Radiator sizing for new build house

[Caecilius]

I'm building a small two-bedroomed end-terrace house, approx 7 x 5M in size. Latest building regs, including Part L, so insulation is pretty good.

I'm going to use a Worcester 25 Si, which is more than sufficient for this house.I want to use the smallest radiators I can, but obviously big enough to do the job.

I've used a couple of different calculators, and I'm getting very different results. The "advanced" calculator on

is giving me a power requirement of more than twice what the Barlo heatloss calculator does.

I'm tending to believe the Barlo heatloss calculator, because although it's very old, the laws of physics don't change. And the Barlo calculator allows me to enter the U-Values, whereas I don't know what assumptions the radiatorsizingcalculator site uses.

For example, with one 2.48m x 2.37m room, the Barlo calculator says I need 308 watts, which seems incredibly low. The radiatorsizingcalculator site reckons I need 686 w.

My U-values are:

External wall: 0.16 (2 x 100mm thermalite + 100mm filled cavity) Windows: 1.5 (Energy Class B uPVC double glazing) Floor: 0.16 (solid floor with 100mm kooltherm K3)

1st floor ceiling: 0.13

I'm assuming:

Room temp 21 degrees Outside air temp -1 degrees Ground temp 10 degrees (guess)

1.5 air changes per hour (guess)

Can anyone see anything obviously wrong with my assumptions, especially the number of air changes and the ground temp?

It looks like the total heating requirement will be under 3KW, which seems crazy low for 70 square metres of living space.

[The Natural Philosopher]

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.

[Phil L]

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

2.4m high so my living room is just over 67 cubes. The room you described in the example above was '2.48m x 2.37m', this gives a floor area of 5.87m, how do you arrive at 70 square metres?....assuming the ceiling isn't 11.9m high

[Caecilius]

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.

[js.b1]

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?

2012 insulation is almost enough to need no heat if South facing, but I am used to living in a barn without even a duvet :-)

[Caecilius]

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!

[Andrew Gabriel]

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).

[Tony Bryer]

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.

[The Natural Philosopher]

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

Ventilation is ..useful.

[Steve Firth]

[NOTHING]

Most intelligent thing you have said to-date harry.

[Steve Firth]

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

[Caecilius]

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.

[gremlin_95]

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 -

18.0kw whilst the 15kw model goes down to 3kw. A wider modulation range will stop the boiler cycling so much when the demand isn't very high.

[Caecilius]

Yes, it's a combi.

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

[Andrew Gabriel]

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.