@sunandair — it is certainly interesting, because we have very different properties. Listed grade II, solid stone walls and concrete slab floor downstairs, and the only insulation I have is in the loft, and on some of the windows (secondary removable glazing, which doesn't fall foul of the Listed Building requirements, and it is also a major draft stopper). The net effect is a high fabric thermal mass. But what is exactly going on in the walls for example? The outside will in the absence of solar gain be near the OAT, and the inside close to the IAT, but what happens to the gradient during a setback and recovery? And does it matter? And as @jamespa pointed out, the immediate environment (eg local ground temp) will also change with the seasons, what influence, if any, does that have?

These and other unknown unknowns are why I prefer a matched control empirical approach, but as you know that has proved easier said than done. I am hoping that if I run without a setback this spring, having run with a setback last spring, then I may find comparable days, one with a setback and one without, and in the meantime I hope to refine my method for selecting matched controls. It seems I am in this for the long game, whether I like it or not! 

@cathoderay interesting house…. Before I retired listed buildings was my thing.

i wish you every success.

Posted by: @sunandair

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Before I retired listed buildings was my thing

They are lovely, and I feel lucky to live in one, but they do have their downsides too! 

Posted by: @cathoderay

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Posted by: @jamespa

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from what I recall your setback and non setback were at different times of year (spring, autumn)

That is correct, the setback period was Feb to Apr 2025 inclusive, the no setback period Oct to Dec 2025 inclusive, and I agree that subtle, or perhaps more accurately less visible, seasonal variations may well have effects.

My efforts may or may not be more or less heroic, but I do enjoy the challenge of trying to make sense of things!     

I am impressed with the depth of knowledge in this thread.. I was not sure what setback is, now I know we have ours at 15C! 

Given the cold weather, we are considering increasing the setback temperatures of our Vaillant gas boiler. That has a programmable thermostat and an external temperature sensor.

I wonder if this would increase running costs or if we should set it back to how it is now come spring?

We live in SE England, the house had several improvements over the last 15 years or so.

We had mixed white/grey EPS installed (~100mm wide) in new aerated block based external walls, a few PIR boards where needed. This was when we had a rear extension built. We converted the loft and had phenolic boards to envelope it all, and it is compliant with modern regs. 

A couple of years ago, we again had phenolic boards installed on pretty much all the external walls, I think 70mm wide. We also had a small section of cavity wall filled. We managed to leave a few wall features untouched as the gov schemes for energy efficiency stopped insisting it would all be upgraded.

I suppose these insulation materials are common, but there´s a bit of a glossary on another post..

Apparently there is lots of useful info that can be picked up from the Vaillant ebus, but as eventually we will have an ASHP, I did not get that connected, so all we would have are gas bills, that includes a bit of hot water and cooking usage.

We have the set back temperature at 15C and the heating is bringing it up to 20.5-22C for the three separate settings. If we increase the setback temperature to 19C or so, is that likely to make much difference to overall costs?

Posted by: @batpred

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We have the set back temperature at 15C and the heating is bringing it up to 20.5-22C for the three separate settings. If we increase the setback temperature to 19C or so, is that likely to make much difference to overall costs?

The intuitive answer is obviously yes, if you heat the house to a higher temperature, then it costs more. Obvs init. But that is the overall picture, with setbacks it gets more complicated, because while you do clearly save energy during the setback, you end up using more energy during the recovery period, and it may be the case that extra energy use cancels out the setback saving. It has even been suggested that the may on occasion they cost money, but that is probably a case of the theory getting ahead of itself. The jury — well, at least this member of the self-appointed jury — is still out on this one. The empirical evidence tends to suggest there is a saving, while the theory (based mainly on the conservation of energy) suggests there isn't. The reason we don't yet have a definitive answer is that it turns out it is extremely difficult to do the necessary experiment. No two days are alike, which makes like for like comparisons tricky, and by extension no two buildings are alike, meaning even if I can demonstrate a real saving, that only applies in the conditions found in my home.

I ran my heating with a setback last spring, and this spring I am going to run without a setback, and I am hoping, probably in a classic case of hope over experience, that I will end up with setback and non-setback days that are similar enough to allow a meaningful comparison of setback vs no setback. We shall have to wait and see whether that happens!   

Posted by: @batpred

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I am impressed with the depth of knowledge in this thread.. I was not sure what setback is, now I know we have ours at 15C! 

This topic will run and run for sure

The misunderstanding many suffer from (and the controls industry almost actively encourage this misunderstanding) is to assume that switching off the heating means that you arent spending money on heating.  In real-time this is true, but you have to make most of the saving up when you switch it back on again because the house continues to lose energy irrespective of whether its being actively heated or not, until such time as it reaches equilibrium with the outside.

Posted by: @batpred

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If we increase the setback temperature to 19C or so, is that likely to make much difference to overall costs?

To answer that we need to know what temperature the house actually drops to currently?

Roughly speaking for a daily setback if you work out the 24 hour actual average house temperature (not what its set to!) the energy saving should be proportional to the change in average delta T inside/outside.  So consider a house normally at 20C with a 8 hour setback during which it drops to a minimum of 15C if the OAT is 0C.  Making the crude assumption that the drop and recovery are linear, then the average during the setback is 17.5C and so the 24 hour average with setback is (20*16+17.5*8)/24 =19.2C  Thus roughly speaking the house will need 19/20 times as much energy = ie a 5% saving.  With a boiler there is probably no 'recovery' penalty*, with a heat pump there is hence the question - does setback actually save money?

Various people have reported much bigger 'savings' which so far have not been reconciled with conservation of energy.  Since conservation of energy is a given and since its almost impossible, without a lab, to make truly accurate experiments, I don't personally think they can necessarily be trusted as being representative of the effect of setback per se, although they may well be some engineering (as opposed to physics) effect that happens coincidentally in some systems.   However if they are that, then the results don't transfer to any other set of circumstances.  Confirmation bias may also be a factor here.  For clarity Im not questioning anybody's observations, Im only questioning the inference from those observations.

* unless, that is, the recovery pushes a boiler which is otherwise set up to condense back into non-condensing mode.  Since few boilers in the UK are actually set up to condense (and thus cost 10% more to run than is necessary in most cases) this is usually academic.  If all boiler owners abandoned any setback or zoning and instead used weather compensation (which many boilers are capable of) and turned down their flow temperature so the boiler condenses, the vast majority would probably save money and be more comfortable.  Unfortunately there is a better than evens chance that the flow temperature would get turned back up to 75C at the next service, because thats what plumbers do in order to minimise the chance of a call out.

Posted by: @jamespa

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With a boiler there is probably no 'recovery' penalty*, with a heat pump there is hence the question - does setback actually save money?

This is an interesting point. Being slightly tongue in cheek, do boilers somehow tweak the laws of physics? Surely the laws of physics are heat source agnostic, ie it doesn't matter where the heat comes from? If there is a recovery period, when extra heat needs to be added, then that is extra heat, wherever it comes from?

Posted by: @cathoderay

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Posted by: @jamespa

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With a boiler there is probably no 'recovery' penalty*, with a heat pump there is hence the question - does setback actually save money?

This is an interesting point. Being slightly tongue in cheek, do boilers somehow tweak the laws of physics? Surely the laws of physics are heat source agnostic, ie it doesn't matter where the heat comes from? If there is a recovery period, when extra heat needs to be added, then that is extra heat, wherever it comes from?

By 'no recovery penalty' I mean no efficiency penalty because it has to work harder to supply the extra energy needed to replace the energy lost during setback.  I don't mean to suggest that it doesn't have to replace that energy still, it does,.  I mean only that there is no obvious reason why it should do so at a lesser efficiency than otherwise would have been the case.

@jamespa Most discussions over savings that may (or may not!) be possible using setback are usually based on the hard-nosed monetary gains or losses to be made. If like myself, being retired and home most of the time, some readers may be more concerned with comfort and whether any possible savings are worth sacrificing this. I realise that one person’s comfort may be another person’s ‘cool night and comfort’ but if 24/7 stable temperatures are required for comfort - I doubt the cost of this choice is likely to be that great. Regards, Toodles.

Posted by: @toodles

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I realise that one person’s comfort may be another person’s ‘cool night and comfort’ but if 24/7 stable temperatures are required for comfort - I doubt the cost of this choice is likely to be that great

The costs, if any, are not that great - see my calculation above and comment about (non-) condensing boilers.

When I abandoned setback and turned my boiler down four years ago so it condensed the costs went down (so far as I could tell and subject to comments I have made about how difficult the measurements are).  The comfort simultaneously went up.  Zoning in both time and space is largely an invention of the controls industry and the implied (but never guaranteed) 'benefits' are way over stated in most cases.  Obviously there are exceptions, if your house has the thermal characterises of a tent then timed setback saves more, and if your house is long and thin then zoning may also save more.  But for the average 3 bed semi the savings, unless the setback is extreme, are modest and much better would be simply to turn down the boiler flow temperature!  The later however doesn't sell smart thermostats etc.

Posted by: @jamespa

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Posted by: @batpred

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I am impressed with the depth of knowledge in this thread.. I was not sure what setback is, now I know we have ours at 15C! 

This topic will run and run for sure

..

Posted by: @batpred

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If we increase the setback temperature to 19C or so, is that likely to make much difference to overall costs?

....

* unless, that is, the recovery pushes a boiler which is otherwise set up to condense back into non-condensing mode.  Since few boilers in the UK are actually set up to condense (and thus cost 10% more to run than is necessary in most cases) this is usually academic.  If all boiler owners abandoned any setback or zoning and instead used weather compensation (which many boilers are capable of) and turned down their flow temperature so the boiler condenses, the vast majority would probably save money and be more comfortable.  Unfortunately there is a better than evens chance that the flow temperature would get turned back up to 75C at the next service, because thats what plumbers do in order to minimise the chance of a call out.

That´s an interesting point. The combi that we have is a condensing model. I assumed it would be getting in that mode most of the time... 

Anyway, we used to have the rad flow temperature set to about 65 in the vaillant boiler and at some stage when I reduced it to like 45 to "see if the rads would keep it warm", simulating ASHP flow temperature, that sparked a general uprising in my household! I sensed (correctly) this would push back any discussion on a potential switch by a few months.. Well, I am an optimist.. 

When I did this, I did not change the weather compensation curve set via the programmable thermostat that uses an external sensor. I wonder if I should have?

Posted by: @batpred

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That´s an interesting point. The combi that we have is a condensing model. I assumed it would be getting in that mode most of the time... 

Never assume it makes an ass of u and me!  You need the return temp to be lower than about 55 for a boiler to condense.  If you start of with a FT of 75, which is common, that's not going to happen other than in  the coldest weather.  

Posted by: @batpred

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Anyway, we used to have the rad flow temperature set to about 65 in the vaillant boiler and at some stage when I reduced it to like 45 to "see if the rads would keep it warm", simulating ASHP flow temperature, that sparked a general uprising in my household! I sensed (correctly) this would push back any discussion on a potential switch by a few months.. Well, I am an optimist.. 

I did something similar with my boiler.  I set it to a fixed 50 for most of the time, bumping it up to 55 in the very coldest weather.  Greater comfort, lower cost.  Then I got my ASHP.

Posted by: @batpred

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When I did this, I did not change the weather compensation curve set via the programmable thermostat that uses an external sensor. I wonder if I should have?

 

If thats the Vaillant and its a weather compensating boiler with a diverter valve for DHW (so it can heats either DHW or space but not the two together) then what you could do is run the whole system on weather compensation by setting all thermostats/trvs to max and adjusting the WC slope value until the house is just at the right temperature.  Same as an ashp but you might need a higher temp depending on your rad size. 

Beware however that its possible to wire the diverter valve (or, wrongly, use a regular 3 way valve) in a way that means the boiler doesnt know its heating DHW, so tries to do so with the lower FT needed for space heating.  Unless the installer set it up for WC, thats probably how its been done.