Minimum operating Gas Pressure on boiler

Depends on which boiler it is and what the manufacturers instructions say.

 

I am no gas engineer but I think it also depends what is on the feed as well - gas cooker, fire etc. As I understand it the danger is one appliance may take all the feed meaning that something like a cooker ring may extinguish under load then when the load eases you have unspent gas leaking into the kitchen ....

May be wrong but that what I was told on one site a few months ago after they had to run a bigger pipe around the house to feed their boiler which had a long twisting path from the meter with a gas fire and 6 ring cooker

 

You can only have 1mb drop standing pressure from meter to appliance, any more than this indicates incorrect pipe sizing. Working pressure is all down to manufacturers instructions, for example if you have 21mb at the meter a valient or baxi combi can run as low as 14mb. Your gas engineer should be "gas rating" the appliance for the safety certificate for zero governed appliances anyway. Your gas engineer is talking junk, every appliance be it system, combi, conventional will have its own minimum working pressure.

 

Not as low as ten, I think with a valiant it's 14.5 or something on max rate. The legal requirement is that it meets manufacturers instructions. If it's lower than M.I. then firstly the meter needs checking and then the size of the pipe supplying the boiler.

 

20mb at meter, no less than 19mb at appliance when running on full load. Some appliance may get way with less, but if not the above, get your meter and instillation pipework sorted.

 

Standing pressure (with no appliances operating) will be the same at the meter and the appliance in a normal domestic house. It is when the appliance is full on and gas is moving down the pipes it starts to matter.

 

Basically the working pressure at the gas meter is 21 +- 2mb. (19-23) you are allowed a 1mb drop across your pipework to the appliance connection point. Modern condensing boilers have a small drop from the connection point to the test point inside the case. This is normally between 1 and 2.5mb depending on model of boiler but normally around 1.5mb is the average.

Manufacturers min working pressures are based on the meter only giving 19mb, the drop on the pipework being 1mb and the drop inside the case being 1-2.5mb. so a boiler with a 1.5mb drop inside the case will have a min working pressure of 16.5mb (this is a typical average value)

Your engineer is correct that the appliance manufacturer requires the min, the main concern is the effect a boiler with a poor supply can have on other appliances. If you put your cooker rings on full, set your combi to max hot water and open the hot tap full you may see a large drop in the flame picture on the cookers rings. If the boiler effect the cooker very badly then its unsafe to leave this tbh.

as for the reading from previous appliances being around 10-11mb that is probably the old appliances burner pressure which is different to the inlet pressure we record on modern boilers.

 

If there is more than a 1mbar loss from meter to appliance than a gas rate is required by law, if it's undersized than its "At Risk"

 

1st step is to check the working inlet pressure at meter 21+/-(2).
This proves supply & regulator.
Then working pressure at appliance is no more than 1 less.
If so then gas rate within 5% tolerance,
Boilers may run as low as 12 or what ever but the law is the law

 

I know this is a very old thread but I feel I need to correct the so-called "legal requirement" of no more than 1 mb pressure drop between the meter and the inlet to the boiler. As far as I am aware, there is no such requirement. For a combi boiler, the supply pipework must be 22 mm or more. For a standard boiler, the sole requirement is that the pipework should be adequately sized so as to meet the boiler manufacturer's specification. 15 mm gas pipework is fine for most standard boilers unless it is a long or convoluted run.

Take my ancient Potterton Netaheat 16-22 (still going strong). There is no means to measure the pressure at the inlet to the boiler. The test nipple measures the burner pressure. Potterton's manual states this should be between 8.1 and 13.5 mb. The pipe run is about 12 metres with several elbows in 15 mm with no issues even with all 4 hob burners and a gas-log fire full on.

If I am wrong (it has been known!) please point me to the relevant section of the legislation re the 1mb pressure drop.

 

I am in the process of getting a new combi boiler and have come across this issue of pipe size and pressure, and have seen various different statements of information and requirement.

This prompted me to decide to write to Gas Safe and Worcester Bosch for an answer. My question was whether I was required to have a 22mm pipe from the meter to the boiler to satisfy gas regulations and boiler guarantee. I have pasted below their answers:

Gas Safe said:
I would not be able to tell you what size gas pipe would be required due to not having attended site and carried out a pipe sizing exercise. What I can tell you is that the gas pipework must be adequately sized to allow no more than a 1 mbar drop in working pressure across the installation. In my experience, a 30 kW Worcester boiler may struggle with this requirement with a 15mm supply pipe. However, this does depend on the length of run, fittings used, and other appliances on the gas run.

Worcester Bosch said:
Provided the correct gas rate (3.15 cubic metres per hour), a minimum working pressure at maximum output at the gas valve inlet of 16.5 milli-Bar (mB), a static difference of no more than 1mB, and a dynamic difference of no more than 2.5mB can be achieved then the gas pipe sizing would be accepted by Worcester Bosch.

I am not a gas safe fitter so those number make little sense to me, but the bottom line seems to be that it is not the pipe size that matters, but the pressure. I hope this helps.

 

What matters is the pressure when the boiler is running at full power, which is related to how much gas can be delivered to the boiler through the pipe work, and what is available at the meter position. This is determined by the size of the pipe work and the number of fittings (eg elbows), all of which restrict the flow of gas.