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Hi All. I have an issue with a boiler in a small flat I purchased that has very old vented system boiler currently fitted.

Both boiler and water tanks are in a need of replacement.

I got 3 quotes from gas safe engineers and all recommended combi boiler to be fitted.

One of the engineers pointed out that the 15mm gas pipe supplying the current boiler is not up to standard with the combi requirement of 22mm.

But replacing the pipe is going to be a mess. It is laid under concrete floor and the meter is in another room. Will have to lay new pipe below the ceiling instead. It’s not going to be visually pleasing and I don’t want to do it. I’m considering going for new system boiler instead.

Engineer suggested that there is a chance new boiler will receive enough gas pressure through existing pipe, to function and meet minimum requirements as this is a small flat without other gas appliances. But there is no way to tell until the boiler is fitted.

If that proved to be the case we would leave 15mm pipe as is and install combi boiler. Otherwise we would have to replace the pipe first.

My question is if that is correct? Is there no way to measure gas pressure without purchasing the boiler?

If the gas pressure was too low and new pipe needed installing I would rather opt for unvented system boiler. Which would be probably much simpler to install and avoid gas pipe going through the living room.

So I wonder if there is a way to tell before purchasing anything, if gas pressure could be estimated/calculated without committing to any work.

And the other question do you think system boiler would be a reasonable option?

Any other advice welcomed!

Mark
 
1. To know whether 15 mm pipe would be adequate you need to know the distance (as the pipe is laid, not as the crow flies) between meter and appliance, together with the number of bends, tees, elbows between meter and appliance. Each fitting is roughly equivalent to 1/2 metre added to the length. Very roughly a 15 mm pipe will support a gas boiler of 24 kW provided it is no more than about 2.5 metres from the meter, including fittings. Pretty unlikely.
2. How has the size of the existing pipe been measured? It might be 15 mm at the boiler, but, given the pipework is under concrete it may well be steel pipe. Quite often this was laid in 3/4" pipe which has an internal diameter of around 19 mm. This would allow around 6 metres from the meter for the same rating of boiler, provided the 3/4" pipe was connected by 22 mm pipe for the final stretch to the boiler.
3. There is no realistic way to measure the pressure drop between meter and appliance without installing the appliance or a similar gas using device.
4. The gas usage, and hence the distance which can be served by 15 mm pipe, depends on the kW rating of the boiler (actually the cubic metres per hour of gas used, but they are equivalent). So you might find that even a system boiler would need new pipework, unless it were very small. Having said that, combi boilers are often rated for hot water supply, which requires a lot of gas for a short period. System (and open vent) boilers are generally sized for the heating requirement as the hot water is stored and can be heated gradually.
 
Does the current boiler still work? If so, an engineer can run it and test the pressure at the meter/boiler. This will give you a ball park.

What size is the pipe off of the meter?

A competent engineer should be able to pretty accurately pipe/boiler size with an on site visit.

Either way, without a new gas pipe, a combi won’t go in.

You’ll be fine with a small 12kw system boiler, if the gas supply copes.
 
Is there no (legal) way a gas installer can run off x lpm gas (equal to the maximum gas rate of the proposed new system boiler) and test the pressure at that flow, assuming the existing boiler is lower output than the proposed and therefore cannot be used as a reference?
 
Yes the pressure at your boiler can be checked buy any engineer or gas safe engineer You need a boiler with a 15 mm gas supply so there should be no need to upgrade gas supply a system Boiler is the way to go Best system you can have to be honest only issue you will have is geting the condensate out of the boiler to maybe the waste of a kitchen sink and the safety valve but that can go out the wall most gas boilers will show a gas pressure of 18 to 20 mbar . Might be a bit of messing around in hot press etc but its all good quite a few boilers on the market that use 15mm gas pipe connection Vokera is one Viessmann is another but they are big money but good boiler the best you can buy to be honest Hope this is of Help
 
Yes the pressure at your boiler can be checked buy any engineer or gas safe engineer You need a boiler with a 15 mm gas supply so there should be no need to upgrade gas supply a system Boiler is the way to go Best system you can have to be honest only issue you will have is geting the condensate out of the boiler to maybe the waste of a kitchen sink and the safety valve but that can go out the wall most gas boilers will show a gas pressure of 18 to 20 mbar . Might be a bit of messing around in hot press etc but its all good quite a few boilers on the market that use 15mm gas pipe connection Vokera is one Viessmann is another but they are big money but good boiler the best you can buy to be honest Hope this is of Help

I just want to make this clear as it’s not in your message.

If a boiler has a 15mm has connection, it does not automatically mean it will be fine connected on to an old 15mm supply.

For example, a 24kw boiler with a 15mm has connection, will not be ok on a 5 meter run of 15mm.
 
And to make it even clearer, the fact that the boiler has a 15mm connexion means the inlet pipe must be 15mm where it enters the boiler. If the pipe run between the meter and the boiler is long, the pipe might need to be 22mm, 28mm, or even larger diameter. But if the connexion at the boiler is 15mm, the pipe will reduce to 15mm fairly close to the boiler connexion.

You'd think that if the narrowest point is 15mm, then the whole run may as well be 15mm. Unfortunately, it just doesn't work that way.
 

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