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

gadgetmind

If your house drops from 21C to 6C that quickly, then you've got bigger problems! But yes, I do have my system set to maintain 16C even overnight.

northernsoul

Gadgetmind
Please make me weep and tell me how much it is to heat up a well insulated 5 bed house. My Kwh have varied, I think, from 13000 to 20000 depending on the winter ! So far this year used 12500.
Is it really successful as to when to turn the heating on earlier on cold days?

northernsoul

Gadgetmind.
It was allowed to drop 6C to 15C during the night (thermostat programmer set at 15 for the night). Not to 6C!
It took so long to increase from 15C to 21C that I think I set the thermostat at 19 . 20 or 21C. Certainly I did not let the temp down to 15C when it was -10C during the day (and probably colder during the night). Years since 2010 have been no problem.

gadgetmind

northernsoul wrote: »

Please make me weep and tell me how much it is to heat up a well insulated 5 bed house.

I'll find some figures, but our insulation is work-in-progress and working from a 1930s draughty house base. We're making progress as the curtains no longer wave in the breeze despite windows being closed, but there is still a lot to do.

And then there's the Aga.

r2015

And then there's the Aga.

How much does that cost to run?

malc_b

lstar337 wrote: »

Yes, but the argument for not leaving your heating on all day, is that you do not need to keep the bucket at the fixed height while you are not in it.

You have this backwards. You're assuming a large hole so if you don't top it up the bucket empties or even dramatically drops in level. An empty bucket is the same as inside house being the same temperature as outside, a freezing cold house! A large drop is house inside being very cold.

Let me explain this with numbers. Heat loss depends on the difference between inside and outside, say 20C inside, 0C outside, heat loss is N kW. Heat 24/24 loss in a day is 24N kWh.

Now suppose we turn the heating off overnight and the temperature falls to 14C in 8hrs. Over that 8 hrs, average temperature is (roughly) 17C so loss is 8N*17/20 = 6.8N a saving of 1.2N kWh.

So in morning we heat back up to 20C and for simplicity we'll ignore the time that takes. Heating is on for 4hrs so loss to outside is 4N. Then work, 8hrs off, same as night time, 6.8N. Back home, heating for another 4hrs, 4N, and that's 24hrs. So over the day 6.8+4+6.8+4 = 21.6N kWh where as if heating had been on 24hr it would have been 24N, saving is 10%.

Not as big as most people think. The bucket hole is not large. But see what effect this has on the boiler. In the 24/24 case the boiler is providing N kW, as are the radiators. In the 8+4+8+4 case the boiler and radiators need to deliver 21.6N in 8 hrs, 2.7N kW. You need the radiators running at 270%. If the radiators can actually deliver that then it certainly won't be at low water temperature, boiler will not be condensing mode and will be running less efficient.

Someone has pointed out that this thread is about leaving the heating on all day or not. I'm sorry if offends some people to state this is the wrong question, it's not that simple. And if will vary depending on the house.

Take the case above, 10% saving running shorter house. That's the similar to the saving running in condensing mode so in that case shorter hours is likely to work out cheaper. But say you insulate the house, overnight temperature falls to 16C rather than 14C. Night loss is 7.2M, over 24 hrs, 7.2+4+7.2+4 = 22.4M kWh. Saving is 7%.

BTW I've used M because house loss is now less, 7.2M must be < 6.8N. Radiators have less demand, say 10% less but they'll still need to run hot so not in condensing mode.

Anyway, it is simple enough to work out the figures for your house. All you need to do is leave the heating off overnight (or during the day) and see how much the inside drops over a long period, when the outside is a cold day (to maximise the change and so get a accurate figure). Once you have that you can calculate the effect of running times.

My suggestion is that cheapest would be run on low water temperature until the house isn't warm enough, then increase the hours, and finally increase the water temperature. In other words maximise condensing mode. That should be cheapest for most people.

victor2

malc_b wrote: »

Anyway, it is simple enough to work out the figures for your house. All you need to do is leave the heating off overnight (or during the day) and see how much the inside drops over a long period, when the outside is a cold day (to maximise the change and so get a accurate figure). Once you have that you can calculate the effect of running times.

As has been said many times, people can work it out for themselves. Your maths is way over-simplified, and even that can be confusing to wade through. Heat loss is not linear and it is a huge error to make the assumption it is.

My "check it yourself" research about 3 years ago found the following.
The house temperature dropped from 19C to 16 in 6 hours. It took 3 hours to get it back up to 19, using 16.5kWh of gas in the first hour, 12.0 the next and 11.4 in the third hour. It subsequently used 9.2kWh per hour to maintain the temperature at 19. Obviously these figures would vary as the outside temperature changed along with many other factors, but they were typical. I do not have a condensing boiler.
My conclusion was that for a small additional cost (which is what this thread is about), I could leave the heating on all day, but from a Money Saving perspective, it was cheaper to have it timed to run when I needed it.

lstar337

malc_b wrote: »

You have this backwards.

Not really.

malc_b wrote: »

You're assuming a large hole so if you don't top it up the bucket empties or even dramatically drops in level.

I'm not assuming anything. I followed on the bucket analogy because it helped somebody else to understand. I'm following the laws of physics, you seem to trying to claim there is some way of breaking them.

malc_b

lstar337 wrote: »

Not really.
I'm not assuming anything. I followed on the bucket analogy because it helped somebody else to understand. I'm following the laws of physics, you seem to trying to claim there is some way of breaking them.

NO, I am not saying the laws of physics can be broken. Read what I wrote. Shorter hours means LESS heat loss.

BUT

Less heat loss does not automatically mean less fuel used.

Condensing boilers run with hot return water no longer condense and lose about 10% efficiency. In other words they use 10% more fuel. If the shorter hours is only saving 5% then it is cheaper to run longer.

This is same as saying it is cheaper to drive a 50mph rather than 70mph even though the car engine is on for longer (and a car uses fuel just standing still).

The key point is that the total energy a house needs each day doesn't vary much. 16/24 could be 5%, 4+4/24 could be 10%. And the better insulated the house the smaller the figure. These numbers can easily be less than the difference between running in condense mode and not. Hence longer hours can mean MORE energy lost, as physics says, but LESS fuel used because the boiler efficiency is higher.

maxmiler

Malc,

I really like the analogy
"This is same as saying it is cheaper to drive a 50mph rather than 70mph even though the car engine is on for longer (and a car uses fuel just standing still)."
which explains it very well.

But am I correct in thinking that the extra energy used by the boiler in non-condensing mode (which does not improve the energy put into heating the house), is lost out via the flue, because in non-condensing mode the return water is not heated by the flue gases as this water is too hot to absorb any more heat? And therefore the flue gases also go out just that bit hotter which is where the increased gas used is wasted.

So the more often I hear my condensate pump running, the better the efficiency of the boiler?


Max