back flow through thermostatic mixer

[ajrobb]

I have fitted an old thermostatic bath/shower mixer. Still waiting for combi boiler installation. The hot and cold pipes are only connected through this mixer but I get a lot of flow out of the basin hot tap. It doesn't seem to be affected by the mixer temperature setting.

Is this a problem?

 

[JohnD]

do you mean the hot water is supplied via a cylinder and a cold water tank?

go and look at the overflow.

 

[kevplumb]

you got a better crystal ball than me john ?

cos i couldnt make head nor tail of it

 

[JohnD]

I reckon the clue was in

Still waiting for combi boiler installation.

 

[kevplumb]

ahh cold main hot tank fed

with a plain bath filler/shower divertor

hence the reference to the overflow

 

[ajrobb]

Sorry, the only connections to the hot water pipes are the bathroom taps. If it weren't for the mixer, the 'hot' pipes would be dry. The mixer is thermostatic with a separate flow-control knob; turn one way for bath and the other for shower.

 

[JohnD]

you mean you only have cold water in your house and there is no hot supply?

and where do these hot pipes go to? unless they are capped off it will be leaking out somewhere

 

[ajrobb]

you mean you only have cold water in your house and there is no hot supply?

The house is uninhabitable. There is no hot water.

and where do these hot pipes go to? unless they are capped off it will be leaking out somewhere

The hot pipe runs from the bathroom basin to the bath mixer and down to the kitchen where it is capped off under the site for the new boiler.

 

[JohnD]

well I wouldn't have guessed that

 

[ajrobb]

The mixer where the 'dry' hot pipe is joined to the cold main:
View media item 26009John Syney Ltd., Milton Keynes, 'Cascade'

This is the flow from the basin hot tap full on:
View media item 26010
The cold tap is much more forceful, full on it would spill out of the basin. Even partial flow is more than hot tap:
View media item 26011

 

[JohnD]

if you turn on both the hot and the cold taps on the mixer, then yes, you would get some cold going up the hot pipe. If you had mixer set to "shower" you would get more than if it was set to the open "bath" spout

If there is any water going into the hot pipes when one or both the bath mixer taps are off, then you have a different cause. For example a Washing machine supply.

You can fit check-valves to prevent water going up the pipe the wrong way, but they restrict flow and are not perfectly reliable so I wouldn't bother.

Once you have the combi, the pressures will be the same hot and cold, so no reason for cold water to go up the hot pipe.

 

[ajrobb]

if you turn on both the hot and the cold taps on the mixer, then yes, you would get some cold going up the hot pipe. If you had mixer set to "shower" you would get more than if it was set to the open "bath" spout

It happens even with the mixer off. I suspect that both hot and cold are permanently connected to the thermostat inputs and only the output is modulated by the demand valve.

If there is any water going into the hot pipes when one or both the bath mixer taps are off, then you have a different cause. For example a Washing machine supply.

The bath mixer is the ONLY connection between the circuits. There is no kitchen yet.

 

[JohnD]

It happens even with the mixer off.

that surprises me. I reckon it must have worn internal seals. Looking at the picture I thought it was the type with separate hot valve and cold valve.

 

[ajrobb]

It has been installed in a hard water area. I might try dismantling the thermostat and soaking in vinegar. As the MI say: "From time to time it may be necessary to remove the thermostatic control for cleaning." "Remove any scale with vinegar and lubricate seals with heat resistant grease."

I have some Castrol LM grease, which states it has a high melting point. Would that be OK?

 

[JohnD]

i would have used plumbers silicone grease (it is white and odorless and does not attack rubber seals)