one mb drop

[stevetheplumber]

i have just repipe a gas carcass in a house that was woefully under sized and the customer asked me why we had to have less than 1 mb drop end to end that was when i realised i didnt know or if i did ive forgotten? anyone enlighten me?

 

[mikegas]

because we get 21mb from the meter and mi states appliance must have 20mb at appliance

 

[migoplumber]

MI's state different minimum operating pressures - some go down as low as 10mbar,
somebody in an office long ago had to decide on minimum pressures in the main line for an appliance to work. and provide manufacturers with this data - so they can build boilers to suit, hence most new appliances have in built governor within gas valve.
now that gas is running out and the type of gas mix being used is changing, the mains pipework is being altered to medium pressure ( up to 75mbar ) many old governors on the new system are now passing up to 35mbars technically id, call transco and they come change the governor.
so i reckon thats the reason, basically supplier has to guarantee an adequate supply to appliance, to secure themselves they insist on 1mb drop that then gets passed to gas regs !!??

 

[Billy Bob Bob]

gas regs basicly,we have to guarantee 20 mb but this is a very out dated rule in some areas at peak time you may only get 18mb and as said most combis run between 10_12mb as you know the rule is there so appliances such as cookers with no safety shut offs(thermocouples etc) are not robbed of gas and extinguish thats why they ask for a 1 mb drop across pipe work

 

[Ockyvines]

gas regs basicly,we have to guarantee 20 mb but this is a very out dated rule in some areas at peak time you may only get 18mb and as said most combis run between 10_12mb as you know the rule is there so appliances such as cookers with no safety shut offs(thermocouples etc) are not robbed of gas and extinguish thats why they ask for a 1 mb drop across pipe work

90% of houses I go to have a drop of more than 1mbar, customers always think you are trying to get more work when you tell them their pipes are undersized

 

[stevetheplumber]

that realy dosent explain why 1 mb why not 2 or 3?or 1/2?if its just a ball park figure
also is it ncs if its more ?
could only get 19 at meter and called transco who said its a district problem and ok to leave at that

 

[Ockyvines]

I have just checked the unsafe situations book and it says that as long as the appliance minimum burner pressure/heat input rating are ok, with all appliances running at full rate then it should be classed as ncs

 

[GreenArmy]

Steve, was that 19mb working pressure or 19mb standing pressure?

 

[fuzzy]

appliances are pre set and designed to have an incoming pressure of 20mb minimum. If we start allowing the drop before the appliance we could have multiple problems

 

[doitmyself]

Does this help?

BS 6891 is the industry standard for mains gas piping installations. It states that in a domestic natural gas installation the pressure drop along the pipeline at periods of maximum demand should be no more than 1mbar.The reason for this is that, if under-sized pipes are installed, the pressure at the burner could become so low that incomplete combustion occurs, which is dangerous as carbon monoxide will be produced! So, when planning a gas installation it is important to ensure that the tube diameters chosen are able to supply the
volume of gas required by the appliances without excessive pressure loss.

It comes from http://www.ukcopperboard.co.uk/lite...ion-Tips/Domestic-gas-planning-and-sizing.pdf

 

[GreenArmy]

appliances are pre set and designed to have an incoming pressure of 20mb minimum. If we start allowing the drop before the appliance we could have multiple problems

Not completely correct. Working pressure should be 21mb +/- 2mb, so in theory you could have a working pressure of 19mb. This will then give an incoming pressure at the appliance of 18mb (if pipe sizing is correct). As far as Worcester boilers are concerned this is absolutely acceptable.

 

[Ockyvines]

Not completely correct. Working pressure should be 21mb +/- 2mb, so in theory you could have a working pressure of 19mb. This will then give an incoming pressure at the appliance of 18mb (if pipe sizing is correct). As far as Worcester boilers are concerned this is absolutely acceptable.

Baxi too and I think, from memory one of the Ideal heat only boilers states 14mb!

 

[newbie1]

working pressure is to be 21mb +/- 2mb ie between 19-23mb at meter with one mb drop between meter and appliance so 19mb at meter 18mb at appliance is acceptable
however on a install with max demand on supply the the minimum pressure can drop to 15mb i think as this is deemed the minimum safe pressure giving 14mb at appliance but is not advised

 

[fuzzy]

Not completely correct. Working pressure should be 21mb +/- 2mb, so in theory you could have a working pressure of 19mb. This will then give an incoming pressure at the appliance of 18mb (if pipe sizing is correct). As far as Worcester boilers are concerned this is absolutely acceptable.

I was working on 21mb at the meter and a 1mb drop as an average setting as opposed to a minimum, sorry should have said

 

[RoryD]

i have just repipe a gas carcass in a house that was woefully under sized and the customer asked me why we had to have less than 1 mb drop end to end that was when i realised i didnt know or if i did ive forgotten? anyone enlighten me?

Because that's as much as "whoever deemed it" allows over any system. If the pipe sizing is correct that will be the max drop allowed through friction and bends etc...