@derek-m Using MELCloud, I have a schedule to change it to flow temp mode which is set to 45 on the local controller, and then at the end of the cheap period the timer triggers it again and sets it back to WC mode.
What happens to the LWT and RWT measurements during the boost period?
I get a good delta:
By increasing the LWT, it means that the heat emitters (UFH loops) are absorbing more heat energy, which in turn increases the DeltaT and puts more load on the heat pump.
By raising the temperature of the floor slab during the boost period, it reduces the heat demand after the boost period. The heat pump pulls back to minimum operation, but is still producing more heat energy than demand requires, hence the cycling.
One way to reduce the cycling frequency would be to set your controller in Auto Adaptation (AA) mode, set the room temperature 'thermo diff' to an upper limit of +5 and a lower limit of -9, and a 'time interval' of 60 minutes. Whilst this will not stop the cycling, it should at least slow it down.
@derek-m ok, I think I understand what you're saying! Is it that the slab has absorbed so much heat form the boost over night that it can't hold any more?
It takes about 7 hours from end of the boost to actually start cycling - does that still seem correct? It doesn't start cycling straight away.
Is it worth not boosting it for one night and see if it can try and match the heat loss continuously?
AA mode - is that the same as setting it to room temperature mode?
@derek-m ok, I think I understand what you're saying! Is it that the slab has absorbed so much heat form the boost over night that it can't hold any more?
It takes about 7 hours from end of the boost to actually start cycling - does that still seem correct? It doesn't start cycling straight away.
Is it worth not boosting it for one night and see if it can try and match the heat loss continuously?
AA mode - is that the same as setting it to room temperature mode?
It is difficult to state definitively what is happening within your system, since I don't know how it is constructed. Is your heat pump actually still running after the boost period has ended?
If the heat pump is actually stopping after the boost period, then when it restarts, the heat demand will be lower at warmer outside air temperatures, and the required LWT will have been adjusted lower by the WC.
AA mode is used when in room temperature control and can help reduce the frequency of cycling when correctly adjusted.
After you have boosted the slab and got it really warm and then switched back to WC control, the LWT is defined by the external temp. At -3 it is fine, the circulating water is still warming the slab, but as it warms up outside the flow temp is adjusted and drops to below the temperature of the slab. In a few weeks I reckon you will see it start cycling straight after the boost period as the nights warm up. On my radiator-based system I only see that absolutely flat temp line when it is in freeze stat mode and circulating the water to take a bit of heat out of the house to keep it above 5C. I'm sure you don't really want to be taking heat back out of the slab!
Effectively you are running an old-fashioned Economy 7 setup, cheap electric to add heat to a thermal store at night and then use that heat during the day. I think ideally you need some guesstimate of how much heat are you putting into the slab in the boost period and consider how it changes the requirement to add more heat during the rest of the day. There'll be a point where that is enough to last the day and you won't need to run the heating – and I might suggest that even now you don't need to run the heat pump straight afterwards when it is -3 and at its least efficient, even if it is adding a bit more heat.
Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop 210l Mitsubishi solar tank Solar thermal 3.94kW of PV
It takes about 7 hours from end of the boost to actually start cycling - does that still seem correct? It doesn't start cycling straight away.
II have a related issue, think the cause is similar to yours After "charging" the UFH over night (100mm concrete slab) when I reduced the flow temperature, heat pump started cycling. (yellow and orange lines).
Still trying to work out how to get the best from the concrete slab and economy 7 tariff, but have discovered you can't reduce the flow temperature very quickly, you have to reduce it in line with the cooling of the slab.
I am not sure if weather compensation takes into account that you may have a massive thermal store in the floor.
Have since wondered if using the concrete slab as a heat store with too wide a temperature range may induce cracks in it. Still thinking about that.
It takes about 7 hours from end of the boost to actually start cycling - does that still seem correct? It doesn't start cycling straight away.
II have a related issue, think the cause is similar to yours After "charging" the UFH over night (100mm concrete slab) when I reduced the flow temperature, heat pump started cycling. (yellow and orange lines).
Still trying to work out how to get the best from the concrete slab and economy 7 tariff, but have discovered you can't reduce the flow temperature very quickly, you have to reduce it in line with the cooling of the slab.
I am not sure if weather compensation takes into account that you may have a massive thermal store in the floor.
Have since wondered if using the concrete slab as a heat store with too wide a temperature range may induce cracks in it. Still thinking about that.
Is your heat pump stopping and starting and what is the indoor temperature?
I dropped the flow so it was 28 and got the following:
So I'm assuming the RWT (a steady 27) is too close to the target LWT (28) and so can't provide it a steady flow. Similarly I dropped it further (to 24) and got the following:
At this point the HP just turned off completely and didn't even try to do anything. Makes sense as if the RWT is higher than the target flow then there's nothing for it to do.
So ideally I think I would like it to turn off like it has now but at a higher temp. So for my example where it was cycling at consistently at at LWT of 28 and RWT of 27 this should be off instead. Is this where the termodiff that you mentioned comes in to it?
Is the thermodiff applied to WC as well as AA? Reason is that I've seen some mentions that AA doesn't work very well with UFH as it is slow to respond, and so might ramp up the flow temp because of this when ideally it should stay low.
I dropped the flow so it was 28 and got the following:
So I'm assuming the RWT (a steady 27) is too close to the target LWT (28) and so can't provide it a steady flow. Similarly I dropped it further (to 24) and got the following:
At this point the HP just turned off completely and didn't even try to do anything. Makes sense as if the RWT is higher than the target flow then there's nothing for it to do.
So ideally I think I would like it to turn off like it has now but at a higher temp. So for my example where it was cycling at consistently at at LWT of 28 and RWT of 27 this should be off instead. Is this where the termodiff that you mentioned comes in to it?
Is the thermodiff applied to WC as well as AA? Reason is that I've seen some mentions that AA doesn't work very well with UFH as it is slow to respond, and so might ramp up the flow temp because of this when ideally it should stay low.
Obviously, no two homes are the same, so what works with one may not work with another.
If you have a temperature sensor in the living area, you could try the following. Set your system to room temperature control with a time interval of 60 minutes, an upper temperature limit of +5 and a lower limit of -9.
Allow the system to operate for some time and record the results.
@derek-m The pump keeps running, seems the compressor switches off. room temp is 20.6 (added to chart below)
The objective would be to allow the concrete slab to heat your home with the heat pump off, until the indoor temperature starts to fall and the heat pump is once more required.
One possible way would be to use a bog standard thermostat, to switch the heat pump off, and then back on again once the indoor temperature has reduced. It could mean a 1C or more variation in indoor temperature, but it should reduce the frequency of the cycling.
Thinking about installing a heat pump but unsure where to start? Already have one but it’s not performing as expected? Or are you locked in a frustrating dispute with an installer or manufacturer? We’re here to help.
