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How do I check hot and cold water pressure on 15mm exposed copper pipe?

We have a Grohe Avensys (GROHE 34222000) from Aug 2018 which I fitted myself, to replace a Grohe Avensys Modern EV from that had stopped regulating temperature properly which I'd fitted in 2008.

The temperature regulation isn't working again, so I'm trying to determine what might be wrong.

I've found that the temperature is significantly affected by the pressure of the incoming water.

We have a PRV on the cold feed - although the needle on the PRV pressure gauge doesn't seem to change regardless of how I set the PRV, the temperature of the shower is affected by changing the PRV screw. A very small change in the screw causes a big change in shower temperature.

I want to check the hot and cold pressures are roughly the same to exclude that as a problem; although according to documentation of the shower, and the current flow limiters in the shower elbows, so long as both are within 1-5 bars it should work OK. I don't actually have a measurement though, so Grohe in the past have not been able to provide support without me being able to tell them the pressures.

I was about to buy Monument Tools Mains Water Pressure Test Gauge 11bar - https://www.NoLinkingToThis/p/monument-tools-mains-water-pressure-test-gauge-11bar/82412 to use, but, apparently this should not be used on hot water, but more importantly, it doesn't look like it will attach to the hot/cold pipe. It's 15mm pipe, and currently has the olive and nut shown as A1 and A2 on.

Screenshot-20201128133523-1522x1266.png


In the past I've tried to replace the wax thermo-element and slide, but wasn't successful at then calibrating the shower (or maybe greasing it properly, I don't know) so I replaced the whole thing in 2018. I'm hoping this time to avoid replacing the whole thing again, since it's only been 2 years!

Many thanks for any tips!
 
If you if you really want to do this yourself when you buy the pressure gauge get 2 washing machine valves remove shower then valves will fit compression nuts and gauge fit strait on .
 
As mentioned above the shower likes balanced supplies although it will work on a 1:5 ratio difference.
Does the PRV just serve this shower with the hot side gravity fed? If that is the case then you will always struggle to get it working well as PRV's generally don't work well at low pressures.
As for the pressure gauge on hot water then I wouldn't worry about that for this purpose, it would need long exposure to excessive temperatures to cause it a problem.
Hope this helps...
 

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