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

orrery

coffeehound said:

I suppose there's the pump having to restart the water circuit flow from static, fan starting, ignition unit, maybe some unburnt gas.  As you suggest I don't suppose it's a big deal but having the whole system ticking along at lower temperature sounds preferable to having it chop on and off.  Almost certainly more wear and tear with that regime too. 

The issue is: how does the boiler behave if it is left on? My boiler is 18kW (completely overpowered) I have it set to run at 9kW, and it can modulate back to 5kW minimum. Long term, it will overheat at 5kW and have to shut down - it will have ceased to be condensing long before that - effectively, the return temperature is so high (above 55C where it ceases to be condensing) that the boiler running at minimum power heats the flow to a higher temperature than it is set to.

The result is that the boiler will short-cycle - heat, shut down, wait a short period, re-ignite and round it goes again, and again - all the behaviours that you are trying to avoid. Much better use a controller thermostat to instigates proportional control - turning the system on and off over a reasonable period, say 40 minute cycles.

coffeehound

That's pretty strange orrery, I don't see the point in that mode of operation.  The boilers I've known set the flow temperature range by the dial setting, e.g. #1 = 30 to 45º, #2 = 35 to 50º, #3 = 40 to 55º, etc.  While the wall stat is calling for heat, the pump runs continuously and the boiler lights for a few minutes to heat the flow to the range maximum temperature, at which point the burner extinguishes, and the radiator temperature then coasts down to the minimum temperature at which point the cycle repeats.  No need to shut everything down due to overheating, and the flow is always below the full condensing limit.  It seems like a good way to run things.

richardc1983

Or just use an opentherm controller which modulates the flow temp based on the temp in the house. 

orrery

coffeehound said:

That's pretty strange orrery, I don't see the point in that mode of operation.  The boilers I've known set the flow temperature range by the dial setting, e.g. #1 = 30 to 45º, #2 = 35 to 50º, #3 = 40 to 55º, etc.  While the wall stat is calling for heat, the pump runs continuously and the boiler lights for a few minutes to heat the flow to the range maximum temperature, at which point the burner extinguishes, and the radiator temperature then coasts down to the minimum temperature at which point the cycle repeats.  No need to shut everything down due to overheating, and the flow is always below the full condensing limit.  It seems like a good way to run things.

 The boiler cutting out in your description is what I'm referring to as an over-heat condition - the boiler has reached its set temperature and can't continue to run, so it shuts down. If the stat is still calling for heat, then the boiler will coast down to a temperature where it can re-start - this is usually called short-cycling and is - as far as I understand - what was being referred to earlier as a reliability issue. A proportional controller will cycle the boiler on and off over longer periods to do what you are describing without so many start/stop cycles.

If you have the flow temperature set below the condensing limit, then the rate of heating is very limited indeed, so takes longer to heat the house especially if it is -10C outside, and wouldn't be much use to people using the boiler to heat their hot water (i.e. those of us with a h/w tank) as the flow temperature needs to be over 60C (for speed and safety, as it ensures bugs can't grow). The proportional controller will facilitate this by running for longer 'on' cycles.

You can get further benefits if you have an intelligent controller that checks the weather and decides not to run the heating as it expects blazing sun imminently, or pulls forward the start time on cold days to ensure you have a warm house, or allows remote control via phone app.

The heating industry (and the industrial control industry earlier) have developed these techniques because they work well and save people money.

David28

My boiler switches on and off about 16 times a day.
After it switches on in the morning at 07:45 when the temp rises to 21 deg C it then switches off until the temp drops to 20 deg C then it switches on again.
It does this about 16 times during the day until 23:30 when the thermostat is programmed to maintain the temperature at 14 deg C during the night.
The boiler has never switched on during the night so far since last Feb when that thermostat was fitted.

richardc1983

Look into opentherm control. This modulates the flow temperature and burner output of the boiler to match the heat loss of the property. So at 21c rather than turning the boiler off, it will turn the boiler down. There may become a point that it is mild outside or the house is so well insulated that the boiler cannot modulate any further. The thermostat will then turn the boiler off and restart when the flow temp drops below a set ammount. 

wooried

This thread seems to ignore heat pump based systems. They have very different characteristics to conventional ones - specially when combined with underfloor pipes rather than radiators. Our hot water is also produced by the  ASHP and has a timed boost facility so that water can be cooler outside showing time. We recently discovered the best time for a boost is late afternoon when the air outside is warmest. This cut the time the pump runs to heat water by a third. 

rp1974

wooried said:

This thread seems to ignore heat pump based systems. They have very different characteristics to conventional ones - specially when combined with underfloor pipes rather than radiators. Our hot water is also produced by the  ASHP and has a timed boost facility so that water can be cooler outside showing time. We recently discovered the best time for a boost is late afternoon when the air outside is warmest. This cut the time the pump runs to heat water by a third. 

Not ignored on purpose,just in the minority as heating systems go at present.
The bigger picture is that it's cheaper to turn heating on and off than leaving it constantly on,whichever method is used to create that heat to begin with.
There's also a section for exactly your type of heating at the top of the page,might find better,more relevant information there.
https://forums.moneysavingexpert.com/categories/lpg-heating-oil-solid-other-fuels.

victor2

wooried said:

This thread seems to ignore heat pump based systems....

This thread started in 2012. Heat pumps were even less of a minority then than they are now!

grendon18

can we have an energy myth-buster for air source heat pumps, please?