UFH mixing valve bypassing hot water directly to cold return

[Andy Lewis UK]

I have a UFH manifold from Wundatrade with a Grundfos pump. The pump feed has a mixing valve which has a hot inlet from the thermal store, and a cold inlet from the return manifold. In-between the mixing valve cold inlet and the return manifold, is a tee which returns the cold water to the store. This cold water return to the store has a higher temperature than what is returned to the cold manifold, suggesting hot water is flowing into the mixing valve hot inlet and out of the cold inlet (i.e. reverse flow). The mixing valve inlets are marked blue and red, suggesting the installation is correct. The phenomena occurs no matter the mixing valve setting. Any ideas?

 

[ShaunCorbs]

Tbh sounds normal as your only needing 35-40 dc for the ufh and your store is at 80 normally so half the flow will bypass roughly

 

[John.g]

Is the "cold" water return to the store at the mixing temperature and what mode/setting is the grundfos pump?

 

[Ric2013]

Hmm. I'd need to see a photo as I'm struggling to visualise.

If this is running off a pumped flow and return with another pump elsewhere, you may need a bypass to prevent pumps 'fighting'.

 

[John.g]

Do you mean it might or should have a low loss header?.

 

[John.g]

A small bit of topic but is correct to assume that a UFH TMV will be calling for full HW (boiler) flow when started up from cold, and if so is the manifold flowrate higher until the TMV reaches its setpoint.?.

 

[Ric2013]

A small bit of topic but is correct to assume that a UFH TMV will be calling for full HW (boiler) flow when started up from cold,

From experience, mine seems to, though I wonder if, like tap TMVs, it also allows a small amount of cold at all times?

and if so is the manifold flowrate higher until the TMV reaches its setpoint.?.

Yes.

 

[John.g]

From experience, mine seems to, though I wonder if, like tap TMVs, it also allows a small amount of cold at all times?

Yes.

This might indicate that some TMVs have some adjustment via a allen key, see post #8.

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[Ric2013]

This might indicate that some TMVs have some adjustment via a allen key, see post #8.

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Ah. I think we're talking at cross purposes. You meant the UFH manifold and I was thinking about a hypothetical LLH manifold from which the UFH manifold may be plumbed, so I've been using the wrong terminology and mislead you.

Start again.

My observations are based on my own UFH at home which I have ample access to.

My blending valve does not have a remote sensor on the manifold. You can turn the valve to a fully shut position and close the hot path off altogether (full recirculation), but I doubt this situation ever occurs in normal use as it would imply a DT of 0°C across the UFH circuit. Whether it ever goes into 100% hot mode (closes the recirculation entirely) or stays slightly open I do not know. There is no evidence that it is mixing in a small percentage of return water at all times, but I have not tested this theory. The only adjustment the manufacturer suggests is to adjust to the correct flow manifold temperature and this is all I have needed to do.

I have observed that when my UFH manifold is cold, the blending valve on the UFH pump inlet seems to take more flow from the boiler primary than when it is hot. Being a thermostatic blending valve, that is exactly what I would expect.

My UFH manifold is plumbed from a non-condensing heat only boiler and so needs to blend the flow temperature down from 70°C to 40°C. The UFH manifold is hydraulically separated by using CCTs on the boiler primaries with the maximum flow rate through the CCTs being regulated by a gate valve (I'm aware ideally it should be a globe valve, but there was not one in my scrap box). The flow through the CCTs is set to be just a little more than required by the UFH mixer valve. Ideally, it would be set to be exactly the flow required by the blending valve, but, being a fixed setting in a world with multiple variables, it needs to be a little more to allow some leeway as I would rather increase the return temperature to my non-condensing boiler than reduce it. Thus, in cold condition, the CCT flow acts as a recirculation path for the return water from the UFH manifold to be fed back to the UFH pump and manifold circuit to maintain whatever flow rate is set; in hot condition, the CCT flow direction is reversed and acts as a very slight system bypass from the boiler flow to the boiler return.

The flow through the UFH manifold pump is, as far as I can tell, a constant.

 

[John.g]

Daughter has a mixed Rad+UFH system which works quite well with no LLH or CCT or any bypass from a SE 20 year old Firebird (oil) Boiler, will probably be changing it sometime (if it ever breaks down) to a HE Grant where the return temp cannot be allowed below 40/45C.
I was thinking of just installing a (auto) bypass in the form of a tapstat where the sensor is attached at the boiler return to control the bypass valve, it might give very tight control of the boiler return temp even though I've never seen it suggested anywhere?.
Two systems compared below, I'm assuming a 5kw UFH system with a dT of 7C.