about 16"-24" granite with dry lining on a free-standing framework. At the living room window the dry lining "steps in" by about 10" with slopes of 45-degrees each side. Below the window the wall is 10" thinner than the rest of the wall. I assume that the stone wall is thinner under the window, and the gap between the dry-lining and the stone is probably less (there seems to be a generous gap elsewhere).

In the dining room the same "bay" has been removed by adding a larger window sill and a new stud-wall below making the lower wall the same thickness as the wall either side.

I don't really want to do the same in the living room, but I don't want a huge draft caused by air cooled at the single-glazed sash window.

There are a number of alternatives as I see it.

a) Get really thick thermal curtains that go all the way to the floor. b) Fit the radiator under the window and have thin curtains that let the heat flow back into the room (seems silly to me!) c) Fit the radiator next to the window and put up with what drafts are caused d) Fit the main radiator next to the window and a small radiator under the window to kill any drafts.

Is d) a silly idea? If not how should it be connected, in parallel (with its own TRV?) or in series (treating both as a single radiator).

Or am I just making work for myself chasing 2 or 3% and I should just fit the system and relax in my nice warm centrally heated room?

[BigWallop]

How much wall space do you have to either side of the bay window ? In some cases it is possible to place two smaller radiators in the room, one at either side of the window, that would make up the same size of one larger radiator under the window. The two radiator along with thick curtains that fit to the floor should keep the room snug and warm at nights when you need it most, and any escape of colder air passed the edges of the curtains is heated by the radiators at either side.

Another alternative is to fit a shelf over the radiators to extend the internal window sills and have curtains that fit tightly between the ceiling and the shelf. The shelf helps to push the heat from the radiator back into the room and the curtains stop any colder air dropping from the windows.

[Andrew Gabriel]

This is what I did, and it works fine. Actually, my two radiators are triple panel in order to get the power output in the width available. In the positions where they are the extra depth is not an issue or particularly noticable.

[Mathew J. Newton]

My lounge is similar in that it also has a bay window that had the potential to cause a cold spot in the room. My solution was similar to yours proposed in (d) in that I fitted a small radiator inside the bay (underneath the window) to combat the cold spot and reduce the temperature gradient, and a 'full size' radiator on the wall at the opposite end of the room as the 'main room radiator'. You can see the former at:

in principle it may seem foolish to put a radiator under a window - as drawing curtains (full length ones certainly) across it would put the radiator on the 'wrong' side of the curtain - as you can see from the above photo this scenario doesn't apply in my case. I fitted recessed blinds on the windows - the full-length curtains are simply to soften the edges. (Don't tell the lads I've made a haberdashery-related posting, okay?) They're never actually drawn in practice.

This dual-radiator combination seems to work well however I've got double-glazing so the cold-spot is less of an issue (and no draughts)

- however before this radiator was put in it was certainly noticeable.

Mat

[Matt Beard]

I'd rather not put a radiator to the right of the window as the door into the room is tight into this corner. This would slightly restrict the space on entering the room and the radiator would be prone to knocks - especially a double panel model.

Actually I may have size problems anyway... my current calculations require about 3000W (about 10kBTU) for the room and either side of the window is only about 8000mm wide! With the position of the door I also have an absolute maximum of 100mm thickness for that side.

Thunks hard...

Matt

[Christian McArdle]

Would vertical radiators solve this? You can get modern looking ones easily, but traditional (i.e. column ones) cost an absolute fortune. For a Myson V20 series, 800mm (I assume you didn't really mean 8m!), gives you a 10 columns. At delta 50C, this gives an output of 2460W. This is for 2m height. To get

3kW, you'd need 2.5m height. Alternatively, your little radiator under the window would make up the difference.

Another alternative is a V10 each side, which at 800mm x 2m will give 3300W. The V10 is 75mm from the wall, which fits behind the door.

Christian.

[Matt Beard]

8m !!! d'oooooooh

Yes, 800mm.

I don't like the look of the vertical radiators - they would somewhat dominate the room too - especially one either side of the window. I had already thought about them and ruled them out.

I think 3 radiators in one room is getting a bit silly!!

My next best bet is to try:

To left of window: Myson 27DC29 (690x740x133) @ 1437W

To right of window: Myson 27DPX29 (690x740x108) @ 1138W

Total = 2575W

But, could I run my system at a delta-T of 60 degrees and get 3219W - I guess this would mean setting the water temperature to about 87 degrees with about 10 degrees drop through the rads.

Note: Myson seem to give the highest outputs in the size range I need, but maybe there is something better...

Matt

[Andy Hall]

Not necessarily if it works for the decor. Don't forget as well that having more will distribute the heat around the room better.

I think that that is pushing your luck a bit. Most boilers will not let you run at much over 82 degrees - you may have problems with the over temperature cut out.

Are you designing for a worst case -3 degrees?

One thing that you could do is to compromise on a higher outside temperature for the design - e.g. +3 degrees to pick a number.

Size the radiators for that and then add in a fan convector which is only used in the coldest weather to boost the heating. These give a relatively high heat output for a given volume but you have the fan. If you oversized on one of these, you could run it on the low setting and it would be reasonably quiet.

Another idea.

Do you have a fireplace where an inset gas fire or something like that could be fitted to add in 1-2kW for when you need it?

There are probably a number of other possibilities along the supplementary heat route. It seems a shame to compromise your room aesthetics to cover a situation that only happens for a smallish number of days typically each year.

.

.andy

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[Matt Beard]

The problem is that I live in northern Scotland and even -3 is a bit optimistic for the winter. I have done the calculations for -3 and -5 and both require over 3kW.

We do have an open fireplace and this can be lit in the winter to give extra heat. However when it isn't lit it will be another source of heat loss - anyone any ideas how much a small open fireplace loses?

[Andy Hall]

Ah.. OK. It was really a thought to give you some things to play around with rather than necessarily needing to cover all losses with radiators.

The losses are almost entirely through air change up the chimney.

If you can find a way to close it off when not in use, that will help.

Another point to remember if you do go for supplementary heating is not to put the room thermostat for the house in the living room, otherwise it will be satisfied too early by the fire and the rest of the house will cool as the CH switches off.

.andy

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[Set Square]

Some of the heatloss programs which you can download from radiator manufacturers' web sites can calculate this for you. The Barlo program

certainly has this feature.

[Matt Beard]

1300W loss through the fireplace??? Is the program serious???

That means I need 4500W for this room... Pretty soon I'm going to need to have radiators on all available wallspace.

Matt

[Andy Hall]

Is this air change loss? (Should be I think). In that case, look at ways to stop it when the fire is not in use.

.andy

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[Matt Beard]

I'm not sure how they arrive at that figure - I tried the Barlo heat calculator to see what it said about fireplaces (I have been using Myson) and it has an option called "Gasflow" which it says to check if their is an open fire. When I do so the heat-loss for my room jumps by

1300W.

It is not really practical to block the fireplace or chimney as the fire is likely to be used often - especially on cold days when the heat loss would otherwise be a problem. Yes, there is no problem with loss while the fire is lit, but the fire would probably be lit early evening so the room will suffer extra heatloss until then.

Matt

[Andy Hall]

It says in the manual that this is to account for an open fire not in operation and the implicit heat loss through air changes from it using approved figures. I believe that the source of these is from the Building Research Establishment, but not certain.

OK, then it seems that you will have to trade off the aesthetics of not having too much radiator area for having a shortfall on heating for some of the coldest days of the year.

Didn't you say that you had thick stone walls? To some extent, that will act to even out the temperature variation because of the large thermal mass.

The only other thing I can think of is to insulate the exterior walls (or some of them) with say 25mm of Celotex. This would cut the heat loss, but there may be aesthetic issues like plaster moulding coving which would stop you, as well as losing, say 50mm of depth to the room.

.andy

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[Matt Beard]

You are ahead of me again... I have thought about adding Celotex to the walls, but maybe not yet. The walls are about 16"-24" granite with a dry-lining that seems not to be in contact with the stonework (I presume to prevent damp transfer. I had been thinking of stripping the plaster off then fitting celotex and new plasterboard. There are decorative covings but I could stop a few inches below these and still get the celotex up behind. Any idea how to acurately adjust my U values?

Matt

[Andy Hall]

OK.

I'm wondering where you got U values for granite walls - I've never found tables with it in. What value are you using, and what are the heat losses that you have calculated for the outside walls, the windows, floor, ceiling and air changes?

Because of the thermal capacity, I think that you are going to get different effects anyway - because of the dynamic behaviour - i.e. it takes a long time for them to heat and cool.

For the Celotex,you can get the R values from their web site for different thicknesses. To a first approximation, you can calculate the U value for the final complete wall by taking the existing U value, take the reciprocal of it (gives R value), add that to the R value of the Celotex and finally the reciprocal of that summation to get the final U value.

If the U value of the wall is very much higher than that of the Celotex, the ultimate U value will effectively be close to that of the Celotex anyway.

For a reference on the summation of U values, there is some info on how to do the sums in the Approved Document to Part L1 of the Building Regulations which you can find on

.andy

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[Matt Beard]

Interestingly the Myson heatloss calculator has a U value for 18" thick stone walls. This is 2.5 Wm^-2K but I'm not sure if this assumes plaster straight onto the stone. If this is the case I should add something for the dry-lining. If I simply assume this is half a basic stud wall then I end up with about 1.6 Wm^-2K which is pretty good, but there is quite a gale blows around behind the dry-lining so I think that is optimistic.

I recon I could get about 0.4 with 45mm Celotex... Hmm, toasty!!

Matt