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

lstar337

malc_b wrote: »

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.

I still don't agree with your assessment of the efficiency increase for a condensing boiler.

malc_b wrote: »

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.

I fully understand what you are saying. But we are talking about an extreme case that applies to practically nobody.

You would need to have a super insulated house (not many do), a top of the range condensing boiler (not many do), that performs to the spec attained in laboratory conditions (I doubt any do), setup perfectly (I doubt many are), with huge radiators (not many people have). Then there is a minute possibility that leaving it on for longer would save you a minuscule amount.

I have most of the things in that list, and I ran my boiler for 1hr a day last winter, maintaining 21c all day. No need to run it 24/7, so why would I.

For almost all people, it is better to not run your heating in your house when you are not there. Turn it on when you get home, or set a timer to just before so it is warm when you come home. Leaving it on while you are out will waste fuel and your money.

orrery

lstar337 wrote: »

..... Turn it on when you get home, or set a timer to just before so it is warm when you come home.....

Or get a controller like Tado which will turn the system on and off as you leave from and and return to the house, and also works hard to turn the boiler on and off to maintain a reasonable condensing performance.

malc_b

maxmiler wrote: »

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

Correct. It's called latent heat. When you heat water it takes 4186 J per litre to raise it 1C, so 90C to 100C is ~42kJ. But 100C to steam (at 100C) is 2260 kJ. And this works in reverse when you condense. Cool 1l of water as steam in the flue gas by 10C and you gain 42kJ of energy. Condense it back to water and you gain 2260 kJ. So if you were to make 1l of condensate per 1000s that would be a constant 2.26kW.

malc_b

lstar337 wrote: »

I still don't agree with your assessment of the efficiency increase for a condensing boiler.

I have most of the things in that list, and I ran my boiler for 1hr a day last winter, maintaining 21c all day. No need to run it 24/7, so why would I.

As I have said before, I am NOT saying run your heating 24/7. The title of this thread is wrong but should that stop me contributing? The advice given is run the heating as short as possible. MSE has just simplified this to either/or.

What I have said is that cheapest is likely to be run the heating in condensing mode as short as possible. When that fails to keep the house to temperature increase the heating time, not the water temperature.

And also, note the likely. It's not cut and dried as it depends on how well insulated the house is and how long the house is occupied. Better insulated, longer occupation will give savings. Poor insulation, short occupation will be better off running non condensing shorter hours.

lstar337

malc_b wrote: »

The title of this thread is wrong but should that stop me contributing?

Sorry Malc, but how can the title of the thread be wrong? It is and always was the starting point.

malc_b wrote: »

What I have said is that cheapest is likely to be run the heating in condensing mode as short as possible.

This I agree with 100%. This is exactly what I do. If this is what you have meant the whole time then Either I didn't 'get it' or you didn't explain it well enough.

malc_b wrote: »

And also, note the likely. It's not cut and dried as it depends on how well insulated the house is and how long the house is occupied. Better insulated, longer occupation will give savings. Poor insulation, short occupation will be better off running non condensing shorter hours.

Yes, agree 100%.

I think many more will fit in to category 2, sadly.:(

Cardew

I once listened to a 'debate'(I used the term loosely) where an argument was put forward that seat belts in cars were unsafe. This was because they knew of a case where an unbelted occupant was thrown clear of a car that subsequently caught fire; the conclusion therefore was that people shouldn't wear seat belts.


In some ways this annual 'debate' on heating is much the same!


The argument put forward by some is that 'it is cheaper to leave on the heating 24/7 because it costs more to warm up a house from cold' That is incorrect! If it were to apply then you would leave heating on if you left the house unoccupied for six months.


Much as I like malc_b's posts, this discussion on the theory of boilers kept in condensing mode is at best a distraction.


Anyone whose grasp of physics is so poor that they can pose the question about 24/7 running being cheaper, will not understand the concept of keeping boilers condensing.

Lily-Rose_3

I would never keep the heating on all the time. Never have, never will.

In winter (the past 2 ones anyway,) we have only had it on about 10 to 15% of the time as it was so warm.

Result: £98 in credit in late June, and the DD that was £36 a month reduced to £25 a month. Even though we use gas to heat the water.

Haven't had it on for more than 3-4 hours (in total) in September and October so far....

malc_b

Cardew wrote: »

Much as I like malc_b's posts, this discussion on the theory of boilers kept in condensing mode is at best a distraction.

That's like saying the speed you make your journey is a distraction to the mpg you get. Condensing mode (in or out of) is the reason you can see a fuel saving running longer.

lstar337 wrote: »

Sorry Malc, but how can the title of the thread be wrong?

Because, even though I have said many times that I am talking about running longer rather than 24/7 or not, I still get people saying 24/7 running is....... In extreme cases 24/24 could be the cheapest but the general case is runner longer rather than as short as possible.

Lily-Rose wrote: »

I would never keep the heating on all the time. Never have, never will.

See, case in point. Many assume because you're arguing that the government advice is wrong that you must therefore be supporting the 24/7 case. I'm saying running longer in condensing mode can be cheaper than running shorter but hotter.

:eek: It's like saying because I've said driving Bristol to London at 70 mph is the cheapest then I must therefore mean you have to drive at 10 mph. In fact I mean 50 mph (ish) is the cheapest even if it is a bit longer.

victor2

malc_b wrote: »

lstar337 wrote: »

Sorry Malc, but how can the title of the thread be wrong? It is and always was the starting point.

Because, even though I have said many times that I am talking about running longer rather than 24/7 or not, I still get people saying 24/7 running is....... In extreme cases 24/24 could be the cheapest but the general case is runner longer rather than as short as possible...

The title of the thread is what it is, and has been discussed annually since it's birth in 2012.
Most people take 24/7 to mean 24 hours a day, 7 days a week. How can you run longer than that?
You have sidelined this thread into a discussion about condensing versus non-condensing. Interesting to many I'm sure, but don't be surprised when the discussion reverts back to what the whole (700+ posts) thread is about.

malc_b

victor2 wrote: »

The title of the thread is what it is, and has been discussed annually since it's birth in 2012.
Most people take 24/7 to mean 24 hours a day, 7 days a week. How can you run longer than that?

You have just proved my point. Gov advice is to run CH as short as possible, so 6am to 8am (leave for work), 6pm to 10pm. This thread is titled 24/7. It's not either or. There is a middle ground, running CH longer than gov advice.

victor2 wrote: »

You have sidelined this thread into a discussion about condensing versus non-condensing. Interesting to many I'm sure, but don't be surprised when the discussion reverts back to what the whole (700+ posts) thread is about.

But the whole point of the discussion on condensing vs non-condensing to explain the anomaly. Physics says heat loss is less with shorter running, but if you read this thread through you'll find posts of people reporting that they find a fuel saving running 24/7. That got me to wondering if they could be right and so I worked out the maths of a house and showed that it could indeed be cheaper to run 24/7. However, it could be cheaper still to run less hours but longer that gov advice. The key point is to run in condensing mode 100% of the time.