Extension doesn’t stay warm

tacpot12

Yes, 100mm of Celotex is pretty good, but as has been mentioned any penetrations through the Celotex will leak heat. 
Your specification for the extension is very helpful It confirms the Type of the Velux Window (GGL) and the Size (S06), but not the actual model number. You can use the Velux window replacement configurator to help identify the model:
VELUX roof window replacement - 4 steps to find your ideal VELUX solution
Velux seem to be more interested in selling a new complete window rather than replacement glazing, but it might we worth a call to them. However, the spec and the windows seem to be relatively new and reasonable for a UK "building regs" building. 

The spec also confirms the construction of the floor : 150mm of hardcore (ok thermal mass), 100mm concrete (great thermal mass), 70mm of insulation (no thermal mass AND it negates ALL the thermal mass underneath it) and 75mm of screed (great thermal mass). So you have 75mm of screed being all the thermal mass of the floor. I had a small terraced house in 1995 where the original floors were quarry tiles laid on bare earth, I had 100mm of EPS laid underneath 150mm of concrete, so I had 1.5 times the insulation and twice the thermal mass. The house had a lot less glass, but temperatures did remain relatively stable. 

It's worse with the walls. The Celcon Solar blocks have lowish thermal mass. The spec doesn't mention plasterboard on the wall; plasterboard has a good thermal mass but at only 12.5mm thick it can't add a lot to the room. The Rockwool stops anything else forming thermal mass.

The spec from the builder is good. I would say they were a better than average builder, but they only did what the building reqs required and didn't think about the thermal mass aspect of the construction. 

JoeLowe

Based on your comments regarding the floor and walls, is there any easy fix yo improve the thermal mass?

grumbler

IMHO, it makes no sense to improve/increase the thermal mass. It's like creating a huge heat storage that needs time to heat up and then slowly cools down. Just keep the heating on and you'll get the same temperature without any extra energy needed. You'll even use less energy because you can let the temperature drop at night and then get it back to normal fast in the morning.

It's good insulation that matters, not some thermal mass.

JoeLowe

grumbler said:

IMHO, it makes no sense to improve/increase the thermal mass. It's like creating a huge heat storage that needs time to heat up and then slowly cools down. Just keep the heating on and you'll get the same temperature without any extra energy needed. You'll even use less energy because you can let the temperature drop at night and then get it back to normal fast in the morning.

It's good insulation that matters, not some thermal mass.

Keeping the heating on doesn’t really work as the thermostat is in the front room (south facing) and when that reaches the desired temperature (and stays warm) the extension loses heat. 

Yes it will kick in and out to keep the front room at 22c but the extension has lost its heat by then.

Thermostat is fixed to the wall so can’t move me it to the extension.

grumbler

Then you have to adjust your heating system in the first place, not increase some "thermal mass". Even for a single-zone system this can be achieved to some degree by adjusting thermostatic radiator valves and lockshield valves. Then there are wireless thermostats and multi-zone systems. And even a wired thermostat can be relocated.

ETA: I assume that there are radiators in the extension. Increasing their size/power is yet another option.

Jeepers_Creepers

JoeLowe said:

grumbler said:

IMHO, it makes no sense to improve/increase the thermal mass. It's like creating a huge heat storage that needs time to heat up and then slowly cools down. Just keep the heating on and you'll get the same temperature without any extra energy needed. You'll even use less energy because you can let the temperature drop at night and then get it back to normal fast in the morning.

It's good insulation that matters, not some thermal mass.

Keeping the heating on doesn’t really work as the thermostat is in the front room (south facing) and when that reaches the desired temperature (and stays warm) the extension loses heat. 

Yes it will kick in and out to keep the front room at 22c but the extension has lost its heat by then.

Thermostat is fixed to the wall so can’t move me it to the extension.

I suspect very strongly that your extension has actually been built perfectly well and is not the cause of this issue. Absolutely check for silly errors like the roof insulation having been cut into too deeply for the lights as others have said, but even then I'd be happy to bet that sorting this will have no discernable effect. (One good reason you may wish to sort it, tho', would be to prevent cold spots and the potential for condensation forming behind the lights. Even then the heat from the lights should sort it...)

You could - would - spend many £ks adding extra insulation or upgrading your windows. And again I'd be happy to bet you won't feel the difference.

I suspect the issue is that you are simply experiencing a temp fluctuation due to hysteresis (no, not hysteria - at least not yet). It's a room which responds quickly to added heat, and ditto when that is removed. That it's North-facing doesn't help.

The actual temp fluctuation in the extension might 'only' be around 1~2oC, but that can very noticeable. Your front room is set to 22oC, and that's nicely cosy. If the extension is only at, say, 20oC at the same time,  then even a 1 degree drop will likely be felt as relatively 'chilly'. Do you have an accurate room thermometer?

Two immediate solutions based on what you've said; (1) is to balance your system a bit better as mentioned by others. Basically, tweak the rad lockshields in the front room closed a fraction so that these rads will get slightly less flow, so will run slightly cooler. That means it'll take the front room a little longer to get to the requested temp, so the boiler will have to keep running a little longer - thereby keeping the other rads fed as required. (2) is to move the room stat into the extension. Yes, that will need a wireless room stat, but then it can be moved between the two rooms if wished. (Hives are great...) Where do you spend most time? Move the 'stat in there.

Seriously, you'd be nuts to consider adding extra insulation or changing the skylights - the outlay would take decades to recoup, and there are easier ways to sort this. Did I mention that each bit you add has less additional benefit? 100mm will not block twice the heat of 50mm.

(Tee-hee)

FreeBear

grumbler said: Then there are wireless thermostats and multi-zone systems. And even a wired thermostat can be relocated.

ETA: I assume that there are radiators in the extension. Increasing their size/power is yet another option.

The levels of insulation would appear to be reasonable. So I would agree, a larger radiator would be worthwhile coupled with a smart heating control system with some element of zoning. Would also be cheaper than ripping off the roof to add another layer of insulation.

Jeepers_Creepers

The cost of sorting this is either zero (tweaking that lockshield) or ~£100, the cost of a Hive. Oh, and a £20 pa increase in your gas bill... You don't get sumink for nuffink.

The extension room seemingly heats up fine with the existing rads, so I wouldn't be looking at increasing their size unless it was found to be necessary (and I doubt it is). The front room is also 'cosy' at 22oC, so these rads are clearly ok too.

Yes, tweaking down the front room rads (if that's where the stat is staying, and fair enough if it is) will mean that the boiler will now have to run 'on' for longer periods, but this won't be as energy-consuming as you might think; every rad in the house (apart from the front room ones) is still individually TRV-controlled, so all these will shut down and adjust themselves exactly as they did before, so no extra demand - or gas - there. Only rads in rooms where their TRVs are saying 'more, please' will be using the hot water, and that's what you want - 'cos that's the extension room :-)  

JoeLowe

Very great feedback, the radiators in the extension are not man enough for the room and I have to heat it with a 5kw gas heater. 

I am planning on swapping these over to something larger.

I'm going to use a thermal camera to see where heat is being lost.

and will do the draught check - is there an easy way of doing this?  

Jeepers_Creepers

JoeLowe said:

Very great feedback, the radiators in the extension are not man enough for the room and I have to heat it with a 5kw gas heater. 

I am planning on swapping these over to something larger.

I'm going to use a thermal camera to see where heat is being lost.

and will do the draught check - is there an easy way of doing this?  

Hmm, that's interesting. Clearly if the existing rads aren't powerful enough, they should be replaced. BUT, make sure it is a case of undersized rads, and not one of the boiler shutting off before demand is met - your very cosy front room is the one controlling the boiler, so when that says 'enough already', the rest of the house has to follow suit. Are you certain it isn't a case of this?

If you really need a 5kW heater to get the temp up, that's quite something - 5kW is a lot. Do you have it going flat out? How long does it take?

Can you give the rough size of the whole combined 'extension' room, and the rads wot's in it?

It would certainly be interesting to see the results of the thermal and air tightness tests should you carry them out. But...