Radiator sizing for new build house

Not at all. My heating requirements are about 10KW for a much much larger space (about 200 sq meters)

HOWEVER what you haven't said is what the house is made of: the real need is not for steady state power but peak power to get a cold house up to temperature. A lightweight plaster board interior over stud and rockwool will heat up a lot faster than cavity filled blockwork and solid concrete floors.

Finally radiators designed fior a 60C flow temperature may be rather pathetic if you decide one day to put in a heat pump and 40C flow.

In short not many people curse too big radiators - you can turn them down. But many curse too small radiators..that cant be turned up beyond fully open.

It's cubic metres you should be working in, not square metres. My living room is roughly 8m by 3.5m, giving a floor area of 28m2, but it's

I'd guess larger houses are more efficient than smaller houses, because the surface area is a square law whereas volume is cube. When I was getting the energy calculations done, the BCO told me that it was more difficult to meet the rules for smaller houses like mine.

It's got a solid concrete floor with 100mm kooltherm K3 insulation. This has got a low U-value, but a high thermal mass.

Walls are thermalite block: 100mm thermalite "turbo" block with 100mm filled cavity. Again, good U-value but maybe fairly high thermal mass.

The calculator includes a 15% fudge factor to allow for getting the room up to temperature, but perhaps that's not enough in my case.

Yes, good point. I'll err on the side of over sizing rather than undersizing.

Can you get ACH that low? I presume you are using heat recovery fan system, if so take care re outlets & prevailing wind location. The central units are better re higher static pressure, the stick-in-a-wall type are poor if facing the prevailing wind.

Have you costed out wet underfloor heating?

The 70 square metres is a total house floor area, which comes from the house dimensions of 5 x 7 metres and two floors. The room details were an example for just one room in the house. I understood what I meant when I wrote it, but on re-reading I can see that I was far from clear.

The ceiling height is 2.4 metres, and I've put that value into the calculator as well.

And that's a pretty big living room - almost as big as the floor area of the house I'm building!

Yes, I oversized the radiators when designing my system. I can steady-state heat the house at around 0C outside running the system at 45/35, which means the boiler is running very efficiently. (I didn't actually intend it to be quite that good, but I'm certainly not complaining.)

I also have the option to go up to max running temp to heat the house up very quickly (which generates about 2.5 times the heat output).

If you're building to recent Part L standards, ventilation losses may be a significant proportion of the total and are hard to quantify. Even when you have all the windows closed air will permeate through the structure. When pressure testing of new dwellings was introduced c.2005 the results obtained showed a range of something like 8:1 between the leakiest and tightest structures.

Rember Viv Stanshall also said "When Sir Henry broke a fast, you cursed double glazing"

Ventilation is ..useful.

Most intelligent thing you have said to-date harry.

Because everything else you write shows you up as a dimwit and a bigot.

I suspect this is what th Barlo calculator is doing. But I think It'll check the figures for one room manually to make sure. It doesn't look like a difficult calculation, given that I know the dimentions, temperatures and U-values already.

Yes, that's right.

The boiler I've chosen is about the smallest I can get from the Worcester Bosch range. It's quoted CH output is 7.2 to 24KW. I guess that means it can modulate down to 7.2KW. With a load below that, as it looks mine will be at steady-state, I guess it'll just cycle on and off.

I think you can turn down the temperature on modern condensing boilers can't you? So if my rads are oversized, I could run the CH at 50 degrees or something.

Is this a combi boiler?

Have a look at the Vaillant range, their new range modulate quite low. The 831 combination boiler has a range of 5.2 - 24.0 kw, same range for the 24 kw system boiler. The 18kw system boiler has a range of 3.8kw -

Yes, it's a combi.

Thanks. I'll take a look at the valliant boilers.

Cycling isn't necessarily an issue - it depends heavily on the design of the software which operates the boiler.

My 20+ year old Potterton Profile cycles on and off with pretty much no energy wastage (and it's designed to - it can't modulate).

Conversely, my 10+ year old Keston condensing boiler is really dumb about cycling on and off, and probably wastes significant energy in doing so when demand is less than 7kW lowest modulation level. (It spends 2 minutes cooling the heat exchanger, regardless if it needs it or not, and in my case it mostly doesn't, so it's just wasting the heat in the circulating water, blowing it out the flue.) Fortunately, it has a large hysteresis, so it doesn't cycle too often.