Advice on air-source heatpump/thermostat settings?

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Sounds like you’ve got the same ashp and cylinder as us then :slight_smile:

Getting the data into the emonpi requires some surgery on the ‘remote’, a bit of soldering, an esp8266, and an Arduino… Happy to give you some pointers on this, but I suspect it’s warranty voiding, so you may be better off watching the meter reading and outdoor temperature to give you an idea of ‘better’ or ‘worse’ for each change.

More volume is always better. On our system, most of the cycling was down to the wall thermostat, which implemented something called TPI - a method suited to oil or gas boilers, which uses fairly short on/off periods to try to keep room temperature more constant (the idea being that a more constant temperature can allow a lower average temperature, reducing consumption) This doesn’t play nicely with a heat pump, we noticed a big improvement by changing this for a thermostat that didn’t do TPI.

We don’t get much dT either, approx 2degC is normal. I’ve not been able to determine if the ‘8k’ route of thumb means ‘max 8k’ or ‘min 8k’, but my own musings suggest that the dT should be minimised (heat out of the ufh/rads is roughly proportional to the mean temp, but heat pump cop will increase as follows temperature increases, so we try to maximise mean temp while minimising flow temp)

The other large improvement we saw was reducing the flow temps - ours were set very high, and reducing them closer to MyForest’s numbers above made a huge difference. The method I used to ‘optimise’ our temperatures was as follows:
Set min and max outdoor temperatures to something reasonable (I’ve got our min as -5, max is +18 - below min the flow is set to max temp, above max the heatpump won’t run)
Flow temps can be set based on estimates of heat required at those outdoor temperatures, and the flow temps required to get that power (based on floor/rad area)
If the flow settings are too low, the heatpump will run constantly and the room won’t reach temperature. If too high, the heatpump will cycle and the floor/rads may get hot then cold.
Over a few hot/cold days you should be able to get pretty close :slight_smile:

Aim for the ‘slightly too warm’ side - the extra power will cope with heating the house quickly (if you choose to have a lower temperature setting overnight, for example) and with windy/dark/etc days where losses might be higher.

Mixing rads and ufh can be difficult, depending on setup. If the ufh was installed assuming a standard boiler, it’ll probably have mixer valves to drop flow temps from 70-80degC from a boiler to lower temps for the floor. This can reduce efficiency a bit with a heatpump. Equally, if flow temps are set to suit the ufh, then the rads may not do much unless they’re very large. As yours are upstairs, that’s probably not a huge issue. I don’t have ufh here yet, so can only really recommend that you do some reading about ufh with heatpumps (I did read a few while doing my own research, but don’t have the links to hand at present, sorry)

Hope that helps get you started :slight_smile:

3 Likes
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Hi, which thermostat did you end up using?

We’re putting in UFH with ASHP in a new build and I’ve been trying to work out the best way to control this - the advice about large zones and buffers is very useful and seems to be the way many people go.

Simon

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I actually built my own - I’d connected an esp8266 to the ASHP remote anyway to get data from it, and this also had some DS18B20’s connected to it to measure cylinder and buffer temperatures, so it was fairly straight forward to add another sensor to measure room temp in the main house. I’d have liked to measure a number of rooms, but a single sensor in the living room has worked fine. A basic thermostat would do the same job though really.

That said, if I was building the system from scratch, I’d be looking for a heat pump that could take a proportional input from a room sensor to modulate heat output. The efficiency improvements would probably be pretty small to be fair, but you could end up with a really constant room temperature, and minimum cycling if the heat pump can throttle far enough.

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Thanks Matthew - looks like the similar esp system I built to measure my heat bank will come in useful in the new house after all!

Simon

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Many thanks, all great advice. I’m pretty handy with a soldering iron, but you are absolutely correct on the warranty - some flying leads from the controller would no doubt be obvious to the grant engineer - who’s due to come out in a few weeks to check on a few things. In terms of delta T, I can’t see how it’s possible to keep that at 8degrees under all circumstances - I can just about get it with all ufh heating zones on, but on average it’s around the 4 to 5 mark. I’m tempted to get rid of the thermostat upstairs for the rads - it’s a basic plumb center rf stat with TPI (not possible to inhibit). I may permanently wire the thermostat input and then leave the ashp run for 3 to 4 hours on timer for the 1st floor rads - that will allow the heat pump to run its own cycling routine! Upstairs only needs the occassional heat top up

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Yeah, let the warranty run out first at least :slight_smile: Some detail here though if you’re interested: Reverse Engineering Grant ASHP Remote Protocol | Hackaday.io (I need to update that - an esp8266 couldn’t cope with the slow serial from the heat pump, pressing buttons, and talking to the DS19B20s, so I used a arduino to talk serial to the heat pump and press buttons, and send data to the esp over SPI)

Yes, I’d be tempted to hotwire the upstairs stat (worth trying ‘off’ too?) and running the heat pump on a timer. If you get the flow temps set nicely, that’ll be all you need.

Have a play with this ods spreadsheet. It helped me work out decent flow temps and settings (the first few sheets were me working out mass flow rates to roughly emulate a heat meter - I know the mass and temperature of the water in the cylinder, and the time taken to heat it, so I know watts in and watts out :slight_smile: )CoPcalcs.ods (96.5 KB)

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Many thanks, will have a play with that spreadsheet :+1:t3: I think you’re right, bypassing the stat is the way to go - damn TPI on an ashp :rofl: unfortunately i didn’t do the research to challenge our plumber beforehand. I normally do my research…

I’ve just set up our first compensation curve - luckily we’ve had a good temperature swing over here in the past month from minus 8 to plus 15 external. I’ve gone for a max temp of 45 and min of 30, between external temp of minus 6 to plus 15. I think that’s a good starter. I have noticed that the ashp controller is displaying the external temp a couple of degrees higher than my independant thermometer. The 2 degree difference is pretty consistent.

Is your integral 50litre buffer tank plumbed as a true buffer (two in two out) or just as a volumizer (one in one out)? Did you get any good advice from the heating engineer who installed the system?

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No problem at all. Forgot to add that is possible to set the flow temps low enough that the floor/rads can’t dissipate the minimum power from the heatpump -may be better in this case to accept higher flow temps and more thermostat cycling, but also may be an indication of too small rads (it is here, the system was specced for ufh which we haven’t installed yet) unlikely to be an issue for you though.

Don’t worry, it’s not exactly an obvious thing to check!

Yup, sounds reasonable, and it’s easy enough to adjust if you need to :slight_smile: ours is a couple degrees off too, not sure if it’s just slightly off, it affected by other great sources, but it doesn’t really matter.

Our buffer is a ‘true’ buffer. A volumiser would be a little more efficient, but not sure where the choice come from. Perhaps just a recommendation from Grant.

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Yes, there comes a point when radiator convection fails if temp is below a certain figure. Was your system designed and installed by grant, or was it all spec’d/installed by a local heating company?

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From the paperwork we have, it looks like the installers spec’d it, but it could quite easily have been copy-pasted from a Grant selection. We didn’t deal with Grant directly at any point.

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:+1:t3: Thanks