winter temperatures: radiator sizing

[Stephen]

Hello,

I downloaded the Barlo heat loss program as recommended here. As part of its calculations, it asks for the outside temperature. When I was using it, I never imagined it could get this cold. I think I used -2C as the worst case temperature. The car said -13C this morning! Luckily, the radiators are coping so far. What worse case temperatures did you use and what will you use next time?

Thanks, Stephen.

[Tim Watts]

I can keep the ground floor of a dormer bungalow OK (19-20C) with about

7kW peak and about 4-5kW sustained average from 4 oil filled rads. That house has most of the insulation missing pending roofing work at -3C day-night average - but I do have to keep that 4-5kW on day and night.

I'd expect to halve that easily once the roof is sorted - noting how warm my bay window ceilings are with only 50mm celotex between the rafters.

Also the roofing work will remove most of the draughts blowing over the ceilings elsewhere.

So I would expect average radiator kW sizing to be smaller than one might expect in a well insulated and draught free house - but of course you need to oversize otherwise it can take a *long* time to recover a warm house from a cold one.

[TheOldFellow]

I used -10C, then thought "that's ridiculously low", however looking at the full model of the current and planned losses before/after renovations I decided to stick with -10C on both rads and boiler. Capital costs were not that much more.

As it happened we got this bit of Global Warming just after I disconnected (drastically) two key rads for some major alterations, then got sick. Great timing, but I'm mighty glad I went for overkill on the rad sizing as leaving a few doors open has compensated.

I expect I pay for some loss of efficiency because the condensing oil boiler will probably cycle more in a normal winter. But, hey, I'm warm.

R.

[The Natural Philosopher]

I used -5c but really should have used -10C. Its rare for outside to AVERAGE worse than that. That was whole house though. For individual rooms you can go to -20C if you like.

[Andrew Gabriel]

Not familiar with that program, but this isn't normally the worse case, but is the worse case with the boiler operating in condensing mode. You have plenty more power output by increasing the boiler flow temperature, albeit the boiler won't be as efficient, but this is supposed to be rare.

I used a beta java version of the Myson heat loss calculator nearly

10 years ago. I used -3C, as is normal for a condensing system (which is what it is, even though that was not required at the time). The program significantly over-estimated the heat losses (I didn't investigate why, but Andy Hall used it too, and said the U values they had for various building materials were a bit pessimistic). This means I significantly over-sized the radiators. (Maybe Myson thought that would be to their advantage, but most if not all of the radiators I bought in the end were not Myson;-)

In retrospect, I'm really pleased about this. I can heat the house at 0C outside with the boiler running at 45/40 (flow/return), which means the condensing boiler is running extraordinarily efficiently. I can boost the heat output by nearly 3 times by cranking boiler up to maximum, which means I can heat the house up from stone cold in about 15 minutes if I need to. (Actually, boiler can't get above about 75C flow, because at this temperature the radiators dissipate all 23kW of the boiler output.)

When running at 45/40, the boiler output is above minimum burner modulation of 7kW, because it can do this without cycling on and off, but probably not by much - let's guess about 8kW. So 8kW is enough to maintain a 20C difference between inside and outside. This means the heating system should be able to maintain the house at 20C at 23kW with outside temperature down to -37C. It might do even better, as I added another 4" of loft and bay roof insulation since I took these measurements. This is a 1900 semi with 9" brick walls, so a more modern house should be able to do better still.