Heatloss of small terrace house 6.5kW ?

[John Stumbles]

I've just done a heatloss calculation for a small terraced house (3.8m wide x 7.4m deep x 2.4m each storey with 2.5m x 4m back addition, mix of carpeted wooden and solid floors, mostly single glazed, 50mm insulation in the roof) and get just over 6.5kW, which seems very low. I've done the calcs on a spreadsheet but cross-checked with the Myson calculator for the floor and air changes of one room, to sanity check.

Does this seem right? I could put it all through the Myson to completely cross-check it but it's a lot of extra work ...

tia

-- John Stumbles

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[Andy Hall]

This seems low to me as well.

Presumably this is an older property with solid walls..??

I would expect the extension to have a fairly large heat loss because of the external wall area.

How have you treated the party walls?

.

.andy

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[Ed Sirett]

Have you allowed for the air changes?

I'd go high on those to allow for the sash windows.

Anyone know of a small output condensing model?

[John Stumbles]

I've assumed 220mm brick + 13mm plaster, U=2.0 (from BS5449 - I see Myson use a figure of 1.8 for this construction).

(The extension shares a party wall with next door.) I make its loss though external walls 712W. (Actually that includes a new 2m extension to the ground floor at the back, which has insulated cavity walls.)

I've assumed next door temperatures of 15 downstairs, 10 upstairs (more pessimistic than the usual 18 up and down, I know).

So for example my big main room (through lounge) has a party wall area of 2 (party walls) * 7.4m (width) * 2.4m (height) = 35.52m^2, and therefore a heat loss of 35.52 * 2.0 (U value) * 6 (21C - 15C temp diff) = 426 Watts

-- John Stumbles

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[John Stumbles]

Yup, 1.5 for the main room, 2 for kit & bath, 1 for bedrooms.

Fortunately (from an energy conservation rather than architectural conservation standpoint :-) they're not sashes. However the front door opens into the main room so I'll probably go for 2 changes there, though that still only gives me a total of a bit under 7kW.

I had one of those Isars in the back of my van the other day... :-)

-- John Stumbles

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[Andy Hall]

OK, so that takes out a fair chunk of outside wall area down to a much lower heat loss.

Seems reasonably conservative.

In the very first house that we had where I installed heating, it was also a terrace, although quite a bit larger than this one - probably about 1.5m wider and quite a bit deeper, with an original 2 storey extension. There was also a fourth bedroom in the loft - basically in the apex area of the main part of the house.

The design was done when it was customary to allow 3kW for the HW cylinder. I can remember that the total heat load, worst case, with

10% allowance came to 20kW and I used a legendary Potterton Netaheat 16-22 wound up to full power. This would have meant that the house peak requirement was probably around 14kW. I did design assuming no heating in neighbouring properties though.

.andy

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[Andy Hall]

Also don't forget the heat loss of the door. This is going to have something of an effect since it is opening directly into the living room, especially if it has glass panels.

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[John Stumbles]

My first house was similar to the one you describe and I remember when I did the calcs it wanted a bit more than a 55K BTUs back-boiler would have given.

However the house in question is heated by night storage and it's generally as warm as toast (during the day, when I've been working there) in the big main room which has only one storage heater, which can't be chucking out more than a couple of kW. From that point of view the figures seem reasonable ... but I'm still not happy with it. :-(

-- John Stumbles

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[bill]

It seems pretty reasonable to me. Most of your wall area is shared with the neighbours (assuming it is mid terrace) and if you have good loft insulation the heat loss would be quite small.

Looking at the heat loss calculations that I did for my house before I renovated it I got values ranging from 15Kw if I installed no insulation to 8kw if I installed as much insulation as was feasible. These included 3.6Kw for air changes. This is for a detached house about 10M square on 3 floors so it should need a lot more heat than a terraced house.

Experience has shown that the calculations were pretty accurate.

Bill T

[Will Dean]

Unless I'm completely wrong about calculating vent losses, then your air change figure sounds extremely low. Even at one change an hour (very low as an average for the house), a three storey 10x10 house is going to need something like 10*10*7 [height, guess] * 1 * .33 * 24 [tempDrop] = 5.5kW.

How did you do the air-change loss?

Will

[Andy Hall]

Mmmm. So in the same range.

Don't forget that there's quite a big difference between how the place may feel when you are there working and moving about during the day and what it could be like sitting still in the evening.

We've also had about a 10-12 degree range over the last couple of weeks and the design is intended for -3. So 2kW or so at a daytime temperature of +8 may be OK.

Realistically, it isn't the boiler that is going to be the problem. You could go for an 11-14kW modulating one for example.

The radiator sizings would be more important. For the living room, having one at each end would be sensible, probably.

.andy

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[John Stumbles]

This is 2 storey, somewhet smaller than your example: I've got roughly 30m^s per floor in the main bit, 10m^2 in the back addition so total 40m^s plan area. Each floor is 2.4m high (call it 5m total), so total house volume approx 40 * 5 = 200 m^3.

With 2 air changes/hour and 20C difference that'd be 200 * 2 * 20 * .33 =

2.6 kW, which is near enough what I've got on my spreadsheet (actually I'v been using a factor of .36 which I got from a post of Andy's sometime back).

-- John Stumbles

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[John Stumbles]

Thanks, that's encouraging

-- John Stumbles

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[John Stumbles]

True ...

Even so it was very warm inside when it was brass monkeys outside

It's not that I'm worried about - I've already got a 23kW combi installed (Hepworth Icon/Ikon)

Natch, in fact I'll probably put 3 since they want one in the front bay where there's not much room and even a double would only give about 600W, so another on the side wall by the front door and a 3rd at the back, to give c.

4-5Kw for the whole room. This room's heatloss is almost 3K (just under half the total for the whole house).

I'm thinking I'll go for oversizing the rads as much as possible to give a decent load to the boiler to reduce cycling (it claims to be 'fully modulating' on CH but I don't have the manual to hand ATM so I don't know what it goes down to) and to give good warm-up.

-- John Stumbles

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[The Natural Philosopher]

It seems a tad on the high side to me.

I have 200 sq meters here (100 meters sq per floor) and that was calculated at a shade over 9KW. But insulation is to specs.

you have 70 sq meters of floor roughly, so you are calculating 100W per square meter more or less. That's about double what I need. Which seems reasonable for solid or unisulated walls and floors. And single glazing.

Anyway, even a small boiler will do 10Kw in all probability, so just bunf one in and let the thermostats take care of the rest.

[bill]

Its over 12 years since I did the calcs, and I only have the results now. I think that I would have used an average of one air change an hour, but I used -1 outside 21 inside so a 22 temperature drop, and the house is actually a bit smaller, than it seems. The bottom floor is smaller than the upper two (built onto a bank), and it has very thick walls - the 10M is the outside dimension.

Bill

[Christian McArdle]

Victorian terraces are actually quite good on energy conservation, despite the lack of insulation and cavities and old leaky windows. This is because the "external" walls are primarily party walls, with only thin slivers of genuine cold outside walls at the front and back. The worst parts are the rear extensions.

The survey SAP rating on mine when I moved was good, despite no insulation (other than the roof) and single glazed sashes at the back. It is a 3 bedroom Edwardian (Victorian style) mid terrace.

I calculated my losses to be around 8.4kW, but this is probably higher than most would calculate, as I calculated the lounge and dining room at 22C and bedrooms at 20C. This is for an outside temperature of -3C.

Christian.

P.S. Have you finished that bathroom yet, John? The old Glowworm Fuelsaver has started making a funny noise.

[Andy Hall]

They do.... you're living on borrowed time ;-)

.andy

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[Ed Sirett]

Let me know how it goes, also if you fit any condensing combi that you feel happy with since I want to have a model of choice for after next spring.

I guess the heat loss from this small terraced house is quite small. A 1kW fan heater in each room would probably make it quite warm.

However it would be good to fit a boiler capable of fully using the indirect coil of the 'Part L' cylinder - so that would be c. 12kW nominal.

Also some spare capacity to deal with both neighbours on holiday in winter (say next door at 12C both sides). This should bring things up to around

10kW maybe.

[John Stumbles]

Wasn't too impressed with it tbh: it looks as if someone's shoved all the controls into the box and slammed the cover shut on it :-). In commissioning it you have to swing down the electronic controls housing so you can rotate the diverter valve enough to get the head off it to actuate it for bleeding the system. The ignition cable goes across the controls housing so you have to unplug the cable to swing the housing down, which means you can't operate the boiler in this state. To get at the gas valve you have to swing down the controls. I notice there's no mention in the manual of measuring the burner pressure, because if you touch the burner pressure test point how are you going to leak-test it afterwards? (You could probably just about manage to swing the controls out of the way enough to see the valve with the boiler running, but it'd be a bit iffy.) Shame really, in other respects it seems a nice enough bit of kit, and it's the only one at a reasonable price.

Another downer is the weight: 45kG. Definitely not a 1-man install unless you're governor of California :-)

-- John Stumbles

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