thanks Derek, we do have a LLH my understanding thats to provide hydraulic seperation of primary and secondary circuits and cope with having multiple pumps on secondary 

Regarding the temp I will try the 50 at -5 ..... will also discuss with the plumber come commissioning 

Posted by: @denevil

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thanks Derek, we do have a LLH my understanding thats to provide hydraulic seperation of primary and secondary circuits and cope with having multiple pumps on secondary 

Regarding the temp I will try the 50 at -5 ..... will also discuss with the plumber come commissioning 

If you have a suitable thermometer, I would suggest measuring the temperature of the pipes coming from your ASHP, and also measure the temperature of the pipes going out to your heat emitters. The temperature of the pipes going to the heat emitters should be about the same temperature as the supply pipe from your heat pump. If there is quite a difference in temperature then this would indicate a flow imbalance, which can adversely affect both operation and efficiency.

As I dont have an electric car and averge less than 30% electric between 00:30 and 6:30 I decided to switch from Economy 7 to single tariff .... now I can heat my DHW in afternoon and not worry what time the dryer is on. The ASHP is failing to reach 55 degrees for DHW heating as it has at least one defrost cycle in an hour  .... almost got there at 49 degrees ....

Posted by: @denevil

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Hi @AllyFish,  thanks for the detailed reply, ... I had no idea of the defrost details .... 

answers follow

"17kW ASHP. Who sized it & on what basis? "

Plumber working with Grant and Stone who sized and recommended unit. Simon at Grant did the calcs and has been here and checked the plumbers implementation of the standard plan in Appendix A.

"The best time of day to generate your hot water from an ASHP is from mid-day to early afternoon. "

My logic was that in afternoon the tariff is 48p/kwh at night its 18p/kwh .... it seemed effective at night when it wasnt freezing ... now its simply not heating the water .... its tepid .....

"Hot water cylinder in the loft. Is the loft converted & the roof itself insulated? "

No the cylinder is new and is factory insulated with 50mm celotex equiv.

There is still a fair amount of unlagged pipework including in loft ... thats on plumber todo list.

"What's the flow rate on the primary side during heating and during hot water generation? What's the ASHP internal pump flow rate dip switch set to? You would benefit from a flow gauge/setter valve on the return of the secondary circuit, because you can balance the LLH that way, and check primary and secondary flows are closely matched, to avoid water passing through the header in either direction. "

Only thing I can see is flow rate on the gauge in primary return to ASHP and its around 32

"What's the ASHP internal pump flow rate dip switch set to"

I believe it was set to max

"TRVs - set them all to at least one number higher than you want the room temperature to be"

They are all on max and the weather compensation is enabled on the ASHP

I had increased TM1 to 45 but  I could see the flow temp going up to 45 then down to 30 .... then backup .... I wasnt given a monitoring option so its just from random checking at the remote control. Took 15 hours to raise temp of kitchen to 19  which fell to 18 overnight even tho thermostat was 19.5. ...... and thats with DHW turned off.

This is becoming somewhat depressing ... last week all seemed good, house was warm and I had hot water in the morning which stayed hot all day ... 

Hi @Denevil, OK, the low rate overnight tariff is a good reason to generate HW overnight, if not the most thermally advantageous time. Defrosts are annoying but unavoidable with ASHPs. I long for a unit designed for the UK climate that has wider fin spacing on the evaporator, and perhaps a hot gas injection defrost rather than reverse cycle. Cold store evaporators are designed to ice up and don't defrost as often, but an ASHP evaporator coil would need to be much bigger. Since most ASHPs are derived from air to air reversible heat pump products, they use 4 port reversing valves. Problem is, they take energy from the water circuit in defrost, which is why you see the water temperature suddenly drop, and then slowly recover.

Do you have any secondary heating? The LLH has a 3kW immersion installed which can be programmed to provide supplementary heating in very cold weather, heat energy during defrost cycles, or as a backup heater. Of course it could be expensive to run at 3kW and CoP of 1. One option if you have a low overnight tariff is to set the parameters at the Grant controller to enable it if the outside temperature falls below a certain value, and put a timer on the 240V supply to it, so that it's only energised during the low tariff.

32l/min flow rate sounds reasonable, perhaps a little low depending on your design temperature:

To get 17kW 'nameplate' out of your ASHP and with pure water circulating, that flow would be a flow and return delta T of 7.6. I've never seem my Grant Aerona 10 achieve anything higher than 6degC delta T, even though the flow rate is optimised and the delta T parameter set to 8. That may be a due to the size of my heat emitters and max flow temperature as opposed to the ASHP itself.

I would recommend checking the balancing of the LLH, which preferably means a flow setter/gauge on the secondary side, or digital temperature probes to do it thermally. I bought these, which are very cheap and reasonably reliable when the probe is secured to the copper pipe under close fitting insulation.

https://www.amazon.co.uk/dp/B07TY6HRL4?psc=1&ref=ppx_yo2ov_dt_b_product_details