Hot and cold water crossflow

[Connor Williams]

Hello, this is an update on the hot water fluctuating in temperature, the cold taps in the bathroom are fed from a tank in the loft and they sometimes run hot!
Somewhere in the system the hot and cold pipework must be connected, which would explain the problems with the hot taps sometimes running cold.
I have replaced the bath tap mixer which rules that out, what else can I check except tracing all the pipework back to the boiler?
Could backflow be the cause?
Driving me insane, any help would be greatly appreciated!

All the best,
Connor

 

[Bernie2]

Hi! Connor

Is the water in the tank hot?

I assume we are talking about a cold storage cistern/tank?

Sometimes if the hot water cylinder (which again I assume you have) which is usually fed from the same cold water tank as the cold taps, overheats, it backs up into the cold storage cistern and can come out the cold taps.

It may not happen all the time depending on usage and a few other things.

 

[Connor Williams]

Thanks for your reply, there is a combi boiler in the kitchen and the cold storage tank in the loft only feeds the whb and bath cold taps so the pipework should be completely seperate, as all the hot taps are fed straight from the cold mains.
The hot taps fluctuate from red hot to cold constantly so I can only assume somewhere,somehow the hot is mixing with the cold supplied from the storage tank.
I am going to trace all the pipework and see if I can find where the pipework has perhaps been crossed over.

All the best,
Connor

 

[Puddle]

well your cold taps sometimes run hot,is this after you have used bath shower mixer,mains hot will go up cold pipework to tank heating tank water,if you leave long enough ,tank over flow will start to run

Hot water going hot and cold,probably down to faulty boiler not modulating correctly,run hot tap,go to boiler see if burner going on and off

 

[Bernie2]

Hi! Connor

Crossflow can occur like Puddle says. Basically wherever the hot and cold combine can be a potential crossflow point.

But there are some in obvious places where this can happen such as dishwashers or twin feed washing machines although I think they are supposed to have an air gap which should make this impossible, its perhaps worth a check.

Other common points are mixer taps, showers, thermostatic mixing valves and the like.

A super long shot could be from a combi with a broken dhw expansion vessel whose water is using the water main as the expansion vessel instead. The heated water may be going into the cold tank. It should not happen of course if the proper nrv's and tee's are in place. But you could check that to see if the tank water was cold or hot.

You could also turn the cold off and see whether any hot water comes out the cold tap. And vice averse for hot tap.

Good Fortune

 

[stevetheplumber]

best cure is remove tank and go all mains

 

[Connor Williams]

Many thanks for all your help guys,am at the property today so will go through everything suggested and let you know the outcome, thanks again.

All the best,
Connor

 

[stevetheplumber]

Hi! Connor

Crossflow can occur like Puddle says. Basically wherever the hot and cold combine can be a potential crossflow point.

But there are some in obvious places where this can happen such as dishwashers or twin feed washing machines although I think they are supposed to have an air gap which should make this impossible, its perhaps worth a check.

Other common points are mixer taps, showers, thermostatic mixing valves and the like.

A super long shot could be from a combi with a broken dhw expansion vessel whose water is using the water main as the expansion vessel instead. The heated water may be going into the cold tank. It should not happen of course if the proper nrv's and tee's are in place. But you could check that to see if the tank water was cold or hot.

You could also turn the cold off and see whether any hot water comes out the cold tap. And vice averse for hot tap.

Good Fortune

most combis dont have an exspansion vessel on the dhw and are designed to use the main to expand same goes for instantaneous water heater

 

[Connor Williams]

Well many thanks for all your help guys, because the whb and bath cold supply was tank fed and the hot was the mains, the hot water on the bath mixer was backflowing down the cold whenver the shower was on.
This was causing pressure/temperature problems and causing the burner on the combi to switch on and off.
I fitted a non return valve on the cold pipe and that sorted it.

Thanks again and all the best,
Connor

 

[Bernie2]

Hi! All

Just a point about expansion vessels on the dhw.

Some boilers have them some don't, you can get kits to fit one if you want too.

The thing is as Steve says heaters less than 15 litters and perhaps many combis usually don't have them.

What this means is that you have got to leave space in the cold mains pipework for expansion to take place. That is why you have to go careful about where you site nrv's and valves on the mains water supply to combi's,