I'm not sure I entirely understand the problem. Your house has been cooling by 1 or 2 degrees per hour every day for the last week? What temperature did it manage to stay at by the end of the week?

It sounds like it could be undersized, but there's more to investigate. How long per day was it running? What temperature is the flow? What's your design peak heat loss? What's the model of your heat pump? Where is the heat pump positioned as in is it near any walls? If so on what sides and how far away? 

Hi

We started at 22C (using a fireplace and electric heaters which we have been stuck using since April as this has yet to be of any use at all) decided to test the heat pump in typical winter conditions (not expecting much of it as it didn’t increase heat during summer or autumn) We were gaining 2C during the heat cycle of about 50-70min, then loosing 3-4C during the defrost cycle up to 40min each time and had to give up with the entire setup when the home reached 14C during the daytime and reheat with the electric heaters. It’s a Fujitsu AOYG45LBLA6 (air to air) running 6 internal blowers with no programmable control so all set to 30C room temperatures and heat only mode full time. The heat loss the installer has us down for is unknown, they never presented this. Another 2 installers since have estimated 16kW and 21kW so we do think it’s drastically underpowered at 13.5kW but trying to work out if there is anything else or if this alone would cause these issues, mainly the defrosting so regularly. 

Thanks

As Osiwu says, short circuiting of the evaporator airflow, where the discharge air ends up back at the coil inlet face, will reduce performance. But the last two weeks have been exceptional, and all ASHPs have been defrosting furiously every hour with low CoPs and, quite often, flow temperatures not getting up to design set point. Defrost reduces the amount of heat over time in kWh the units can supply, and they take some heat from the water side to provide heat for the defrost cycle. Not ideal, but one of the characteristics of the current generation of ASHPs, you see the flow and return and feel the radiator temperatures drop when this happens, and you see that more with circuits with a smaller circulating volume.

Is the ASHP a monobloc outdoor packaged unit, or a split system with interconnecting refrigerant pipes? Refrigerant leaks are quite rare with monobloc units as all charging and leak testing is in a factory environment. It's more common with splits where refrigeration pipework is connected on site. A refrigerant leak is unlikely as they tend to cause the ASHP to fault out on low charge, monobloc units have very small charge quantities of refrigerant.

The unit could be under-powered, but if it drawing 23A it's a larger kW capacity unit I guess. So...

Make/model of unit?

Original heat loss calculation estimate to size the unit?

Design heating flow temperature?

Type of emitters? Wet rads, UFH?

Are rads sized for flow temperature?

Design or actual flow rate and flow and return temperatures? Not all units measure flow, but a flow setter gauge might be on the circuit. Most controllers can display flow and return temperature.

Control method? Does the ASHP run 24/7 or for long periods? Is a master room thermostat switching it on and off frequently?

Any thing else like a low loss header, buffer, secondary pump, etc., that might be contributing to poor performance?

@huwsy I'm not too familiar with A2A heat pumps, nor do I think are most here. Why did you not use the system at all from April to now? If it's very underpowered that's one thing, but wouldn't you use it alongside supplemental heat to get a more efficient system? I also wonder if it's possible to lower the output temperature of the refrigerant to lower time during defrost.

You can calculate your own heat loss if you're so inclined. It involves a few hours of measuring windows and wall lengths then entering the figures into any one of several free online tools or Excel spreadsheets. If you did that then you would know if your system really is underpowered.

How's it doing in this much warmer weather? 

Hi.

The unit is about a foot to 18inch from the wall at the back of the unit on a wall mount bracket, but nothing in front that should cause a short circuit if i understand correctly.

As its air to air a defrost cycle doesn't just cool water in a system it results in cold air being blown into the property at about 10-15C. But it does mean the intended flow output of refrigerant is 50C so by the looks of it this impacts COP and defrost substantially more in the cold.

The outside unit has one large radiator component but 6 separate flow and return pipes from this to each internal unit, all piped outside the property and all the insulation on the 50m+ external pipework is only 8mm and sitting at over 20C when running even in sub zero outside conditions.

The unit is a Fujitsu AOYG45LBLA6, 13.5kW nominal.

Original calculation unknown, more recent calcs (no property changes) 16kW+ and 21kW heat loss.

Flow temperatures, being air to air are between 25C and 50C of refrigerant along all pipe runs.

Output are 2x 12kBTU and 4x 9kBTU internal blowers.

No record of flow rate or return temperatures known or ability to access without specialist Fujitsu computer interface.

Control is non programmable, runs 24/7 at 30C heat only mode and auto fan speed, when we bother attempting to run it although only capable of heating the home when above 12C outside so no use in winter or autumn.

No additional parts in the setup.

Thanks

Hi

We didn’t use it from April until now as running it even in summer temps it was running at anywhere above 4kW average input power but using electric heaters we could get the same effect off 2 electric heaters at 2kW (averaging 1-3kW with their cycling through a day in summer, autumn and winter we have to also light a fire)

Output temps aren’t adjustable only room temps and it varies the temp depending on how far off the rooms are from the temp.

I will be doing my own calc tomorrow, a rough calc using the online plum base calculator for radiator sizing gave me a total 22kW. Mainly down to solid stone walls.

In the warmer weather it doesn’t defrost as much, although still every few hours. But runs at excessive cost to justify not bothering and using electric heaters or a fire until resolved.

Thanks

Posted by: @huwsy

↑

Hi.

The unit is about a foot to 18inch from the wall at the back of the unit on a wall mount bracket, but nothing in front that should cause a short circuit if i understand correctly.

As its air to air a defrost cycle doesn't just cool water in a system it results in cold air being blown into the property at about 10-15C. But it does mean the intended flow output of refrigerant is 50C so by the looks of it this impacts COP and defrost substantially more in the cold.

The outside unit has one large radiator component but 6 separate flow and return pipes from this to each internal unit, all piped outside the property and all the insulation on the 50m+ external pipework is only 8mm and sitting at over 20C when running even in sub zero outside conditions.

The unit is a Fujitsu AOYG45LBLA6, 13.5kW nominal.

Original calculation unknown, more recent calcs (no property changes) 16kW+ and 21kW heat loss.

Flow temperatures, being air to air are between 25C and 50C of refrigerant along all pipe runs.

Output are 2x 12kBTU and 4x 9kBTU internal blowers.

No record of flow rate or return temperatures known or ability to access without specialist Fujitsu computer interface.

Control is non programmable, runs 24/7 at 30C heat only mode and auto fan speed, when we bother attempting to run it although only capable of heating the home when above 12C outside so no use in winter or autumn.

No additional parts in the setup.

Thanks

So it would appear that you have a 13.5kW heat pump supplying 15.6kW's of fan units, which may not be adequate if your calculated heat loss is 21kW.

Check your heat loss calculations.

Is your home large and/or poorly insulated? The first thing that I would suggest is to draft proof and insulate as much as possible.

I have a small A2A heat pump which is mainly used for heating during the Spring and Autumn period. One thing that I have noticed is that if I set it for an indoor air temperature of 30C, it does not get to that temperature but the outdoor unit tends to run almost flat out. If I set the desired indoor temperature at 22C, I get a temperature in the region of 21C, but the outside unit is running much slower. I would therefore suggest that you turn all your fan units down to 22C and see what happens.

If all that fails to improve the situation then you may have to get an 'F gas' Engineer to check your system and the quantity of refrigerant gas.

50C refrigerant across 50m+ of pipework will lead to monumental losses across the pipes with only 8mm insulation. I suspect (without calculating) possibly over 2kW of losses just to the air when below freezing. I think you might get better performance turning the fans on the indoor units to full and decreasing the refrigerant temp by turning the thermostat of the indoor units down if as you say that's what controls the refrigerant temp. 50C also sounds very hot for air to air.

At the end of the day depending on the accuracy of your heat loss calcs it just sounds woefully undersized. I would look into lagging your pipe runs, running at lower temps, then considering doubling up the system or some major insulation.

Try the MCS heat loss calculator if you want to be sure you're using the right methodology. Estimating the U value of solid stone walls might be difficult though. 

Hi

Thanks both, Setting lower temps like 22C knowing it doesn’t achieve 22C when set at 30C seems counter intuitive but I will retry in the next cold snap on high fan speeds and lower room temps.

Out of interest should these or air to water ever be able to get to 25/26C within rooms over winter or is that too high to achieve as most things I see are for around 21/22C

The home is only slightly above average size but solid stone dating early 1800s amongst other designs ideas from the time so the heat loss isn’t great. 

Thanks

@huwsy I see no reason why air to water heat pumps should not be able to get to 25C room temp, it's just a matter of balancing energy in and energy out. It's a very high temperature to aim for though which is why I imagine you've never seen people discuss it.

Hi

We stipulated to our installer that we needed 24-26C out of medical necessity, sever risk to life, so it’s rather concerning that this isnt currently possible with our setup without additional heaters and if it may not even be achievable if it was correctly specified is somewhat upsetting. 

Thanks