Mixer tap making cold pipe warm

[Hic777]

Just fitted a new basin with a Grohe mixer tap. The tap has 2 flexis pre-installed which I have connected to hot and other to cold.

Our system is a Worcester 28i Junior boiler, cold tank still present in loft feeding all cold taps in Bungalow (except Kitchen). No hot water tank. Hot water pressure is higher than the cold, due to tank on loft? Can this be bypassed?

Anyway, after plumbing the new basin, I noticed the cold pipe behind the basin was getting warm. Switched mixer to cold and after 30 seconds it gets cold again. Ran it at part hot and part cold and same again, cold pipe gets warm.
Do I just need to fit non return valves? On both or Cold feed to basin only? I have isolation valves 10 inches off the floor, so non return valves between these and basin?

I will also be fitting a thermostatic shower mixer so would I put non return valves on the shower feeds also?

Thanks

 

[terryplumb]

Non return valve on the tank fed cold.
The thermostatic mixer should be plumbed in with cold from the mains ,they need balanced pressures to work well.

 

[just pumps]

As said different pressures so one over powers the other.

 

[Hic777]

Much appreciated. I will double check the shower plumbing and see if it definitely does come from tank or is mains fed already.

The cold pressure is quite low in main Bathroom and en suite so if it does come from tank I might look at getting the tank removed. Would it have been left in for a reason or lazy plumbing?

 

[bernardgreen]

Non return valves will stop hot water flowing into the cold pipe. BUT as long as the pressure in the hot is higher than the pressure in the cold then the non return valve in the cold will remain closed and no cold water will flow.

 

[Hic777]

Non return valves will stop hot water flowing into the cold pipe. BUT as long as the pressure in the hot is higher than the pressure in the cold then the non return valve in the cold will remain closed and no cold water will flow.

Thanks. I will fit one and see if it fixes the problem and if still get water from the cold. I'd say hot is definitely higher pressure going on the main bathroom bath and basin.

 

[Madrab]

It would be a relatively easy process to convert cold to mains pressure if the cold water pipework is sound enough to take it and you have enough mains pressure/flow.
There will be a cold mains feed to the cistern in the loft and the cold feed to the house is running from the cistern so it just needs the mains connected over, up in the loft.

 

[Hic777]

It would be a relatively easy process to convert cold to mains pressure if the cold water pipework is sound enough to take it and you have enough mains pressure/flow.
There will be a cold mains feed to the cistern in the loft and the cold feed to the house is running from the cistern so it just needs the mains connected over, up in the loft.

Would that just be a case of trying it and seeing if the pipework is sound enough as most of the pipes run under the suspended floor so can't tell if there is a leak? Any kind of pressure test we could do?

 

[terryplumb]

Your loft tank may have been left connected for a reason. If your incoming mains pressure is low ,for instance . You should test your incoming mains pressures ,with a pressure gauge,before considering connecting all cold outlets to mains.
The mixer shower however must be.

 

[Madrab]

Well, normal process would be to isolate the mains and pump the pipework up to 1.5 times the normal static pressure for an hour and see if it drops.

Another way to test though would be to cut a pressure gauge into the distribution pipework, make the connection to the mains, isolate and let it stand for an hour or 2 and see if there's any drop. Will also tell you the mains pressure at the same time

 

[John D v2.0]

You could do a pressure test, the bodgy way if you have no equipment would be to make the connection and prime the pipework, then turn off the mains and from the system drain a small amount say 0.1l.
Then open the stop cock up again with the air in the system and turn it off again.
At this point the system is under pressure so opening a tap should emit a few tens of ml under pressure.
Leave it like that overnight and when you open a tap in the morning, you should still get the splutter of water. If not there's a leak.
It's a bit like a gas drop test only less precise as you don't have any measuring guage.
Edit: posts crossed over, also see the previous posters!

 

[Hic777]

Non return valves will stop hot water flowing into the cold pipe. BUT as long as the pressure in the hot is higher than the pressure in the cold then the non return valve in the cold will remain closed and no cold water will flow.

As it stands at the moment I put a NRV on the cold as the hot is higher pressure. What if I do as above and 'convert' cold supply to mains pressure for the basin, put a NRV on both? Or will it then be balanced so no need?

 

[Hic777]

Your loft tank may have been left connected for a reason. If your incoming mains pressure is low ,for instance . You should test your incoming mains pressures ,with a pressure gauge,before considering connecting all cold outlets to mains.
The mixer shower however must be.

Well, normal process would be to isolate the mains and pump the pipework up to 1.5 times the normal static pressure for an hour and see if it drops.

Another way to test though would be to cut a pressure gauge into the distribution pipework, make the connection to the mains, isolate and let it stand for an hour or 2 and see if there's any drop. Will also tell you the mains pressure at the same time

You could do a pressure test, the bodgy way if you have no equipment would be to make the connection and prime the pipework, then turn off the mains and from the system drain a small amount say 0.1l.
Then open the stop cock up again with the air in the system and turn it off again.
At this point the system is under pressure so opening a tap should emit a few tens of ml under pressure.
Leave it like that overnight and when you open a tap in the morning, you should still get the splutter of water. If not there's a leak.
It's a bit like a gas drop test only less precise as you don't have any measuring guage.
Edit: posts crossed over, also see the previous posters!

Much appreciated. I will let you know how I get on, will be over the next week or so.

I will check the shower first and if it is already mains fed leave as it is. I tested the old shower before ripping it out and seemed good pressure and. temperature. Am I correct in that shower doesn't require any NRV as it is balanced? I do have an access hatch to the pipes from behind the shower in hallway if when it gets plumbed in, something is amiss.

Thanks

 

[Madrab]

If the shower hot and cold supplies are fed from the same place - both being fed from the mains - then they would be balanced. Most mains fed showers have NRV's fitted into the inlets these days.

 

[dilalio]

Much appreciated. I will let you know how I get on, will be over the next week or so.

I will check the shower first and if it is already mains fed leave as it is. I tested the old shower before ripping it out and seemed good pressure and. temperature. Am I correct in that shower doesn't require any NRV as it is balanced? I do have an access hatch to the pipes from behind the shower in hallway if when it gets plumbed in, something is amiss.

Thanks

Depending on make/model of shower valve, it may have NRVs built in, lots do.

You can easily test which of your outlets are mains fed by closing your main stopcock and seeing which no longer run cold water.

It does beg the question as to why the Cwsc was left in place.

If you have isolation valves on both the cold feed to the Cwsc and the cold outlet AND in convenient locations, you can cut in a bypass from supply to outlet, using some pushfit fittings and pipe, temporarily.
This means you could leave the Cwsc in situ and still full of water, whilst you see how it performs over a few days.
If all is good, you could then drain and remove the Cwsc.

NOTE: if you go over to mains then any toilet fill valves will need to be converted to high pressure inserts (if you can get hold of them) or the complete fill valve will need replacing, in each.