@derek-m I suppose that accuracy of the measuring of ambient temoerature in the locale of the heat pump will partially depend upon the frequency of the ‘polling’; if the sensor is polled every few seconds or several minutes or ten minutes for arguments sake might produce very different reports, perhaps a measurement that tells the pump to start needs to be followed just seconds later to sense that true OAT as seen when ‘new air’ surrounds the sensor. Regards, Toodles.
The frequency of measurement is indeed an important factor in obtaining accurate and precise values. Temperature is one of the variables that can cope with a slower sampling rate, since it tends to change less rapidly than other variables.
Industrial systems often sample the sensors at a rate of 50 times each second, but may only update the display once each second.
I have no idea how often heat pump controllers sample their sensors, and also often display the result as an integer, which is not too helpful.
The heat pump merely tries to supply sufficient energy to meet the demand.
@jamespa I think I am right in saying is of this view, and I do get this point: the heat pump responds to the feedback contained in the RWT. Indeed i was looking into this earlier today, in the context of cycling, trying to discern what triggers the cycle:
On first glance it makes sense that it is the LWT going over a certain point above the set LWT, but in fact it may be the RWT reaching the set LWT. Comments welcome.
I would suggest that you measure the flow and return temperatures at each radiator as accurately as possible, to see if they are each receiving the full flow temperature and to see if the system is correctly balanced.
I would also suggest that you ask Freedom to provide details of the PHE, its make and model. It should then be possible to ascertain if it is actually capable of transferring 12.5kW of thermal energy from primary to secondary, without placing a large burden upon the operation of the heat pump.
Measuring the flow and return temps on the rads is far from easy or reliable, I have tried. Access problems, being consistent with where the IR thermometer is pointing, and with or without black masking tape? More to the point, although the balancing is good enough for me, it is far from perfect, but I am well into diminishing return territory if I do any more to try and balance them. Furthermore, two rads are stubbornly cooler, even if I close off all the other rads, they are reluctant to warm up. It is probably no coincidence they are bother at the ends of their respective pipe runs. The pipework was power flushed, in an attempt to get those remote rads to warm up, but it is still the original pipework, decades old.
I will certainly ask Freedom, nothing to lose, and I might strike lucky.
I forgot to ask. Did the OAT actually increase by something like 9C over a 3 hour period?
Not sure which period you are referring to? If it was on a chart, then that is what the OAT sensor says happened.
To be quite honest, if it was my system I would make a complaint to MCS and any other body responsible, that Freedom Heat Pumps have designed and installed a system that is 'not fit for purpose', since it continually fails to heat your home to even the lower IAT that you require. I do believe the standard design should be able to achieve 21C in most rooms.
To be quite honest, if it was my system I would make a complaint to MCS and any other body responsible, that Freedom Heat Pumps have designed and installed a system that is 'not fit for purpose', since it continually fails to heat your home to even the lower IAT that you require. I do believe the standard design should be able to achieve 21C in most rooms.
Putting aside the toothless qualities of MCS, it is complicated by other factors. If it had been a privately funded installation, my contract would be with my installer, but it wasn't privately funded, it was grant funded (and I am eternally grateful for that, without it I would not have a heat pump), and so in effect there was no contract as such between me and the installer.
I have always been very satisfied with my installer. He moved heaven and earth to get the installation done against some very tight grant deadlines, the short time frame being down to some 'approved' installers who hadn't got a clue and he has been very amenable and helpful throughout. But his modus operandi, which isn't unreasonable, it is a common one, is to be guided by Freedom, in that he got trained by them, uses their spreadsheet, and follows their installation instructions. The root cause of the difficulty comes from Freedom, and the industry model of making installers heavily dependent on suppliers, but I don't have a contract with Freedom, and so can't act against them. All I can do is air my concerns in publicly accessible spaces like this forum...
BTB, I sent that email about the PHE to Freedom a short while ago. If they monitor/read this forum, I don't expect I will get a reply!
Midea 14kW (for now...) ASHP heating both building and DHW
On the subject of control strategy, some units can be set to control on LWT or RWT….
Mine used to be switchable between the two but a recent software update removed that option. You could always strap your buffer temp sensor to the return line I guess.
Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.
My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.
@jamespa I think I am right in saying is of this view, and I do get this point: the heat pump responds to the feedback contained in the RWT. Indeed i was looking into this earlier today, in the context of cycling, trying to discern what triggers the cycle:
On first glance it makes sense that it is the LWT going over a certain point above the set LWT, but in fact it may be the RWT reaching the set LWT. Comments welcome.
Yes. My view is simple, to think about how much energy the heat pump supplies, first think about how much energy is demanded by the emitters.
In more detail:
The heat pump knows: RWT, OAT, LWT and (maybe) IAT. It also has a target LWT determined by the WC curve (or a fixed number).
The starting point for the control system is clearly to achieve RWT = target RWT, which happens by supplying just enough energy to make up the difference between LWT and RWT, ie the energy lost by the emitters at the chosen LWT. That's a very simple feedback loop and, unless there is an auto adapt mode which has been turned on, the only one necessary to achieve what the heat pump promises to do.
If there is an auto adapt mode that presumably operates by adjusting the target LWT based on measurements (and history) of measured IAT/OAT. However that still results in a target LWT which the simple feedback look can then operate on with the same result.
The upshot of this is that the energy supplied by the heat pump is determined by the energy lost from the emitters. This is in turn determined by IAT, OAT and LWT.
Some heat pumps also change flow rate but that has only a very small influence on the energy lost by the emitters at any given LWT. I'm not actually sure whether there is any advantage in modulating flow rate in this way, leaving it fixed actually straightens out the WC curve a bit (for radiators), and since most heat pumps feature linear wc curves that would seem to be an advantage. It may help with the transfer in the heat pumps heat exchanger though.
Various things will trigger the heat pump to shut down (cycle) including LWT > target LWT plus permitted hysteresis, (very likely) RWT > target LWT minus permitted hysteresis, and various fault conditions which leads the pump to conclude that its control loop cannot achieve what is being requested of it or its in danger of being damaged.
On the subject of control strategy, some units can be set to control on LWT or RWT….
Mine used to be switchable between the two but a recent software update removed that option. You could always strap your buffer temp sensor to the return line I guess.
Interesting.
It is of course in principle equally valid (although presumably achieved in practice by turning the 'wick' up or down to change LWT, or by modulating flow rate - which would not achieve the required end result). However it has an inherent delay in its response, which is undesirable in control loops, so I'm not sure why manufacturers would choose this option!
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
@jamespa I’ve spent ages tweaking the P, I, and D on mine to try and get a decent response without overshoot. It’s not easy, especially when 1. We’re about 2kW oversized, 2. The heat pump is on the end of 15m of primaries.
Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.
My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.
@jamespa I’ve spent ages tweaking the P, I, and D on mine to try and get a decent response without overshoot. It’s not easy, especially when 1. We’re about 2kW oversized, 2. The heat pump is on the end of 15m of primaries.
What kind of control system are you using, is it a single loop or a cascade (master - slave)?
@derek-m single heat pump, just run on weather comp, no stats, no setback.
The Carel platform that my heat pump is built on allows full tuning of the PID settings, along with full control of the compressor ramp speeds. It’s close to maintaining setpoint, but I’ve still got some work to do. I’d like to deal with the initial overshoot next, I think I’ve got the oscillation damped quite well.
When it’s around freezing outside, it runs happily for a long period of time, it’s dealing with the overshoot when it’s cycling that’s the next area of tuning that needs my attention.
Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.
My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.
@jamespa I’ve spent ages tweaking the P, I, and D on mine to try and get a decent response without overshoot. It’s not easy, especially when 1. We’re about 2kW oversized, 2. The heat pump is on the end of 15m of primaries.
Interesting, is this to get the house temp right or just the flow temp. Intuitively would have thought that the latter should be quite straightforward, but maybe compressor/fan responses complicate things.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
@jamespa flow temp.... get that accurate first, then tweak the WC curve up/down as required. Think I've got the curve vaguely correct at the moment, standard GH lizzie curve but with the top end down at 47.5@0
Although there's no such thing as 'too hot' in this house - Wife and Sister-in-law (who live there) are happy as long as it's >22
Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.
My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.
@derek-m single heat pump, just run on weather comp, no stats, no setback.
The Carel platform that my heat pump is built on allows full tuning of the PID settings, along with full control of the compressor ramp speeds. It’s close to maintaining setpoint, but I’ve still got some work to do. I’d like to deal with the initial overshoot next, I think I’ve got the oscillation damped quite well.
When it’s around freezing outside, it runs happily for a long period of time, it’s dealing with the overshoot when it’s cycling that’s the next area of tuning that needs my attention.
I suspect that it will not be possible to accurately control the LWT during milder weather conditions, simply due to the fact that the heat pump will be starting to operate outside its control envelope. i.e. it cannot turn-down any further.
As far as controlling IAT is concerned this should not be a major issue, though it could take some time to optimise for maximum efficiency.
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