Gas supply size help

[fender]

Hi Guys,

My nieghbour has a Potterton Flamingo 20-30, which is now getting a bit old.
She's had a heating engineer to obtain a quote. She is unsure whether to have it changed for a modern equivalent, or go the route of a combi.

In any event, she's been told that the gas supply will need to be changed from 15mm to 22mm, which involves a fair bit of work.

I'm just interested as to whether this is correct. Why would a modern boiler of the same duty require a larger supply?

Thanks for your time.

 

[Nixt]

Because the combi might not be the same duty or the existing supply to the old boiler is initially undersized.

 

[doitall]

It's an interesting question that no-one can answer.

You change the boiler for an all singing all dancing suppa duppa condensing boiler, and you need a bigger gas pipe, so assumably you use more gas.

One for the gas boys I think

 

[fender]

Thanks so far.

On further investigation, the supply is in iron pipe (about 25mm) all the way to the kitchen. It's only the last 3m in 15mm, which is buried in the kitchen floor.

 

[doitall]

Hopefully you would only need to get the 15mm up-graded.

The actual size depends on the total length, the number of bends and the output of all the appliances at each section.

Your RGI should be able to work it out, but 22mm or 28mm is the norm for most combi's

 

[robertito]

If you do a search you'll find a big history of discussion on this subject. Personally I reckon that in most cases it would only make a marginal difference, bearing in mind that modulating boilers will not often be required to run at their full rating anyway.

 

[fender]

...Thanks,

I did a quick search, but did not turn up anything specific.

More info...

The iron pipe is 7m from the meter to the kitchen, with one bend.
The 3m of 15mm has 3 bends.

It's just that I would have assumed modern boilers are more efficient than the older ones and would therefore use less gas. That's why I'm curious that a larger supply has been advised. The only other gas appliance is the cooker, which is only about 500mm from where the iron pipe changes to 15mm.

 

[gas4you]

If you do a search you'll find a big history of discussion on this subject. Personally I reckon that in most cases it would only make a marginal difference, bearing in mind that modulating boilers will not often be required to run at their full rating anyway.

Yes but the gas pipe must be sized for the maximum demand for the appliance resulting in a maximum of a 1mBar drop across the carcass.

 

[Corgigrouch]

If you do a search you'll find a big history of discussion on this subject. Personally I reckon that in most cases it would only make a marginal difference, bearing in mind that modulating boilers will not often be required to run at their full rating anyway.

An undersized gas supply is potentially very dangerous.. When the boiler does fire up and run at full output, the drop in pressure in the gas pipe can be big enough to extinguish other appliances... gas fires, ovens, hobs and the like. When the boiler requires less gas, the supply will return to the appliances and unburnt gas can be released in to the room...

In addition if a boiler of a specific output has been specified, say 30Kw and the gas supply is too small, then the customer doesn't really get a 30 Kw boiler does he? .... Then he is on to the phone to you telling you to come back and fix the crap boiler that you fitted.

Any gas appliance that has been fitted and registered with CORGI runs the risk of being inspected by CORGI, it is possible that they may check the pressure drop or even gas rate the boiler... Where does that leave the installer?

Just out of interest, I once ran a newly installed 30 Kw boiler on the old 15mm supply (Before I replaced it with 15m of 28mm) and recorded a 10Mb pressure drop

In short, it is important to get the pipe size right.

 

[Nixt]

Yet more examples of peoples utter inability to grasp the simple equation of pressure and flow. Sometimes I think a pipe sizing chart should be branded on the forehead of those who continuously challenge this law of physics purely because they don't like the sound of the cost.

 

[Gasguru]

If you do a search you'll find a big history of discussion on this subject. Personally I reckon that in most cases it would only make a marginal difference, bearing in mind that modulating boilers will not often be required to run at their full rating anyway.

Please refrain from posting nonsense otherwise we may as well rename the forum to screwfux.

As has been mentioned it's a legal requirement to have a correctly sized gas pipe with the gas appliance running at maximum output. The majority of existing gas pipework I see is undersized for the existing boiler. Once you change over to a combi you are often using 3 or 4 times the gas (that the original heat only boiler consumed) so a considerably larger pipe is required to avoid a pressure drop due to friction. A conventional heat only boiler burns gas at a "low" rate but takes perhaps 1/2 hour to an hour to heat up a hot water cylinder. A combi has to heat the water instantaniously so uses vastly more gas (but only whilst the tap is open).

How about the mods allow the genuine rgi's to have some sort of logo under their name to try to differentiate between the correct and garbage advice (that seems to have proliferated recently) by some diyers/illegal installers.

 

[Corgigrouch]

Och Jim... Ya cannae change the laws of physics... The Dilithium gas pipe cannae take na moor...Shes gonna blow!!!!!

 

[fender]

Thanks guys, interesting stuff.

What if she goes the non combi route?

 

[Nixt]

Thanks guys, interesting stuff.

What if she goes the non combi route?

What would you like to hear? Pipe size would be correct, no need to re-calculate perhaps. Why not install whatever boiler suits the existing pipework to save changing it.

 

[fender]

Very helpful comment mate, thanks.....

I'm just interested, is that ok? - that's how you learn.

I can now see why a combi would require a larger supply, that was explained quite well. However, I can't see why a modern equivalent of the 20-30 would require a larger supply if they are meant to be more efficient.