Gas supply pressure

Until recently, I understood that the regs say the pressure after the gas meter must be 21+/-2mb. I didn’t dream that, I’ve seen this figure various places, including on this forum.

But having been corrected, I see that the pressure after the meter can be minimum 15mb, so allowing maximum 1mb pipe drop, minimum 14mb at the boiler. Do the regs say a boiler must work, at design output, when the pressure is 14mb? Or is that considered an upset condition, as the attached link suggests?

If they do, how do manufacturers get away with specifying minimum pressure into the boiler 20mb, as many of them do? Even based on 21+/-2mb after the meter, and pipe pressure drop 1mb, it could be 18mb at the boiler. In practice, I’m fairly sure all boilers would be OK at 18mb, but there does seem to be a case of left hand and right hand.

Can 21+/-2mb be assumed for normal conditions?

gas pressure worcester-bosch

This is because appliance manufacturers and Gas transporters work to different regulations, Gas safe engineers work to GSIUR where transporter engineers work to their own regulations, nothing new has been a problem for many many years

ianmcd said:

This is because appliance manufacturers and Gas transporters work to different regulations, Gas safe engineers work to GSIUR where transporter engineers work to their own regulations, nothing new has been a problem for many many years

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I'm sure most of the time in practice there's not a problem, but that doesn't answer my questions. My guess is boiler makers specify 20mb because they haven't bothered to think about it, and just quote the "nominal" supply pressure. But they might be giving themselves a get-out. If the pressure is 18mb we all know a boiler should work fine, but they could point to their installation manual and say sorry, not our problem. There's plenty of regs out there, it doesn't seem unreasonable to specify a minimum pressure boilers must work at.

fixitflav said:

it doesn't seem unreasonable to specify a minimum pressure boilers must work at.

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Totally agree, but it wont happen

I haven’t seen where they specify a minimum of 20mbar, they state a minimum inlet working pressure. This is different to standing pressure. Also, what is taught at a lot of centres, is 21mbar plus or minus 2mbar at the meter test nipple.

CBW said:

I haven’t seen where they specify a minimum of 20mbar, they state a minimum inlet working pressure. This is different to standing pressure. Also, what is taught at a lot of centres, is 21mbar plus or minus 2mbar at the meter test nipple.

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A particular example from a Ferroli, in a thread November last year "Appliance inlet working pressure must be 20mbar MINIMUM, for NG". And one from a Potterton about the same time "Gas Supply Pressure 20 mbar". Those pressures must be in operation, meaningless if anything else.

Good to see you've also heard of 21+/-2mb!

CBW said:

I haven’t seen where they specify a minimum of 20mbar, they state a minimum inlet working pressure. This is different to standing pressure. Also, what is taught at a lot of centres, is 21mbar plus or minus 2mbar at the meter test nipple.

Click to expand...

Just noticed this from an Ideal manual.

"It is the responsibility of the Gas Installer to size the gas
Installer to size the gas installation pipework in accordance with
BS6891:2005. Whilst the principle of the 1:1 gas valve ensures
the Logic range is able to deliver it’s full output at inlet pressures
as low as 14mb, other gas appliances in the property may not be
as tolerant. When operating pressures are found to be below the
minimum meter outlet of 19mb these should be checked to ensure
this is adequate for correct and safe operation."

Looks like they have put some thought into it recently, the manual for my old Minimiser says 20mb minimum. The 14mb suggests they've taken account of the possible minimum 15mb after the meter, but they also seem to recognise a normal range 21+/-2mb.