Baffled By Cylinder Hot Water Draw Off Leak After Removal Whilst Cold Water On

[old duffer]

As said above ,mixer tap is allowing crossover ,cold water into hot pipework.

interesting theory. What pressure would be required to do that and why would he not have experienced this before?

 

[terryplumb]

interesting theory. What pressure would be required to do that and why would he not have experienced this before?

Most domestic mains pressure would be 1bar minimally,and more than enough.
With the system fully operational ( domestic hot water in use constantly) the symptoms have probably not manifested themselves and gone unnoticed.

 

[old duffer]

Most domestic mains pressure would be 1bar minimally,and more than enough.
With the system fully operational ( domestic hot water in use constantly) the symptoms have probably not manifested themselves and gone unnoticed.

I operate my cold feed @ 2.5bar and my hot water feed is 4.5bar. I use the grohe shower mixers (the simple ones with no cartridge) and on baths I use the same simple mixer (no temp control cartridge)
If I turn the valve to full cold setting, the cold water comes out yet this is a lot less pressure that the competing hot water
On every occasion, the hot water comes out of the tap but with your suggestion, it will also be flowing back along the cold water feed pipe.
Would this mean that I'm losing pressure at the outlet (the showers have very large rainfall heads so hard to notice)
Just how far would the hot water push the cold water back? Would it push back into other showers cold supply and raise their temp or would it just push against the main supply?

 

[terryplumb]

What supplies your hot water at 4.5 bar ?
Are there non return valves on the cold supplies ?

 

[JackK]

If cold water was constantly running into the hot distribution pipework ,of course the water level in the CWSC would rise ,to overflow level ,it won't come out the vent pipe as that is higher than the CWSC ,and bends down into it, it would back fill via the cold feed pipe which goes into the bottom of the cylinder.
Cold must only be entering hot pipework when tap is running ,with both hot and cold valves of tap open.

What happens if cold only is switched on and no hot required from kitchen tap - Under your explanation water would come out of overflow correct? I never saw that happen so I am still confised

Mains is 3.5 bar - Hot water is gravity

 

[terryplumb]

If the hot valve on the tap is closed ,no cold water could enter the hot pipework,as it couldn't get past the valve.

 

[JackK]

If the hot valve on the tap is closed ,no cold water could enter the hot pipework,as it couldn't get past the valve.

That is true - I follow you

So the conclusion is when hot and cold open at different pressure cold water can go up the hot water tank and cause CWST to fill up and overflow - I have to try that when my system is filled up - Seems really scary actually

 

[terryplumb]

I didn't get to see your tap ,but try this .
Turn cold tap on full and open hot tap partially ,let water flow for 1 minute and close both taps. Then open hot tap only ,See if water flows out the tap spout.

 

[JackK]

I didn't get to see your tap ,but try this .
Turn cold tap on full and open hot tap partially ,let water flow for 1 minute and close both taps. Then open hot tap only ,See if water flows out the tap spout.

Shamefully I bought it on amazon - Here it is https://www.amazon.co.uk/gp/product/B08XZ2KYQP/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

 

[terryplumb]

Thanks ,I assumed tap had 2 handles . With yours ,put lever in central position ( as though mixing hot and cold ) let tap run 1 min. Turn off.
Then turn lever to hot position only and turn on ,does water flow from spout ?

 

[JackK]

Thanks ,I assumed tap had 2 handles . With yours ,put lever in central position ( as though mixing hot and cold ) let tap run 1 min. Turn off.
Then turn lever to hot position only and turn on ,does water flow from spout ?

i will try that tomorrow - dont want to try it at night when plumbing could go. i expect answer when tap open will be yes but will confirm

That is the sure test to see if backwater theory is true - If it is true I will have to buy non return valves for the tap isoltaion valves ?

 

[terryplumb]

A non return valve on the hot supply pipe to tap would certainly prevent cross contamination ,however it may cause another issue. I suspect the flow of hot water from that tap, previously , was pretty poor compared to the cold, unless your hot water was pumped ?

 

[JackK]

The hot water pressure was always low. With the new tap it’s dreadful. I thought that was because of the tap tails which are 10mm

 

[terryplumb]

The hot water pressure was always low. With the new tap it’s dreadful. I thought that was because of the tap tails which are 10mm

You are correct ,the small diameter tap tails are restrictive ,but the real issue is the low pressure ,gravity fed hot water. The cold is good is it not ?

 

[JackK]

You are correct ,the small diameter tap tails are restrictive ,but the real issue is the low pressure ,gravity fed hot water. The cold is good is it not ?

The cold is good correct - I always will have gravity fed hot - What does everyone with a hot water cylinder end up doing?