Can a TMV fail on such a way as to create a 'short circuit' from cold to hot?

[Philip Saunders]

Hi, everyone - I hope someone can point out the obvious thing that I'm sure I've overlooked.

I have a DHW problem.

I have a 210 litre indirect cylinder (Megaflo) with secondary circulation.

There are 9 'drops' off the hot loop - all at the far end of the loop. Not all of the pipe run is accessible without breaking into walls - which I'd like to avoid.

Problem - when hot water is run from any outlet, all is well for about 5 minutes. It seems to be volume Independant - shower or bath tap it's still about 5 mins.
After that the water turns cold - at all outlets.
When in the 'failed' condition, the cylinder outlet pipe is HOT so it's not a 'not enough water' problem - however the hot return is cold.

Just to make diagnosis a pain, several of the tmvs have no isolation valves - either on the feed pipe or built in.

How do I diagnose this? Anyone have any ideas?

 

[ShaunCorbs]

could be mixing the water luke warm so would try a new tmv valve first

 

[Philip Saunders]

Thanks - but the water turns cold at ALL outlets simultaneously - not just the one being used. Once it's gone cold, there's no hot water anywhere for an hour or so - even though there's hot water in the tank.

 

[ShaunCorbs]

Thanks - but the water turns cold at ALL outlets simultaneously - not just the one being used. Once it's gone cold, there's no hot water anywhere for an hour or so - even though there's hot water in the tank.

would say its an issue with the tank heating then

 

[Last Plumber]

Hi, everyone - I hope someone can point out the obvious thing that I'm sure I've overlooked.

I have a DHW problem.

I have a 210 litre indirect cylinder (Megaflo) with secondary circulation.

There are 9 'drops' off the hot loop - all at the far end of the loop. Not all of the pipe run is accessible without breaking into walls - which I'd like to avoid.

Problem - when hot water is run from any outlet, all is well for about 5 minutes. It seems to be volume Independant - shower or bath tap it's still about 5 mins.
After that the water turns cold - at all outlets.
When in the 'failed' condition, the cylinder outlet pipe is HOT so it's not a 'not enough water' problem - however the hot return is cold.

Just to make diagnosis a pain, several of the tmvs have no isolation valves - either on the feed pipe or built in.

How do I diagnose this? Anyone have any ideas?

Hello,
It sounds to me as though you have correctly diagnosed this one.
You need to find the faulty valve.

 

[snowhead]

In answer to the original question, Yes a Non return valve in the TMV can fail and cause Cold to flow to Hot, but only if the pressure of the Cold water is higher than the Hot pressure.

Is the Cold to the taps the same pressure as the Hot, i.e fed from the balanced connection on the inlet control valve?

Is the Secondary pump working and moving water?
Is the pump in the Return or Flow?
Where does the return pipe join into the Cylinder, 1/3rd of the way down or ?, or does it join into the cold supply to the cylinder?
Is there a Non return valve in the Return close to the Cylinder?

Can you get access to all the TMV's at the outlets, not to shut them off but to feel the temperature on the hot and cold pipes to them?

 

[Philip Saunders]

In answer to the original question, Yes a Non return valve in the TMV can fail and cause Cold to flow to Hot, but only if the pressure of the Cold water is higher than the Hot pressure.

Is the Cold to the taps the same pressure as the Hot, i.e fed from the balanced connection on the inlet control valve?

Is the Secondary pump working and moving water?
Is the pump in the Return or Flow?
Where does the return pipe join into the Cylinder, 1/3rd of the way down or ?, or does it join into the cold supply to the cylinder?
Is there a Non return valve in the Return close to the Cylinder?

Can you get access to all the TMV's at the outlets, not to shut them off but to feel the temperature on the hot and cold pipes to them?

Cold taps fed directly off the cold main, cylinder fed from a separate connection to the cold main. Balanced connection is unused (I know - but I didn't install it)
Secondary pump (new) is definitely running although I've no way to know what the flow is like other than 'some' based on the fact that the return does get hot. Pump is in the return right next to the tank.
Hot return connects to the cold feed via a non return valve
Yes I can access the feed pipes in all but one case - and I can isolate that one for testing.

I think i know where you're going with this - problem - if it's failed cold to hot, both feeds will be cold - and they are - both at the outlet that's open and all the rest.