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

wantanswers

malc_b wrote: »

You need a thermometer to start with. A DVM with a thermocouple can be got quite cheaply. You then need patience and a cold day. Run the heating with the themostat set max, all trvs set to max, and wait for the CH water temperature to get up to normal (ish) levels. Go round and record the temperature in, and out for all radiators and across the boiler. Ideally the all have a 20C difference (I think 30C for condensing, you'd have have check). Those radiators which are <20C need the lockshield shutting off, those >20C need it opening. However if the lockshield is near shut you'll see <20C diff but a much lower input water temperature than the other radiators so you'll know the flow is restricted to this radiator.

Of course shutting one lockshield will divert flow to other radiators so you need to take some measurement. Tweak some valves. Wait for it all to settle and measure it all again. I find it's a job you do over a few days. Taking a reading, tweaking then doing it again another day (otherwise the house gets too hot).

20C is the design figure for radiators so its when they give out their specified heat output. More drop is a lower average temperature so less output. Less drop is thus more heat output. But the bigger issue is that a lower drop means too much the water is flowing though just one radiator. You can't heat a house with radiator and you can't get rid of all the boiler heat with one radiator so the whole system performs very badly.

Another good post malc....sometimes i would use a IR gun which i borrowed, nowadays I follow a similar procedure by hand (touch) when i think an imbalance is occurring....thanks for your reply.

Perry525

Cardew wrote: »

Well your FIL is completely wrong!

You say that you leave heating set at 20 when away for the weekend.

If you were away for two years, would you leave it set to 20C?
How about a month? A week? At what point do you feel the laws of physics don't apply?

Is life just about saving money? What about, you only live once?
And being cold all the time is not fun!

We are at home 90% of the time, we have our boiler set at 80C, our wireless thermostat set at 22C from 6.30am to bed time and 21C during the night.
This translates to a room temperature of a little above 21C during the day and a little above 20C at night.
Like a lot of modern thermostats, it is intuitive and averages out the changes in the days temperature and keeps within half a degree.

We have a comfortable air temperature, while the walls feel cold to the touch as 20/21 is a lot lower than body temperature.
We have had experience of lower temperatures as children living in homes where only one room was heated and where ice would form on the inside of our bedroom windows.

We experimented with turning the heating off when going away for Christmas some 40 years ago, the home was freezing on our return and it took three days to return to comfort level.

Since then, we have had our heating on 24/7 all year round - every year, if its cold its on, if its warm or hot the thermostat looks after it for us.

We like a comfortable life, we know when the government and the power companies are trying to create a consensus of opinion where cold is good, we know that last year less power was used and that the power companies are saving money by getting businesses to cut down on power use, so the power companies and the national grid can save money by delivering less power at higher prices.

Sooner or later, the public will realise that the future is....everyone use less power but - see their bills go up as the power companies insist on greater profits every year for delivering less power.

We look forward with interest.

We have 5 to 8 inches of polyurethane foam and polystyrene in the ceilings, and cavity wall insulation, the place is draft proof.

wantanswers

Tell you what Perry, I wish I was sitting somewhere in Florida at this moment, temperatures currently 25C, Sunny, just getting prepared for that Turkey Roast outside on Xmas Day.

Ah well...some of us can dream! Sod the Bills!

grahamc2003

malc_b wrote: »

To answer grahamc2003

The 6kWh comes from the fact you've only saved 2kWh, not 8kWh. The house is still warmer inside than outside between 11pm and 7am. Hence there is still a flow of heat energy to the outside. That depends on the temperature difference. If we maintained 20C then it would be 1kW for 8 hours, so 8kWh. Assuming 10C at 7am and taking that as linear gives an average temperature of 15C so an average loss of 750W for 8hrs. So we have lost 6kWh to the outside. That has come from the fabric of house. When we turn the heating on in the morning we have to replace that 6kWh.

Sorry, I don't understand that. I would understand if it made sense. Please could you take my reply to your previous post, and type comments on individual points in a similar manner I did to yours? That way, we can narrow down where you and physics part company.

The only thing you need to know for the myth is that heat loss is a function of temperature. That's all, nothing else at all. It pretty much follows in a single step that the myth is a myth. You are confusing yourself bringing in 'fabric of the house' and other irrelevant factors - just as most others are confusing themselves by bringing in comfort/burst pipes and other varied and wonderful irrelevant factors.

Could I ask Cardew's favorite question, which is really good in these circumstances - do your maths again using a month with the heating off rather than 8 hours. Do you still think it's cheaper to keep it on all the time rather than turning it off for a month then on again?

grahamc2003

Perry525 wrote: »

Is life just about saving money? What about, you only live once?
And being cold all the time is not fun!

.

It's hard not to bring money in when the question is 'Is it cheaper to keep the heating .......'.

If the question was 'Is it fun being cold' then your answer would have some merit, but as an answer to 'Is it cheaper .....' is seems to miss the point.

oldskoo1

I might have posted this already in this thread, it's so long now.

I tested all these theories quite extensively and as fairly as possible. All tests show that running 24/7 used more gas for me. 17c all day / all night used the most, 17c all day and 12c all night less gas and 17c timed 2 hours morning and 5 hours evening used the least, around 7% - 10% less. If the outside temperature is low like 3c and lower, the colder it is the quicker the heat loss which increases the gas use significantly when on all day.

You just can't argue with that. You find even heating from "cold" it still uses less. But interestingly not much less. We are talking maybe £5-£15 pm in my house depending on the winter month.

These figures are specific to my house and set up. A older draftier house might be significantly different.

I didn't factor in heat absorption as I can't calculate that. The only thing I can add is timed twice a day leaves us feeling cold. We don't feel comfortable and where this theory breaks down is people feeling cold put the fire on, an electric heater or turn the stat up or boost for longer. Some even run it at 20c on timed instead of 17c on all the time.

My test was as measured and fair as possible. I specifically picked a period where weather was almost identical.

I couldn't live on timed, we run it 24/7 but it goes down to 12c overnight. Ignore my temps as they are specific to my house and thermostat location. The rooms temps are exactly as they should be.

The conclusion is, whilst timed is ultimately the cheapest like for like, all day and low simulated off overnight produces the most comfort for us at a slightly higher but worth it cost.

I think keeping it at the same temp is quite mad. Typically overnight is when you loose the most heat. There might be a case for running it at say 14.5c in my case. Heating may come on once, possibly twice. Less of a gap to bring backup in the morning but still ultimately I think it will cost more but only slightly again but may improve comfort.

malc_b

grahamc2003 wrote: »

Sorry, I don't understand that. I would understand if it made sense. Please could you take my reply to your previous post, and type comments on individual points in a similar manner I did to yours? That way, we can narrow down where you and physics part company.

Actually I'm in step with the physics but OK see below

The only thing you need to know for the myth is that heat loss is a function of temperature. That's all, nothing else at all. It pretty much follows in a single step that the myth is a myth. You are confusing yourself bringing in 'fabric of the house' and other irrelevant factors - just as most others are confusing themselves by bringing in comfort/burst pipes and other varied and wonderful irrelevant factors.

Temperature difference but otherwise I agree. So you tell me what happens at night when the heating switches off? Does the house loss to the outside drop to zero? The temperature inside is still higher than outside so there is still more or less the same heat loss. Not until the inside is the same temperature as the outside is the heat loss zero. So where does the heat energy come from? The structure aka fabric of the house.

Could I ask Cardew's favorite question, which is really good in these circumstances - do your maths again using a month with the heating off rather than 8 hours. Do you still think it's cheaper to keep it on all the time rather than turning it off for a month then on again?

The answer to that is obviously it would be cheaper to turn the heating because over a month the house would stabilise to the same temperature as the outside. Over a single night or day that doesn't happen.

Quote:
Originally Posted by malc_b
My 2p.


For those who like physics we can do a thought experiment. Consider a one room house, inside 20C, outside 0C, losses 1kW. Heat it 24hrs at a constant 20C and it takes 24kWh.

Agreed


So what happens if we turn off the heating 11pm to 7am. What are the house losses at 11:01pm? 1kW since the inside temperature is still 20C.

No, losses will be 1kW-delta, where delta is a very cmall amount. This due to the temperature being 20C-delta.

That's why I said 11.01pm so near enough 20C (19.999 if you want).


If we assume the house reaches 10C (brrr!) by 7am and take as linear (which is isn't) then average temp overnight is 15C so losses are 1000x15/20 = 750.

Crude, but OKish, we'll say average rate of heat loss is 750W for that period.

We have saved 0.25*8 = 2kWh out of 24kWh or 8.3%. During the night we have used 6kW rather than 8kWh which has come out of the fabric of the house.

OK- You've 'saved' 2kWh of energy compared to heating at 20C all night. It hasn't 'come out of the fabric of the house' - you just haven't burnt that 2kWh of heat which you would have done if you maintained 20C

But with the heating off we haven't burnt 8kWh of gas/oil/elec yet the losses have been 6kWh. Where has that 6kWh come from? It has to have come from the structure of the house.


In the morning we have to put that 6kW back as well as providing the 750W-1kW to balancing the losses.

No idea wher 6kWh comes from - do you mean 2kWh?
If you do, then this is where your error is. If you simply heat at a rate of 1kW, the house will come back to 20C, where it will stay. The losses at 11C will be much less than those at 20C, hence why supplying at a rate of 1kW rises the temperature until the losses balance again at 20C. You don't have to 'put in' the 2kwh of losses you have avoided!

In the real situation, you will actually heat at a higher rate than 1kW to get back to 20C as quickly as possible, but the total energy used will always be less (much less) than the 2kWh (the savings (2kWh in this case) will be the integral of the heat loss curve between 11C and 20C over 8 hours), and the 'extra' energy used to bring back the temp to 20C quickly will be the same integral over just 15/20 minutes

You're not looking at the whole picture. You have to consider what is average U factor, the house loss per C of difference. What is the average (or integral) of the heat loss over the whole day. You need to put that energy into the house. The question is, is it more efficient to put that energy is steadily or in bursts? Yes outside of the bursts you will have a lower house temperature and so a lower loss but the drop is quite small. I've assumed 10C in the morning which is very cold and that gives an 8% saving. Is the boiler 8% less efficient running in burst mode? If so then that system on a day to day basis would be better on 24/7.

If our heater was 2kW then it would take ~6hrs to put back that loss during which time the house feels colder than we'd like.

Completely wrong!

No, right, you've just missed my point.

And a factor of 2 over design on system is not unrealistic. Most probably don't have that. Boilers might be x 2 but radiators are usually not that much over.

? No idea what this is about


The way you design a house heating system is to work out the losses for each room, usually based on a -1C outside and 20C lounge, 18C bedrooms etc.. That tells you the minimum radiator sizes you need. You then pick the next up or a larger size. AFAIR if you want to go condensing you go 30% bigger as condensing needs a lower return water temperature so radiators are less efficient. You then pick a boiler larger than the house loss plus some for water heating.

For example, in my house (slightly out of date figures) the house needs 7023W, the radiator total is 13664W, that is in part small rooms and large radiators (for the room size/loss). 150% is minimum oversize. Boiler is 15kW (smallest in the range). So in theory I can put in heat at roughly double the loss rate in the worst case. That's more because I have some oversized radiators.

So on those figures, overnight assuming a drop from 20C down to 15C in the morning the average loss is 7023 x (15--1)/(20--1) = 5351W. For 8hrs 7023 x 8 = 56.2kWh, 5351W x 8 = 42.8kWh, a saving of 24% overnight. Over 24hrs, 7023 x 16 = 112.4kWh, 112.4+56.2 = 168.6 kWhr (24hr heating), 112.4+42.8 = 155.2kWh a saving of 8%. But note that just the day I have to put in 42.8kWh to replace what is lost during the night and this is on top of the 112.4kWh I need to balance to current loss. I need 40% more heat out of my radiators and the boiler has to work 40% harder. In my case the system can do that but not all can. Hence I can well imagine that some people can use less on 24/7. It of course get worse if you consider just heating 4hrs in the morning and 4hrs at night. You then save perhaps 16% but need 80% (?) extra capacity (in depths of winters, spring/autumn losses are less).

HappyMJ

malc_b wrote: »

But note that just the day I have to put in 42.8kWh to replace what is lost during the night

There is no way my house would need 42.8kWh of heating in the morning to bring the house up from 15 to 20 degrees. For starters it only takes half and hour with the boiler on full to bring my house up to temperature in the morning. There is no way my heating uses that much energy in half an hour considering the boiler in my house only has an 11.72kW output (14.65kW input) boiler.

wantanswers

HappyMJ wrote: »

There is no way my house would need 42.8kWh of heating in the morning to bring the house up from 15 to 20 degrees. For starters it only takes half and hour with the boiler on full to bring my house up to temperature in the morning. There is no way my heating uses that much energy in half an hour considering the boiler in my house only has an 11.72kW output (14.65kW input) boiler.

Obviously everyone's house is not the same so it is difficult to make a fair comparison.

If assuming we are talking outside temperatures of -1C Night and zeroC day then in my case when my house drops to or below 15C then it is not unusual to consume 42 Kwhrs to bring it back to a comfortable temperature 20C.

I never let it drop below 17C Lounge temp. in the Winter Months.

malc_b

HappyMJ wrote: »

There is no way my house would need 42.8kWh of heating in the morning to bring the house up from 15 to 20 degrees. For starters it only takes half and hour with the boiler on full to bring my house up to temperature in the morning. There is no way my heating uses that much energy in half an hour considering the boiler in my house only has an 11.72kW output (14.65kW input) boiler.

As wantanswers has pointed out houses are different. Mine is 5 bed. Also, don't confuse air temperature with house structure temperature. The air in the house changes 1-2 times per hour so naturally that warms up pretty quick. The walls take longer. The walls suck heat out of the air during the day and give it back at night. They are what maintains the temperature during the night when the heating is off. Otherwise inside temperature would be the same as outside at least after 30-60 min when all the air has been changed.

And the figure of 1-2 changes per hour is the recommend value YMMV.