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 Monday at 16:15 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 23 hours ago 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 2 hours ago by SimonD