Energy myth-busting: Is it cheaper to have heating on all day?

victor2 wrote: »

Anybody care to summarise it?

Some Facts:

1. House heat loss depends on the temperature difference, inside to out (sun gain is immaterial, it is a gain so can be considered a second boiler).

2. House heat loss continues whether the boiler is on or off. As long as the inside temperature is higher than outside the house is losing heat.

3. Running shorter hours means this inside temperature falls, hence average inside temperature over 24hrs is lower, average heat loss is lower.

All of that is common sense and basic physics so hopefully nobody will disagree with that.

Therefore if outside is 0, inside 20C is 100% heat and we then run shorter hours and that results in an 18C average inside temperature we have saved 2C, 10% so our heat requirement is 90%. That is just roughly and assuming outside temperature is constant. I'm just showing how it works not a specific house on a specific day.

If we are saying heating is off 8hrs at night, 8hrs during the day, then it is off 2/3 to 1/3 on. 20C during on, average of 17C during off, i.e. falling to 14C (20C cooling to 14C-> 17C average). And for the pedants yes is isn't a straight line fall so these figures are slightly off, I'm showing the process.

Hopefully, and given the figures posted by people who have measured their house then this too can be seen as factual.

This is key point. Running the heating 8hrs per day does NOT save 66% of the heat loss (compared to 24hrs). The saving is more like 10% but that depends on the house. A better insulated house will save LESS, and worse insulated will save MORE. A tent would save 66%.

The second key point is if you still need 90% heat but only have 8hrs to do that then you need to put heat into the house at 270% of the 24hr rate. The heat has to come from somewhere. You will get some solar gain, when the sun shines, but unless you like in a passive house it won't be a lot and not in winter.

The third key point is that in order to get more heat out of radiators you have to run them hotter (download a radiator catalogue and compare the figures for T30 (50C CH) vs T50 (70C CH)). Running hotter makes the boiler less efficient, wikipedia reckons about 10%.

Starting from cold is a red herring. If you need higher output from the radiators they aren't giving the heat you need until they are hot so you quickly get the water hot and lose the benefit of cold start. Again, this is assuming the system is running on a cold winter's day. If the house is designed for 0C outside and it is actually 10C then heat loss is halved, 270% becomes 135% which with oversized radiators looks more possible at lower CH temperatures.

The best advice for best efficiency is to run shorter but not to be afraid of running longer rather than turning up the boiler temperature. Modern smart thermostats will likely do this as they will move the start time earlier automatically when it is cold.

Regarding combi boilers, I see many new properties are fitted with these. Many have on suites plus standard bathrooms. If as stated all were in use at the same time and there is a slower water flow it's a problem. Surely fitting these as standard is mistaken apart from the fact it's cheaper!

orrery wrote: »

Well, it saves space too.

Sadly, without a water tank it precludes heating the water with solar so - in my view - having a combi is a bad idea. Better a very big water tank.

Hi

Totally agree ... we have a very big water cylinder, mainly to compensate for poor days for solar, just like today's been!

HTH
Z

Cardew wrote: »

However just how much money does diversion of solar generated electricity save? I suspect diverting 1,000kWh PA saving around £30 a year would be a good result and it would cost £100+ for the diversion device(Immersun etc)...

Probably much more than £100 cost as most people will need someone to install it, unless it gets added at the time of the PV install. So, potentially a 7 year payback period.

Saving? No real idea. We get a bath and and a shower almost every day for about half the year, and a top-up any sunny day and then it drives a convection radiator with any surplus over that (more cost there). I can also swap the connectors so that the radiator becomes the primary if I'm away on holiday to offset frost protection heating during the cold half of the year.

I'm very happy with what it delivers, but I have no method of quantifying it. All this will be superseded when we get house batteries.

jk0 wrote: »

Spam reported

Are you sure? - they appear to have MSE's blessing