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

lstar337

malc_b wrote: »

You don't say if your boiler is condensing or not. That makes a difference.

Not as much as you seem to think.

Unless you live in the laboratory of the boilers manufacturer. The real world numbers are not so impressive.

Smiley_Dan

gadgetmind wrote: »

Empirical evidence that backs up solid theory based on real physics, no, I'm guess that won't convince everyone!

No, it won't because the thread is about "cheaper" which I assumes means financially cheaper and is another layer of complexity over the physics.

r2015

You don't say if your boiler is condensing or not. That makes a difference.

Not as much as you seem to think.

I agree.
I used to have gas warm air heating with a separate boiler to heat the hot water tank and used 17 kWh of gas a day with no heating in the summer so did not need the "but it heats the fabric of the house etc. so it's not wasted" but it was.

This included a pilot light wasting 3 kWh a day again did not need the "but it heats the fabric of the house etc. so it's not wasted".

Now I have a condensing combi boiler and use 12 kWh a day of gas for the same hot water usage.

A saving of 2 kWh a day if you disregard the pilot light.

Which was what was probably keeping the airing cupboard warm.

malc_b

"you don't say if boiler in condensing or not, that makes a difference"

lstar337 wrote: »

Not as much as you seem to think.

Unless you live in the laboratory of the boilers manufacturer. The real world numbers are not so impressive.

Which is because people don't use condensing boilers the right way. The return water temperature has to be low, continuously, to be continuously in condensing mode. Naturally in testing the boilers are set up right. I would agree that retrofitting to an old house is not going to achieve the same as test figures because the radiators are likely too small, but that can be offset a bit by increasing insulation. The issue relative to this thread is that running longer can work out cheaper than running hotter.

r2015

The return water temperature has to be low, continuously, to be continuously in condensing mode.

My boiler is supposedly in condense mode when it supplies hot water, it still uses the same amount of gas as my so called inefficient non condensing boiler and HWC.

northernsoul

What I have found interesting about this is that after looking at the graphs on condensing/non condensing that one person put a link to I noticed the following.
In condensing mode ie return less than 55C, it was a steep graph with a big difference in efficiency for every degree below 55C, whereas in non condensing it was much less steep, with a much smaller drop in efficiency for every degree more than 55C. For example, the difference of efficiency in condensing mode between 45C and 55C was much greater than between 55C and 65C in non condensing mode. So this often quoted figure of keeping below 55C is really a bit misleading. There is not much difference in efficiency between 55 and 60C return temperature, but there is a big difference for every 5C less than 55C. Hmmm, not as bothered now if I have to run it at above 55C return.
In my 100 year old terraced house, with some insulation, depending on outside temperature, I have to run it in non condensing mode to keep it warm. It loses heat faster than it is output by the radiators if the boiler is in condensing mode when it is, say, 0C. It’s no use running it in condensing mode if it is not heating up your house. When it was -10C in 2010 for several days, it struggled to heat the house up, even on max! I live in Yorkshire. With hindsight I should have got larger radiators when I had a new boiler and radiators in 2010.
Also, if it heats the house up in condensing but takes 2 hours to do so, is this cheaper than having it on full and heating it up in 30 minutes? (then turning it down just enough to keep the house to temperature J ). From what I have read it seems the answer is yes. I am not entirely convinced though. On low temperature maybe, just maybe (not being up on the technical side of heat losses), it loses say 75%, of the heat every 30 minutes, whereas, at full it may lose only a quarter (but is 75% of a little more than quarter of a lot, if at all this makes sense?). At lower heat (condensing), is it always catching up trying harder to replace lost heat (ie on longer)?
Hmmm. The more water in a leaking bucket, the faster the rate of flow out of the hole. I think I will go with that (never tested though), but, with a house you have to get it to a temperature you are happy with, so, the level of water in the bucket has to be kept at a fixed height (to retain the temperature), ie supplying enough new water (or heating) to retain this height. Too low, and the analogy is your house does not get to the required temperature. So, it is immaterial what height is in the bucket, it has to be at a fixed height to retain house temperature. Maybe I am completely missing the point with the bucket and will get shot down in flame .
The car analogy. The car always gets to its destination, whether at 1mph, 50mph or 100mph. The heating does not always get to it destination (set temperature). Too low and losses outway input (heat).
Best to way oversize your radiators, then at any outside temperature you can easily keep your house to the required temperature.
Please, if I have upset anybody, please keep contributing, because every comment or thought could be important.

lstar337

northernsoul wrote: »

Hmmm. The more water in a leaking bucket, the faster the rate of flow out of the hole. I think I will go with that (never tested though), but, with a house you have to get it to a temperature you are happy with, so, the level of water in the bucket has to be kept at a fixed height (to retain the temperature), ie supplying enough new water (or heating) to retain this height. Too low, and the analogy is your house does not get to the required temperature. So, it is immaterial what height is in the bucket, it has to be at a fixed height to retain house temperature.

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.

Put it this way, you use your bucket (complete with hole) to water the plants once a week. Do you leave the tap running Monday through to Saturday so that the water is ready for you on Sunday?

Do you think you would have wasted more water by leaving it on all week, or if you turn it on 5 minutes before you need it?

It's really quite simple. The length of this debate is what's puzzling!

lstar337

malc_b wrote: »

The issue relative to this thread is that running longer can work out cheaper than running hotter.

I agree that running longer *can* work out cheaper than running hotter, but I don't agree that the issue is relative to this thread.

This thread is about timed heating vs. 24h heating.

You can still use your boiler in condensing mode with timed heating, I do it myself.

The argument is that timed will use less energy than 24h, condensing or not.

gadgetmind

I don't decide how long to run my heating. I tell it what temperature to maintain each floor (of 3) at what times of days and what days of the week. If it needs to come on sooner to achieve this (it learns thermal inertia of house) then it does, if later is fine, later it is.

Rooms then have TRVs, walls and loft have lots of insulation, windows and doors are getting there, boiler a modulating condenser, so bills not too bad despite this being a rambling five bed detached.

Of course, the Aga isn't quite so efficient ...

northernsoul

Istar337
yes I agree. I missed the point ) I was thinking more of when its on.
As far as I remember, when it was really cold at -10C I gave up turning it off at night or more correctly, therm set to 15C for nights, because it took hours and hours and hours to increase 6C to 21C, even at max.