UFH manifold problem

[joe90]

I appear to be having a problem with my Wunda UFH manifold, despite water approx 35’ going into the manifold the water going into the top rail (see pic below) is only approx 23k ?, the TRV is set wide open and the pipework is hot both sides of it. I can only presume that the water is being blended with the cool water from the bottom rail. What stops water from the bottom rail being sucked into the pump rather than returning to the heat source???

 

 

[JohnMo]

It looks to operate in a similar way as a radiator.  But with a pump in the middle.

 

So water comes from source to TRV. 

 

TRV fully open - hot water goes into pump, through UFH loops and out of the right hand manual valve.

 

At a part open position for TRV, some hot water will flow to pump, the rest of the water will be taken from the return water from the UFH heating loops.

 

So if the loops are not getting hot water, the TRV must be closed more than it should be.

 

Loops like the TRV has a capillary tube, so would suspect either the top rail or TRV end is broken and needs to be replaced.  I think the TRV comes as a complete unit, for about £20 to £30.

[joe90]

Thanks for the reply but the TRV is fully open as temp both sides of it are the same, I wonder how the pump can pull hot water from the source  (through the TRV) rather than from the return rail as it’s closer, I wish I had a stop cock between the pump and return rail to limit this ? and see if the temp went up!!!

[ProDave]

I think it relies on the hot feed to the manifold being driven by a circulating pump of it's own, so the hot water arrives under slight pressure and the return is relatively under slight vacuum.

 

The position of the manifold circulating pump means it is pulling water into the bottom of the pump so it will be a blend of fresh hot and recirculated cold going through the manifold pump.

 

Assuming it was working but has stopped, I would suspect either the manifold pump, or the main circulating pump has failed with my bet being the manifold pump has failed.  If it's not on it's full speed setting already, try increasing it?

[joe90]

1 minute ago, ProDave said:

I think it relies on the hot feed to the manifold being driven by a circulating pump

No it does not have one, the only pump is on that manifold, hot water comes from the buffer tank. The manifold pump is working (flow gauges show this). The only other pump is on the pipework from the ASHP (which does  not have a pump in it) then diverter valve to DHW tank or buffer tank.

3 minutes ago, ProDave said:

Assuming it was working but has stopped,

Funnily it has always worked at 23’ it’s only this latest cold snap that made me look and your comment on another thread about yours working at 35’ (I think ?).

[joe90]

Been thinking about this and I am tempted to remove the TRV (top bit) and block the bottom rail where that lower temp gauge is then it will circulate the water at the temp set by the buffer tank stat and not blend any from the bottom rail ?.

[JohnMo]

This how your manifold works.

 

Can only really be TRV issue.  Gone wunda a call, may be covered by warranty.

 

 

 

[joe90]

20 minutes ago, JohnMo said:

Can only really be TRV issue.

Unfortunately I still disagree, yes I know how they work but because unlike a normal boiler system the flow and return from the heat source (a buffer tank) does not have a pump, it relies on the manifold pump to circulate the water from that tank and round the UFH loops. As the loop of pipe from the buffer tank is much longer and hence more resistive than that from the lower manifold it’s easier for cold water to be recycled than to use warm water.

 

22 hours ago, ProDave said:

I think it relies on the hot feed to the manifold being driven by a circulating pump of it's own, so the hot water arrives under slight pressure and the return is relatively under slight vacuum.

I think @ProDave is correct. I have removed the TRV head so this is fully open and temps both sides of it are the same. Another solution would be to remove the buffer tank as the UFH pump would then pump water into the manifold but I don’t really want to do this as I would loose the immersion facility to back up the ASHP. I may do what I said above, block the lower connection from lower rail to blending valve and TRV and rely on the buffer tank stat to control the temp of water in the UFH loops ?‍♂️

[Bramco]

15 hours ago, joe90 said:

I may do what I said above, block the lower connection from lower rail to blending valve and TRV and rely on the buffer tank stat to control the temp of water in the UFH loops

 

So if you were about to install this system again with 20:20 hindsight, you'd leave out the TRV and simply route the lower rail connection to the buffer tank return?

 

Would this be advisable for anyone starting an installation, or is the problem you're seeing only due to the length of the pipes to the buffer tank?

 

Simon

[joe90]

13 minutes ago, Bramco said:

So if you were about to install this system again with 20:20 hindsight, you'd leave out the TRV and simply route the lower rail connection to the buffer tank return?

Probably, my Initial plan was a buffer tank at 35’ and blended down to 25’ at the manifold, however it’s been suggested a water temp of 30’ is needed to get a floor temp near 25’.

 

13 minutes ago, Bramco said:

 

Would this be advisable for anyone starting an installation, or is the problem you're seeing only due to the length of the pipes to the buffer tank?

I don’t think the problem is length of pipe,(10m) more the lack of pump from buffer tank to manifold. Later today I am going to remove the  blending valve and block it some way, I have already removed the TRV head. I can then control the temp of the UFH water with the stat on the buffer tank. ? @Nickfromwales you got any advise on this?

Edited December 30, 2021 by joe90

[Nickfromwales]

54 minutes ago, joe90 said:

Probably, my Initial plan was a buffer tank at 35’ and blended down to 25’ at the manifold, however it’s been suggested a water temp of 30’ is needed to get a floor temp near 25’.

 

I don’t think the problem is length of pipe,(10m) more the lack of pump from buffer tank to manifold. Later today I am going to remove the  blending valve and block it some way, I have already removed the TRV head. I can then control the temp of the UFH water with the stat on the buffer tank. ? @Nickfromwales you got any advise on this?

The tank stat option will likely be too crude as a control, due no linear hysteresis, and result in under / over-shoot IMO. 

[joe90]

So @Nickfromwales do you think @ProDave is correct that no pump from the heat source is causing the problem? What would you suggest ?‍♂️ I could remove the buffer tank but would rather not as I would then need to wire the stat to control the heat pump and the manifold and I don’t want to hack into a new build to add wiring.

Edited December 30, 2021 by joe90

[epsilonGreedy]

@joe90 is it possible your pump has failed prematurely because the low flow temp has caused excessive run duration?

 

I ask because after looking at some industry tables on UFH flow temps, +35 is the norm.

[joe90]

1 minute ago, epsilonGreedy said:

joe90 is it possible your pump has failed prematurely

No the flow gauges are performing normally.

[Bramco]

6 hours ago, Nickfromwales said:

The tank stat option will likely be too crude as a control, due no linear hysteresis, and result in under / over-shoot IMO.

 

Wouldn't it be possible to have the buffer tank working like a heat bank (albeit small).  So while the slab is soaking up the heat, the ASHP will stay on.  As soon as the 35' threshold is reached it would turn off the ASHP.  Then, when a lower limit is reached, it would turn on again.

 

Simon

 

 

[joe90]

4 hours ago, Bramco said:

 

Wouldn't it be possible to have the buffer tank working like a heat bank (albeit small).  So while the slab is soaking up the heat, the ASHP will stay on.  As soon as the 35' threshold is reached it would turn off the ASHP.  Then, when a lower limit is reached, it would turn on again.

 

Simon

 

 

That is how I planned it but the manifold won’t give a decent temp to the top UFH rail and I must assume this is because of no pump pressure into the TRV.

[Temp]

I have a different make of manifold that uses a three port TRV. I originally  questioned the need for a store to manifold pump. Supplier said one was needed because they couldn't guarantee the manifold loop pump would draw water from the store when the TRV was half open. I was surprised as obviously the resistance in the floor loops should be a lot higher than in the short 22mm run from store to manifold.

 

PS My concern about fitting a pump was that it would stir the store when the TRV were recirculating. To make matters worse my electrician wired this pump on permanantly! I've two manifolds and ended up making an OR gate from relay so that it was only turned on when one was calling for heat.

 

PPS May need a bypass as well. In case rooms call for heat turning on the pump but both TRV are shut/recycling.

Edited December 31, 2021 by Temp

[dpmiller]

I wonder if adding a restrictor (disc with hole in) at the joint between return rail and the bottom pump manifold might upset the flows enough to force more return out and back to the buffer?

[joe90]

4 minutes ago, dpmiller said:

I wonder if adding a restrictor (disc with hole in) at the joint between return rail and the bottom pump manifold might upset the flows enough to force more return out and back to the buffer?

Exactly what I was thinking this morning, shame it’s not a piece of pipe there as I could insert a stop tap so I could adjust it. Going to drain it and take apart tomorrow, will report back ?

[ProDave]

Yes I think the manifolds with a 3 port mixer are better at "drawing" the hot water in.

 

At our old house, the heating is UFH upstairs and down and UVC for HW.  I was called by the tenant to fix no heating upstairs.  That fault was the useless 2 port valve head had failed, for about the third time.  i replaced the head but I also observed the circulating pump was not working.  But no other fault had been reported and the heating worked again with the new 2 port valve head even before I replaced the pump.  So i concluded both UFH manifolds were circulating the water in on their own and the HW tank must have been working on gravity flow.

 

P.S. replacing the pump was a mare as the the cheap pump isolating valves both leaked after changing the pump requiring a drain down to replace the isolating valves.

[dpmiller]

I've the opposite issue with my system- *too much* flow from the manifold to the ASHP. Thankfully the pump set has a bypass and a restrictor, so you can throttle back the return flow even if the thermo valve is wide open; I'm playing with that currently to control the DeltaT at the ASHP.

[joe90]

Result ???, I made a rubber diaphragm from a bike inner tube and blocked the non return valve and temp immediately shot up ?. I am going to try running it like this to see if it works relying on the buffer stat to control the temp. If as @Nickfromwales says it overshoots I will consider a small hole in the diaphragm and restore the TRV. Thanks guys. @ProDave what temp did you say you run your UFH water at?

[ProDave]

I think my temperature is 35 degrees.  I could run it lower probably but it needs to be close to thet when it's -10 outside.  Perhaps I will have another go at getting weather compensation to work and reduce the temperature when it's mild.

 

12 degrees outside right now.  All rooms up to temperature and heating has shut off.

[joe90]

11 minutes ago, ProDave said:

12 degrees outside right now.  All rooms up to temperature and heating has shut off.

Yes, not ideal to test mine but it’s due to get cold next week.

[joe90]

Just noticed that the pump on the manifold is much more noisy whether turned up or down . It does go quiet if I shut the TRV but I guess not good for the pump as it’s not able to circulate the water now  the “bypass” is blocked?

Edited January 1, 2022 by joe90