Pool Heater - gas pipe sizing

[Stephen Mawson]

I'm planning to replace an ancient and inefficient gas-fired pool heater (120,000 BThU/hr input, 84,000 output(!)) with a slightly larger (130,000 BThU/hr input, 122,000 output) condensing one. The existing heater is about 75 metres from the gas meter and pressure regulator in the house and is connected with 3/4" galvanised gas barrel. Is this likely to be of sufficient capacity to run the proposed new heater? I suspect that the pipework may be theoretically inadequate even for the existing heater, though it seems to work well enough. Many thanks in advance.

Stephen

[John Rumm]

130K BTU/h equates to about 38kW. Which will be a gas rate of around 3.43 m^3/hour

With 22mm pipe (similar internal diameter) the maximum discharge rate of

3.4m^3/h will be met at only 15m - you will get less than a quarter of that discharge rate at the max design pressure drop of 1mbar.

Even 28mm pipe will not hack that pipe run (officially) on the regulated side of the meter. You may however find the actual minimum inlet pressure in the boiler spec is lower than nominal 20 mbar though.

(conversions from [1])

[1]

[Stephen Mawson]

John,

Very many thanks for that. Both the old boiler and the proposed new one are specified as 20mbar input so it looks as though I shall have to install a fatter pipe or possibly use the existing pipe run at mains pressure and have a second meter near the new boiler, with all that that implies about two standing charges. Thank you again,

Stephen

[Toby]

If you run the current one for a set time without using gas for anything else, and work out how much gas it has used by reading the meter before and after (to the smallest unit), you can tell if it is using the appropriate amount of gas for the rating...

[John Rumm]

Most of them are nominally - but often if you look at the detailed spec many can run a bit lower (I think mine goes down to 16).

The other option is a larger pipe obviously. You can get tracpipe in diameters up to 50mm quite easily, and you can get 40mm in a single 75m length as well (in fact 32mm might actually be adequate - according to BS 6891, 32mm corrugated stainless pipe has a discharge rate of 7.6m^3/h at 30m - so at 75m you will probably get a bit over half that (the reduction in rate with length is not quite linear)...

(although sit down before checking the price!)

[dennis]

Is there anything that says he can't run a second pipe and just parallel them up? It might be easier and cheaper.

[John Rumm]

The short answer is I don't know... I was going to include that in my initial response, but could not ascertain if it was an "allowable" solution. Logic suggests it ought to be, but I can't find any reference to it.

BS 6891 seems silent on the matter, and I can't find any mention in Tolley's domestic book either...

Any of our GasSafe contributors out there are the mo?

Yup, quite possibly.