Interesting couple of days with the new heat pump.

[JohnMo]

You can work it out knowing your heat loss and then W/m²

 

Just use this as a baseline, you may need to extend the W/m² and the MWT - mid way between flow and return temperature.

[SimonD]

24 minutes ago, MikeSharp01 said:

because there is a hint the heat goes up when the temperature goes up

 

This could just be the shift settings need adjusting but at the moment, if you've got min set to 28 and max at 32, your curve is definitely too flat. What's the calculated flow temp and min outdoor air temp? And what is your calculated heat loss at this temp? You can simply calculate your heat co-efficient W/K to establish a baseline curve at all relevant temps.

[TerryE]

Mike, it looks very much as I'd expect, that is no surprises.  I did warn you about the consequences of the 185s before you poured concrete but that conv obviously went into the information overload bin, but this falls into the 'it is what it is' category rather than something that needs fixing.  The main consequence is that the flow speed is double what it would have been, with the increased circulation noise, and hit on your pump life. 

 

Your return temp is still 5°C more than room temp, so the avg slab surface temp is prob 7⁺°C warmer than the room env, and so you are dumping ~100W/m² from the slab into the house, which seems to be heating the house by maybe 5°C per day.  That's a hell of a lot for a passive class house, and you'll need nothing like that once the internal fabric is at target temp.

 

You might want to experiment setting the house set-point to say 18°C and let the system reach equilibrium, leave a day or two; then step up the set-point another °C; rinse and repeat.  That way you aren't going to be stressing anything too much.

Edited February 9 by TerryE

[MikeSharp01]

3 hours ago, TerryE said:

I did warn you about the consequences of the 185s before you poured concrete but that conv obviously went into the information overload bin, but this falls into the 'it is what it is' category rather than something that needs fixing. 

Yes I recall now you mention it. To be fair the heat pump is, for the most part working well. The house is over warm now, so I have turned it off until I can get some more info from the manufacturer about controlling the curve. As it stands although I thought I had control of the two endpoints on the 'custom' curve I don't so I am awaiting an update on how to get control of it. I did think today, in the midst of my frustration with this, that now I have my MCS certificate I should just pop a Willis heater in to run it when the output required is less than the heat pump minima - how cheeky would that be?

[JohnMo]

14 minutes ago, MikeSharp01 said:

should just pop a Willis heater in to run it when the output required is less than the heat pump minima - how cheeky would that be?

Think you maybe over thinking stuff.

 

A heat pump cycling is a reasonably normal thing. If it runs for 10 to 15 mins it over comes any startup losses. As long as it's not 5 mins later you have no issues. Quick restarts are down to setting startup dT correctly. Don't get bogged down, thinking it's bad in mild conditions. It isn't.

[TerryE]

Mike, one thing that you might try is get an idea of the time constant for the house at some point.  Heat the house to some set-point and keep it at that for a couple of days, then tiurn off the heating for 24 hrs, say, and track how the house cools. 

 

We have low overall U-value loss, but high internal fabric total specific heat ("thermal mass"), so we lose maybe 1°C / per day with the heating off.  This means that we can shift the heating windows to pretty much any time that the electricity costs are cheap and make maximum advantage of a ToD tariff.

 

You might find the same, but some build techniques use a smaller slab volume inside the thermal envelope, plus low SH insulation so the time constant is shorter and the house loses  2-3 °C / per day.  This impacts your best heating strategy, so it's a characteristic to understand. 🙂

Edited February 10 by TerryE

[JohnMo]

20 hours ago, TerryE said:

we can shift the heating windows to pretty much any time that the electricity costs are cheap and make maximum advantage of a ToD tariff.

 

You might find the same

I have done all sorts of modes of heating, including just heating during E7 period, Cosy periods etc, and by far cheapest (with battery and ASHP) is just letting the heat pump run on WC. A 5 degree day today and it's ticking away at a CoP of 5.78 so far today. Any sort of batch charging, can drop CoP to the mid 3s

[Nickfromwales]

On 09/02/2026 at 19:27, MikeSharp01 said:

I should just pop a Willis heater in to run it when the output required is less than the heat pump minima - how cheeky would that be?

Not cheeky, just daft, sorry. 
 

You’d need a second circulating loop into a low loss header, and a re-plumb, as you’re reliant on the HP for circulating the water. 
 

Note that the Willis are 15mm too, so cannot simply ‘go inline’ as it would choke the flow. 
 

Whenever I fit Willis I always fit 2 so the 22mm flow goes through “30mm” of Willis heater(s), which some goons just don’t understand or appreciate.

 

Some say 1x will work, yes it will, but 2 is less than £100 more, can go inline, and will offer redundancy and fail safe. Jet is to plumb them hydraulically identically so they both see the same throughput; so one doesn’t bypass the other and so both share the duty.

 

Mike……”No” 👎. Not needed, and your COP will be 1. 

[SimonD]

On 09/02/2026 at 19:27, MikeSharp01 said:

I did think today, in the midst of my frustration with this,

 

Stop fiddling!!!!! Be PATIENT 😉😊 You are barely into this by a week.

 

This is not enough time for the system to balance itself out. Get the information from the manufacturer technical support you need to input a baseline WC curve correctly. Calculate your heat co-efficient for your house in W/K and use those to calculate your flow temps at the controller inputs. Then leave the system alone. And if you're tempted for reach out and make adjustments, slap yourself on the wrist and go do something else. Yes, of course look at the data to make sure it's running OKish, but let it do it's thing over at least 24 hours or preferably a week - as long as the WC curve is nice and low. Use this period not as a fiddling period, but one of research - gather data to understand how the heat pump is working in context - then you'll understand when it starts to cycle (if at all), what it really modulates down to etc. etc.

 

I know you want to play, but heat pumps, big specific heat capacities, new building and all that do not like this. They want to chill out and relax, take it easy and watch the world go by 😉 

Edited Wednesday at 09:47 by SimonD

[PhilT]

23 hours ago, JohnMo said:

I have done all sorts of modes of heating, including just heating during E7 period, Cosy periods etc, and by far cheapest (with battery and ASHP) is just letting the heat pump run on WC. A 5 degree day today and it's ticking away at a CoP of 5.78 so far today. Any sort of batch charging, can drop CoP to the mid 3s

Surely by far the cheapest would be running continuously with Cosy? You get 8 hours plus battery hours at the reduced rate and only 3 hours, or no hours with a big enough battery, at the premium rate.

[TerryE]

8 hours ago, Nickfromwales said:

your COP will be 1. 

 

No, that's me. 🤣

[MikeSharp01]

10 hours ago, Nickfromwales said:

Mike……”No” 👎. Not needed, and your COP will be 1. 

Yes I get that and I was only Joking anyway. 

 

9 hours ago, SimonD said:

Stop fiddling!!!!! Be PATIENT 😉😊 You are barely into this by a week.

Yes I am being now as the house is too hot for heavy plaster boarding so I have left it off while I await the technical curve setting details. Even leaving it off is quite instructive though. the slab has lost 1 degree in 24 hours with an external temperature (OAT) of 10 degrees form 21.5 - 20.5 while over the same period the room temperature has dropped slightly less - could be because I was working in there today.

 

[SteamyTea]

70 tonnes of concrete holds about 15.5 kWh.K^-1.

The losses are about 650W in total, with about 500W going usefully into the house.

[TerryE]

My friendly Gemini AI gives me 17 kWh/K so (1) is the right ballpark.  However, I can't recall Mike giving details on his slab dimensions and profile. 

 

You need some  assumptions about the profile, so mine has ring beams cross beams etc with a 10cm fill-in for the slab itself.  If you simplify this to a cuboid slab with area 11×7½  then the average depth is 35 cm which is too high. 

 

I am at a loss as to where (2) comes from.  If the slab is sitting on 20cm PIR then the downward loss would be ~ 120W for a slab at 25 °C sitting on a subbase at 10 °C, plus the edge losses which should be smaller.  The amount going usefully into the house is ~ 7×A×ΔT which more in the 2-3 kW pallpark if is ~ 5°C

[MikeSharp01]

I think the 70T is the whole structure without the cladding or insulation. The slab is 100m² at an average depth of 200mm so 20m³ of concrete. Which interestingly is what it says on the slab concrete bill IIRC. So 50T inc the steel or there abouts. Its been too hot in here for the last two days. I have added no heat in since the 10th and what with solar gain, high local temps, and me working at plaster boarding, the temperature today is 0.7 ^oC lower than it was on Tuesday when I turned it off. 19.9 when I turned the heat pump off and 19.2 now! Can I be impressed with that? The slab has lost 2^oC over the same period and is now at 19.44^oC so perhaps some charging will be needed as the whole house drops together.

 

[Beelbeebub]

1 hour ago, MikeSharp01 said:

I think the 70T is the whole structure without the cladding or insulation. The slab is 100m² at an average depth of 200mm so 20m³ of concrete. Which interestingly is what it says on the slab concrete bill IIRC. So 50T inc the steel or there abouts. Its been too hot in here for the last two days. I have added no heat in since the 10th and what with solar gain, high local temps, and me working at plaster boarding, the temperature today is 0.7 ^oC lower than it was on Tuesday when I turned it off. 19.9 when I turned the heat pump off and 19.2 now! Can I be impressed with that? The slab has lost 2^oC over the same period and is now at 19.44^oC so perhaps some charging will be needed as the whole house drops together.

 

With that level of heat retention your best bet is to maintain the slab at a fairly constant temp, prob 20C or thereabouts.

 

Basically as it is. You probably could try take advantage if cheaper overnight electricity to charge the slab up. Your cop will suffer a little bit but your flow temps will still be pretty low so it won't matter. 

[MikeSharp01]

3 hours ago, Beelbeebub said:

You probably could try take advantage if cheaper overnight electricity to charge the slab up.

I need to look at the Agile tariffs to help once I get it settled down and have the full BMS integration details. As it is I only have the read codes for the Modbus link. So although I can turn it on and off using the call for heat contact I cannot switch modes to DHW other than at the controller itself so not much good for the agile tariffs. 

[JohnMo]

On 09/02/2026 at 19:27, MikeSharp01 said:

have turned it off until I can get some more info from the manufacturer about controlling the curve

I have been tuning my system over the last couple of weeks and kept hitting a lot of instability. I think what is occurring, is outside temp increases, heat pump tries to change flow temperature per compensation curve, but return temperature stubbornly stays where it is, due to the big slab of concrete not changing temperature readily (in like a radio system). So it backs off some more, then some more, and nearly trips itself up, so loads on more power to protect itself and move operating parameters out of trouble zone - this keeps repeating.

 

The old ASHP cycled, so just stopped and waited as temperature outside changed to get a new return temp to it's liking and started up again. The new one doesn't it's determined to keep running, so isn't playing ball with WC. 

 

So I have reverted to a fixed flow temperature, mild temps I cover off with a thermostat as a high stop. I chose 28.5 degs flow temp as that should be ok for my house most of the time, thermostat will stop over heating when mild if heat pump doesn't modulate enough.

 

Suspect your 200mm thick slab (mine 100mm), will cause more issues.

[JohnMo]

To add to the above, if set the circulation pump to automatic modulation, the heat pump will change flow rate to help give a full range of modulating output, so for a fixed flow temperature you get a full range of kW outputs. So the flow meters on the UFH manifold need to be wide open. So low heat required, circulation pump runs slowly and ASHP compressor needs to do less work, high demand, circulation pump runs faster and compressor works harder. So in effect floor becomes self regulated.

[MikeSharp01]

1 hour ago, JohnMo said:

Suspect your 200mm thick slab (mine 100mm), will cause more issues.

It's been a busy weekend family wise so have not had a chance to talk about any issues but you may have hit upon one I saw on Saturday. I was running with normal settings 28 degree flow and trying not to fiddle so as to limit the number of variables as @SimonD encouraged me to. (Remember I am not using WC as I cannot control the end points of the curve yet and I am awaiting a response from the manufacturer as to how to set them - hopefully tomorrow.) and for some reason at 12ish the system just stopped. Well when I say stopped I mean it stopped producing heat and just ran the pump.

Those dips at about 12:30 & 13:15 are me turning off and on again - making no changes. It was a sunny day so the room temperature is rising as is the outside air temperature (OAT) but the heat pump (HP) knows nothing of the Room Temperature and it has no 'stat' anywhere. Initially I thought that the problem was the OAT sensor of the HP because it is in direct sun - something that will be sorted when the bin house is complete but for now I may have to shield it. It was up at 24 deg C. So I left it and went out for the afternoon with my Valentine! Came back at 5 and tried to start it, now the OAT sensor was normal, about what the EMON trace shows (remember that the EMON trace is derived from the local weather station). Still it would not start and I was now quite worried - I have broken it! However at 18:00 I thought OK lets see what happens if you call for a domestic hot water (DHW) cycle. That started first time so the main workings of the HP are in order something else must be preventing it from delivering heat. Anyway while watching some telly across the evening I just prodded it with reductions in the flow temperature, a few times to no avail.

 

I slept on it and this morning I decide to go the other way - upped the flow temperature setting to 31 and off it went. Nothing broken but something odd going on - now all I need to know is why this occurred. Reflecting on it all I think there are aspects of two things hitting the issues in this chart. The first is the OAT sensor - If it gets too hot it won't allow the HP to run anything but the pump, so I am going to have to shield it for now.  ( I do assume it will allow the DHW cycle to run however even though I did not try this.) This ties up with your experience above @JohnMo. Secondly if the delta (Δ) T is less than some value, I think around 5, it won't allow the unit to start although it might allow less if it was already running. So as I lowered the setpoint down in the evening it was too close to the return temperature because the slab (hovering across this whole period at around 22 deg C) was feeding back return water at too high a temperature. If I increase the setpoint enough it would start without issues.

 

I managed to trip the same behaviour on Sunday evening and it is clear that for this heat pump it won't start if the return temperature, after perhaps couple of minutes of the pump starting up, gets too close to the set point the pump won't start but that you can get it to start by upping the setpoint, letting it run for a few minutes and then backing the set point off to where you wanted it to be in the first place. So now all I have to do is find out what setting(s) causes this behaviour, perhaps its some sort of overlap between dead bands somewhere in the system or some such.

9 hours ago, JohnMo said:

if set the circulation pump to automatic modulation

I don't think we can turn this off on ours but I have to say I have not seen much, if any, modulation in output below about 3kW, I will hunt through the trends data and see. I already have all the flow meters wide open.

 

 

 

[JohnMo]

Your seeing pretty normal behaviour amplified with a thick screed.

 

Your set point is 28, heat pump runs until it gets X degs above set point, then trips off. Will not restart until it gets to X degs below target.

 

So your adding heat to slab it's is very slowly released, leave it long enough heat pump will restart. If your not running WC, outside sensor has no input to ASHP running.

[dpmiller]

certainly with the older coolenergy units there is a selectable overshoot as well as the PID settings being accessible