Central Heating economy of running 24 hours

[jj4091]

For medical reasons we need to keep the heating on for 18 hours a day. With the room stat. set to 17.5 degrees C it takes the boiler approx. 45 to 60 mins to heat the house up to temp.depending on the ambient temp.. Once up to temp. the boiler cycles on/off throughout the day to maintain house to the temp. on the room stat.
However I've noticed it only takes perhaps another 10/15 mins. to increase temp to 20 degrees C. & then cycles for what appears to be a similar amount of time as at the lower temp.. Therefore by having it set to lower temp. we are only saving 10/15 mins. extra gas at full power & am wondering if it really worth it for the reduced comfort & I haven't noticed much of a reduction in gas usage.
Do my observations sound normal?
I also wonder if leaving it running 24 hours a day would really use much more gas, as there seems to be different trains of thought when you read about which is the most efficient way to run C/H.
If it is relevant we have a Logic 35 combi boiler. Thanks for any advice.

 

[motorbiking]

It does depend to some degree on the age of your house. A modern build will do a much better job of keeping the inside warm, while an older property will leak heat more easily. The fundamental issue is not the energy required to heat your home another 2 degrees, but the difference between the inside and the outside. The greater the difference the more heat loss and the more energy is needed.

Letting your house cool over night does save energy as does reducing the temp to 18. But it mostly depends on the thermal efficiency of your home.

 

[ericmark]

My first thoughts is why is a modulating gas boiler cycling, it could be it's too big, but also could be poor control. I have no option with an oil boiler, it will not modulate, it has to cycle.

However the majority of gas boilers are designed to gain the latent heat from the water produced when burning gas, to do this they must modulate (turn down) and we only turn off the boiler when it is either not required as when we leave the house or the weather gets warmer, or it can't turn down low enough.

We can get modulating thermostats, as with the OpenTherm option, but in the main we use the TRV (thermostatic radiator valve) these do not switch on/off, but slowly open or close, these two graphs are to show advantage of OpenTherm but the same applies using the TRV instead of wall thermostat, if the boiler is allowed to modulate, it will use less fuel.

Your boiler can go down to 8 kW it seems, so as long as you need more that 8 kW the boiler should not switch off.

As the TRV's close, first they force more though the ones still open, and then it opens the by-pass valve, and as the warmer water returns the boiler reduces output.

However this means timing the time between cycles tells us nothing, as we have no idea how much the output is, with a well insulated home likely heating 24/7 will not cost much more than for 16 hours, but a poorly insulated home it will, so all down to design of your home, not the central heating.

I also need the warmth for medical reasons, but find I simply can't sleep if too warm, so boiler does turn off over night. I have tried to limit which rooms are heated with programmable TRV heads they were cheap enough £15 each in 2019, but cau'r drws (shut the door) seems to fall on deaf ears with my wife, and since I heat with oil, the boiler does not modulate, and once the boiler is not running, the TRV does not work. But with gas modulating boilers they work very well.

As to hysteresis the main control is the lock shield valve, any TRV takes time to open and close, and if the radiator gets stinking hot before it closes you will get more hysteresis, so it is important, but to be frank I used the other TRV heads far more expensive, although do the same thing, but link to the PC and shows and if the current exceeded target I just closed them a bit, and in mothers house with a modulating boiler rooms were spot on.

However the higher you are in a room the warmer it is, so a thermostat on the wall at 1300 mm from floor will show higher than the TRV at 100 mm from floor, so often the TRV needs setting lower than a wall thermostat.

 

[JonathanM]

As stated above, it’s about the heat loss, which depends on the temperature difference between inside and outside. If the temperature outside is zero, like it is here right now, then keeping the house 2.5 degrees warmer, at 20C rather than 17.5C, will take 2.5/17.5 x 100% = c.14% more gas than at 17.5C. But if the temperature outside is 10C, then the percentage difference is 2.5/7.5 x 100% = 33% more gas than at 17.5C.

You probably won’t notice the difference in the amount of cycling. It’s also extremely difficult to accurately compare one day to the next in terms of consumption. Having been through these thoughts myself, in the end I decided I just had to trust the maths.

 

[Harry Bloomfield]

Once up to temp. the boiler cycles on/off throughout the day to maintain house to the temp. on the room stat.
However I've noticed it only takes perhaps another 10/15 mins. to increase temp to 20 degrees C. & then cycles for what appears to be a similar amount of time as at the lower temp..

To add to what the others have said....

A combi boiler has to be sufficiently large in Kw, to cope with serving the HW, so that limits how low they can modulate, to avoid the inefficient on/off switching. The result is that in most cases the heat output will be too much for most homes when called to serve the CH.

In your case, I would suggest that if you don't have them already, you fit TRV's to all but the radiator where the room thermostat is. It might even be worth fitting those programmable TRV's which Eric seems to be familiar with, because they can switch heating in rooms on and off individually on a timed basis.

My own way of dealing with the energy cost at the moment, is to have the CH set to just provide a general background temperature of 16C, during the day and make use of a radiant gas fire in the living room. I also use an electric blanket, to preheat my bed, switched on via Alexa and a Smart Plug. I take the wireless stat into the living room with me, so when I light the gas fire - the thermostat sees the heat from the fire, and it fools it to turn the CH off.

My regime has made quite a difference to my overall gas consumption.

 

[vulcancontinental]

Read your gas meter at a set time run the heating as you normally do for a period then read the meter again. Next period run the heating the way you speculate may be better, same period hopefully similar weather then read you meter. The results are your own and not a comparison with someone else's home that may be completely different construction as the occupancy and comfort expectations may be too.

 

[Dereekoo]

Read your gas meter at a set time run the heating as you normally do for a period then read the meter again. Next period run the heating the way you speculate may be better, same period hopefully similar weather then read you meter. The results are your own and not a comparison with someone else's home that may be completely different construction as the occupancy and comfort expectations may be too.

Only way to do it to get meaningful results (?) but use a full day between readings and do it for a week then try and interpreate what factor the outside weather threw into the mix

 

[Johntheo5]

Well, we are led to believe that the relationship between outside and inside air temperature is linear ie x kw/degc difference so maybe carefully note the actual room(s) temperature and the outside air temperature for exactly one hour and note the gas consumption for this period. If the difference between the in/out temperature is say 15C and the boiler consumption is 7.5kwh then the fuel consumption is 0.5kwh/degC difference so you will get a good feel for your expected energy requirement by just noting the outside air temerature.

 

[jj4091]

As stated above, it’s about the heat loss, which depends on the temperature difference between inside and outside. If the temperature outside is zero, like it is here right now, then keeping the house 2.5 degrees warmer, at 20C rather than 17.5C, will take 2.5/17.5 x 100% = c.14% more gas than at 17.5C. But if the temperature outside is 10C, then the percentage difference is 2.5/7.5 x 100% = 33% more gas than at 17.5C.

You probably won’t notice the difference in the amount of cycling. It’s also extremely difficult to accurately compare one day to the next in terms of consumption. Having been through these thoughts myself, in the end I decided I just had to trust the maths.

Thanks I think that this supports my observations in so far as mine were made by wetting my finger & sticking it in the air. In the main it is difficult to accurately work out definite figures because the outside temp tends to fluctuate constantly depending on the weather over any particular period. The only thing I am now confused by is the relevance of boiler modulation to the cycling on & off of the room stat. surely it just gets to the set temp then stops supplying the boiler power & reconnects a supply to the boiler once it has dropped below that set temp.. Our stat seems to open & close in half a degree increments.

 

[JonathanM]

The only thing I am now confused by is the relevance of boiler modulation to the cycling on & off of the room stat. surely it just gets to the set temp then stops supplying the boiler power & reconnects a supply to the boiler once it has dropped below that set temp.. Our stat seems to open & close in half a degree increments.

Many modern thermostats use an algorithm called TPI (or similar) to provide more accurate room temperature control. This means that once you get close to the set temperature, they start turning the boiler on and off in short bursts of a few minutes several times an hour, to keep the room temperature steady. I know from personal experience that this can sometimes be confusing. Do you know which brand and model of thermostat you have?

 

[ericmark]

The problem is we get poor information. This report seems good, but it shows when my thermostat called for heat, not when the boiler was running, the same applies with a modulating boiler, it tells you how long it was running for, but not at what output it was running at.

Simple fact the warmer the home the higher the loses, but it is not linear, but a logarithmic increase, so preventing hysteresis will save energy.

However also the boilers efficiency is not static, at maximum output the boiler is not as efficient as when it has started to modulate, but the details are not published in a way we can use it. Also when the house has been warmed it stores some energy, I have noted this when being on holiday it takes a long time for the house to reheat.

I tried to work out how much gas I was using with my mothers house, but to read the gas meter same time every day, and compare with outside temperature was near impossible. I have decided I don't like pain, and a little extra oil is worth it to stop my pain.

 

[Johntheo5]

Simple fact the warmer the home the higher the loses, but it is not linear, but a logarithmic increase, so preventing hysteresis will save energy.

I thought it was linear?, I'm allways a bit skeptical of this myself, however thats whats used in calculating heat losses.
But then a rad does not give up its heat in a linear fashion, a 50deg rad with a mean rad temperature of 70C and a room temp of 20C will give 100% of its rated output, one might think that a 25deg rad with a mean rad temperature of 45C and a room temperature of 20Cwould emit 50% of its rated output but it doesnt, it emits ((25/50)^1.3)X100. or 41% of its rated output.

 

[Mr Chibs]

I've been doing a test since November, I'm running my boiler 24/7.
In November I used 30% less gas.

I set the heating flow temp of 45° and hot water temperature of 62°.
Powering 10 rads and UFH (single room)

My usage is less than last years, but I have set the temps lower too, they are at 18°
House is permanently warm, (the rads feel barely warm) in a nice way, doubt I'll set the thermostat higher, there's no need.

House is old, but insulated, and boiler is 4 year old Combi.

 

[Johntheo5]

As stated above, it’s about the heat loss, which depends on the temperature difference between inside and outside. If the temperature outside is zero, like it is here right now, then keeping the house 2.5 degrees warmer, at 20C rather than 17.5C, will take 2.5/17.5 x 100% = c.14% more gas than at 17.5C. But if the temperature outside is 10C, then the percentage difference is 2.5/7.5 x 100% = 33% more gas than at 17.5C.

You probably won’t notice the difference in the amount of cycling. It’s also extremely difficult to accurately compare one day to the next in terms of consumption. Having been through these thoughts myself, in the end I decided I just had to trust the maths.

Evohome works by firing the boiler on/off multiply times/hour and I never see any complaints re frequent cycling from a ecomomy point of view.

 

[Johntheo5]

I've been doing a test since November, I'm running my boiler 24/7.
In November I used 30% less gas.

I set the heating flow temp of 45° and hot water temperature of 62°.
Powering 10 rads and UFH (single room)

My usage is less than last years, but I have set the temps lower too, they are at 18°
House is permanently warm, (the rads feel barely warm) in a nice way, doubt I'll set the thermostat higher, there's no need.

House is old, but insulated, and boiler is 4 year old Combi.

Its difficult to get monthly average temps but it averaged ~ 10.8C here (Cork) last november so reducing the room temp to 18C from say 20C will result in a 22% reduction in energy consumption.