Single or Double check NRV

[fezster]

I understand the physical difference between the two, but wanted to know in the context of plumbing and heating, when is one used vs the other?

Also, in terms of pressure differential, would either stop a buildup of pressure on one side resulting in an increase in pressure the other side?

 

[ShaunCorbs]

Back flow prevention

WRAS Backflow Prevention Devices

 

[fezster]

I get that. But I want to understand how it works in relation to an unvented setup. The NRV is after the balanced cold to prevent hot water expansion to the cold outlets. I wanted to understand if this is how an NRV works - does it stop not just backflow but pressure build up also?

 

[ShaunCorbs]

Doesn't stop pressure build up its just a one way valve

 

[fezster]

Thank you for that.

The reason I am asking this is in my unvented setup I have:

Cold Water Main -> Pressure Reducing Valve (3 bar) -> NRV -> Pressure Relief Valve (8 bar) -> Megaflo inlet.

I'm finding every 3-6 months I need to recharge my Megaflo air bubble. Whether this is right or not is a different question - I'm waiting for funds to allow to get a plumber in to fit an external expansion vessel, as others have recommended.

But what I do see is my Pressure Reducing Valve showing a build up of pressure when the air bubble needs recharging. I'd always wondered why this was as I thought the NRV was supposed to stop this. Looks like that is not the case. Whilst it is a good indicator to tell me when the air bubble is depleted, it worries me that 6/7 bar means potentially my showers could have premature failure because the MIs state they should not exceed 3-4 bar.

 

[ShaunCorbs]

Sounds like you need to get it checked over and sorted by a G3 engineer or contact megaflow direct as the cylinder has a warranty

 

[fixitflav]

Cold Water Main -> Pressure Reducing Valve (3 bar) -> NRV -> Pressure Relief Valve (8 bar) -> Megaflo inlet.

Some types of Pressure Reducing Valve only work when there is a flow through the valve. If there is no flow the downstream pressure gradually rises to same as upstream. Other types maintain the set downstream pressure at zero flow. I believe (but worth checking) that the latter type needs a bleed-off pipe.

 

[snowhead]

But what I do see is my Pressure Reducing Valve showing a build up of pressure when the air bubble needs recharging.

When the bubble has gone, or the precharge in a vessel has gone there's no room for expansion when the water heats up.
The pressure within the system will rise, as seen on your gauge, until a tap is opened to reduce the pressure.

But then you've let cold water in (by drawing off hot) so the cylinder will reheat and the pressure rise again.

 

[Rob Foster]

THe nrv will not stop air that is dissolved in the incoming water which then is liberated when its the other side.

 

[fezster]

Some types of Pressure Reducing Valve only work when there is a flow through the valve. If there is no flow the downstream pressure gradually rises to same as upstream. Other types maintain the set downstream pressure at zero flow. I believe (but worth checking) that the latter type needs a bleed-off pipe.

Yes, the Pressure Reducing Valve is "drop tight" - certainly, the incoming pressure never exceeds 3 bar on the gauge, even though my mains pressure fluctuates between 5 to 7 bar.

It sounds like, though, from what Shaun said above, that the NRV does not stop the pressure increase on the megaflo side (when the expansion bubble has been depleted) from crossing it to the cold water side.

 

[johnduffell]

that the NRV does not stop the pressure increase on the megaflo side (when the expansion bubble has been depleted) from crossing it to the cold water side.

That's not correct, he is saying that the pressure on the downstream side of the prv will increase because the pressure reduction is impossible if it happens downstream due to expansion
If a nrv or dcv is passing backwards then it needs replacing. Dcv should be more reliable, but neither are 100%, you'd need a break tank for that with an air gap.