UVC cooling down very quickly

[oldbutnotdead]

Do you have a water meter? If yes, turn every tap off in the house, see if it's still showing flow. If it is, turn the main stoptap off, see if it's still showing flow.
Is the pressure relief valve continually operating (should be a tundish on the outlet pipe so you can see it).
Is the return from the secondary circulation teed into the cold water supply at/near the tank or does it go into a separate boss.
Do you have a group valve (to equalise hot and cold pressure in outlets where it matters- usually mixer showers), does that group valve incorporate a backcheck valve in the cold supply?
A few pics of the tank & valve gear would be handy, as would a sketch schematic of the dhw pipework.

 

[Johntheo5]

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

Have you isolated the cold mains overnight as suggested above, ensure no heating can occur for the 6 hours or so.

 

[osmononame]

I have an instant read thermometer so will try and measure the temp between tonight and tomorrow morning with the cold mains on and then the following night with the cold mains off to see what the difference is.

There is no water meter so I can't check that although it is mains pressure cold I have. The tundish is not dripping. All the mixer taps have non return valves on both hot and cold sides but the thermostatic shower valves do not - maybe this is where the issue is? I will try and feel by hand to see if I can feel any of these are any hotter than they should be.

Will report back with that and pictures!

 

[oldbutnotdead]

I have an instant read thermometer so will try and measure the temp between tonight and tomorrow morning with the cold mains on and then the following night with the cold mains off to see what the difference is.

There is no water meter so I can't check that although it is mains pressure cold I have. The tundish is not dripping. All the mixer taps have non return valves on both hot and cold sides but the thermostatic shower valves do not - maybe this is where the issue is? I will try and feel by hand to see if I can feel any of these are any hotter than they should be.

Will report back with that and pictures!

If that's an infrared non contact thermometer they're not very good on shiny surfaces (like pipes). Mixer showers are prone to that sort of failure and ought to be fitted with double check valves as a matter of course.

 

[osmononame]

I meant the temperature of the water coming out of the tap with the thermometer - it's a food probe but should work!

Regarding the temp of the actual taps and mixers I have just realised I have a infra red camera that I can attach to my phone. That should show if any of the mixers are hotter than ambient easily enough.

Lastly I have no idea how the plumbing is routed - the plumber who did it has gone AWOL. I honestly think he's crossed the hot return into a cold mains somewhere and that's allowing cold water to be forced back in to the tank. Nothing to go on with this theory mind!

 

[JohnD]

I honestly think he's crossed the hot return into a cold mains somewhere and that's allowing cold water to be forced back in to the tank.

A shower mixer could easily do that. The pipes will be surprisingly cold, not hot.

Isolation valves are very handy.

 

[Madrab]

If that's an infrared non contact thermometer they're not very good on shiny surfaces (like pipes). Mixer showers are prone to that sort of failure and ought to be fitted with double check valves as a matter of course.

Not often I disagree but in this case .... .... Most shower IME would normally have NRV's already built in. They are usually non verifiable valves but they perform the same function to avoid crossflow (like a single check valve). If they jam they are usually removable and serviceable.

Double check valves, IME, are primarily used for backflow protection and are installed to avoid contamination, I have found they can be restrictive to flow, even with mains pressure so wouldn't use them on outlets where maximising flow is of benefit.

 

[osmononame]

Would a thermostatic shower do that as well rather than just a mixer?

 

[oldbutnotdead]

Not often I disagree but in this case .... .... Most shower IME would normally have NRV's already built in. They are usually non verifiable valves but they perform the same function to avoid crossflow (like a single check valve). If they jam they are usually removable and serviceable.

Double check valves, IME, are primarily used for backflow protection and are installed to avoid contamination, I have found they can be restrictive to flow, even with mains pressure so wouldn't use them on outlets where maximising flow is of benefit.

Yeah they are very restrictive. Do I really not need them on mixer taps- have I misunderstood water bylaws somewhere? (I can think of one that'll be coming out straight away- flow and pressure are ok when dhw is coming from the plate hex on the thermal store but rubbish if I switch over to the combi).

 

[Madrab]

Do I really not need them on mixer taps- have I misunderstood water bylaws somewhere?

The only bylaw with regards to backflow protection IME, as far as the water regs are concerned (if any are actually being observed), is to install protection of the external mains. There should be a verifiable double check valve on the mains just after the stop tap and before the drain off, that then protects the mains supply upstream of the property.

What happens inside the property is really only for the individual to worry about.