Gas supply and pipe sizing: the check most quotes skip

The physics, briefly

Gas is supplied at the meter at a regulated pressure. As it flows through pipework, friction causes a pressure drop, and that drop increases with:

• Flow rate – more appliances running simultaneously means more gas moving.
• Pipe length – every metre adds resistance.
• Number and type of fittings – each elbow, tee and valve counts.
• Reduced diameter – and this one is dramatic. Pressure drop rises very sharply as bore reduces, so going down one pipe size increases the drop by far more than proportionally.

There is a maximum permissible pressure drop between the meter and any appliance, and the design job is to size the pipework so that the drop stays within it with every appliance running at once.

Why replacements are where it goes wrong

The single most common scenario: an older, lower-output boiler is replaced with a modern one of higher output, on the existing pipework.

The old appliance was within the capacity of that pipe. The new one is not. Nothing about the pipe changed, so nobody thought to check it, and the new boiler underperforms from the first day. It then gets blamed on the appliance, the manufacturer, or the installer’s choice of model.

Other recurring causes:

1. Appliances added over the years – a new hob, a gas fire, a second boiler – each seeming small, cumulatively exceeding the pipe’s capacity.
2. A kitchen extension with a new range cooker on a supply that was at its limit.
3. Long runs in larger properties, where the length itself dominates the drop.
4. A boiler relocated further from the meter without upsizing the run.
5. Microbore or undersized sections hidden in a run that is otherwise adequate.
6. Old steel pipe with internal corrosion reducing the effective bore.

How the check is done

1. Standing pressureAt the appliance, with nothing running. Establishes the baseline.
2. Working pressure at the applianceWith the appliance running at full rate. The difference from standing is the drop caused by that appliance alone.
3. Working pressure with everything runningEvery gas appliance in the property at full rate simultaneously. This is the condition the installation must satisfy and it is the test most often not done.
4. Compare the drop against the permissible figureWithin it, the pipework is adequate. Beyond it, it is not.
5. Check the gas rateBy timing the meter, to confirm the appliance is actually consuming what its data plate says. An appliance under-rating is not achieving its output.
6. Check the meter’s capacitySeparately from the pipework. A meter has a maximum throughput and it is a different constraint.

The whole thing takes a few minutes with a manometer and it should be part of every boiler survey and every service. Its absence is one of the more informative things about a quote.

When the pipework is inadequate

The meter point is worth understanding. Meter capacity is separate from pipe capacity, and increasing it is a matter for the gas transporter rather than a plumbing contractor. It takes time to arrange, and it is best discovered at survey rather than on installation day.

How pipework must be installed

• Correct materials and jointing. Copper with appropriate fittings, or steel. Certain fittings permitted on water are not permitted on gas, and the distinction is not cosmetic.
• Protection where the pipe could be damaged, and sleeving where it passes through walls.
• Support at the correct intervals.
• Ventilation of voids. Gas pipe must not run in an unventilated void where a leak could accumulate. Pipe in a sealed floor void or a boxed duct needs ventilation at both ends.
• Cavity walls – generally pipe may only cross a cavity by the shortest direct route, not run within it.
• Electrical bonding at the correct point near the meter, with an adequately sized conductor. Very commonly found missing or undersized on older installations, and routinely picked up at safety checks.
• Tightness testing and purging on any new or altered installation, with results recorded.
• Accessible isolation to each appliance.

What to ask

1. Has the gas rate been checked against the data plate?
2. Has working pressure been measured with every appliance running?
3. What is the permissible drop and what did you measure?
4. Is the existing pipework adequate for the new appliance?
5. Is the meter capacity adequate?
6. If upsizing is needed, is it in the quote?

These are not difficult questions and any competent installer will have the answers. A quote for a higher-output boiler on existing pipework, with no mention of the supply check, is a quote with a risk in it that has not been priced.