Commissioning a combi on a reused 15 mm supply: 19.5 kW instead of 24 kW
You are commissioning a new 24 kW input natural gas combi boiler in a terraced house, replacing an old back boiler. The existing gas supply is 15 mm copper with an equivalent length of about 14 m from the meter, and you have reused it. With the boiler at maximum rate (hot water tap fully open, no other appliances running) the meter outlet working pressure is 19 mbar. The data badge and instructions require 20 mbar at the appliance inlet. The supplier's declared calorific value is 39.0 MJ/m³. Over a timed 2-minute period at maximum rate the meter registers 0.06 m³. All other gas appliances are off and the test dial has been checked for zero return. The homeowner says the old boiler "was always a bit slow" and asks whether the new one can be "turned up" to compensate. The installation has been notified and the commissioning record is blank.
1. Calculate the measured gas rate and heat input, and state what the result tells you.
Read the worked answer
Gas rate 0.06 m³ in 2 minutes = 0.03 m³/min × 60 = 1.8 m³/h.
Heat input 1.8 m³/h × 39.0 MJ/m³ = 70.2 MJ/h. 70.2 ÷ 3.6 = 19.5 kW.
Interpretation The appliance is running at 19.5 kW against a rated input of 24 kW - about 81% of rating, a shortfall of roughly 19%. That is well outside normal commissioning tolerance (manufacturers commonly allow around ±5%), so the boiler is not achieving its rated input and cannot be signed off as commissioned.
A shortfall of this size means either not enough gas is arriving at the burner, or the appliance is not actually at maximum rate. Before condemning the supply, confirm the boiler really is at maximum: on a combi, hold a full hot-water draw with the heating off, or force maximum rate as the instructions allow, and allow the appliance to stabilise for several minutes. Then recalculate. If the figure stands, the cause is a gas supply or pressure problem and the commissioning stops here.
The customer's request to "turn it up" is not an option - input is fixed by injector sizing and supply pressure; altering it would wreck combustion and produce CO.
2. Explain how you use the 19 mbar reading to decide whether the restriction is upstream of the meter or in your own pipework, and what you check next.
Read the worked answer
The two readings answer different questions, so take both.
- Meter outlet working pressure, appliance running - 19 mbar against a nominal 20–21 mbar suggests the pressure available at the meter is already marginal while gas is flowing. Repeat it with the appliance off: if standing pressure is around 21 mbar and it collapses to 19 mbar under load, the restriction may be upstream (meter governor, ECV not fully open, service pipe, or a supplier issue), or simply the whole installation drawing more than the supply can hold.
- Appliance inlet working pressure - measure at the boiler's test point with the appliance at maximum rate. The difference between meter outlet and appliance inlet is the pressure loss through your installation. If meter shows 19 mbar and the appliance inlet shows 16–17 mbar, the pipework is undersized: 15 mm copper over a 14 m equivalent length for 24 kW will not carry the load with an acceptable loss.
- Checks before blaming the pipe - confirm the ECV and any secondary isolation valve are fully open, check for a partly closed appliance service valve, confirm no other appliance is running, verify the gauge is calibrated and correctly zeroed, and check the meter governor is not passing or creeping.
- Then act - upgrade the run (typically 22 mm, sized by calculation for the actual equivalent length), or refer a persistent low meter outlet pressure to the gas network operator; you cannot adjust the meter regulator yourself.
Document both readings - the boiler inlet pressure is what the manufacturer's commissioning record actually requires.
3. The supply is upgraded and the boiler now achieves rated input. What must appear on the commissioning record, and what do you tell the homeowner about "turning it up"?
Read the worked answer
On the record
- Appliance make, model and serial number, and the rated input (24 kW) with the data badge values.
- Gas rate measured after the upgrade, with the time, meter reading and calorific value used, plus the calculated heat input now within tolerance of rating.
- Working pressure at the meter outlet and at the appliance inlet, both with the appliance at maximum rate, and confirmation they match the data badge requirement.
- Combustion analysis - CO/CO₂ ratio comfortably below 0.004 for natural gas, CO reading, and the flue gas temperature or other values the manufacturer asks for.
- Flue flow and spillage/terminal checks as applicable to the appliance type, plus confirmation of ventilation requirements and any condensate disposal checks.
- Tightness test result for the whole installation after the supply change, purge details, and the operative's details and notification reference.
- The homeowner's instructions handed over, including the benchmark/commissioning section completed and signed.
What to tell the homeowner Explain that the boiler was starved of gas by the old 15 mm pipe, not set too low, and that its output is fixed by the injectors and the supply pressure - there is no "turning up" screw to compensate, and altering the gas pressure would raise CO and make the appliance unsafe. Point out that the old boiler's sluggishness was probably the same undersized supply, and explain how to recognise a gas escape and who to call.