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

malc_b

Cardew wrote: »

I really cannot believe some of the posts on this thread.

Can some proponent of 'on all day is cheaper' answer this question that was posed earlier:


The EST - a Government sponsored organisation state unequivocally that the longer heating is off - the cheaper. So they must be wrong?(surely a sacking offence if they can't get it right!) As must every technical organisation be wrong - yet these hairbrained theories are still put forward to demonstrate that the laws of physics don't apply.

If you read my post #251 page 13 you'd understand the physic is not that simple. Yes, if you heat 16/24 you save some energy but not 8/24. On the simple model the numbers came out to 2/24 saving (~8%), on a real house that's likely to be less. At these differences the boiler efficiency comes into play, especially for condensing boilers. Keeping the house at a level temperature with the boiler running at max efficiency could well save power over running it flat out at lower efficiencies.

And as for goverment departments always being right... Well, we have hillsbourgh, BSE, WMD, etc...

HappyMJ

malc_b wrote: »

If you read my post #251 page 13 you'd understand the physic is not that simple. Yes, if you heat 16/24 you save some energy but not 8/24. On the simple model the numbers came out to 2/24 saving (~8%), on a real house that's likely to be less. At these differences the boiler efficiency comes into play, especially for condensing boilers. Keeping the house at a level temperature with the boiler running at max efficiency could well save power over running it flat out at lower efficiencies.

And as for goverment departments always being right... Well, we have hillsbourgh, BSE, WMD, etc...

What model? What numbers? My boiler can only ever output 11.72kw of heat per hour. If it's on for 2 hours then the maximum output is 23.44kWh per day consuming 29.3kWh of gas to produce that heat. How come then the daily consumption figures in winter (over 4 months) are about 100kWh? It's physically impossible. To me that's at least a 70% saving by having it off 22 hours per day.

147718

malc_b wrote: »

Are the lounge radiators sized right for the lounge losses? Are the radiators balanced? Balancing is setting the lockshield valve so the when running flat out and up to temperature the temperature drop across each radiator is ~20C. That way all radiators are getting their fair share of the heating water. TRVs do not mean you don't have to balance. If the system is not balanced then in the extreme just one radiator will get hot while the rest don't, until it's TRV turns off, then next one will get hot and so on.

Also, I'd advise against a thermostat in the lounge. I used to have but one Christmas will all the people and candle the lounge reached temperature with no heating. The rest of the house got very cold. IMO TRVs for the lounge and main thermostat in the hall is the right approach. The ideal would thermostats everywhere where anyone could call for heating but that's far too expensive.

I'd suggest turning down the lockshield valve on the hall a bit, say for a 25C drop across the radiator inlet to outlet. That will reduce the radiator output a bit as it sounds like the hall radiator might be oversized (f the system is balanced).

Thank you very much for your comment . I have been seeking an answer to my question for a long time and your reply is the most comprehensive I have ever seen and I am grateful I will follow your advice Have a warm contented joyful Christmas

grahamc2003

To answer Malc_b

malc_b wrote: »

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.


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


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


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!

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

And for those that have read to the end of this long post - congrats :j

wantanswers

malc_b wrote: »

Are the lounge radiators sized right for the lounge losses? Are the radiators balanced? Balancing is setting the lockshield valve so the when running flat out and up to temperature the temperature drop across each radiator is ~20C. That way all radiators are getting their fair share of the heating water. TRVs do not mean you don't have to balance. If the system is not balanced then in the extreme just one radiator will get hot while the rest don't, until it's TRV turns off, then next one will get hot and so on.

Also, I'd advise against a thermostat in the lounge. I used to have but one Christmas will all the people and candle the lounge reached temperature with no heating. The rest of the house got very cold. IMO TRVs for the lounge and main thermostat in the hall is the right approach. The ideal would thermostats everywhere where anyone could call for heating but that's far too expensive.

I'd suggest turning down the lockshield valve on the hall a bit, say for a 25C drop across the radiator inlet to outlet. That will reduce the radiator output a bit as it sounds like the hall radiator might be oversized (f the system is balanced).

Now thats a good/practical post Malc, you might like to follow it up with a simple method/instruction of how to balance radiators.

Oh and don't forget to bleed the radiators occasionally!

Sorry for the addition again, but another practical tip, if you have TRVs fitted check them out annually, sometimes the small spindle can seize or the fixiing may come loose.

bwg

macman wrote: »

Like so many proponents of this fantasy, you are confusing cost with comfort. Leaving the heating on 24/7 is of course more comfortable, but it is not certainly cheaper, and never could be, even if your house had no thermal loss whatsoever.
Two completely different arguments.
Yout test was entirely unscientific, as it failed to take into account the differing ambient temps from day to day and week to week.

Fantasy? I am not confusing cost and comfort. The cost of my heating is my fundemental consideration. The warmer house is a pure bonus!

Have you actually tried the 24/7 system? Unlike many other detractors, I was originally sceptical but unlike most detractors ( I suspect) I have taken the time and trouble to see whether it works for me. In addition, I've taken the trouble of revalidating my original findings over the years. The latest was last winter, after I had a new condensing boiler installed because (after reading up on the comparative efficiency ratings of modern boilers with my 1990s model) I felt my old boiler was probably now uneconomic.

As to whether my tests were scientific, I'm not really interested . As an accountant I am only interested in whether 24/7 costs me money over the usual twice daily system.

I readily admit that it is a rough and ready means of getting an indication of what effect 24/7 usage would have on my bills (as these tests were carried out week on week over the 6 months winter periods I am happy that the ambient temperature factor was probably averaged out across the whole period).

Originally, it was only as I monitored the first few weeks (in the 1970s) of that winter that I realised that I wasn't consuming any more, week by week despite alternating weekly between twice daily and 24/7.

Constant weekly monitoring of my consumption in more than 3 properties across the country (from the Shetlands to Cornwall) has proved to me that I don't use any more gas using the 24/7 system. I pay the bills so that's good enough for me.

Also, we never have to use secondary heating during the day, as stay at home families sometimes do under the twice daily system. In my experience, most people don't factor that additional cost into their overall heating bills.

As I said in my original posting, I'm not trying to convert people I'm only giving an alternative viewpoint. I couldn't care less whether people adopt 24/7 or not - all I wish to point out is that in my experience I don't consume any more gas on 24/7 usage, so it doesn't cost me any more. That's all I'm interested in.

PhilTilson

Hmmm - came across this thread today and have just read through it (though missed out the middle eight pages, I admit!).

Some of the physics may be a little shaky here and there, but I'm in general agreement. However...

As always, the theory tends to break down when we're dealing with the real world! People talk about "modern, almost-hermetically-sealed houses" - presumably ignoring the fact that probably 60% or more of us live in older, rather draughty and only moderately insulated houses! This can completely screw the neat and tidy theoretical formulae.

In an early post, someone commented "...but this thread is about saving money...". Well OK, let's start by saying that you will save most money by switching your heating off altogether! Now, I'm not being facetious - the point is that we are surely looking to find the best balance between saving money and comfort.

So, my observations: I have a modern condensing boiler and a 30-year old barn conversion with good loft insulation but 2ft thick solid walls and concrete floors. Nothing much I can do to improve insulation without spending a fortune, though I will be improving the draught-proofing. There is no room thermostat on this rather basic system.

At the beginning of the season, we ran the heating for three hours in the morning and three hours in the evening. We were uncomfortably cold in the middle of the day. For the last month or so, we have been running it from 7:00am until 10:00pm, with the boiler heat control turned almost to minimum. The house is a pretty even temperature, the radiators rarely get more than pleasantly warm, we are comfortable all day, but we are using less gas than we were previously.

Now, to my mind, given these specific circumstances, this is a good solution. More comfort, less expense.

Physics, schmisics! :rotfl:

alan7157

i leave mine on but at a lower temp............i got fed up coming home to a greenhouse 'cos better half was cold and turned temp right up (pity i can't do the same with the toaster - cos if she wants toast in a hurry...........you guessed it...........up goes the control, i only find out when i make burnt breakfast):(

wantanswers

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

OK MSE can you be more specific re the title ie what hours are you classing as All Day?

HappyMJ

wantanswers wrote: »

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

OK MSE can you be more specific re the title ie what hours are you classing as All Day?

From the time you leave home to go to work/school to the time you arrive back home.

Stay at home parents should have the heating on all day as they are actually getting a benefit from the heat.

To me this is quite easy to compare. Do the timed method Monday-Friday when everyone is out then leave the heating on all day on Saturday and Sunday when everyone is home. Read the meter every morning at the same time such as when you leave home and you should have a good idea on how much more it costs to have it on all day and set to the same temperature.