Cycling ASHP twice an hour

[Ambaz79]

Thought I would start a new topic as my last post focused on my dhw tank loosing heat very fast. Solved by the members here and a bit of detective work. 

 

On another subject I'm noticing my hp is cycling around twice an hour. 

 

Running in weather compensation mode with the following. 

 

Low is 40 Degree Flow at -1 Degree outside

High is 25 Degree Flow at 15 Degrees outside

 

External thermostats in every zone set to 30 degrees and rooms hit around 20-21 degrees. 

 

Flow on manifold is slightly higher then the designed flow at around 1L per minute to around 1.5l.

 

I've tried to increase flow to 3L but it's still cycling. 

 

Outside tempreture is currently - 2 in Leicester so we expecting it to run more without having to shut down. 

 

Is there anything I could change or look at to reduce this cycling. 

 

[PhilT]

defrost cycle could be once or twice an hour in this damp cold air. You can watch it stop for a few minutes then a lot of steam comes out and blows away when it restarts

Edited December 2, 2023 by PhilT

[Ambaz79]

If I reduced the flow temp to maybe 37 or 38 then would we expect a cycle less often.

 

Ideally would like it to run more constant as we have the UFH in thin layer of screed at around 60-70mm and this means it does not retain much heat for long. Slab just about warms up and then a cycle where it cools the slab down again due to the UFH still running. 

 

Should the UFH actually still be running during a cycle. I do notice the supply to manifold does get cool during cycle. 

[JohnMo]

10 minutes ago, Ambaz79 said:

If I reduced the flow temp to maybe 37 or 38 then would we expect a cycle less often.

 

Possibly not - a normal condition all heat pumps do it. If you want to compensate for it increase the flow temp a degree or so. Or install a buffer in the flow line (2 port)

 

60 to 70mm isn't thin screed. 

 

 

[Ambaz79]

Forgot to mention I've got a 100L buffer tank. 

 

[PhilT]

is your system still achieving your desired room temps despite the increased defrost cycling?

[sharpener]

1 hour ago, JohnMo said:

Possibly not - a normal condition all heat pumps do it. If you want to compensate for it increase the flow temp a degree or so. Or install a buffer in the flow line (2 port)

 

 

 

IIRC though I can't provide a reference it is better to have a 2-port buffer aka volumiser in the return to the HP so the stored heat is directly available for defrost.

 

I don't think two cycles an hour (though not ideal) is much to worry about if you are achieving the target indoor temperatures.

[dpmiller]

I'd be surprised if your screed was cooling that quickly. Ours is around the same thickness, and takes the whole E7 period to fully charge with the downstairs not needing any more heat gain through the day. Warmth is certainly obvious to bare feet until noon or so?

[ReedRichards]

From time to time your flow temperature is lower than your return temperature.  The only time that happens in my case is when the direction of flow is reversed in order to defrost the heat pump.  I cannot think of another reason that could cause this to happen.  My heat pump reports that it is defrosting when it is.