pressure at inlet with a zero governor

[Simon F]

http://www.ukplumbersforums.co.uk/g...forum/66714-operating-pressure-appliance.html

I agreed with the point TB made in the above post about not getting an accurate reading due to the gas being fanned in rather than just the gas pressure doing the job, but no one else seemed to agree with him.
So I thought at the first opportunity I would do a test.

- wp at meter with hob on 3 rings - 23 mbar
- wp " " boiler on 60 percent power - 22 mbar
- wp " " " on service max power - 21 mbar
- wp at boiler inlet on 60 percent power - 21 mbar
- wp " " " on service max power - 19 mbar

the pipe was sized and calculated at 0.7 mbar pressure loss.

so I would say the working pressure was 23 mbar as that was the positive pressure of the gas at the meter.
open to opinions, but I agree with the point TB made that a zero governor does create a suction on the pipe/valve and so the pressure taken at the boiler inlet valve, even at the meter is only an approximate figure, that will be lower than the reality.

 

[AtlasGas]

Your working pressure is 19mb. The zero governor is Is set to zero, ie fan draught -4mb then gas pressure 4 mb, differential pressure 0mb. The operating pressure is 0mb.

 

[AMGasServices]

I'm totally confused. Because I went to a glowworm combi 38 cxi. And the standing pressure at the boiler was 23mbat but when the fan kicked it the pressure shot up to 26 mbar than shot down to 20 mbar.

 

[AtlasGas]

Probably fluctuation in the meter governor on demand, don't read too much into standing pressures there pretty much irrelevant in most cases.

 

[AWheating]

the boilers should only suck so to speak if the gas supply is incorrect.

 

[Ermintrude]

Gas rate and inlet will give u good idea on a zero gov. Say u gas rate 30kw appliance at 34kw....

Check inlet 15mbar


Gas pipework undersized!

 

[stevetheplumber]

in the example you give you have a 2 mbar drop on pipework there fore its undersized
if there is ample gas at the inlet of the gas valve it cant be sucked through the guvnor in the gas valve

 

[TBServices]

Better turn off all the boilers and upgrade to a 35mm supply then!

 

[AWheating]

are these figures tested at the gas valve? you will loose 1-2.5mb from nut to nipple remember.

 

[AtlasGas]

I could do with loosing a bit between nut and nipple. :rolleyes2:

 

[Reg Man]

WP loss over pipe run under max flow 1mbar at inlet to boiler. Restrictions allowed are from the boiler inlet to the gas valve on in the boiler. So unless, in most cases, you fit a test point at the inlet to the boiler you will not get an accurate reading.

 

[wattsie1987]

WP loss over pipe run under max flow 1mbar at inlet to boiler. Restrictions allowed are from the boiler inlet to the gas valve on in the boiler. So unless, in most cases, you fit a test point at the inlet to the boiler you will not get an accurate reading.

Totally agree vaillant now saying a possible 5mb loss from appliance isolation valve to air/gas ratio valve. Test tee under boiler is the only way should be mandatory to have a test point built in to the iso valve I think.

OP There is never a negative pressure on the inlet of the air/gas ratio valve providing the correct volume of gas is present at the boiler.