Underfloor heating and hot water priority - potential issue?

[Jon C]

While tinkering with the flow meters on my ground floor UFH manifold today the hot water came on. I've heard of hot water priority and I think this is how my system is set up. I noticed, however, that the manifold pump was still running, the actuators were all energised but the flow meters all showed no flow. At this point the manifold pressure was higher than normal too. This seems wrong to me as the pump is working but unable to circulate water through the system as the heating zone valve is closed (as it should be in).

 

The micro switch on the heating zone valve seems to be working as the pumps in the heating and hot water tank cupboard come on when it energises - as an additional point i'm curious about, there are two red Grundfos pumps in that cupboard and they both come on when the hot water or heating are on - is this correct? I've always assumed that one was for the hot water system (indirect unvented cylinder heated by ASHP) and the other for the heating (all UFH heated by ASHP) but maybe they are meant to be on at the same time? Perhaps one is a send pump, one a return pump? I've attached a photo in case anyone is kind enough to take a look.

 

So, in summary should the UFH manifold pump and actuators be de-energised if the heating zone valve is closed due to the hot water priority? And should both grundfos pumps be running at the same time when the hot water or heating are running? 

 

Thanks for any help anyone can give me. 

[Marvin]

So many questions to ask...

So UFH on the ground floor. Anything up stairs?

 

You would expect no flow but your system has a bypass valve the black tubey thing with the numbers on it between 0.1 and 0 5. When the pressure reaches above the set level indicated the valve lets the water bypass the UFH system avoiding over pressuring the pump.

I see 3 white boxes ( these are 2 port valves) which implies to me 1 system for hot water tank, 1 for UFH and I guess 1 for other heating?

What's the name and model of your ASHP? This will help others answer better.

 

Good luck.

 

Marvin.

 

P.s. like the wellies.

[Jon C]

Thanks @Marvin. Yes the wellies are cool aren't they. Wish I could claim as mine but they're my wifes! I have UFH throughout which is a manifold each for ground, first and second floors. Each manufold gas its own black Grundfoss pump. No rads at all. There are two drayton 2 port valves, not sure what the other white box you're seeing is but it could be the cylinder stat. The ASHP is a Mitsubishi Ecodan. I checked today when just the hot water was on and the UFH manifolds all had a higher pressure than normal "resting" and the gauge needles were vibrating suggesting to me that there is a pump running but its not the black Grundfos at the manifold (all room stats were off). 

[RHayes]

Hello @Jon C, looks like the top pump is on the return leg judging by which side of the bypass valve is connected.  I can't workout where the bottom red pump connects to.

 

Hot water priority is typical with a heat pump so i expect you do have this set up.

 

Regarding whether the pumps should both come on or not depends on what they are leading to.  Not so easy to tell in the photo but it does seem like the bottom pump is on the flow and the top pump is on the return.  I can only assume this was done this way as the pump duty was too much for the one pump.

 

If the 2 port motorized valve for the UFH closes when the hot water comes on then there must be something that intercepts the signal from the UFH stats when the hot water comes on.  The issue is that this doesn't then tell the UFH manifold pump to close and therefore it is effectively pumping against a closed valve, and can show a small increase in pressure gauges.

 

To resolve this the grey and orange (micro switch) on the UFH zone valve will need to connect back to the UFH wiring centre (I assume you have one) to de energise the pump at the manifold.

 

Do you have a UFH wiring centre?  It should be possible to de energise the UFH manifold pumps this way but will be much more difficult to close all the valves too.