Water Pressure Query

[diy mad]

Hi,
I have fitted a water pressure restrictor but am puzzled that when I turn the cold tap on and then off the pressure stabilises at 3.5 bar as expected.
However, when I do the same with the hot tap it reads almost 6 bar!
I know that hot water expanding will cause the pressure to rise but I have a combi boiler and the water is cold when I quickly turn the hot on and off.
Is this normal?
Many thanks.

 

[ianmcd]

What you have installed is called a pressure reducing valve, how far away from the boiler did you fit the valve (pipe length)

 

[diy mad]

The boiler is in the loft, the PRV is at ground level.
There is a 0.16L. mini expansion vessel about a metre on the cold water supply from the boiler.

 

[Johntheo5]

The PRV is supplying all the cold, including the cold to the boiler, set to 3.5bar?.
If you run the boiler on DHW does the pressure remain at 3.5bar and does it stay there for awhile when HW turned off or does it rise instantly to 6.0bar?

 

[diy mad]

Correct Johntheo, the PRV supplies all cold supply.
If I turn on hot tap and off immediately, before any water has chance to heat up, pressure rises to 6bar.
If I then turn cold tap on and off it will drop to 3.5,

 

[Johntheo5]

If I then turn cold tap on and off it will drop to 3.5,

If it then stays at 3.5bar and doesn't creep back up to 6bar then the PRV must be drop tight, could imagine the pressure increasing to 6bar if the boiler fired up for a brief period or had DHW preheat but the mini E.Vessel would have to have failed as well, ie full of water/ no air end precharge.

 

[diy mad]

Yes, it's a real puzzler.
Even if the PRV wasn't drop-tight I still don't understand why the hot tap off gives 6bar and the cold tap off gives 3.

 

[Johntheo5]

What might explain it is if the boiler supply is unregulated with full mains pressure and there is xover in a mixer.

 

[dal5band]

Just a thought - do you have to have some flow for the PRV to work ? Is there a difference between static ( no flow) and flowing pressure. I.e. if you actually run the hot the PRV should do its job.

 

[diy mad]

Thanks Johntheo but the boiler is supplied by water that has passed through the PRV.
Am I missing your point?

 

[diy mad]

Thanks dal5band, but at the end of the day, when the hot supply is turned off the pressure still reads 6.5 bar, which is worrying to say the least.

 

[Johntheo5]

Thanks Johntheo but the boiler is supplied by water that has passed through the PRV.
Am I missing your point?

No.

 

[Johntheo5]

Thanks dal5band, but at the end of the day, when the hot supply is turned off the pressure still reads 6.5 bar, whic0h is worrying to say the least.

If you open a HW TAP only and leave it open, does the pressure fall to 3.5bar?

 

[diy mad]

Yes Johntheo, in fact while the hot tap is switched on it drops below 3.5bar but shoots up to 6.5 as soon as it is shut off.

 

[Johntheo5]

Pressure cycling like that isn't good for the Combi PHE, even if the PRV is operating properly and isn't lazy then the water in the PHE will expand by 1.7% in heating from say 10C to 60C, ~ 0.051L assuming 3L in the PHE so would advise installing a 1L (if available) EV precharged to 3.0bar, if space allows.

You might consider replacing the PRV as well, what is the make/model of the present one?