Not got the answer but sounds like it could be similar to one of my posts.  Have a look?  My HT Daikin freezes far too often when cold, then struggles to warm up the leaving water temp / refrigerant.  Then typically enters a defrost again.

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/problems-with-daikin-18-kw-high-temp-ashp/paged/3#post-26858

Thanks but what im observing is that it doesnt get up to say 60 or 65 even when no defrost cycles. Today for example it has been running continuous with no defrost cycle for nine hours and is only at 54 degrees (refrigerant 44) despite the system set for 65 degrees. 

Hi @signorjim, the slow rise in leaving water temperature from your ASHP [I presume you mean, rather than refrigerant temperature], may be due to the kW capacity of the ASHP being equal or less than the total emitter capacity. An ASHP typically raises the circulating water temperature by 5-8degC. If the return water is coming back quite cool, and the house heat loss is about the same in kW as the ASHP rating in kW, it won't be able to raise the LWT. That's what is probably making it go v slowly. The leaving water temperature will only rise when the return water temperature starts to rise. They will track each other at a fairly constant delta T, then when your house heat loss starts to reduce, and rooms get up to set point temperature, the RWT will start to rise.

It also depends on how you run and schedule your ASHP. If you switch the heating off overnight, you have a cold system and a cold house in the morning. ASHPs starting from cold don't have the massive heat emission capacity a fired boiler has. Boilers are usually oversized, ASHPs are usually sized with a very small capacity margin. They are intended to run 'low and slow' and continuously. (That said, we turn ours off overnight between 9pm and 4am, but it takes quite a while - several hours - in the morning starting from cold to recover the heat energy lost overnight from, the house, the hydronic water circuit and the refrigerant circuit and get to within 3or4degC of target LWT. Our 10kW ASHP is marginally sized, with little in hand, due to DNO supply issues to our property meaning we could not install the next size up 13kW without major and costly supply upgrade work.)

You have a HT heat pump, but do you need a LWT of 65degC for central heating, unless this is just to try it out and see if it can achieve 65degC? It would be much more efficient and cheaper to run to have the unit running continuously at a significantly lower LWT and with weather compensation enabled, rather than heating for part of each day at 65degC. Every 1degC reduction in set point LWT reduces and ASHP electricity consumption by about 2.5%

Thanks for that Allyfish. thats really interesting. I thought the problem was that the refrigerant wasnt heating up. I thought the refrigerant took the lead rather than the delta T. At the moment the delta T is set at 10 degrees and on the Altherma hydrobox that figure cant be altered on the radiator setting. Delta T can only be altered on the fancoil setting which gives a max LWT of 55 degrees. It can set be between 1 and 10 degrees c. 

The ASHP is I believe 13kw. In all other regards the system works well apart from when it is interrupted by a dhw or defrost. I need it to get back to heat quickly between defrosts/after dhw cycle.

The altherma is 12kw and its a big victorian five bedroom house 270sqm

@signorjim delta T flow and return of 10degC is quite high for an ASHP. At 10degC delta T and 17l/min flow it's theoretically giving out 10x4.2x(17/60) = 12kW. A very high LWT setting is going to take a long time to reach from a cold start.  Defrosts adversely impact that as well. Set the unit back overnight if possible rather than switch off. (I switch off because my Grant Aerona has no set back function that doesn't start to cycle the unit on/off on a room thermostat, and her who shall be obeyed wants a cool house overnight!)

DHW cycle: schedule it when it is least disruptive to your heating needs, and schedule it just once a day, twice at the most - you don't want to be charging the DHW cylinder several times a day just because the cylinder stat is calling for top up, a timer should limit how many times DHW cycle operates and to when it best suits. Best time is usually early afternoon when the outdoor air temperature is highest, which means the heat loss from your house is lowest and the enthalpy [energy] available in the outdoor air for the ASHP to capture is highest. If you have a low rate EV tariff or something, then you may prefer to take advantage of off peak DHW generation, but the wee small hours of the night are not the best time for an ASHP to create DHW in terms of COP.

Oh, you've thrown me slightly mentioning 'fan coil'. Is this an ASHP connected to radiators, fan assisted radiators, or fan a coil? 65degC is traditional radiator temperature, too high for fan coils!

We have normal radiators in the house (apart from UFH in the kitchen which has the water temp lowered by a valve)but the Altherma has three possible settings - UFH, Fancoil (restricted to max 55c but you can alter the delta T between 1-10c) and radiators (max 70degree lwt but you cant change the delta T - its stuck at 10). 

Yes the DHW is set on schedule for an hour mid-afternoon and an hour at midnight and mostly works fine but occasionally have to top it up.

Is it possible12kw is not powerful enough? I imagined that a defrost cycle knocks the temp back for 8-10 mins or so when the unit defrosts but then should get to temp again (we also have the additional small reservoir to minimise water temp drop during defrost). If it cant get up to temp in between defrost cycles then the whole thing is a bit useless for three months a year...

Posted by: @signorjim

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Is it possible12kw is not powerful enough?

Depends on the heat loss of your house and how warm you are wanting it. Defrost can effectively de-rate a heat pump's output capacity if the time taken for the defrost cycle and for the LWT to recover is, say, 15minutes, and it is defrosting every hour. It will always be delivering heat to the emitters, but not in sufficient quantity to offset the heat losses, or reach target LWT. It's a fine balance, and one the industry doesn't fully acknowledge yet. ASHP OEMs typically don't give de-rated performance charts when their units are in scheduled defrost, because the frequency of defrost is dependent on the outdoor temperature and relative humidity. We have mild but damp & humid winter weather here in the UK, which isn't great for the current designs of mostly imported ASHPs to be honest. (I have my views on whether a defrost method taking heat out of the radiator circuit is good design and at all appropriate for the UK, and those views are shared elsewhere on this forum!)

Too small a ASHP may struggle (mine does, as I know it's a marginally sized 10kW and Grant / Chofu Japanese ASHPs do have a propensity to defrost frequently). The amount of time it is regularly defrosting per year is probably 4-6 weeks, but only for 2-3 weeks does the indoor air temperature noticeably struggle to reach an acceptable 19 or 20degC. So I sometimes use secondary heating - oil filled rad or two, maybe light the log burner. Not ideal I agree, but in my case because I should really have a 13kW unit fitted. Too large an ASHP can be bad as well, as it won't be as efficient when you don't need large amounts of heat output, it will reach minimum turn down frequency and then start cycling on and off. Too high a LWT can increase the defrost rate as well as the evaporator coil, the bit that frosts up and that is collecting heat from the ambient air, has to run colder with a lower evaporating temperature.

Air to water ASHP technology is simple and incredibly efficient (my total energy bill for a 150m2 4 bed EPC C all-electric house is under £1K a year) but there are a lot of variables the impact the unit performance on the primary side - the weather, which we have no control over, and many variables on the secondary water side. It takes time, skill and a bit of commissioning trial and error to find the sweet spot in all this. But you're absolutely right, we should just switch it on and it should do all that itself. A massively oversized gas boiler with a LWT of 75degC will heat any house quickly, and pre-heat it from cold quickly, by brute force. ASHPs are not designed or sized to work that way. 

Posted by: @signorjim

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Its pushing 17 litres per min

What model is the heatpump, have you checked what the minimum flow rate is for your heat pump 17l/min doesn't sound like a lot for a 12KW unit especially as you have a big house and therefore presumably a large water volume in the heating circuit.

Posted by: @signorjim

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The altherma is 12kw and its a big victorian five bedroom house 270sqm

That's revealing. The house may be super insulated, but typically a Victorian property would have solid walls and non-insulated floors. Do you know much about the house's current insulation standard, and current EPC? (EPCs can be inaccurate however). As a benchmark, a 12kW Altherma heating 270m2 of floor space is 44W/m2. By a typical benchmark, that's quite a low amount. 75W to 100W might be more typical, depending on the insulation. Have you access to the heat loss survey calculations that determined a 12kW ASHP was suitable?