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• MSE Guy
• By MSE Guy 4th Dec 12, 6:43 PM
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MSE Guy
This is the discussion for the following MSE guide.

Energy myth-busting: Is it cheaper to have heating on all day?
Page 47
• malc_b
• By malc_b 9th Nov 17, 8:51 AM
• 999 Posts
• 385 Thanks
malc_b
Bricks you're missing a key point.

Can I take it as accepted that the difference between the heat lost in a day is less if you heat intermittently than say 24hr but that this difference is not that much (under 10%). This comes about from knowing that heat lose depends on how warm it is inside and experiencing that after turning the heat off the house takes a long time to cool down.

So, from that then lets say in 24hrs you need to put into a house say 100 kWh running 24hr or 90 kWh if you run 16/24. That heat comes from the boiler via the radiators. For the 24hr that is 100/24 = 4.2kW continuously out of the radiators. For 16hr running it is 90/16 = 5.6kW. The 16hr is lower energy but is asking over 30% more from the radiators. The only way to get more power out of radiators is to turn the water temperature up. For a standard radiator the factors I have are 30C = 0.51, 40C = 0.76, 50C = 1.0, 60C=1.25, where the temperature is the radiator above ambient figure. If we are looking at 55C return (for condensing), 10C drop, 65C out then that is 60C average radiator temperature so dC of 40, 0.76 factor.

So, if our house has been installed with radiators that just balance the heat lost in middle of winter while running at 65C out and we then run 16/24 the only way we can keep the house warm is to increase the boiler temperature up 75C or more. We'd be better to run longer rather than have to put the temperature up.

Every house is different but the old calculation method was just to size the radiators for the calculated heat loss and for a radiator factor of 1 (no condensing back then). I don't know what the new method is, probably to assume a lower radiator factor but I doubt it considers running shorter hours too.

I would agree that intelligent thermostats help with this. They should learn the house heat up time, and if the boiler is limited to 65C so it is in condensing mode the stat should learn the curve for that mode. The stat will then start the heating earlier and earlier as the weather gets colder. That's exactly what I have said, run longer not hotter.
• jet104
• By jet104 12th Feb 18, 4:52 PM
• 1 Posts
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jet104
I think it's better just to control each room with the radiator valves. The problem with having a 'house' thermostat is they're usually in a communal area or hall and unless you have it permanently high some rooms never get warm even if the radiator valve is fully open. I've experienced this with a downstairs toilet that was always baltic.
And if you permanently have the thermostat high you need to control each room with the radiator valves anyway.
• Hengus
• By Hengus 12th Feb 18, 5:11 PM
• 5,931 Posts
• 3,651 Thanks
Hengus
I think it's better just to control each room with the radiator valves. The problem with having a 'house' thermostat is they're usually in a communal area or hall and unless you have it permanently high some rooms never get warm even if the radiator valve is fully open. I've experienced this with a downstairs toilet that was always baltic.
And if you permanently have the thermostat high you need to control each room with the radiator valves anyway.
Originally posted by jet104
Welcome to the forum. Sadly, no one solution suits all. For example, I have a TPI thermostat which is designed to get a property up to temperature and keep it there. Fiddling with the system uses more gas than just leaving it to do its own thing.
• orrery
• By orrery 12th Feb 18, 5:24 PM
• 556 Posts
• 463 Thanks
orrery
I think it's better just to control each room with the radiator valves. The problem with having a 'house' thermostat is they're usually in a communal area or hall and unless you have it permanently high some rooms never get warm even if the radiator valve is fully open. I've experienced this with a downstairs toilet that was always baltic. And if you permanently have the thermostat high you need to control each room with the radiator valves anyway.
Originally posted by jet104
This is a very wasteful solution as the system will be running all the time. Most modern thermostats will operate in 'proportional' mode as they get close to the set temperature i.e. they will turn the heating on and off proportionate to the heating required, which helps keep boilers in condensing mode. Not having a main thermostat means that the boiler is likely to be out of condensing mode for the majority of the time and delivering the worst possible efficiency.

The problem you have might be best approached by restricting flow to other radiators so that the downstairs toilet gets more flow and is up to temperature before the system is shut down and/or fitting a double radiator instead of a single, or adding a towel rail in addition to the rad, if there is space..

A thermostat also helps shut the system down if the outside temperature is high and no heating is required or there is ample gain from the sun.
Last edited by orrery; 12-02-2018 at 5:26 PM. Reason: Add to explanation
4kWp, Panels: 16 Hyundai HIS250MG, Inverter: SMA Sunny Boy 4000TL, SolarImmersion
Location: Bedford, Roof: South East facing, 20 degree pitch
Nissan Leaf, TADO Central Heating control
• Hengus
• By Hengus 12th Feb 18, 7:08 PM
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Hengus
I think leaving your heating on all the time will drain a lot more energy from inefficient homes, as the heating works a lot harder to replace lost heat. So you actually need to make sure that your home is well insulated and drought proofed in order to minimize heat loss.
Originally posted by Abi_J
In general, I would agree, however, it depends on your property, boiler and heating controls. Modern, modulating, condensing boilers are at their most efficient when operating in what is known as the low temperature condensing mode. My boiler has been ticking over throughout the day with recorded usage averaging 0.3 cubic metres/hour (3.5kWhs/hour). If I turn it down and go out for, say, 3 hours then the gas usage to get the house back up to temperature exceeds the gas saved. I know as my gas usage is recorded every 15 minutes. For most of the day, my boiler has had a flow temperature of 45C. When there is a significant heat demand, my thermostat will demand a flow temperature of 70C which takes the boiler out of its most efficient operating mode.
• malc_b
• By malc_b 13th Feb 18, 8:09 AM
• 999 Posts
• 385 Thanks
malc_b
I think it's better just to control each room with the radiator valves. The problem with having a 'house' thermostat is they're usually in a communal area or hall and unless you have it permanently high some rooms never get warm even if the radiator valve is fully open. I've experienced this with a downstairs toilet that was always baltic.
And if you permanently have the thermostat high you need to control each room with the radiator valves anyway.
Originally posted by jet104
Sounds like your system is not balanced. Google balancing radiators to see what this. I reckon most houses have unbalanced radiators as it is just a time consuming PITA to do :-). Some people also have the idea that TRVs will fix an unbalanced system (which they won't) so you don't need to balance a system which has TRVs.

The only true house wide control would be to sense each room temperature and then work out if heat is needed or not and use that to decide boiler on or off. As that is complicated then the cheaper option is to sense a typical location to control the boiler and use TRVs to give finer regulation of each room. The hall is meant to be typical, centre of the house, no activity or people which might heat the location and give the impression no boiler heat is needed.
• Hengus
• By Hengus 13th Feb 18, 8:38 AM
• 5,931 Posts
• 3,651 Thanks
Hengus

The only true house wide control would be to sense each room temperature and then work out if heat is needed or not and use that to decide boiler on or off. As that is complicated then the cheaper option is to sense a typical location to control the boiler and use TRVs to give finer regulation of each room. The hall is meant to be typical, centre of the house, no activity or people which might heat the location and give the impression no boiler heat is needed.
Originally posted by malc_b
That is exactly what a zoned heating system such as Evohome, Tado or Wiser does. The controller adds the various zone demands together and sends a calculated total heating demand to the boiler. Finer granularity can be achieved with Opentherm boiler control which adjusts the gas valve.
• Hearty74
• By Hearty74 14th Feb 18, 11:44 AM
• 1 Posts
• 0 Thanks
Hearty74
Interested in thoughts/opinions on merits of replacing standard gas boiler & hot water tank with combi boiler for central heating / hot water. Hot water tank fitted with thermostat which can be regulated so that when temp of water is reached heating system bypasses coil in tank which has certainly improved efficiency / cost saving but plumber / heating engineer maintains further savings could be made by converting to combi. Not sure if his advice has merits or is he just looking for further work
• JDogg
• By JDogg 14th Feb 18, 12:03 PM
• 1 Posts
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JDogg
Should I have the gas fire on in the living room, or all the radiators in the house?
When I worked for British Gas, I was a member of one of the few teams trained as energy experts. We were advised by the engineer running the course that it took just as much energy to use the gas fire as it did to heat the whole house with radiators, with the major difference that if one just used the fire, every time you left the room you'd be cold, while the C/H kept the temperature the same throughout the property.
• blimeyharri
• 14th Feb 18, 5:27 PM
• 4 Posts
• 3 Thanks
blimeyharri
It's like having the kettle constantly boiling all day just in case you want a cup of tea at some point...
• Cardew
• By Cardew 14th Feb 18, 7:48 PM
• 27,366 Posts
• 13,390 Thanks
Cardew
Interested in thoughts/opinions on merits of replacing standard gas boiler & hot water tank with combi boiler for central heating / hot water. Hot water tank fitted with thermostat which can be regulated so that when temp of water is reached heating system bypasses coil in tank which has certainly improved efficiency / cost saving but plumber / heating engineer maintains further savings could be made by converting to combi. Not sure if his advice has merits or is he just looking for further work
Originally posted by Hearty74
Welcome to the forum.

If you do a search the combi v standard boiler discussion/argument has appeared many times in this forum.

There is no 'one size fits all' answer! It depends on a host of factors - type of usage, length pipe runs etc etc. One thing is certain, even in the cases where a combi might be marginally cheaper(and how would you prove it anyway) it would never justify the installation costs of a new boiler, removal of tanks etc.

Modern well insulated Hot water tanks lose very little heat - pence per day - and any heat 'lost' warms the fabric of the house. That is why they are often in airing cupboards.

Costs apart, a standard boiler is a far better system for many(most) people. Copious supplies of Hot water from the tank.
• Gromitt
• By Gromitt 14th Feb 18, 8:24 PM
• 4,984 Posts
• 2,383 Thanks
Gromitt
I've tested this theory out several times over the last couple of weeks (as doing it just once is pointless) and I've come to the following conclusion:

When outside temp is 2 - 3 degrees C:
It costs about £2/day for a quick burst before I get up (typ 19c), day heating (13c), evening heating (21c) and overnight heating (16c)

If I stay at home all day it costs about £2.20 and the only difference is that the temperature is set to 20c during the day, 21 evening, and 16c after I go bed.

So a 20p difference, but 365 * 20p = £73. If only it was that easy, because before November the difference was more like 5p or so.

So I'd guess it would cost me £20 - £30 a year extra if I had my heating on all day at 20c.
• Gromitt
• By Gromitt 14th Feb 18, 8:35 PM
• 4,984 Posts
• 2,383 Thanks
Gromitt
It's like having the kettle constantly boiling all day just in case you want a cup of tea at some point...
Originally posted by blimeyharri
Well, it depends. We have an urn at work which is basically exactly that - it keeps the water inside the 'kettle' above 90c. We used a killawatt meter and found out it was negligible difference in cost as when the urn did boil it did so in such short bursts because the water was already near boiling rather than freezing cold straight out of the tap. But yes if you only have one cup of tea per day it would be a costly exercise, but then again you are unlikely to be using an urn.
• Cardew
• By Cardew 15th Feb 18, 8:14 AM
• 27,366 Posts
• 13,390 Thanks
Cardew
updated version!

https://www.moneysavingexpert.com/utilities/energy-saving-myths
• malc_b
• By malc_b 15th Feb 18, 12:54 PM
• 999 Posts
• 385 Thanks
malc_b
Interested in thoughts/opinions on merits of replacing standard gas boiler & hot water tank with combi boiler for central heating / hot water. Hot water tank fitted with thermostat which can be regulated so that when temp of water is reached heating system bypasses coil in tank which has certainly improved efficiency / cost saving but plumber / heating engineer maintains further savings could be made by converting to combi. Not sure if his advice has merits or is he just looking for further work
Originally posted by Hearty74
Any heat you lose from the HW cylinder during the 9mths of the year when the heating is on just goes to heat the house so isn't wasted. The main benefit of a combi is you saving the airing cupboard so combi suits a small house which benefits from more floor space. Of course you then need somewhere to store your towels.

The disadvantage of a combi is the limited flow rate. Hot water is heated from cold so the flow can only be as fast as you can heat the water. Again this is not so much of problem in a small house but a house with 2 bathrooms with showers say might have a problem giving adequate supply to both showers at the same time. Big houses with 3, 4, 5 bathroom are obviously worse again. You then end up in the silly situation of having a lot of bathroom but you can't use them all at the same time.

Cost wise you can work out the saving by looking at the label on the tank, if it is a newish tank, or looking online for a similar sized and type of HW cylinder so see what figures that gives. It should tell you kW/day loss. Multiple that by how much you pay for gas, and say 100 (100 days when you aren't heating the house) and that will give you an idea of the saving.
• Hominu
• By Hominu 15th Feb 18, 8:05 PM
• 1,650 Posts
• 868 Thanks
Hominu
This is exactly why I don't leave the heating on all day, in pretty graph form:

House is heating in the morning to raise temperature to 19c. It then gradually decreases to 15c during the day and pretty much stays there, then comes on at 15:30 and raises the temp back to 19c for 16:30.

It probably wouldn't cost much difference to leave it on to be honest, but since I'm not there it seems a little pointless, plus I'd imagine starting from "cold" would be more efficient for the boiler.
• malc_b
• By malc_b 16th Feb 18, 8:02 AM
• 999 Posts
• 385 Thanks
malc_b
Interesting graph. It shows what I've tried to explain. The heat lost from the house depends on the difference between inside and outside. As we can see from this graph the inside temperature is still pretty constant even though we aren't heating the house for a long time. That means the heat lost is pretty constant whether you heat 24hr or 16/24 or 4+4/24. You save a little energy of course but not as much as people think.

It's a mistake to think that starting from cold is more efficient. Look at this way, if the energy you need to put into a house is pretty constant, let's say 100% is 24/24 heating and 90% is 4+4/24 heating. Hence when you heat for just 4+4 hours you need to get in 90% but in just 8hrs, i.e. the radiators need to be at 270% power compared to 24/24. To get more out of radiators you need to run them hotter. Running hotter is less efficient.

Of course it depends on how hot. If you radiators are oversized and the weather isn't too cold then maybe you stay in condensing mode so then the boiler is still running efficiently and you'll save gas. But if that doesn't work out and the boiler has to run too hot for condensing mode then it will be running less efficiently and that might cost you more gas than you save by running short hours. The key is to keep the boiler in condensing mode even if that means running longer. 24/24 is the extreme case of running longer.
• Hengus
• By Hengus 16th Feb 18, 8:30 AM
• 5,931 Posts
• 3,651 Thanks
Hengus
It's a mistake to think that starting from cold is more efficient. Look at this way, if the energy you need to put into a house is pretty constant, let's say 100% is 24/24 heating and 90% is 4+4/24 heating. Hence when you heat for just 4+4 hours you need to get in 90% but in just 8hrs, i.e. the radiators need to be at 270% power compared to 24/24. To get more out of radiators you need to run them hotter. Running hotter is less efficient.

Of course it depends on how hot. If you radiators are oversized and the weather isn't too cold then maybe you stay in condensing mode so then the boiler is still running efficiently and you'll save gas. But if that doesn't work out and the boiler has to run too hot for condensing mode then it will be running less efficiently and that might cost you more gas than you save by running short hours. The key is to keep the boiler in condensing mode even if that means running longer. 24/24 is the extreme case of running longer.
Originally posted by malc_b
As I have said previously, I have been monitoring gas usage throughout this Winter season. I have a modern modulating boiler controlled with an Opentherm connection that adjusts the size of the gas flame. With this type of set up up, my data shows that when the heating comes on at 7.30am, it is cheaper just to leave it on until 10pm. A key to efficiency is the need to avoid fiddling with the thermostat and TRVs. Any adjustment will invariably result in an increase in the boiler flow temperature and a loss of efficiency.
• Cardew
• By Cardew 16th Feb 18, 8:38 AM
• 27,366 Posts
• 13,390 Thanks
Cardew

It's a mistake to think that starting from cold is more efficient. Look at this way, if the energy you need to put into a house is pretty constant, let's say 100% is 24/24 heating and 90% is 4+4/24 heating. Hence when you heat for just 4+4 hours you need to get in 90% but in just 8hrs, i.e. the radiators need to be at 270% power compared to 24/24. To get more out of radiators you need to run them hotter. Running hotter is less efficient.

Of course it depends on how hot. If you radiators are oversized and the weather isn't too cold then maybe you stay in condensing mode so then the boiler is still running efficiently and you'll save gas. But if that doesn't work out and the boiler has to run too hot for condensing mode then it will be running less efficiently and that might cost you more gas than you save by running short hours. The key is to keep the boiler in condensing mode even if that means running longer. 24/24 is the extreme case of running longer.
Originally posted by malc_b
We go round and round in circles on this subject. It was discussed here: http://forums.moneysavingexpert.com/showthread.php?t=5090074

In all the publications I have read, and enquiries I have made, I have yet to discover just how much more efficient a boiler is when running in condensing mode.

In any case the key to getting the boiler to run in condensing mode is to keep the return water temperature low. With a cold house, with all radiators on full, the boiler will initially be in condensing mode regardless of the set water temperature.
• SwanJon
• By SwanJon 16th Feb 18, 10:30 AM
• 2,388 Posts
• 934 Thanks
SwanJon
Interesting graph. It shows what I've tried to explain. The heat lost from the house depends on the difference between inside and outside. As we can see from this graph the inside temperature is still pretty constant even though we aren't heating the house for a long time. That means the heat lost is pretty constant whether you heat 24hr or 16/24 or 4+4/24. You save a little energy of course but not as much as people think..
Originally posted by malc_b

If heat loss is pretty constant, but internal temperature remains the same without heating, where is the extra heat coming from?
In fact, the heat lost to bring it down to that constant is what needs to be replaced, and replaced continuously, but if on timer it only needs to be replaced once.

It's a mistake to think that starting from cold is more efficient. Look at this way, if the energy you need to put into a house is pretty constant, let's say 100% is 24/24 heating and 90% is 4+4/24 heating. Hence when you heat for just 4+4 hours you need to get in 90% but in just 8hrs, i.e. the radiators need to be at 270% power compared to 24/24. To get more out of radiators you need to run them hotter. Running hotter is less efficient.

Of course it depends on how hot. If you radiators are oversized and the weather isn't too cold then maybe you stay in condensing mode so then the boiler is still running efficiently and you'll save gas. But if that doesn't work out and the boiler has to run too hot for condensing mode then it will be running less efficiently and that might cost you more gas than you save by running short hours. The key is to keep the boiler in condensing mode even if that means running longer. 24/24 is the extreme case of running longer.
Originally posted by malc_b
All your numbers are based on this arbitrary assumption.
Why 33% of the time need 90% of the energy put in? (or 66% for 90% in your earlier post) Not saying the condensing mode could not make a difference, but your initial premise undermines any later calculations.
Once the house is up to temperature there should be no difference between on 24 or on 4+4 (or 16 etc), so the actual comparison should be between the 16 (or 8 etc) hours of heating when not required and the two hours (one hour twice) it takes to get back up to temp (from Hominu's chart, 1530-1630). Yes that warm up hour will be more expensive than any one keep warm hour, but is to 800% more expensive?
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