Low gas pressure at boiler

[Chris French]

So, fitted the new gas valve to the boiler and the house is warming up nicely now.

I set the output pressure on gas valave (abiout 12 mbar), and while I was at it checked the supply pressure on the valve as well.

It was about 14-15 mbar. So somewhat low..

I checked the pressure at the meter, it was fine, 21 mb.

Now this doesn't entirely surprise me. We have lived here since 2004, there is the boiler and the gas hob on the same pipe (they are on oppsite sides of a wall), if the hob was on full, you would notice a change in the burner sound when the boiler fired up. So I suspect it maybe has been like it for a long time (we've never had aplumber in, it's never been checked). Everythign wroked ok, and doing anythign about the supply would be a PITA until we rdid the kitchen and utility room area. so I let well alone .

A ssume this is down to the pipe run, most of it runs under the floor, the total run is probbaly about 25m, the first 4m or so is 1inch OD steel pipe, somewhere between there and the boiler/hob it tunrns into 15mm copper, there are probably 4-5 bends on the way.

The boilers rated gas consumption (it's a basic open flued, on/off boiler) is

4m^3/hr.

so given that it has worked in this fashion for presumably some years I am minded to carry on, we hopefully will get round to a full kitchen refit in the next few years.

what are the implications of that sort of low pressure?

[Andy Burns]

21mbar is spot on, but maximum permitted drop in pipework to boiler is 1mbar

[Brian Gaff (Sofa]

Would that really matter though as long as the draw off was not enough to significantly affect the burner in the boiler? I'm sure in a friends house you can hear the boiler rumble alter when they start cooking on the gas cooker sometimes. Brian

[John Rumm]

ISTR from that a quick look at the manual the other day, the input pressure latitude was quite generous on that boiler. Probably helped by not being fanned flue with forced combustion etc. So although likely well out of spec it will probably work fine.

The bigger worry would be could the boiler firing momentarily extinguish a burner on the hob? If so, does that have flame failure detection?

As above, plus possibly lower than expected boiler output (although at

35kW+ you may not notice!)

You could always run through the calcs to see what the setup ought to be:

[Chris French]

It does seem to have been working fine all these years. and I don't think is is a new situation.

It doesn't (or at least if it supposed to it doesn't work) , which is a bit of a concern, but then again it's never happened so far, and at one point we had a gas oven as well (now electric).

The system seems to perform ok. it's a pretty large victorian house so needs plenty of heat (hence 35Kw :-) ) and soem of the rads are undersized I think (system probbaly installed in the 1970's, maybe eary 80's I think), though the boiler was a more recent replacement. ISTR that a while back, after moving in I did heat clacs for the rooms/house and overall the boiler was sized big enough with spare capacity for the building)

It seems like it should be sufficient with a quick calc, goign on the table on there.

It looks like most of the run is 1 inch steel pipe. maybe about 25m given the likly route, though most hidden away under wooden and concrete floors. Probably 7-8 elbows. Say an effective length of 30M, which should be able to supply 5^3 m/hr. so plenty for the boiler. There is a bit of copper after that, but only maybe a metre or so and a few elbows..

I may redo my measuremnts to check. but whilst everything works I'm not really goign to do anything about it givem it wold presumably mean replacing some of the gas pipe.

thanks

[John Rumm]

With big lump non modulating boilers you can run into problems with too much capacity for the majority of the time when its not -5 outside. Not so bad if you set it up with pump overrun, otherwise if it short cycles, you waste lots of heat out of the flue every time it shuts down.

Is that 1" the actual OD of the pipe? (normally 1" has an OD of about 1.3").

It does seem odd getting that much pressure drop though...