UFH mixing valve bypassing hot water directly to cold return

[John.g]

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.

Could you please post a simple schematic of your systems please including the CCTs, have you 3 circulating pumps in all?.

 

[Ric2013]

Could you please post a simple schematic of your systems please including the CCTs, have you 3 circulating pumps in all?.

2 pumps, 3 circulating valves.

The CCT is just below the UFH blending valve. It would arguably be better to have the motorised valve for the DHW coil as a 3-port, with the zone closed position being through the bypass as, if there weren't an additional feature (not shown), this system might potentially reverse circulate as it does, in effect, break the 3 tees 'rule'.

Unfortunately, the cast-iron heat exchanger in the boiler needs a certain amount of heat dissipation, so a longer bypass run is preferable. Unfortunately, this is one of those systems where you just just can't meet the requirements of everything due to the MIs of the old components being along different lines of thinking from what is now 'best practice'.

 

[John.g]

The main difference I see between the systems that give trouble ie the UFH robbing the Rads and your system is the CCT? (which looks more like a bypass?) Is the UFH return throttled in?, what are the makes/models and modes/settings of the primary and manifold circ pumps.

 

[Ric2013]

The main difference I see between the systems that give trouble ie the UFH robbing the Rads and your system is the CCT? (which looks more like a bypass?) Is the UFH return throttled in?

It is throttled. That is the gate valve used to limit maximum flow through the CCTs and adjusted to just more than the flow required by the warmed-up UFH as previously described.

In fairness, I previously ran this system without the CCTs and there was no evidence of it causing a problem, so not sure if this discussion is relevant to the OP's problem (which wasn't directly related to radiators being robbed) or not. What I would say is that it is possible my previous system did rob the radiators at start-up, but since the UFH circuits content only takes around 7 minutes to change, it was a non-issue. In a sense, the UFH was previously able to make use of the (oversized) 12kW output of the boiler, whereas it's now limited to the 1.4kW which the gate valve is allowing to pass, so the revised system is not an improvement on all fronts.

, what are the makes/models and modes/settings of the primary and manifold circ pumps.

UPS2 15 50/60 on the primaries, set to PP3

UPM3 AUTO 25/70 on the UFH manifold, set to CP1

 

[John.g]

Very interesting.

Is the modified system below the same as your system?, if so, what, in your opinion,do the CCTs achieve.

 

[Ric2013]

Very interesting.

Is the modified system below the same as your system?, if so, what, in your opinion,do the CCTs achieve.

Essentially yes.

What do they achieve? On my system, not a great deal. It was an idea Heat Geek had about how having two 'smart' pumps essentially in series would confuse/upset the pumps as the effect of one pump responding to feedback from its circuitry would affect the other pump and reduce their lifespan and I wanted to try the CCTs to see what happened. I hadn't originally put CCTs in as I was of the opinion that the UFH would essentially only take what it needed from the primaries, and so it seemed to. Whether the pumps will last longer is impossible to say as there is no control experiment.

I can see they might be needed if the maximum flow through the UFH exceeded what the system pump or boiler could allow, but as my boiler is running at 12kW output (IIRC) and my UFH load is around a tenth of that (between 1 and 2 lpm), I suspect the question is academic. If they do anything, I suspect they extend the UFH warm-up time but reduce the radiator warm-up time when both zones call for heat in the morning.