Hi Batalto,

The easy answer is 'how long is a piece of string'.

Any potential energy reduction, achieved by lowering the indoor temperature setpoint overnight, is dependent upon many variables.

1) The heat loss due to the difference between the indoor temperature and the outdoor temperature.

2) The thermal mass of the building, which will dictate how quickly it will cool down and heat up due to (1) above.

3) The variation in efficiency of the ASHP at differing outside air temperature.

4) How the system responds to the selected weather compensation curve if enabled.

5) When and for how long the indoor temperature setpoint is lowered.

Lowering the temperature setpoint does not immediately lower the indoor temperature, which would probably take many hours to fall from 21C to 17C. It should therefore be remembered that raising the setpoint will not immediately increase the indoor temperature, which could take several hours to get back to the desired value.

I'm doing a similar thing. I just can't bring myself to heat the house or parts of it while I'm not there.  Also, I want it to be warm when I'm getting up and dressed but not when I'm asleep.  I don't have any comparative figures; leaving it on all the time could be better.  At the moment the heating occasionally comes on for an hour or so but not longer.     

@kev-m 

Hi Kev,

From calculations that I have carried out, it may be possible to reduce energy consumption by about 5% by lowering the setpoint overnight. Of course at this time of year energy consumption for heating is virtually nothing, so 5% of virtually nothing is not a great deal.

Since your heating system is running for approximately 1 hour per day, it would be more efficient to make that 1 hour when the outside air temperature is at its peak value for the day. Even better, if you have a solar PV system, would be to heat your home when your solar PV can power your ASHP.

As you are aware at this time of year the heating demand is quite low, so if you were to run your heat pump continually at its lowest continuous operating parameters, its output heat energy would exceed the heat demand, and the indoor temperature would increase, until the heat loss was equal to the heat energy produced by the heat pump. The indoor temperature would therefore increase during the warmer daytime hours and fall during the night. The heat pump is therefore switched on and off to keep the indoor temperature around the desired setpoint.

As we go into Winter and the heat demand increases, there will be a point where the heat demand exceeds the minimum output of the heat pump, so if left on it would run continuously, but at the lowest output temperature that is required to meet the heat demand.

If the heat demand for a home throughout a 24 hour period is 120 kWh, giving an average heat demand of 5 kWh per hour. If the heat pump is now switched off for 4 hours, then it needs to provide the 120 kWh of heat energy during 20 hours of running time, so the average heat output would now need to be 6 kWh per hour. Running ASHP's at higher outputs uses more electrical energy and therefore reduces efficiency. This is particularly bad if the heat pump is switched off during the day and has to work harder during the night.

@derek-m,

I think I'm going to have to do some trials when it gets colder.  I'm in the process of getting the full MMSP monitoring package so that should help track usage.

If my ASHP is controlled by a wireless room thermostat (and it currently is) then it will tell the ASHP that there is a heat demand when the room temp drops below the level I have set.  The water pumps will start up so the ASHP will detect the return water temperature and the weather compensation will tell it what the output temperature should be. So it will just heat the water to that temperature and pump it round until the room thermostat tells it to stop. I don't know how it decides to heat at high or low power (it definitely varies though; I can hear the difference). Is it the difference between output and return temps or does it also use the internal temperature?  (I know this sort of thing was discussed on another thread).  

Interesting stuff.