Heat pump plumbing thumps on change over from heating water to heating the UFH.

[MikeSharp01]

Been running the new heat pump today testing and commissioning the EMON heat meter. The heat meter is working well and telling me I get about 4.2 COP on heating the slab - with a cold slab at about 12^oC.

 

When I switch to heating the hot water the changeover happens smoothly and the heat meter starts reporting HW correctly BUT when I switch back at the end of the valve close we get an awful thump, I guess water hammer? Is this common and what might the / a cure be?

[JohnMo]

How are you controlling the 3 port valve and pump - is all via the heat pump controller or something else?

 

What valve are you using?

[MikeSharp01]

Its controlled by the heat pump and its the valve that came with the heat pump - this one I think: 

The ZVM28

[Nickfromwales]

You may need a differential bypass valve, across flow and return before the valve.

 

Are you 100% sure that the flow and return are the correct way around, as the thump is often associated with the valve 'paddle' being slammed shut in the flow.

[MikeSharp01]

1 hour ago, Nickfromwales said:

as the thump is often associated with the valve 'paddle' being slammed shut in the flow

Interesting thought and entirely plausible. Here is the basic circuit I used from the install manual - except that the Underfloor Heating (UFH) is where the buffer tank is as we have no buffer tank. This circuit makes sense this way round although I guess you could put the valve in the return junction. The valve is normally open to the UFH and the Heat Pump sends a signal to the valve to change over when Hot Water heating is called for. So the flow comes up the middle of the valve and goes out either to the UFH circuit or the Tank circuit. Hence as the valve closes, on the spring, to the tank circuit it will have the flow helping it close.  If that is the cause of the thump then it's a control problem. Perhaps the Heat Pumps circulating pump should be shut down while the valve changes over although that would be a firmware issue in the controller? 

 

 

[JohnMo]

40 minutes ago, MikeSharp01 said:

Perhaps the Heat Pumps circulating pump should be shut down while the valve changes over

Had a look at mine, and pump doesn't shutdown, it may slow down but doesn't stop. As the paddle moves it doesn't cut flow off, flow basically transitions from one port to the other, with both ports open until the DHW port closed.

 

Assuming the UFH manifold is fully open it shouldn't see an issue.

 

Do you have the piping all clamped up as it should be?

 

No air in the system?

 

If it's water hammer, the pressure wave need something to hit, and bounce back. A photo of the valve and the piping to UFH may help.

 

Assume you don't get any fault codes?

[MikeSharp01]

7 hours ago, JohnMo said:

Assume you don't get any fault codes?

No fault codes.

 

7 hours ago, JohnMo said:

Do you have the piping all clamped up as it should be?

I think so it is all 28mm, one clip at least every 500mm, more often usually inside, although not on the risers to the tank entry points albeit they are fixed at the top of course - can look at that and see if there is any flexing. Outside runs are clipped outside the insulation to cable trays with a flexi at the end to the heat pump.

 

As to air I cannot be sure but there is a trap at the highest points of both the tank and UFH loops (see pic for UFH) and I bled the stubs that will be running to the fan coil units upstairs. (Although I could check those again now its been running for a bit.)

 

Here is a pic of the valve and the UFH pipe work