Help with UFH / TMV

[Andrew]

Having moved into our self-build in June, we are only just this weekend using the heating in anger for the first time. Unfortunately it isn't working very well.

 

The system has two manifolds, an 8 port and a 4 port both fed from an ASHP. The manifolds are fed from a plate heat exchanger, there is pump on this side of the heat exchanger which runs when the ASHP is in heating mode. 

 

Each manifold is set-up as a single zone (i.e. no actuators on the manifolds) for simplicity, with a thermostat, pump on the manifold and a 2-way zone valve controlling each one. All of the control side seems to work fine with the thermostat calling for heat at the appropriate time, the ASHP is firing up, the pump near the PHE is running, the zone valve opening and the pump on the manifold running. The problem seems to be related to the TMV on the manifold - by twisting from 'Min' to 'Max' (this TMV does not show the temperature) I can either have full flow through the manifold but no heat (i.e. the water seems to just be circulating through the pump - the flow and return on the manifold are the same temperature) or by turning it to 'Max' I can have some of the incoming heat but it seems to throttle the flow completely (the flow meters show zero). Btw I have opened the flow meters and the white caps on the manifold fully so these are not constraining the flow at all. 

 

I have tried to find a 'sweet spot' on the TMV where the flow and temperature are balanced but it seems to be a knife edge, as soon as any heat gets through the TMV the flow just seems to disappear. I have tried a variety of supply temperatures from the ASHP, starting at 30 degrees and up to 40 degrees but it doesn't seem to make any difference. I should also note that both of the manifolds do the exact same thing so I don't think it's a faulty TMV. I've attached a photo of the manifold. 

 

I'm tearing my hair out as I can't get enough heat into the floor to raise the temperature, at the moment the ASHP is running constantly but there is very little of that heat reaching the house. Any ideas or help would be very gratefully received. Thanks. 

[ProDave]

Is anyone familliar with that particular TMV? or can you post a link to the make and model or instructions?

 

My money is on the TMV being connected incorrectly, but I can't confirm that for sure as i don't know that one and how it should be connected.

 

The flow should not vary.  all that should change is is when turned up, it should allow more hot water in to mix with what is circulating.

[Andrew]

Thanks Dave. 

 

It's the Wunda premium pump set (as of about 24 months ago when I bought it). The instructions are here - https://cdn.wundatrade.co.uk/wp-content/uploads/2017/08/M06-Wunda-Pumpset-1.pdf

 

I'm not sure if you can see from the photo but the red / blue dots are on the correct sides, so it looks to me like the plumber put it on the right way! 

[ProDave]

Check the arrow on the side of the pump to ensure it is really pumping upwards.

 

Are you SURE flow and return are correct, i.e the pipe feeding into the left red valve really is the hot flow?

[Andrew]

Arrow on the pump is pointing upwards, so seems correct. 

 

Yes, I'm sure the flow is on the correct, when I turn the TMV and restrict the flow then I can feel the elbow by the red isolation valve heat up which then is reflected in the temp gauge on the top rail. Problem is the flow meters are then at zero so no heat goes round the loops. 

[JOE187]

Can you confirm all the white valves are fully open and the top flow gauge adjusters are also open and the system has no air in it??

[Andrew]

Yes all the white values and flow gauges are all fully open. 

 

I'm starting to wonder if there may be an airlock on the flow side somewhere - I guess this could cause the issue if there is very little flow getting to the manifold. The flow and return pipe run does go from knee height in the plant room up to first floor joist level before coming back down to the manifold. 

 

 

[JOE187]

One way to prove system is circulating would be to connect a  standard  washing machine hose on to the  drain valves at the  end of manifold making sure no air is left in hose before switching on!

  This would be like adding a  short loop on the manifold  but would show up if the main pipe work is airlocked or the loops.

[Andrew]

The plumber came yesterday and we flushed everything out to ensure there is no air in the system but it's still not working as expected. 

 

 

What seems to be happening is that the manifold nearest to the plate heat exchanger is taking all of the flow, so when that thermostat is calling for heat the other manifold gets little flow. When the zone valve of the near manifold is closed the other manifold then seems to get decent flow. I've attached a very crappy diagram of how the system is set-up. Apologies I don't know the correct symbols for everything and I've left out the actual UFH loops. The branches to the manifolds are just teed off the pipework. 

 

 

I know little of fluid dynamics so I don't really know how the water would be expected to behave in this situation. Would beefing up the pump near the plate heat exchange improve things or could pumps be added to each of the runs to the manifolds (after the zone valves). If these were added would the pump near the plate heat exchanger still be needed?  

 

[Fly100]

Im no expert, but water is lazy and will take the easiest path.I cant help thinking those feeds should really come off a buffer tank or some sort of manifold with some shunt pumps and some balancing valves. Are all those 3 manifold fed via one pipe ?

 

As I say im no expert.