@emceed

Hi. Im not an installer but as no one else has answered you, I will try and help.

28mm copper pipes can move about 10 kW of heat at 0.9 m/s so would seem unsuitable for a 16 kW heat pump. Adding a buffer vessel should help your flow rate issue but will have a knock on effect of lowering your efficiency and increase your bills.

Do the 28mm pipes connect your heat pump to the cylinder/heating system? If so they are too small if you actually need 16kW, but if you think your house doesn't need that much heat it could maybe work.

The energy usage you quote is eye watering, and no the heat pump should not be heating the hot water and space heating at the same time!

Do you have any kind of monitoring on the system? That would be very helpful in trying to find out the best way forward. If you dont then investing in some might be a good idea. Something like this would tell you what was happening and costs about £550. There are other cheaper alternatives.

@bontwoody  Hi, thanks for reply.  System has settled to 0.9 kW, but is difficult to be sure which, if any, of our many fiddlings worked and so how likely to recur. I wonder if maybe was sticky zone valve somehow released?

Do you happen to know (for combinations of LWT and RWT), the logic ASHP uses to decide when to switch on/off and what power to apply?

@emceed I assume you have a 3 way valve controlling the direction of water flow to either the space heating or DHW. Its possible that they can stick I think. There is also something called a mid position valve which can do both at the same time but thats not what you want. I believe a mid position valve can be used as a three way if its connected correctly. Might be worth checking if you think it was heating both at the same time.

If that was occuring then it would account for the ridiculous energy usage as you would be sending hot water around your heating pipes. If it happens again feel the pipes leaving the valve and see if all are hot or not. The space heating pipe should be cooler. 

Have a serious think about some kind of monitoring as a badly set up system can waste a lot of money very quickly.

@emceed A heat pump will have sensors monitoring the LWT and the RWT and when the difference is close to zero, the pump should stop applying energy to the water. A good Delta T for a heat pump is 5 degrees C.; when the returned water is nearly the same as the LWT, this indicates (all else being equal) that the emitters (radiators in many cases) are no longer emitting sufficient heat to cool the returning water. At this point, the Delta T is probably less than 1 degree C. I think. Hope that clarifies things a little. Regards, Toodles.

Posted by: @emceed

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Do you happen to know (for combinations of LWT and RWT), the logic ASHP uses to decide when to switch on/off and what power to apply?

I am glad to hear that your consumption problem has resolved. On starting, it seems that the compressor ramps up to close to 100% and then modulates down after 20-30 minutes, depending on the target LWT. As long as the emitters can match the minimum ASHP heat output, then the ASHP should just continue to run at target LWT. In mild conditions, if the emiiters cannot match the minimum ASHP heat output, then either LWT rises above target or Delta T falls below an acceptable limit and the compressor will stop. From my observations, I think it is the rise in LWT that triggers the compressor to stop rather than a low Delta T, but this is probably academic.

If cycling on and off frequently due to the above mechanism is a problem, then it is worth altering your FSV settings (2091/2092 if using an external stat and 2093 if using the wired remote controller's stat). You can set these so that the compressor only switches off if the stat is no longer calling for heat. LWT will rise with these settings if the emitters cannot match the minimum ASHP heat output because the compressor cannot modulate any lower, but in my experience the electricity consumption stays at its minimum value.

Again thanks for replies.

Is not a 3-way valve. ASHP output tees to 2 Honeywell zone valves, HW directly to HW coil and returning via buffer. The heat zone goes thro buffer then tees to rads/UFH. Not so easy to be certain what water flows are doing. I know we should buy/install/learn monitor but are so cheesed off with poor design and installation that we may have to make other costly decisions first.

We have 2091/2092 so already. It would make sense that RWT and Δ(5) eventually try to drive the LWT over target which stops ASHP. Presumably there is then some hysteresis setting to trigger restart. Can anyone confirm this logic, especially in situations where (1) heating slab from cold and (2) LWT/flow just balancing emission so maintaining heating at maximum efficiency ?

(We do need LWT over 33 or inadequate emission causes cycling - at 7kW !)

@emceed even something like these taped to your pipes with insulation over the top will give you some idea what’s going on if you don’t want to spend a lot.

Thlevel 4x Digital Thermometer Temperature Monitor with External Probe for Fridge Freezer Refrigerator Aquarium (4x Black)