Pressure Reducing Valve on DHW Cold Supply to Boiler ONLY

[kevkdg]

Hi,

Gas engineer has advised fitting a Pressure Reducing Valve to the cold supply feed just before my combi boiler and setting it to 1.5 bar. I already have a mini expansion vessel/arrestor fitted near to the boiler, and he has suggested putting the PRV valve between that and the boiler so the mains will still get the benefit of the mini expansion vessel.

I asked about my mixer showers, and what the effect would be having cold water fed to them at mains pressure (~5 bar) and hot water at 1.5 bar. He said that it will make no difference as the flow will be unaffected.

Looking around this forum I get a different picture.

Whilst access and available pipework will make it easy to fit near the boiler, yet next to impossible near to the mains stop tap due to layout of pipes and access. I'd like to understand the affects of reducing only the hot water pressure down if possible. Do mixer showers have non return valves on their hot and cold supplies to prevent hot water for example finding its way back into the mains supply?

 

[Johntheo5]

Why is he suggesting the installation of a PRV?.

 

[kevkdg]

@Johntheo5 To protect heat exchanger. Only lasted a year or so. Third one I think in as many years. Boiler DHW Inlet is rated at 6 bars max though so am unclear why it's needed. By fitting the expansion vessel I have done away with the spikes that used to occur (up to 7 bar spikes) when shutting off taps etc.

 

[ianmcd]

Gas engineer has advised fitting a Pressure Reducing Valve to the cold supply feed just before my combi boiler and setting it to 1.5 bar. I already have a mini expansion vessel/arrestor fitted near to the boiler, and he has suggested putting the PRV valve between that and the boiler so the mains will still get the benefit of the mini expansion vessel.

Do that and you will definately need a new heatexchanger, it must be PRV then Expansion vessel, then boiler
A PRV by design is also a Non-return valve and the boiler uses the cold water incoming main as a method of expansion, by fitting a PRV you eliminate that so the small EV must take up the expansion

 

[kevkdg]

@ianmcd I have a water meter, but see your point. Does a boiler's cold DHW inlet not already have a non-return valve?

My maths suggests if the DHW inlet is rated at 6 bar, then a 5 bar static mains pressure (4 bar under flow) is well within tolerances. If the boiler is rated to take up to 6 bar DHW cold inlet (manual specifically states "Max Inlet Pressure 6 Bar" under the DHW section in the table), then allowances must have been made on top of this for expansion within the boiler right? So why is a Pressure Reducing Valve even needed I am asking myself. There are no longer temp pressure spikes since fitting the mini expansion vessel (tested with gauge), so the temporary spikes of up to 7 bar when shutting off taps has been eliminated.

I'm not convinced a PRV on the hot water side would not impact the mixer shower with the unbalanced pressures you'd end up with!

On a separate note. the AAV (Automatic Air Release Valve) was passing water every so often. I notice the black plastic schrader like cap on this has been fully tightened to close it off. Any repercussions of this? I assume air will make it's way to the highest radiator anyway and can be bled out!

 

[Johntheo5]

Most combi boiler manufacturers now state that a E.vessel must be fitted on the cold feed especially the ones with DHW preheat as the expanding water could damage the plate heat exchanger otherwise.

 

[kevkdg]

Most combi boiler manufacturers now state that a E.vessel must be fitted on the cold feed especially the ones with DHW preheat as the expanding water could damage the plate heat exchanger otherwise.

My boiler doesn't have pre-heat, BUT I have fitted a mini expansion vessel.

 

[Johntheo5]

You shouldn't, require a PRV as no spiking now but if deemed prudent maybe install it to reduce all the cold mains to say 3.5bar, no problem with mixing then.