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

orrery

Smiley_Dan wrote: »

I think there is an interesting nuance to that though which I think you touched on in your first para (tell me if I'm wrong).

If the TRV closes as it reaches the desired temperature (lets call this T1), than means that setting the temperature higher (T2) will achieve T1 quicker because you don't get the closing effect as T1 is approached. Yeah?

Indeed. It also mitigates in favour of having different settings for spring/autumn and winter - the TRV should be set a little higher in winter. For someone who is prone to shout "For $%&*£ sake, set the thermostat to the right temperature and then leave it alone!" that takes some admitting.

orrery

matelodave wrote: »

It's therefore a balancing act to get the optimum water temperature to run the system efficiently - too low and the boiler will run virtually continuously, you still wont be warm and it won't save you money either. Too hot and the boiler & pump will cycle on & off, the room temperature will overshoot and it will cost more than it needs to as there will be a lot of wasted heat in the boiler & pipework.

There are (at least) two things that could be done here.
- My room stat/controller is a relatively bog-standard wireless device and will resort to turning the heating on and off at 10 minute intervals when within 1.5C of the set temperature (called proportional control). I don't know how sophisticated it is in doing that though
- some condensing boilers can be set to regulate on the return water temperature (to maximise the condensing efficiency). I've just switched my Vaillant boiler over to this mode, but it isn't for the feint-hearted - just trying to understand what it is doing is a nightmare.

matelodave

We've got an air source heatpump (ASHP) and it has weather compensation so when it's cold outside, say -5 degrees it increases the flow temperature to 42 degrees and when it's about 15 degrees then the flow temperature decreases to about 25. This maintains the flow temperature at just about the right level for the room stats to do their job.

The programmable room stats control the room temperatures - we've got u'f heating and each room has it's own stat so any extra heat in those room like solar gain, cooking or lots of people will shut it down.

It took a couple of winters to optimise the weather compensation slope and adjust the stats to suit our lifestyle - we are at home all day but we have different temperatures in the rooms depending on how and when they are used.

The ASHP is usually gently idling away most of the time in the winter but I've managed to get our total annual electricity consumption down from 8500kwh in the very cold winter of 2012/13 to just under 7000kwh this year (the last mild winter probably helped there as well). We are all electric in a fairly large 3-bedroom detached bungalow which is adequately but not excessively insulated (none under the floor)

oldskoo1

I like the idea of air source heating and I would like to move to that option once my boiler needs replacing. However for now I'm not sure my current radiator set up is compatible and might need underfloor heating.

For now though what should my boiler star be set to? It's an unmodulating old system boiler. It feels like if I put it to 4 it turns off earlier and cycles more but is this inefficient than have it on longer and getting screaming hot.

Not sure as all the talk is about maintaining a 45-55c return in these modern condensers

tuk585

Probably not quite the place for this but I have had 5 houses in my long life. 3 of these had a 'room thermostat' (usually in the hall) and TRVs. In all 3 houses the TRVs were usually broken and not working. In my current house one of them I have had to sellotape up the adjuster as, if moved, water pours out. I have no faith in them.

I am a believer in only turn the boiler on during the day in winter and twice a day in spring/autumn - definitely not 24 hours!.

Food for thought - just as you would turn your heating to come on a little earlier than when you need it, so it is warm at the time you get home, for example; do you turn it off a little time before you don't need it, as it takes time to cool down also? I have my boiler going off an hour before bedtime and it is still nice and warm for bed.

Mary_Hartnell

Air to water then into the under floor should be comperable with modern gas, only if well designed. - electricity is very expensive to create and more than half of the energy in the fuel goes missing by the time it gets to your meter.

Heat pumps were all the rage in Austria in the 1970s (no oil) - I doiubt any of those systems are still in use now.

If you have (fracked?) gas the advice is to stick with it and in due course get a condensing boiler.

oldskoo1

For now is it better to run my old system boiler on max boiler temperature regardless of the time of year?

I've been running it mainly on setting 4 out of 6 which equates to roughly 72c

In the middle of winter at 0c to -10c i put it to 6.

But is it more efficient to have it cycling less and keep it on 6? It does not modulate the gas output

Robwiz

malc_b wrote: »

And for those who are wondering what the maths are here is a little thought experiment. Imagine a one room house, 0C outside, 20C inside. The loses are 1kW so it has a 1kW fire. Total loss over 24hrs is 24kWh. Now imagine the owner goes to bed, the fire is turned off. At that instant the inside temperature is still 20C so the loss is still 1kW. As the temperature falls the loss drops. Say 8hrs later it is 10C inside, loss is now 500W (and 10C is very cold house!). So 750W on average overnight. Total daily loss in this case is 16hr x 1kW + 8hr x 750W = 22kWhr. The saving is 8.3%. But also during the 16hrs the heating is on we need to put in 22kW so the fire must run at 1375W. If you use a more reasonable 15C after being off overnight then the figures are 4.2%, 23kW, 1478W.

This will happen in any home other than a tent. In any house when you turn the heating off the temperature stays higher than outside due the stored heat. Turning heat off saves losses but puts up the heating power needed when it is on. Hence it is quite probable that running in low power condensing mode 24/7 will use less fuel than 16/24 (in mid winter). The aim should be to run in condensing mode as much as possible (i.e. low radiator water temperature which mean low radiator output, at 60C diff (80C-20C) rad = 100%, at 40C diff (60C-20C) rad = 60%). So as the weather gets cold increase the heating hours rather than run not in condensing mode.

I would appreciate maths based arguments as to where that is wrong. Arguing that physics says it is wrong when I'm using physics to prove it is a waste of your time and mine. I'm not saying you lose less heat, I'm saying heating efficiency can exceed the extra heat lost so less fuel is used.

Your maths is true for the assumptions you have made. However, I don't think those assumptions reflect the real world, in which the outside temperature follows a diurnal pattern.

For example, take this week, when the daytime peak has been 18º C with the overnight low of maybe 11º C for the southeast. As heat losses are proportional to the delta t, most of the heat input to the house overnight (when the delta t is greatest) will go straight through the walls, windows and floor. If you allow the inside temperature to fall overnight, then the heat losses will be reduced. The rule of thumb is that when outside temperature is above 15.5º C no heat input is required so the heat requirement during the day will be far less than you calculate.

orrery

oldskoo1 wrote: »

For now though what should my boiler star be set to? It's an unmodulating old system boiler. It feels like if I put it to 4 it turns off earlier and cycles more but is this inefficient than have it on longer and getting screaming hot.

I'm not sure what 4 represents (sorry, hadn't seen the subsequent posting). I'd turn it down to the lowest level that will give you hot enough radiators and water.

The boiler cycling is seriously wasteful - you're cheapest solution might be to get a new controller that will switch into 'proportional control' when the house temperature is within a degree or so of what you have set.

malc_b

Robwiz wrote: »

Your maths is true for the assumptions you have made. However, I don't think those assumptions reflect the real world, in which the outside temperature follows a diurnal pattern.

For example, take this week, when the daytime peak has been 18º C with the overnight low of maybe 11º C for the southeast. As heat losses are proportional to the delta t, most of the heat input to the house overnight (when the delta t is greatest) will go straight through the walls, windows and floor. If you allow the inside temperature to fall overnight, the the heat losses will be reduced. The rule of thumb is that when outside temperature is above 15.5º C no heat input is required so the heat requirement during the day will be far less than you calculate.

The example wasn't meant to be accurate it was meant to prove that it is more complex. What the simple model proves is that by looking at something you can easily measure, the morning temperature after the heating has been off all night, you can estimate the amount of heat stored in a house and thus the energy saving between 24hrs and 16hr running. I've kept it simple with fixed temperatures. You could easily record inside and outside temperatures if you want a more accurate figure. However, the key fact is that the difference between 24hr running or not, is less than most people expect. When you look at that figure and compare it with the gain from running in condensing mode then it explains why some people say they find a fuel saving running at a low boiler setting (i.e. condensing) and 24hr running.

If you want to get complex then yes you need to look at passive gains, heat stored in the house, heat lost to outside (delta T). You also need to look at the boiler, conditions for condensing and the radiator sizing compared to the room loses. And do that for the whole heating season. Too much like hard work so I reckon it is easier to simplify it to some rules.

1. Run condensing boilers in condensing mode i.e. low return temperatures. 2. If house is cold then run boiler for longer, not turn up water temperature. 3. If longer running doesn't work then switch to higher temperature but this will reduce efficiency so then it might be better to set to max temp and run shorter hours (depending on whether you can still get any significant condensing mode operation).

In a typical house then you would run 1 and 2 most of the heating season switching to 3 when it got really cold.