Sense check

[Stones]

We have friends who built round about the same time as us, with 200mm SIP.  They have a GSHP and UFH.  Problem is they have never been able to get the house much warmer than 19C, but still have quite high energy bills.  In an effort to get to the bottom of why they can't heat the house up, I spent a few hours there earlier this week.

 

The GSHP is plumbed to a buffer tank.  A circulation pump in the GSHP runs a very small loop between the heat pump and the buffer tank.  There is then a 20m loop from buffer tank to the UFH manifold position.  2 manifolds, each with own circulation pump (as pictured). Every room has a thermostat and as you can see motorised actuators on the manifold.

 

 

The GSHP registers the return temp (in this case 32C) and according to the manual, will produce a flow temp 5C higher (a default differential). 

 

First alarm bell was the circulation pump in the GSHP (circulating between heat pump and buffer) was red hot to touch - basically too hot to hold your hand on for any length of time.  Second alarm bell was that the temperatures registered at the manifold were lower than the return temp measured at the heat pump - one of the manifolds, despite being active with the pump circulating, was registering 25C on both flow and return.  Circulation pumps on the manifold both set to medium.

 

So the GSHP seems to be doing its job and heating water, the buffer tank being at the target temp.  However, the hot water from the buffer is not getting to the manifolds, and as a consequence, excess heat is being lost via the red hot circulation pump.  

 

The only way I could get more heat to the manifolds was to crank up the call for heat so both manifold circulation pumps were active, and switch them to their fastest setting.  This seems to have created enough pull to get more of the hot flow through the 20m loop, and we managed to get the temp at flow temp at the manifold to 30C.

 

The good news is this resulted after a few hours, with the house temp rising to 21C.  

 

The problem as I see it is that the buffer is in effect acting as a large low loss header, and that the circulation pumps at the manifold, whilst doing a grand job of circulating around the UFH loops, are not there to pump water around the 20m circuit between buffer and manifold / or the flow is being outbalanced by the manifold pump blending / recirculating too much. My various testing of variables and settings showed that both manifold circulation pumps need to be on and on the highest setting to get hot water delivered to the manifolds.

 

Whilst this can be managed in the short term, our friends want a longer term solution.  They have tried by getting plumbers out, but so far the fixes have been additional controllers or an additional expansion tank.  Nobody has taken the time to actually see what is going on and test different variables.

 

As I see it there are a couple of options - an additional circulation pump on the 20m loop from buffer tank to manifold.  I can see that a different or adjusted pump arrangement at the manifolds to reduce or eliminate the circulation blending that must be going on may also be an option, although I'm not as convinced this would be as effective.

 

Would appreciate any thoughts.

 

 

[ProDave]

The circulation pumps on the manifold are just to circulate the water around the loops.

 

The pump between the GSHP and buffer is doing it's job of circulating the water between them.

 

But there is nothing to circulate the water from the buffer to the manifolds.  the manifold pumps as you can see do a very poor job of getting heat from the buffer.  So you want an additional pump between the buffer and the UFH manifolds.

[JFDIY]

Think a pump between the buffer and the manifolds would be what's needed.

 

I think the pump on each manifold is ok at pushing it round the loops, but not able to pull it from the tank.

 

 

[Stones]

Thanks both.

[Nickfromwales]

Those manifolds promote excess bypassing too, with difficult loops not being serviced as they would with a different manifold eg without the return having direct lino-of-sight with the flow  

[Stones]

32 minutes ago, Nickfromwales said:

Those manifolds promote excess bypassing too, with difficult loops not being serviced as they would with a different manifold eg without the return having direct lino-of-sight with the flow  

 

Certainly from the time I spent there, that was the case.  In addition to the primary loop circulation pump that's needed, do you think a different arrangement at the manifold would be advised, (in my non plumbing mind, replacing the bottom T with an elbow so the pump is only taking flow in/through, and blank off the end of the manifold, so there is direct flow/line of sight)?

[Nickfromwales]

I’d fit the pump between the buffer and the manifolds first and try that, as it’s the easiest and cheapest path. That pump needs to be activated by either manifold wiring centre, so will need to be brought on by a pair of relays, each one connected to the pump on the output, and each individually triggered by each manifold. Ideally that needs to be off a 2-port zone valve at each manifold, so one that is off cannot allow return water to be pulled backwards ( sucked ) by the other ‘flowing’ manifold.

I’d just try the pump install first and see what results that yields.