I have an 8.5kw heat pump and its currently running at 36C and pulling 2kw so not miles away from yours, when it runs at 45C its will pull 3.5kw, there is a big consumption penalty for high flow temps.

I've got a 7kW Arotherm Plus which is currently using 1133W with a 31C flow temp and -2C outside. This is with a 0.5 heat curve setting and target room temperature of 20.5C (actual temp 20.1C). I don't have any UFH but had many of the radiators upsized when I had the heat pump installed. The bedrooms are plenty warm enough (I've fitted smart TRVs help the rooms get a bit cooler at night). 1970s 4-bed detached house with cavities insulated, double glazing, etc.

Posted by: @jeegnesh

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This is not so great for the radiators although the temperature upstairs is not bad, likely from the heat rising from downstairs, however if I try to run the upstairs radiators at a normal temp so say a flow temp of 35-40c the electric consumption is around 3.2kw to 4kw 

Is this really normal, as thats a hell of alot of kw consumption if I have it running like this thru the day, I understand setup etc makes a difference, but I just wondered what is normal for for a 10kw ASHP to consume, maybe this is normal I have no idea right now.

Like any heating system the consumption will depend on outside temperature.  At the design minimum temperature you need to deliver roughly twice as much energy to the house as you do at the typical temperature during the heating season.  So if you need to deliver 10kW at whatever your design temp is (eg -2 in south east England), much of the time you need to deliver only 5kW or less

Unlike other heating systems, with an ashp the COP (heat to house/electricity consumption) varies strongly with outside temperature).  At your design outside temperature the COP is likely 3 or less, so the consumption will around be 10/3 = 3.3kW.  At a more typical outside temperature your COP should be more like 4.5 so your consumption will be much lower, around 1-1.5 kW.

Unfortunately a mix of rads and ufh generally means that the heat pump has to run at the higher flow temperature (= poorer COP) needed by the rads.  Essentially you lose the COP benefits of ufh if you also have radiators, unless they are sized for the ft of the ufh.  I picked the COP figures above assuming rads and a ft at the design temp of 45 or thereabouts.  

It's not really valid to give a typical consumption for a 10kW heat pump because the consumption depends on the house loss not the heat pump capacity.  They should, in principle, be fairly close but it's by no means unknown for heat pumps to be significantly oversized, by much as a factor of 2.  From your figures it sounds like yours may be correctly sized, but without knowing your house loss we don't really know.  What does the heat pump claim the cop is (look in myvaillant if you have it))?

Hope that helps.

So according the design info,

Heat Loss: 9.99kw @ -2c

Target: 440% efficiency at 40^oC max flow temperature.

I’ve not been running nowhere near the 40c as it scares the hell out of me (kw cost), but I would like to be able to run closer to that  

Heating COP from app:

Nov - 4.9

Dec - 4.0

Today probably been the coldest day so far temp ranged 3c to 0c and app COP is at 3.6

Posted by: @jeegnesh

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So according the design info,

Heat Loss: 9.99kw @ -2c

Target: 440% efficiency at 40^oC max flow temperature.

 

I’ve not been running nowhere near the 40c as it scares the hell out of me (kw cost), but I would like to be able to run closer to that  

Heating COP from app:

Nov - 4.9

Dec - 4.0

Today probably been the coldest day so far temp ranged 3c to 0c and app COP is at 3.6

Those cops sounds pretty close to what I see (7kW house, 7kW Arotherm).  My energy delivered is currently tracking fairly close to what my measured gas consumption was.

Taking these together and with a bit of electric tarrif manipulation (I've signed up for eon next drive which gives 7cheap hours overnight, I also have an EV) I reckon that means its going to be a little little cheaper than gas over a season.  If I didn't have an EV then the OVO heat pump tarrif 15p for everything used by your heat pump) would easily bring it in cheaper than gas. 

How does your energy delivered (as a function of outdoor temp) compared to your previous (if you know the latter)? Unless there is a big disparity then I can't see what you have to worry about, unless of course your previous fuel consumption scared you too (which is if course possible, but that's just the cost of energy!).

If I take out what the water heating from the gas, then I would say yes the generated kw is about right from previous, I also have an EV and play with the tariffs which is really what saves the day in terms of cost.  So far its def cheaper than my gas bill and we are in the worst months right now, so there is hope 🙂

Couple of things I am interested in knowing

1 - what water pressure do you keep it at, mine is sitting on 1.4 bar

2 - From cool 24/25c how long does it take to ramp up to the flow temp say of 35/40c?

I noticed overnight the call was for 42c flow temp, but after 2 hours it only reached 36/37c then it ramped down onto my lower setting, it seems to love sitting on 30/31c calls for higher seems to take forever and not sure it ever reaches its target - I have not been brave enough to keep higher flow temp for more than 2 hours.

Posted by: @jeegnesh

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Couple of things I am interested in knowing

1 - what water pressure do you keep it at, mine is sitting on 1.4 bar

2 - From cool 24/25c how long does it take to ramp up to the flow temp say of 35/40c?

Vaillant's specs indicate an operating pressure between 0.5 and 2.5 bar for the 10kW Arotherm Plus https://professional.vaillant.co.uk/for-installers/products/arotherm-plus-air-source-heat-pump-10kw-146562.html. Inconsistently, the specs for my 7kW unit say 0.5 to 3.0 bar.

The time to ramp up the flow temperature depends on both system volume and the temperature in the building. If you've let the floor slab cool down then it will need a lot of kWh to warm it up again and that will suck the heat out of the circulating water. You may find it more efficient overall to let the heat pump run for more hours a fairly low temperature sufficient to keep the temperature of the concrete close to constant.

Yes I am finding that running it long and low is the better outcome also more comfortable throughout. 

with regards the water pressure what’s the positive or negative of it being higher or lower considering there such a range from the manual?

Posted by: @jeegnesh

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1 - what water pressure do you keep it at, mine is sitting on 1.4 bar

abut 1.5 bar, but I dont think it matters much unless you have a very tall house (it merely needs to be sufficient to ensure that negative pressure doesnt occur anywhere in the plumbing, there is nothing 'ashp' about this at all the same occurs in any unvented CH system)

Posted by: @jeegnesh

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2 - From cool 24/25c how long does it take to ramp up to the flow temp say of 35/40c?

Here is a shot of some defrost cycles today.  You can see wat the FT does.  Obviously this is with the house already warm.  If the house were cold it would take several hours.

Thanks, I'm away at the minute so cannot get to my HA flow temp graphs, but I have it it running on a more constant basis, and I get this feeling that it never really reaches the target flow temp, I will post some graphs on my return likely Wednesday just to get views on if this is the expectation.

I for sure know that 440% efficiency at 40c at -2 is more like 290/300%

I think that conditions over the last couple of days are about the worst possible - cold but very wet = lots of defrost.  I too am interested in what others see, mine is defrosting more than once per hour (but in fairness is just about keeping up).  I suspect defrost behaviour might vary quite considerably depending on

• Available (central heating) system volume
• Whether the heat pump is oversized or right sized (this is a case where oversizing probably brings benefits)
• The specific heat pump model and how sophisticated its defrost routines are

I may start a thread!