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

malc_b

ASavvyBuyer wrote: »

That is a rather strange scenario that re-enforces the doubts in your assumptions. Who would be daft enough to drive around for 30 minutes if they arrived early at a destination ? Most people would park up and just wait. The above statement does not do much for the credibility of your "proof".

It's a metaphore, but as you ask I would say someone dropping the wife and kids off in central london for a premier. No parking anywhere close and what parking there is is loadsofmoney. Cheaper to circle for 30 minutes than pay for parking. But it's not the point. It using something else that have varying efficiency to explain CH.

ASavvyBuyer wrote: »

Your assumption in the calculation is that the boiler will not run in condensing mode if it is switched back on after being off for a period of time. However, the return temperature of the water will be well below 50 degrees when it is switched back on, and with proper controls, will stay below 50 degrees. Therefore, it will be running in condensing mode whenever it is running, providing the user has not used the controls to set it at too high a temperature.

My point is that a house over 24hrs loses a certain amount of energy. If you turn off the heating over night or more often then this doesn't affect the total 24hr loss by that much. I've done the maths and for a fall to 10C I've come up with ~8%. I've seen figures of 5% and the 8% would be 4% with better insulation.

Can we agree on this?

You might be able probably google this. I have notes from "16293: Environmental Engineering Science 1" which gives factors of 1.0 for 24hrs, 0.85-0.95 for heavy building with night time shutdown, but note this is for a factory with an 8hr working day. Houses have a shorter night which is why I worked it out myself from basic physics.

Once you accept that it doesn't make much difference 24 vs 16/8 then you realise that the energy must come from somewhere. It must come from the boiler and radiators. Hence they must be working harder and the radiators must be hotter.

Some other points that have been raises elsewhere:

1. Yes running at lower return temperatures improves efficiency however there is step change going from non condensing to condensing mode followed by a slow increase. We need to be in condensing mode. If we can also run cooler than that as well then good but the gain going cooler is smaller so might not offset the cost of running longer.

2. "My system already run in condensing mode all the time as I force the return water to be below 57C". Good. This is exactly what I have said. Run in condensing mode for as short a time as you can. My only point is that if that doesn't work out for you then running longer hours is likely to be cheaper than not running in condensing mode. That's what the maths show.

3. "The saving due to running in condensation mode is small compared to the saving by turning the heating off". No. This is exactly what the maths show. The overnight saving is small and comparable with the saving for running in condensing mode.

4. "Your assumptions don't match the real world." Unless you live in a 1 room house where the outside temperature is a fixed 0C then you are correct. It's not meant to be a specific house it's meant to explain the process so that we have better understanding of how it works.

5. "You have simplified the maths too much". I have covered this. Feel free to do more accurate calculations but I found it made around 10% error (that is 10% of the 8%) which I mention in the proof. My approach btw was to use a spreadsheet to do a minute by minute calculation. Once you have worked out one equation to give the new temperature then you can just drag it down to generate the temperature points for the entire night.

6. "I don't need to run 24hr, my system just starts a bit earlier". I didn't say you did. I did the proof to square the circle as some people were posting that they saved money running 24hr. Rather than dismiss them as idiots I decided to see if there was any possibility they were correct. I found that there are conditions where they could be correct, but, rather than being either 24hr or at little as possible, the heating has a sweet spot for lowest cost.

gazza007

I am on the current EDF energy club switch so cheapest fuel but after a lot of research I installed the Tado smart system Sept 15. One of the best purchases I have ever made. It is so fire and forget. It looks after itself, very clever and well engineered like most German things. One thing that most people forget even without a smart thermostat and geofencing app is getting a Balance across your radiators, using TRV's where possible and ensuring that the temp thermostat doesn't cut out before the other rooms reach their required levels.

lstar337

I have grown bored of stating the same points over and over, but hopefully those with a basic grasp of physics will read the majority of posts and save money by not pointlessly running their heating 24/7.

Those who wish to waste money are free to.

Malc, sorry but you still haven't provided any proof (not based on assumptions that in my opinion need not apply), so I'm unconvinced.

Those who want to save money, if you have a condensing boiler then turn down the output as much as your house allows, and heat for as short a period as possible. Don't heat for 24/7 if you don't need to, and don't heat while you are out of the property.

orrery

lstar337 wrote: »

My boiler won't run at high power. The output is set at 55c so the return is always less...

For many people who use the same boiler to heat a h/w tank, the output needs to be 60C or more to ensure that bacteria can't develop.

Also, setting such a low temperature will cause my own Vaillant boiler to frequently shut down during the ignition sequence, as it will be unable to perform it's ignition test sequence (always starting on full power) before going over temperature.

lstar337

orrery wrote: »

For many people who use the same boiler to heat a h/w tank, the output needs to be 60C or more to ensure that bacteria can't develop.

Also, setting such a low temperature will cause my own Vaillant boiler to frequently shut down during the ignition sequence, as it will be unable to perform it's ignition test sequence (always starting on full power) before going over temperature.

Well I was simply talking about my own setup at that point. Obviously if individuals have less equipped/capable boilers and/or setups then they would need to account for that.

As I said in my last post:

lstar337 wrote: »

Those who want to save money, if you have a condensing boiler then turn down the output as much as your house allows, and heat for as short a period as possible. Don't heat for 24/7 if you don't need to, and don't heat while you are out of the property.

So go as far as you can get away with basically.

magicsnaps

A question. We're lucky enough to have underfloor heating on the ground floor, but cannot agree between us how to run the system. On all the time, controlled by thermostat or switched on and off by timer for set periods? Please save the relationship.

richardc1983

Underfloor heating is better to be left on all the time with night set back due to the long response times. Set your thermostat to say 20C when occupied and 18C when non occupied eg in bed or out at work.

orrery

magicsnaps wrote: »

A question. We're lucky enough to have underfloor heating on the ground floor, but cannot agree between us how to run the system.

If it is electric, then is it on economy 7? That would mitigate in favour of it being on for the E7 period.

It should always be on a thermostat - that just prevents the heating being left on when the room is hot enough.

With any appliance, I'd always try to keep it on for the minimum period required to do the job. If you want it on when you get up in the morning, set it to come on for a period before you need it, then try increasing and decreasing that period until you find the shortest period that works. Then try the same experiment turning it off before you need it off - the floor will store a lot of heat.

The general rule is - the longer you keep heating turned on, the more time there is for that heat to leak out and the more it costs you.

daveyjp

My parents have hot water underfloor heating throughout their bungalow. Every room is seperately controlled so thermostats are set for comfort in each room.

Even at this time of year once the concrete is warmed through the heat soon builds to well above the stat temp so the boiler is hardly firing.

Cardew

magicsnaps wrote: »

A question. We're lucky enough to have underfloor heating on the ground floor, but cannot agree between us how to run the system. On all the time, controlled by thermostat or switched on and off by timer for set periods? Please save the relationship.

You haven't stated whether it is electric UFH or by hot water through pipes.

As stated above, the principle of UFH is to have the heating medium at much lower temperature than conventional radiators. This entails having the heating on for longer periods.

Like any form of heating, heating is a compromise between comfort and cost. The longer heating is off - the cheaper to operate. Nobody can state what is best for your conditions; you need to experiment to get the acceptable level of warmth.