Water balance issues since going over to an unvented cylinder

[Robbot]

A year back I had a new unvented cylinder installed to replace an old vented one. As you can see from the attached diagram, no balanced cold feed was taken from the back of the PRV. The cold feed that supplies the kitchen and bathroom remains as before, and the fitter took the existing feed to the loft tank for the cylinder.

When the bath is being run, the kitchen taps dripple and Vice versa. It appears as if there is not enough flow. The PRV was checked during a recent service and was found to be fine. Any ideas?

 

[oldbuffer]

1. Did you have the incoming mains water pressure and flow rates measured before the installation. If so, what were the pressure and dynamic flow rate readings?
2. Do you know the diameter of the cold water pipes?

 

[Robbot]

1. Did you have the incoming mains water pressure and flow rates measured before the installation. If so, what were the pressure and dynamic flow rate readings?
2. Do you know the diameter of the cold water pipes?

The installers didn't make those checks. However, Anglia Water took a measurement a few years back (before the unvented tank install) and they said it was just over 3 bar and normal for the area.

The cold water pipe is 15mm from the stopcock to the kitchen tap, bathroom tap, and cistern. Then just before the PRV it has a small section of 22mm. Hot is 22mm.

I did measure the flow prior to the install as I was trying to balance a shower system with the old vented tank. I cant remember exactly what it was, but again it was within a normal range.

I gutted, and fitted a new bathroom a year before the unvented tank went in. I replaced all the pipework under the floor and never had any issues with the cold balance between all the taps. I was looking forward to having a decent balanced shower with this install !

 

[Madrab]

An unvented cylinder should really have a 22mm feed all the way from the mains to the combination valve to maximise flow. Then there should be a balanced feed either from the combination valve itself or at least after the Pressure Reducing Valve to ensure there are no issues with monobloc mixers, thermo mixer showers etc.

It's kinda unvented 1'o'1.

There needs to be a dynamic pressure and flow survey done to see exactly what is available and what can be achieved, ideally with an outside tap and 2 or more outlets open at the same time.

 

[Robbot]

To replace the 15mm with 22mm would have required pulling up (or at least cutting a channel in) the subfloor across three rooms. The budget didn't run to that amount of work unfortunately. I can tap into the 15mm cold feed under the airing cupboard floor, blank of one end and connect 22mm from the bathroom to the balanced out of the PRV (coloured red) if that would make any difference?

 

[Madrab]

Just to clarify when you say

When the bath is being run, the kitchen taps dripple and Vice versa

Are you talking about the cold supply or the hot?

 

[Johntheo5]

Why not stick a pressure gauge on the (unused) balanced cold out, if you (or your plumber) shuts the isol mains valve then the incorporated non return valve in that combination set will/should stop any water coming back from the cylinder, just slacken the blank after shutting the islol vale to check.

 

[Robbot]

Just to clarify when you say


Are you talking about the cold supply or the hot?

This is directly related to the hot supply. The cold supply does have an issue, but it's of my own making. I installed high pressure plain mixer taps in the basin, shower and sink, which all work together fine -even when the hose pipe and cistern are using water. However, the bath has low pressure taps, so when the bath cold is on it robs all the other cold taps (I did temp fit a high pressure bath tap and that issue resolved). Whereas, turning on any hot tap (kitchen, basin, ect ) it robs all the other hot taps regardless of what bath tap (low or high pressure) was fitted.

 

[Robbot]

Why not stick a pressure gauge on the (unused) balanced cold out, if you (or your plumber) shuts the isol mains valve then the incorporated non return valve in that combination set will/should stop any water coming back from the cylinder, just slacken the blank after shutting the islol vale to check.

View attachment 410705

The installer fitted the PRV, so the back of it is against a solid 6 inch brick wall

 

[Madrab]

I'd also be checking the combination valve output, if you say the other taps are all fine.

As @oldbuffer mentions though, I would be taking, dynamic pressure and flow reading @ the mains to give a starting point, the rest is all guess work really until there is a known baseline and therefore what's actually available.

 

[Robbot]

I'd also be checking the combination valve output, if you say the other taps are all fine.

As @oldbuffer mentions though, I would be taking, dynamic pressure and flow reading @ the mains to give a starting point, the rest is all guess work really until there is a known baseline and therefore what's actually available.

I'll get that organised. I'm more curious now as to what is going on.