House heating; most efficient way.

Agentgonzo said:

Cold air falling and hot air rising are two sides of the same coin.

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Yes, but which comes first?

hoppyscotty said:

If you keep your heating ticking over then you're able to maintain temperature very efficiently

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Citation needed. I dispute this claim.

hoppyscotty said:

Due to thermal mass there is a lag for the walls to warm up and cool down, but if they are already warm then all you're doing is keeping the heat in the room topped up.

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Incorrect. If the walls are warm, they are still leaking heat/energy into the outside walls, so you need to maintain this as well.

A simple explanation is that the total amount of heat energy in that you house looses to the outside world will need replacing. It doesn't matter if you replace that heat energy immediately (keeping a constant temperature in the house) or replacing it later (letting it cool down, then heating it back up again) - the heat will still need replacing.

The rate at which your house looses energy to the outside world depends on the temperature difference between your house and the outside world. The larger the temperature, the higher the rate at which it looses heat (energy).

So the larger the temperature difference between your house and the outside world, the more energy you need to put back into the house (whether immediately, to keep a constant temperature, or later by letting it drop then come back up to temperature). And if you maintain a constant temperature, the temperature difference will be higher (than if you let the temperature drop), so you'll need to put more energy (in total) back into your house, which means more gas/electricity, which makes it more expensive.

Admittedly, not a lot, but it's always more expensive to maintain the temperature than to let it drop.

And it's certainly more comfortable to maintain a constant temperature rather than having the heat raging when you get home from work.

Agentgonzo said:

Citation needed. I dispute this claim.


Incorrect. If the walls are warm, they are still leaking heat/energy into the outside walls, so you need to maintain this as well.

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Yes they are leaking heat but at what rate...if the leakage is very fast due to low levels of insulation then you're always going to struggle, but if the leakage rate if very slow because you are well insulated home then it will take several hours for the walls and floor to chill down, so less energy you need to put back into the walls and floor when you come back. Simple physics and frequency response...low levels of insulation and you have a high response, have a very well insulated home and you have a slow response and in between the peaks and troughs its best to keep your heating ticking over...it is literally the basic fundamentals of why underfloor heating is so much more efficient than radiators. You're heating up a massive block of concrete that has a very slow response, it gives up its heat slowly so acts as a big thermal battery. If you turn off your UFH and let that massive concrete slab chill down it takes a huge amount of energy to bring it back up to temperature and then once upto temperature heat your room.

Your premise suggests that there is no benefit or purpose of insulation. The whole point of insulation is to create as much of a thermal lag in between your wall and ceiling and floor and the temperature inside the room and outside the house. Good insulation means the temperature 'leakage' to outside is slowed down...it takes far less energy to warm a wall up a few degrees vs warming a wall up ten or more degrees. So if you're going out for a short enough time then leave your heating on, if you're going our for longe then turn the heating off.

Like the same old thing about flourescent tubes..it was once thought unless you're leaving the room for a while then leave a fluorescent tube on as it takes more engergy to turn one on than it would consume if you left it on while you were out of the room....though turns out that this time is about ten seconds, but the principle still holds. And stop start on your car...if you're stopping for less than seven seconds then don't bother, just let your engine continue to run as it take a bit more fuel to start an engine, but if you're stopping for more than 7 seconds there is a fuel saving benefit to stop start.

hoppyscotty said:

it is literally the basic fundamentals of why underfloor heating is so much more efficient than radiators

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Again, citation needed. I have never heard anyone say that underfloor heating is more efficient than radiators. More comfortable, yes. More efficient? No.

hoppyscotty said:

If you turn off your UFH and let that massive concrete slab chill down it takes a huge amount of energy to bring it back up to temperature and then once upto temperature heat your room.

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The slab will still leak energy into its surroundings. So if you don't let it chill down, you're still putting the same amount of energy into the slab, just at an earlier time.

hoppyscotty said:

.it was once thought unless you're leaving the room for a while then leave a fluorescent tube on as it takes more engergy to turn one on than it would consume if you left it off....

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That's a completely different thing (same with the car), and to do with the energy required to start the fluorescent lamp/car. Fluorescent strips take a big spike of energy just to turn on (the starter). Same with the car's starter motor. This is not the case with starting a central heating boiler, so no, the same principal does not apply, sorry.

Agentgonzo said:

Again, citation needed. I have never heard anyone say that underfloor heating is more efficient than radiators. More comfortable, yes. More efficient? No.

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This is well known. UFH uses lower water temperatures than radiators. Typically 40° as opposed to 70° in radiators. As we all know from brewing, getting water temperature from 18° to 40° takes a lot less energy than from 18° to 70°.

UFH slabs are very very well insulated. UFH is more efficient because it operates at a lower temp, the water in my UFH is only at 35 degrees C vs about 60 degrees C in my radiators. It works by heating the concrete slab in the pipes are set within and exploits the thermal lag of the concrete. Before I got a smart stat my UFH was on 100% of the time and saved me a ton of gas. The smart stat now manages that better and does spend some time off but rarely completely off for any significant length of time unless I'm on holiday and the house is vacant for a week or so.

Go on, I've explained the principle, it's a widely regarded principle and utilised in industry. You want to keep chilling down your walls only to heat them up again if you pop out for a pint of milk then its your bill.

The fluorescent tube analogy is not a completely different thing it's EXACTLY the same thing. You missed the point of my analogy its not about firing up the boiler its about sinking energy into your walls - until your walls and ceiling and floors are upto temperature you wont heat your room as they act as big heat sinks. Walls have a big specific heat capacity...takes alot of energy to raise the temp of a wall by one degree C. If the wall is already upto temperature it requires alot less energy to maintain the walls temperature IF you have a good level of insulation - like I said if you have poor insulation you're on a hiding to nothing. Really I struggle to see what is so hard to grasp about this principle.

hoppyscotty said:

If the wall is already upto temperature it requires alot less energy to maintain the walls temperature IF you have a good level of insulation - like I said if you have poor insulation you're on a hiding to nothing.

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If the wall are up to temperature, then you've already put a lot of energy into them. I could counter with "it takes a lot more energy to maintain the temperatures of the walls than to let them cool down". If you maintain the temperature, putting the lots of energy into the walls sooner in the day. If you let them cool down, you're putting the energy into the walls later in the day. You still need to put the energy into the walls.

hoppyscotty said:

Really I struggle to see what is so hard to grasp about this principle.

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You seem to be under the impression that if you leave the heating on, your walls don't lose heat to the outside world. But if you turn the heating on, they do (as they cool down). I seem to be struggling to make it any simpler. I'll leave it to Martin Lewis to try

The 'Official Answer' – leaving the heating on all day leads to greater heat loss and means higher cost​

The main UK public body for reducing energy use and carbon emissions is the Energy Saving Trust. Its formal answer is that leaving the heating on all day consumes more fuel, leads to greater heat loss, and that means higher costs.

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Agentgonzo said:

I'm curious at to how did you measured this, because it violates some pretty basic laws of physics (thermodynamics)

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Religious recording of KHW and experimentation over 3 years.
I can only speak re the thermal behaviour of my building
I put this sweets-pot of 16c down to the internal mass of the building i have a lot of cinder block internal walls.
If im away for a few days i drop my set temp down to 10c
As for dropping my boiler temp down lower.
I get amore even heat im my rooms. And the way it works is this. If my thermal loss over a period is 10kwh. I can either blast 10kwh into my house fast at a higher temp and then have yhe room cool down creating a peak/ trougb of got and cold. . Or i can turn the boiler down a bit and deliver that 10kwh evenly throughout the same period. Once the thermal loss is higher than yhe heat delivered thats when yhe boiler needs turned up. As i dint have weather compensation on my system i have to do that manually

Frisp said:

Or i can turn the boiler down a bit and deliver that 10kwh evenly throughout the same period.

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This might be the root of what you are seeing. Boilers (especially modern condenser boilers) are more efficient operating at lower temperatures. This is probably one of the reasons underfloor heating might be more efficient - the boiler is more efficient as underfloor heating operates at a lower temperature than radiator systems. It would be interesting to see whether underfloor heating is indeed more efficient than a radiator system set to the same lower boiler temperature. (otherwise we're comparing apples with oranges)

I perfectly comprehend that walls lose heat to the outside world....but insulation slows down that energy transfer and that is the key point - the walls hold onto heat for longer and until your walls are warm you're not going to heat your room precisely because they leak heat into the outside world...that is the fundamental point.

Also I'm not advocating leaving your heating on ALL day if you're out as the Martin Lewis article is talking about, of course that doesn't make sense...I'm saying if you're going out for a small number of hours or so it will be more efficient to leave your heating on (the fluorescent tube analogy). And when I say on I mean thermostatically controlled. I know that in my house with my levels of insulation then unless I'm going out for 3 - 4 hours I'll leave the heating on any longer than that I'll leave turn it off...well not manually as the smart stat takes control of that if I'm going out as part of my usual routine. I leave everything upto my smart thermostat to manage and that knows my routine and when I leave the house and wont always turn the heating off.

But if you remain unconvinced, then that is fine. It's your bill and you know your home better than anyone and what works for you. But what works in one home doesn't translate into a universal constant for every home in the land.

Agentgonzo said:

It would be interesting to see whether underfloor heating is indeed more efficient than a radiator system set to the same lower boiler temperature. (otherwise we're comparing apples with oranges)

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Are you asking if setting radiator temperature to 40° is as efficient as UFH at the same temperature? In practical terms, the boiler for UFH will not be set to 40°, it will be set to its optimum operating temperature and the temperature in the UFH pipes will be set to 40° by means of mixing valves reducing the water temp to 40° using the return water as the balancing mechanism.

Reducing rad temp to 40° will require reducing the boiler output temp which in itself will be less efficient. And since rads use convection to circulate heat, average temps will be a lot lower than rad temp. But maybe I've misunderstood your point.

Agentgonzo said:

This might be the root of what you are seeing. Boilers (especially modern condenser boilers) are more efficient operating at lower temperatures. This is probably one of the reasons underfloor heating might be more efficient - the boiler is more efficient as underfloor heating operates at a lower temperature than radiator systems. It would be interesting to see whether underfloor heating is indeed more efficient than a radiator system set to the same lower boiler temperature. (otherwise we're comparing apples with oranges)

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This is true...and if you have UFH then you are best running your boiler down at a lower temp as if not all you're doing is warming the water to a higher temp and diluting with cold water to bring it down. My house is a mix of UFH and rads, so not the most efficient from the UFH perspective, but if I had more UFH then it would be far better for me to turn my boiler down to a lower temp...also I have a hot water storage system and they work better at a higher temp so need to keep my boiler temp up for more efficient water heating. It's all a balance.

But if you're running your boiler down and you have oversized your boiler...like most households in the UK have, then its not more efficient to run your boiler cooler...there are alot of factors to consider, but we rely on plumbers to fit a system for us and some are better at specc'ing systems than others.

Same for heat pumps...alot of cowboys out there and most problems with them are that they are not well specc'd in the first place.

Larse said:

Are you asking if setting radiator temperature to 40° is as efficient as UFH at the same temperature? In practical terms, the boiler for UFH will not be set to 40°, it will be set to its optimum operating temperature and the temperature in the UFH pipes will be set to 40° by means of mixing valves reducing the water temp to 40° using the return water as the balancing mechanism.

Reducing rad temp to 40° will require reducing the boiler output temp which in itself will be less efficient. And since rads use convection to circulate heat, average temps will be a lot lower than rad temp. But maybe I've misunderstood your point.

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If you run a lower temp in your rad you need a bigger rad - or alot longer to heat the room. It's all about w/m2 so a cooler temp requires a larger surface area to get the energy into the surrounding air. This was the problem with older heat pump systems that required refitting your rads for larger ones to output the same heat energy. Newer systems can run rads at higher temps so you can, in theory, run with existing rads.

Larse said:

Are you asking if setting radiator temperature to 40° is as efficient as UFH at the same temperature?

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Yes. That was what I was asking.

Larse said:

In practical terms, the boiler for UFH will not be set to 40°, it will be set to its optimum operating temperature and the temperature in the UFH pipes will be set to 40° by means of mixing valves reducing the water temp to 40° using the return water as the balancing mechanism.

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I didn't know that. Thanks . Good explanation

Larse said:

...the temperature in the UFH pipes will be set to 40° by means of mixing valves reducing the water temp to 40° using the return water as the balancing mechanism.

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And of course the same is often done on VT radiator circuits in commercial buildings, using weather compensation. The radiator flow temperature varies along a linear scale based on outdoor temperature, using a mixing valve. In some cases a room temperature sensor can also feed into the compensator loop to reset the calculated setpoint therefore adjusting the flow temperature up or down to meet the losses from the space.

This is from the moneysavers website -

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Is it cheaper to leave the heating on low all day or only turn it on when I need it?​

We wish we could give you a definitive answer, but this question is far more complex than it first seems and we're not heating engineers or physicists. So please ignore clickbait publications erroneously crediting Martin with an answer – he knows a lot (too much) about energy bills, but this one isn't his or our bag.
Ultimately, there are arguments for and against it, and it'll likely depend on the type of heating system, your usage pattern, and what type of home and insulation you have.

The 'Official Answer' – leaving the heating on all day leads to greater heat loss and means higher cost​

The main UK public body for reducing energy use and carbon emissions is the Energy Saving Trust. Its formal answer is that leaving the heating on all day consumes more fuel, leads to greater heat loss, and that means higher costs.
The concept is simple: pump heat into your home when you need it; don't pay to keep pumping it when you don't. It says this is the best way to save energy and money (using a timer's best, so your thermostat turns your heating on and off to keep your home at the temperature you want).

Heat Geek, a help service which gives advice on heating, offers training services and installs heat pumps, says it's much more complicated than this. It argues that if you spend a decent amount of time at home (all evening and weekends), even if not there all day, every day, you could be better off leaving the heating on constantly if you have:

a) Heat pumps: A low-carbon form of electric heating which captures heat from outside the home.

b) A modern condensing boiler (installed since 2005): These recover some waste heat before it's lost.

Rather than turning it on and off, it could be more efficient to lower temperatures to around 18 or 19°C and keep it on (you could increase slightly during the day if you're cold, but don't let it dip below this) and have as low a 'flow temperature' (radiator temperature) as you can safely – it'll mean your heating system won't be as responsive, but as it's on constantly, this won't be as big an issue.

Heat Geek calls this the 'low and slow' method – low temperatures and slow heating responsiveness – and it says this means the boiler can recover more waste heat and so operates more efficiently, which it thinks will outweigh the cost of heat loss from having it on all day. It says there are other variables to consider though, including:

• Not in a lot? If you're out all day, every day, and often go straight to bed when getting home, then heating throughout the day would be wasteful.
• Newer plasterboard walls are quicker to heat up. Modern plasterboard walls are quicker to heat up, so turning off the heat during the day may be a decent option. Yet older brick buildings will likely take longer to warm up, but will release heat back in, so low and slow is good here.
• Do you have radiators and/or underfloor heating? Radiators are more suited to on and off heating; underfloor to constant heating.
• What insulation do you have? If you have cavity wall and loft insulation up to modern standards, then low and slow would work better as you'll lose less heat. If not, on and off heating might be better. (Also, see if you qualify for free insulation.)

- Some argue you should keep the heating on constantly for an entirely different reason...

Here, the argument is that keeping the heating on low all day, turning all radiator valves up to the max and the boiler down to the minimum will prevent condensation collecting within the walls each time the heating is turned off. This condensation can help to conduct heat outside the home – meaning you lose heat more quickly and use more energy as a result.

Some councils also recommend this to prevent damp and the growth of mould.

- Oh come on MSE, that didn't help – I still don't know what to do...

We get, and share, your frustration. Sadly there doesn't seem to be a firm answer. Hopefully the info above will tell you which is a good starting point for your circumstances.

So perhaps the best advice is to try an experiment. Try a few days or a week with your heating on a constant low setting and then try only having the heating come on as and when you need it over the same time period and compare the energy use.

Take meter readings at the start and end of each period and compare. Though of course if you're comparing a particularly cold week with a fairly mild one, it'll skew the results. And please let us know how you get on.[/B]