Mucka sizes gas pipework to BS 6891 — the equivalent-length method, with a maximum 1 mbar drop from the meter outlet to any appliance inlet. It handles a run that tees off to two appliances, which is where sizing by eye usually goes wrong.
You need the Gas Calculators skill installed. It sits alongside Gas & Oil Certs.
What it does and does not do
Does: copper tube, 10 to 35 mm, on natural gas. One run or a branched system.
Does not: steel, or LPG. Those size to tables Mucka does not carry, and rather than reuse the copper figures it says so and asks you to size them from the standard. It will also stop if a run works out longer than the published table goes.
The bit worth understanding
The 1 mbar is a budget for the whole path, meter to appliance — not for each run. So if the pipework tees, and there are two sections between the meter and the boiler, each section only gets half a millibar. Mucka does that by doubling the effective length before reading the table, which comes to the same thing.
That is why sizing each leg on its own is the classic mistake: every leg looks fine on a full millibar, and the shared feeder ends up undersized.
Where a section is shared by two branches of different lengths, Mucka judges it on the longer path, so the appliance furthest away still gets its share.
Doing it
For each run, say:
- What feeds it — the meter, or another run. That is what makes the shape.
- Size and length — the measured length; Mucka adds the fittings.
- Elbows, tees and bends — counted, not measured.
- The appliance gas rate, in m³/h, but only on a run that ends at an appliance. A run that feeds another run does not need one: Mucka adds up everything downstream of it. If you have the appliance in kilowatts, the gas rate calculator will give you the m³/h.
Each run then shows its own verdict, with the working underneath — measured length, what the fittings added, the section split, and which row of the table was read.
What it tells you
- Whether each run carries what hangs off it.
- If it does not, the size to go up to.
- The total at the meter.
Lengths land between the rows of the table, so Mucka always reads the next length up. That is the cautious direction: it never calls a run adequate by treating it as shorter than it is.
A note on the fittings
Mucka adds 0.5 m for an elbow, 0.5 m for a tee and 0.3 m for a bend, and 1 m for elbows and tees at 35 mm. Those are the allowances stated with the standard's own table, and they are printed with every answer.
If you work to different allowances, put your own effective length in as the run length and leave the fitting counts at zero. The figure Mucka shows is always the one it used, so you can check it.
Sizing it by voice
You can ask: "What size pipe for a boiler at 3.6 cubic metres an hour, ten metres from the meter with four elbows?"
By voice it does one run, meter to appliance. If the job tees off, it will tell you to use the screen — because sharing the millibar between sections is exactly the part that cannot be done a leg at a time.
It feeds the installation volume too
You have already told Mucka every length and diameter on the job, which is what the installation volume calculator needs. It carries across, so you are not describing the same pipework twice.