Mains cold water knocking out gravity hot water in bath shower mixer

[The Plumber Guy]

Hi all. My customer has a bath shower mixer that gets cold from the mains and hot from a vented cylinder and the cold knocks out the hot. I'm thinking about putting in a thermostatic bath shower mixer but would appreciate any other ideas! Cheers all/

 

[Ric2013]

Bath shower mixers are generally not designed to run on 'unbalanced' pressures. If you are running unbalanced pressures to a mixer, you should have (at least) single check valves on both pipes. Some thermostatic mixers will accept unbalanced pressures (though it is not how they work best), others will not.

The real solution in your case may be to run the cold from a cistern as well and then the pressures will be balanced, or consider adding a pump the on the hot.

 

[Guyven]

Bath shower mixers are generally not designed to run on 'unbalanced' pressures. If you are running unbalanced pressures to a mixer, you should have (at least) single check valves on both pipes. Some thermostatic mixers will accept unbalanced pressures (though it is not how they work best), others will not.

The real solution in your case may be to run the cold from a cistern as well and then the pressures will be balanced, or consider adding a pump the on the hot.

Thanks very much for this. One issue I have is that it's a rented property and you need a crowbar to open the landlady's pocket. What do you think of putting a pressure reducing valve on the cold feed under the bath? I know that's not as good as adding a pump to the hot but it'll mean the mixer will work.

 

[Ric2013]

Thanks very much for this. One issue I have is that it's a rented property and you need a crowbar to open the landlady's pocket. What do you think of putting a pressure reducing valve on the cold feed under the bath? I know that's not as good as adding a pump to the hot but it'll mean the mixer will work.

Provided you can get the pressures to equalise, I can't see a problem with that, in principle, anyway.

 

[John.g]

If you install NRVs as suggested above then the manual mixer should work fine? but the flow is mainly governed by the hot water pressure, I had a shower like that one time but could only get ~ 5/6 LPM of mixed due to this.

 

[Ric2013]

If you install NRVs as suggested above then the manual mixer should work fine? but the flow is mainly governed by the hot water pressure, I had a shower like that one time but could only get ~ 5/6 LPM of mixed due to this.

The NRVs don't really help the mixer work. They actually tend to limit flow. The requirement to fit them is to prevent contamination and wastage of water, not to help the mixer work.

 

[John.g]

I had a swing check (horizontally only on the hot as the spring loaded type will destroy the head from a CWST/HW cylinder on a upstairs shower and I only fitted it in case the high pressure cold back flowed into the cylinder/CWST which I think can happen with the normal TMV? but probably not with the manual.

 

[Ric2013]

I had a swing check (horizontally only on the hot as the spring loaded type will destroy the head from a CWST/HW cylinder on a upstairs shower and I only fitted it in case the high pressure cold back flowed into the cylinder/CWST which I think can happen with the normal TMV? but probably not with the manual.

It would seem that, so long as the mixer is indeed supplying a mixture of hot and cold water (i.e. it's mixing!), then backflow is not actually occurring and therefore, the backflow devices are useful in protecting installations in which the mixer works correctly under normal circumstances but which might backfeed under special or fault conditions. It would seem that, so long as the mixer is indeed supplying a mixture of hot and cold water, then backflow is not actually occurring at that point in time.

My feeling is that a decent swing valve on the hot may still prevent the cold water backfeeding (and thus overflowing or reducing the air gap=backflow protection at the cistern) even if it does not satisfy the letter of the law. A swing check valve would not however, solve the OP's problem, I think. The cold will always be higher pressure than the hot and that may make it very difficult to get the mixer to mix properly.

As it is a bath/shower mixer we are talking about, I would imagine there will be some level of back siphonage protection incorporated to prevent the bathwater being sucked back up the mains in the event of mains failure. As such, the mixer might (but will not necessarily) incorporate a spring check valve in the outlet which leads to a postive pressure forming in the tap body that is higher than the hot water pressure. If this is the case, then this manual mixer will probably only allow hot water to flow if there is no high pressure cold water being mixed in at all.

John - Can you explain why you feel a manual mixing valve would not allow backfeed into the cylinder to occur, but that a thermostatic mixing valve would? I don't think you are correct, but I'm not sure!

 

[John.g]

You are probably correct in that a manual is no less likely than a thermostatic to back feed, I have never seen either opened up to see what's inside but assume something on the principle below.

 

[Alan Wright]

A ShowerPowerBooster on the hot for £125 is the obvious solution. Its a small micro-pump which fits on the hot pipe feeding the shower or to boost the whole house if fitted near the hot water cylinder. This will equalise the pressure.

 

[John.g]

Hi Alan, have you any actual pump curves for this single booster?, I have seen some output info but no actual real pump curves. If the pump power is 12 watts? then a boost pressure of 1 bar (assuming motor+pump efficiency of 60%) will give ~ 4 LPM?.

 

[alanderekwright]

Hi Alan, have you any actual pump curves for this single booster?, I have seen some output info but no actual real pump curves. If the pump power is 12 watts? then a boost pressure of 1 bar (assuming motor+pump efficiency of 60%) will give ~ 4 LPM?.

I would personally recommend the double boost for a gravity hot and mains cold system. The double boost SP21S is £155 and gives double the pressure of a single boost SP2B so I now regret suggesting the SP2B (£125), although many have fitted the single boost SP2B with great results.

We have sold over 30,000 pumps since launch, in the past year 95% of our reviews are 4 & 5 star on Trustpilot with 82% 5 star. We get rave reviews from customers with gravity hot and mains pressure cold.

The pump curves are more of a straight line than a curve and I have included a link to give you the flows and pressures for a single boost at the end of this reply.

You quoted a flow of 4 litres a minute at 1 bar but I believe that only flows of real importance is that from a shower head and for a single boost pump and for a WrightChoice shower head the flow is 5.9 litres a minute with a residual pressure of 2.3 metres.

https://showerpowerbooster.co.uk/wp-content/uploads/2019/05/Pump-Output.png

 

[Alan Wright]

The important flow is that from a shower head and for a WrightChoice Shower Head the flow is 5.9 litres a minute and a residual head of 2.4 metres.

I suggest you double boost the hot with a SP21S (£155) rather that a single boost SP2B (£125)

We have 95% 4 and 5 star reviews and we have lots of rave reviews from customers doing exactly what you want to do so I suggest you give it a try.

https://showerpowerbooster.co.uk/wp-content/uploads/2019/05/Pump-Output.png

 

[burkecontractors]

Hi all. My customer has a bath shower mixer that gets cold from the mains and hot from a vented cylinder and the cold knocks out the hot. I'm thinking about putting in a thermostatic bath shower mixer but would appreciate any other ideas! Cheers all/

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