Cold tap is warm when recirculating pump is on

[phykell]

Hi,

I'm having a problem with my plumbing - the builder responsible has long since "departed" and I've only noticed a problem when I've increased the time/duration of my recirculation cycle. It seems that the cold water tap is now producing relatively warm water and I'm concerned that warm water from the return feed is being forced into the cold supply by the pump.

I've attached a diagram (best endeavours, etc.) to show the layout of the system and I suspect it might be down to the position of the check valve being immediately after the pump before it joins the pipe between the hot water return and the cold cylinder inlet if that makes sense? I've also attached a photo of the section that looks wrong to me - any help is appreciated.

Thanks

 

[oilhead]

I'd be inclined to look at any thermostatic mixers as found on showers etc.

 

[oldbutnotdead]

If that check valve wasn't where it is you'd get cold going up the hot water return.
More likely the increased recirc time is keeping the hot dhw pipe hot for longer and thus giving more radiant heat into the cold pipe (almost certainly run next to each other, quite likely not insulated under the floor).
Test hypothesis- run the furthest cold tap for long enough to clear pipe volume, does it run cold eventually?

 

[phykell]

I'd be inclined to look at any thermostatic mixers as found on showers etc.

Everything is brand new - I doubt it's the mixers.

 

[phykell]

If that check valve wasn't where it is you'd get cold going up the hot water return.
More likely the increased recirc time is keeping the hot dhw pipe hot for longer and thus giving more radiant heat into the cold pipe (almost certainly run next to each other, quite likely not insulated under the floor).
Test hypothesis- run the furthest cold tap for long enough to clear pipe volume, does it run cold eventually?

I think the check valve should be moved above the next junction and just after the "Cold Cylinder" gate valve which means it can only pass into the cylinder's CW inlet. Having it immediately after the pump achieves nothing other than preventing cold water from entering the pump and on into the hot water return - that's bad enough of course so perhaps the solution is to have an *additional* check valve in the existing position and the new position I suggested.

My thinking is that the cold water is only lukewarm because the mains pressure is relatively high. The pump can't overwhelm this but can mix just enough hot water in to warm the cold up.

 

[MeldrewsMate]

Everything is brand new - I doubt it's the mixers.

Your call, but you've been given good advice that you are willing to ignore.

 

[phykell]

Your call, but you've been given good advice that you are willing to ignore.

I didn't say I was willing to ignore it - I just said I doubted it as everything's new (I should have said that in my original post of course) and mixers tend to fail over time (caveat - only what I've read on Google) rather than being faulty out of the box (touch wood). The cold water is consistently warm throughout the property - three showers, seven sinks and one bath - when the recirculating pump is running. Consequently, and notwithstanding my lack of experience with heating/DHW systems, I think my suggestion is more likely at this point but if someone is able to definitively rule it out, then I'll start looking at the more unlikely causes.

 

[phykell]

If that check valve wasn't where it is you'd get cold going up the hot water return.
More likely the increased recirc time is keeping the hot dhw pipe hot for longer and thus giving more radiant heat into the cold pipe (almost certainly run next to each other, quite likely not insulated under the floor).
Test hypothesis- run the furthest cold tap for long enough to clear pipe volume, does it run cold eventually?

Sorry, I meant to say that I have run the furthest cold tap enough to clear the pipe volume but it still doesn't run cold. I'll give it another go tomorrow though and run it for a good five minutes. I only ran it for two minutes but we do have very high water pressure...

 

[oldbutnotdead]

Have you tested any of the various theories promoted here- it's very easy to discard equipment failures 'it can't be that, it's brand new' but new gear fails often enough that it's worth testing.
Since your cylinder is unvented, there should already be a double-check valve on the cold supply to the cylinder- yes it'll be before the group valve but should be in a location such that water cannot be 'pushed back' up the cold line to other taps.
You can add another check valve in your suggested location but it'll cost you about half a bar in hot water pressure and will mess up the balancing performed by the group valve.

 

[oldbutnotdead]

Sorry, I meant to say that I have run the furthest cold tap enough to clear the pipe volume but it still doesn't run cold. I'll give it another go tomorrow though and run it for a good five minutes. I only ran it for two minutes but we do have very high water pressure...

Fair enough- 2 minutes is probably enough to disprove my first theory.

 

[oldbutnotdead]

There's a couple of oddities on the cold piping on your schematic.
First one is the link that apparently bypasses the group valve to tie both controlled and uncontrolled cold to the expansion vessel.
Second one is that little green zigzag that appears to link cold with UFH return (if that's actually a filling loop with at least 1 check valve then fine but there's no good reason for it to be on that side of the group valve).

 

[Bod ...]

I'd say possible heat transfer.

Secondary return and cold water touching each other.

I take its all in copper? Is it insulated?

 

[MeldrewsMate]

Fair enough- 2 minutes is probably enough to disprove my first theory.

Sorry, I meant to say that I have run the furthest cold tap enough to clear the pipe volume but it still doesn't run cold. I'll give it another go tomorrow though and run it for a good five minutes. I only ran it for two minutes but we do have very high water pressure...

The check valves in the new shower mixers are just as prone to having debris trapped in them when new, perhaps moreso.
If you repeat your 2 minute cold challenge but with the hot water turned off at the cylinder then you will be in a position to see if hot water is leaking across one or more mixer valve(s).
You can then home in on the offending mixer if the cold feed pipe (to it) gets warm/hot when repeating the test with the hot water on again.

 

[phykell]

I'd say possible heat transfer.

Secondary return and cold water touching each other.

I take its all in copper? Is it insulated?

All of the pipework other than in the "plant room" is in plastic but it's not insulated. Of note is that the temperature is consistent no matter which tap you try, from the sink closest to the heat source, all the way to the kitchen which is the furthest point.

 

[Bod ...]

All of the pipework other than in the "plant room" is in plastic but it's not insulated. Of note is that the temperature is consistent no matter which tap you try, from the sink closest to the heat source, all the way to the kitchen which is the furthest point.

What's the brand of the plastic?