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

[JackK]

I had my indirect hot water cylinder removed a few days ago and the central heating boiler and pump is turned off at the fuse spur right next to the boiler.

I have been without hot water for a few days which is fine - The plumber left the flow / return to the cylinder uncapped as well as the hot water draw off from the cylinder (This is connected to a vent pipe which runs into the cold water storage tank (as it is meant to on a low pressure system).

The cold water to the storage tank is also off as the cold water storage tank is being replaced.

Today I was astonished - I had my cold water kitchen tap open (its a mixer tap) and saw water coming out of the (open) hot water draw off pipe.

See attached

How is it possible for cold water switch on to interfere with hot water ? - If pipe was capped off (and it should have been) it would have gone into the cold water storage right?!

I am quite experienced with heating systems and plumbing but cant figure this out

I d be willing to buy a drink or make a donation to charity if anyone can explain his

 

[cross thread]

Mixer tap passing through cold to hot

 

[terryplumb]

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

 

[JackK]

Is that really possible - There would have been no water coming from hot water tap when on as it is empty - Is it possible that mains pressure cold would have backed up the empty hot pipe? I find that difficult

 

[terryplumb]

Yes it is possible ,thermostatic shower valves can also do the same thing as well as mixer taps.
If there is an isolator on hot supply to problem tap turn it to off.

 

[oldbutnotdead]

Is the hot tap switched off- it's usually the kitchen tap used to drain vented systems cos it's the lowest

 

[JackK]

Is the hot tap switched off- it's usually the kitchen tap used to drain vented systems cos it's the lowest

Well that I cannot be sure - I cannot remember if the mixer tap was opened on all cold or some hot and cold. So far the only explanation is pressure from mains was so high that it could not be maintained by tap and therefore backfilled up the hot water pipe to first floor level and out of hot water open connection.

 

[oldbutnotdead]

Well that I cannot be sure - I cannot remember if the mixer tap was opened on all cold or some hot and cold. So far the only explanation is pressure from mains was so high that it could not be maintained by tap and therefore backfilled up the hot water pipe to first floor level and out of hot water open connection.

Mmm. Good odds hot was open- I've done exactly the same thing . If it was a thermostatic mixer then you can get problems with large pressure differentials but a standard mixer- highly unlikely.

 

[JackK]

its a standard mixer kitchen tap - not trv

 

[gas112]

edit

 

[JackK]

This got me thinking all weekend - If the cylinder was in and the system running then when I open up the kitchen tap with mains in and hot water from cylinder would this result in water backing up to the hot water and therefore water appearing up the hot water vent pipe and into the cold storage ?

 

[terryplumb]

With the hot water pipework connected ,and filled with water,there may not be a noticeable difference when hot and cold are running at the same time on the problematic tap.
Can you show us a pic of the tap in question ?
If cold water can enter the hot water distribution pipework ,the water level in the cold water storage cistern would raise ,but it wouldn't come out of the expansion pipe ,although the level in the expansion pipe would raise to the same level as the water in the cold water storage cistern.

 

[Johntheo5]

I have two taps going to one mixer in the kitchen, the cold (mains) is at ~ 3.0bar and the hot from a vented HW cylinder is ~ 0.4bar but the mixing takes place as the water comes out the spout, thermostatic mixers are supposed to have non return valves to prevent cross feeding.

 

[JackK]

With the hot water pipework connected ,and filled with water,there may not be a noticeable difference when hot and cold are running at the same time on the problematic tap.
Can you show us a pic of the tap in question ?
If cold water can enter the hot water distribution pipework ,the water level in the cold water storage cistern would raise ,but it wouldn't come out of the expansion pipe ,although the level in the expansion pipe would raise to the same level as the water in the cold water storage cistern.

I will get a pic shortly - However I dont understand this - If we believe that cold water backed up and came out of the hot water then if this was all filled up and running i would expect the hot water cylinder to be filled up and therefiew water backing up would cause the water up the vent pipe and into the cold water storage

 

[terryplumb]

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.