Cold from hot taps

[Barryscotland]

Unvented cylinder heated from a gas boiler. All hot taps and showers are fed from the one outlet off cylinder. All mixer taps are putting only cold water out, showers and normal taps are fine. Mains water has a pressure regulator on it that was set at 4 bar, I turned that down to 2.5bar and I get hot water from mixer taps as normal.

 

I've not noticed an increase in pressure so pressume that it's always been at 4 bar and nothings changed there but for some reason the cold water is now over powering the hot?   system is no longer balanced? Any plumbers point me in right direction?  

At first I presumed it was a failed cartridge in the mixer taps but three wouldn't go at once but if one failed could it cause the problem? 

 

[ProDave]

It's been plumbed wrong.

 

The cold taps should be fed from the same pressure reduced feed as feeds the cylinder.  Instead it sounds like yours are fed direct from the mains and since you turned the pressure down, as you say the cold feeds are now at a higher pressure.

[Barryscotland]

15 minutes ago, ProDave said:

It's been plumbed wrong.

 

I don't think it has, it's been fine for two years, I think something has just broke. Just need to figure out what, if the trend continues  it'll be the most expensive bit. 

 

[Nickfromwales]

55 minutes ago, Barryscotland said:

I don't think it has, it's been fine for two years, I think something has just broke. Just need to figure out what, if the trend continues  it'll be the most expensive bit. 

 

I went to a house with exactly the same symptoms a few years back, and this took some Sherlock level fault-hunting.

 

The cause, imo, is a failed non return valve in the shower mixer valve. This means that hot and cold are mixing when you open a hot tap, so the cold pressure overwhelms the hot and wins the tug-of-war for it being freed at the outlet you've opened.

 

Try listening to the shower mixers when you run a hot tap on the kitchen sink or adjacent basin sink and see if that shower has the sound of water flowing (backwards) through the thermostatic cartridge.

 

That is your problem methinks  

[Barryscotland]

9 hours ago, Nickfromwales said:

cause, imo, is a failed non return valve in the shower mixer valve. This means that hot and cold are mixing when you open a hot tap

It appears that I have mains cold in the hot side permanently not just when a taps open. When I close the cold feed into the water cylinder open a hot tap then open the valve at the tun dish I have a constant pressurised flow where as it would normally spit n gurgle a bit then stop. Can a normal mixer tap failing cause this or is it only a shower body that would? Suppose I can eliminate the taps easy enough by isolating them

 

[Nickfromwales]

1 hour ago, Barryscotland said:

It appears that I have mains cold in the hot side permanently not just when a taps open. When I close the cold feed into the water cylinder open a hot tap then open the valve at the tun dish I have a constant pressurised flow where as it would normally spit n gurgle a bit then stop. Can a normal mixer tap failing cause this or is it only a shower body that would? Suppose I can eliminate the taps easy enough by isolating them

 

It all depends on the tap type, and whether or not the two bodies of water mix before being let out of the spout. So if you open the hot and cold tap at the kitchen sink, running the water slowly, the excess cold pressure that cannot force its way out of the spout contacts the hot water and pushes back, if the hot and cold are not balanced correctly.

 

Mixer outlets usually come with a factory fitted non return valve, at least one in the hot, or instruction to fit some during installation if deemed necessary. Typical example is where mains cold is on the cold side and a gravity hot supply is on the other side of the same mixer tap.

 

You say that you have cold mains permanently in the hot side, so It sounds like either the UVC install hasn't been done correctly, as in the cold feeds to all mixer taps haven't been taken from the control group (balancing valve) at the cylinder, so high pressure cold from the mains is one side, and balanced hot (at 3bar) is on the other, or there is defo a mix of the two bodies of water somewhere; (I suspect) a failed or failing non-return valve somewhere at one of the mixer outlets. So, when you turn off the isolating valve at the UVC control group, there is still cold mains in the property fed from the stopcock. When you open a hot tap, the cold mains is free to force its way into the hot pipework and then it travels backwards up it until cold water comes out of the hot tap you just opened. It is at this point I suggest you go into all rooms and listen to taps and shower etc to see if you can hear water "passing".

 

Usually this is where there's a thermostatic cartridge, hence I say to check the showers first, just a PITA as isolating them is tricky. Most should be able to be disassembled in situ for servicing, but depends on make and model obvs.  A lot of these come with factory-fitted integral non return valves, and if any crud gets into them then they cannot close fully, and then the issue presents itself after a period of time, a-la your symptoms.

 

Is the entire system new as of the ~2 years ago?

[Barryscotland]

1 hour ago, Nickfromwales said:

 

Is the entire system new as of the ~2 years ago

Yes, new build 2 years ago. Probably going to end up being a shower body as only way to access is remove tiles an cut a hole. Will investigate further after work. 

[Nickfromwales]

1 hour ago, Barryscotland said:

Yes, new build 2 years ago. Probably going to end up being a shower body as only way to access is remove tiles an cut a hole. Will investigate further after work. 

Take a pic of the shower(s) and post it here before you fire up the chainsaw

[Barryscotland]

If I remember there is an adjustable Allen key in the face of each cartridge behind the dials once there removed, unsure what they did tho. Put pressure back up and isolated each tap in turn but that made no difference so must be a shower. Couldn't hear any flowing noise from behind tiles. One shower does drip awhile after switching off if that could indicate a fault 

 

[Nickfromwales]

1 hour ago, Barryscotland said:

If I remember there is an adjustable Allen key in the face of each cartridge behind the dials once there removed, unsure what they did tho. Put pressure back up and isolated each tap in turn but that made no difference so must be a shower. Couldn't hear any flowing noise from behind tiles. One shower does drip awhile after switching off if that could indicate a fault 

 

Yes, that type are concealed but should have forward facing plugs which allow access to the NRV's etc, plus you should be able to remove the cartridge too. Get the dials off, slide the cover plate off, and take a pic.

 

I'd bet its the shower, lets put a 4-pack on it  

[Barryscotland]

If I remember there is an adjustable Allen key in the face of each cartridge behind the dials once there removed, unsure what they did tho. Put pressure back up and isolated each tap in turn but that made no difference so must be a shower. Couldn't hear any flowing noise from behind tiles. One shower does drip awhile after switching off if that could indicate a fault 

 

[Nickfromwales]

You may need to remove the plasterboard so you can see the whole valve, and then look again. The Allen head plugs usually give access to the NRV inserts, so you can change them.

[dpmiller]

check tinternet for piccies of the bare valve so you know where to aim for

[marshian]

On 04/07/2025 at 23:46, Barryscotland said:

One shower does drip awhile after switching off if that could indicate a fault 

if you have a rainfall type shower head that isn’t 100% level then that’s pretty normal for the head to drain itself after a shower (the body of water in the head and riser above it just needs a small air return to set off a self drain.

 

Both our showers can do this sometimes up to an hour later after a shower!!!

[marshian]

On 04/07/2025 at 22:12, Barryscotland said:

<snip>

 

Is it just me that has teeth itching looking at that picture?

 

ok yep just me 🙂

[Nickfromwales]

4 hours ago, marshian said:

Is it just me that has teeth itching looking at that picture?

 

ok yep just me 🙂

Sometimes the OCD has to be left in the van so you can get to 5pm mate   Great if you’re retired or on day rate, but a bugger when you’re s/e and on a price.

 

Very few people will notice or care, but if you give them an uplifted bill then they 💩 the bed. 

[Barryscotland]

On 06/07/2025 at 07:11, marshian said:

Is it just me that has teeth itching looking at that picture?

 

ok yep just me 🙂

All I see is I need to give it a clean. It is straight I have the camera squint. You would have it centered in a tile? 

 

[Barryscotland]

Shower valve is a rhoper Rhodes event sv1406. All I can find is a flow cartridge and thermostatic cartridge. No non return valve or parts diagram

 

[Nickfromwales]

53 minutes ago, Barryscotland said:

Shower valve is a rhoper Rhodes event sv1406. All I can find is a flow cartridge and thermostatic cartridge. No non return valve or parts diagram

 

Maybe a IR thermometer will help. If you start at ambient and run the water to recreate the issue, if cold (street temp) water mixes through that valve you’ll be able to watch the temp of the brass dripping all the way down.

 

Grab one locally in Screwfix or cheaper on Amazon etc. 

 

[marshian]

2 hours ago, Barryscotland said:

All I see is I need to give it a clean. It is straight I have the camera squint. You would have it centered in a tile? 

 

 

I'd have centered it on a grout line so effectively a small cut out of 4 tiles but as I said it's probably just me Please ignore me

[Nickfromwales]

1 hour ago, marshian said:

 

I'd have centred it on a grout line so effectively a small cut out of 4 tiles but as I said it's probably just me Please ignore me

Ok, we'll ignore you lol. 

 

Sometimes with a tile like that centring the gadget over the tile can mean that you then end up with a skinny rip of tile in the corner or end of wall, so you favour the shower being centre, and the tile cut being whatever, depending on your level of OCD; freakish to stratospheric etc.

 

That make sense, #strato-boy?

[Barryscotland]

4 hours ago, marshian said:

 

I'd have centered it on a grout line so effectively a small cut out of 4 tiles but as I said it's probably just me Please ignore me

To be fair the shower was fitted about 6 months before tiles where even picked. Now the shower in the other bathroom has no excuses, it's proper squint

[marshian]

5 hours ago, Barryscotland said:

To be fair the shower was fitted about 6 months before tiles where even picked. Now the shower in the other bathroom has no excuses, it's proper squint

 

Now that I find perfectly fine - I think it's the off centre that makes it OK

[marshian]

8 hours ago, Nickfromwales said:

Ok, we'll ignore you lol. 

 

Sometimes with a tile like that centring the gadget over the tile can mean that you then end up with a skinny rip of tile in the corner or end of wall, so you favour the shower being centre, and the tile cut being whatever, depending on your level of OCD; freakish to stratospheric etc.

 

That make sense, #strato-boy?

 

I know I have a problem - acknowledging it is half the battle and I agree skinny cuts suck big wang...............

 

I centered the shower controls and ended up with skinny cuts on the window - I spend less time looking at the window than I do at the shower controls - I'm at peace with the world