Thanks for the additional information. 

As I say 2 hours to get up to temperature after a long shut off is not totally surprising (albeit that the LLH may well be contributing - we may circle back to that later).  However I would initially focus on performance when defrost is occurring which, as I understand it, is when your house is not warming sufficiently (please correct me if I have misunderstood your problem).

Looking at the plot most recently posted, it seems that the system keeps up initially (I can be certain about this because I dont know the target flow temperature, but it stabilises so I presume this is at or around the target), but then there are again extended periods when the compressor frequency is zero and the flow temperature thereafter is a bit all over the place.  Why are there extended periods when the FT is zero and what is happening to the room temperature (which I think is the problem) in this period and the OAT.

Im basically trying to find the evidence in the plots of the problem you refer to.  If you hadn't said there was a problem the plots appear fairly normal except for the unexplained periods when the compressor is off for an extended time.

Hi, 

I'm saying this from my experience, not as a heating expert:

Your compressor is turned off for too long periods. You may think the UFH is at temp, but low down its cooling. and when its OAT is cold, needs constant flow going through it. I would turn down the flow temp and run it constantly.  DT 10 the unit straining like mad to heat the screed. I too though my unit was undersized, but after I turned to flow down and ran it constant, I could keep the house topped up with heat for 8kw an hour produced at freezing (heat loss 11) and the pump had lots more in the tank. 

My graphs looked just lie yours when was aiming for a temp that the screed could not get to. 

In mild weather, I tend to batch heat a bit more until I get a weather comp that I'm happy with

at DT10, your little unit is putting in over 10-11kw of energy, then is idle or just ticking over for large parts in the day. You need to even this out more

I did think about trying that this year just need to convince my financial voice to try it, as it will be significantly more expensive, it will pull 25% less energy but at 4 times the price

@gary

I get where you’re coming from. But I don’t think you’re undersized . You’re trying to cram in all the heat your house needs into certain periods of the day. This is a perfectly reasonable thing to do, but with the nature of heat pumps, you’ll defrost a lot. 

Posted by: @davidnolan22

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Hi, 

I'm saying this from my experience, not as a heating expert:

Your compressor is turned off for too long periods. You may think the UFH is at temp, but low down its cooling. and when its OAT is cold, needs constant flow going through it. I would turn down the flow temp and run it constantly.  DT 10 the unit straining like mad to heat the screed. I too though my unit was undersized, but after I turned to flow down and ran it constant, I could keep the house topped up with heat for 8kw an hour produced at freezing (heat loss 11) and the pump had lots more in the tank. 

My graphs looked just lie yours when was aiming for a temp that the screed could not get to. 

In mild weather, I tend to batch heat a bit more until I get a weather comp that I'm happy with

at DT10, your little unit is putting in over 10-11kw of energy, then is idle or just ticking over for large parts in the day. You need to even this out more

 

Can I just say that this is what I think too, but I was waiting for @gary to answer the questions before being certain about the conclusion. 

Heat pumps need to be on 24*7 during the height of the heating season, assuming that they have been correctly sized.  The FT needs to be turned down as low as possible (to be done with thermostats, TRVs etc set well above desired temperature).  In the shoulder season, or with a well oversized heat pump, batch heating may be possible and may work out cheaper, although there is no guarantee. 

Operated this way its reasonable to expect the running cost to be similar to gas or oil, or a little cheaper.  If you qualify for OvOs 15p tariff then definitely much cheaper, particularly if you have UFH and so can operate at a max FT of 35C or below. 

As a concrete example I am operating with radiators and a max temp of 42C, the heat pump is working out ~12% cheaper than gas.  Im buying electricity for it at an average of 19p by using EON Next Drive which gives 7hrs cheap rate at night in return for a 10% daytime price hike.  I operate 24*7 without setback or set forward, so I'm not exploiting the overnight cheap rate to any great extent.

Posted by: @gary

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he issue is that the heat pump cannot get to the set flow temperature during the coldest periods

Hello, I confused about why this is a problem for you.  Is your house not getting warm enough?

Thanks Bob

Correct it’s designed to 45 at -2C and can’t get to 45C house isn’t cold but 20 rather than 21C

@gary mine was “designed” to 45, bit system can’t get above 36. That bit was nonsense. 

from looking at your graphs of one day: you’ve got loads of scope to get more energy into  your house, if you want/need to,  by changing your control strategy. 

Yes the examples I gave weren’t when it was at its coldest that week when it didn’t get above zero and dropped to -4 overnight I don’t have data for then but it was running 24/7 

@gary some the trends you've posted show the LWT is at the setpoint.  They also show the setpoint is changing and they show the compressor  frequency at zero (not running).  This makes me think you're running with a control scheme that's changing the LWT to meet a room temperature demand and then switching off when the demand is met.  Do you have an EcoDan installed with room temperature auto adapt?

Bob

No not auto adapt running hotter at cheaper times cooler at expensive times.  Off is when demand is met on milder days.  System won’t run below 32c without cycling 

Not sure if you have the defrost cycle and what happens correct. The heat pump reverses the refrigeration cycle not the heating water. The circulation pump runs as normal. Hot and cold refrigerator gases swop roles at the condenser only. So instead of being in heat mode it moves to cooling mode.

So cold water from defrost does not enter the return it enters the flow side. But a big volume of water on the flow side will allow some mixing and less very cold water will hit the radiator. Just slightly cooled water.