UVC cooling down very quickly

[osmononame]

Hi All,

I have an issue which my plumber has been struggling to get to the bottom of so thought I'd post here for some ideas!

I have a 300l UVC which loses all its heat overnight. It's been like this since day one. The house is a self build.

Initially we thought it was because there wasn't a NRV between the pump and tank (you could feel the pipework become cold to touch when the pump wasn't running). The NRV solved the cold pipework at the pump but not the loss of heat in the tank.

The secondary return pump works on motion now (we thought maybe we were losing the tank due to it running 24/7). Now if motion is detected in the house the pump switches on and if no motion is detected it switches off. This means at night it's not running. We continued to lose a full tank each night.

In any case I have been experimenting over the last few and as I said before if the pump is switched off but the secondary return not isolated then I still lose the whole tank in one evening. If I isolate the secondary return pipework the tank stays hot overnight.

So the issue is something to do with the secondary return. There are two theories. The first is that the secondary return has maybe been mixed with a cold mains at some point in the house. However if this is the case when I isolate the hot water at the tank and isolate the secondary return shouldn't I get cold through the taps? That's not the case. The second is maybe one of the mixers is faulty and letting cold back up hot (but then wouldn't this be a problem even with the secondary return isolated?).

Any thoughts on this? The plumber who did the initial install is AWOL so I can't even check with him to make sure he tested the pipework / to ensure nothing was crossed!

 

[cross thread]

Have you isolation valves on you taps ? If so turn them off and check if anything changes

 

[osmononame]

No isolation valves on the taps I'm afraid and if I was to go down that road it might be easier to just install check valves on the hot side rather than isolation valves?

 

[Madrab]

Is the secondary circuit fully insulated? If not then it will lose the heat through the secondary circuit. Does the circuit run over several levels, if so then it will probably still flow, through convection, even when the pump's off.

 

[osmononame]

Is the secondary circuit fully insulated? If not then it will lose the heat through the secondary circuit. Does the circuit run over several levels, if so then it will probably still flow, through convection, even when the pump's off.

Not fully insulated but maybe 70% of it is. It runs over 3 floors (tank on the second/top floor). So with that in mind would convection be enough to empty a 300l tank in circa 6 hours? That might explain it then.

 

[Madrab]

Well, if the cylinder stays hot overnight with the secondary circuit isolated then it's pretty obvious that there is some circulation going on that's leaching the heat out of the cylinder's HW, when it isn't isolated.

Normally there is a NRV installed into the return pipework, this is usually to ensure that the water flows only in the one direction but it also acts to avoid any reverse/convected circulation.

 

[osmononame]

There is a NRV installed between the cylinder and the bronze pump already but that doesn't seem to help with the heat loss. If it is being lost through convection would a motorised valve (designed for drinking water) opening and closing the secondary return when motion is detected maybe be the answer?

 

[picasso]

I would fit an anti gravity loop off the top of the cylinder after checking the check valve is not stuck open .

 

[flameport]

a motorised valve

Probably, but that's just fixing the symptoms, not the actual problem.

Where is the cylinder located relative to the outlets - above, below, same level?
How is the return pipework configured and what size is it?
Is the pump on the return, or the flow?
Does the pump operate in the same direction as the hot water would flow with a tap on, or is it in the reverse direction?

 

[osmononame]

It's been a while since I responded on this thread but life has been busy this last year!

The cylinder is in the loft so higher than all the outlets. The return pipe is 15mm and it's flow is towards the cylinder and the pump flows this way too.

I have now had double check valves fitted on every basin mixer and isolators too. I've tried going basin by basin to isolate each one to see if the problem resolves but no luck.

Contrary to what I thought before with the secondary return pipework closed I'm still losing near enough a full tank each day.

One thought I had was that if the secondary return pipework has been crossed with a cold mains somewhere when the house was built would that cause this symptom? It's the only thing I can think of for how the cold is entering the tank! Is there any easy way I could.maybe test this theory?

I've had a couple of guys out to look at it now and everyone is stumped!

 

[lloyda]

Turn off cold mains overnight?

 

[JohnD]

Do you mean you have a secondary circulation system pumping hot water from the cylinder to a distant bathroom, and back again? And you say it is not fully insulated?

 

[osmononame]

The tank is still losing heat overnight with the secondary circulation system isolated so the lack of insulation shouldn't have an affect?

 

[JohnD]

Feel all the pipes during the night, see if any are warm.

 

[JohnD]

(Or, indeed, cold. After an hour or so they should be around ambient temperature unless water is flowing through them. There might a leak, drip or overflow somewhere. If you have mains pressure cold, it can force its way into hot pipes in a mixer, especially a shower mixer or a joystick ceramic tap.)