Gas water heaters: how they fire, commissioning and faults

Two kinds of instantaneous water heater

• Mechanical water section, as in the Main Multipoint BF. Water flowing through a venturi creates a pressure difference across a diaphragm. Sufficient flow moves it to open the gas valve against a spring; closing the tap lets the valve shut. A permanent pilot, proved by a thermocouple, lights the burner, and a slow ignition valve helps it light smoothly. This model's temperature selector adjusts the water flow.
• Electronic control, as in the Rinnai KM range. A turbine detects water flow and the burner lights electronically. The control board modulates the gas supply and stages the burner to suit demand. A flow-control valve limits the draw if the heater cannot maintain the selected temperature; the system also uses a bypass valve to mix water as required. Closing the tap stops the flame. The Main Multipoint FF is also electronically ignited and does not use the BF's permanent-pilot lighting procedure.

Flow and temperature

For a given useful heat output, a greater temperature rise leaves less flow available: flow (litres per minute) = output (kW) × 60 ÷ (rise (°C) × 4.2). At 20 kW, the calculation gives 20 × 60 ÷ (25 × 4.2) = 11.4 litres per minute for a 25 °C rise, or about 5.7 litres per minute for a 50 °C rise. A modulating heater can vary its output, but once it reaches its maximum it cannot maintain the same temperature rise at an unlimited flow.

Commissioning

Start with the correct model's instructions. Complete the required tightness test, then purge by the applicable procedure before lighting. Connect the inlet pressure gauge with the gas isolated and refit the case as instructed before running the appliance.

• Main Multipoint BF: hold the gas control at ignition while using the igniter; release it about 10 seconds after the pilot lights. Its pilot should envelop about 5 mm of the thermocouple head. At first light-up, air left in the supply pipe can delay the pilot lighting. Follow the lighting procedure and investigate if it keeps failing; repeated ignition attempts do not replace the required purge. Once the pilot is established, select burner operation and fully open a hot tap for the 20 mbar dynamic inlet pressure check.
• Main Multipoint FF: use its main switch and temperature control, with a hot tap fully open as instructed. Electronic ignition starts the burner. This model also specifies 20 mbar dynamic inlet pressure, but its burner-pressure check uses its own test setting. Do not apply the BF's pilot procedure to it.
• Burner pressure is a separate check. On the BF, confirm it after at least 2 minutes of running. The FF's service procedure sets maximum before minimum using non-magnetic tools; minimum adjustment is needed only for the stated symptom of repeated burner extinction as the water flow approaches its specified minimum.

After testing, remove the gauge with the supply isolated, secure the test-point closure and check it for tightness as instructed. Complete the remaining commissioning checks and records. At handover, leave the instructions, demonstrate operation and shutdown, and explain servicing and frost precautions.

What the symptoms point to

The Main Multipoint BF fault table gives the following leads. Confirm the actual fault before repairing or replacing a part; another fault can also cause these symptoms.

• Main burner will not light: low gas pressure, a partly closed gas isolation valve, a low water rate, a faulty water section diaphragm or a faulty gas valve.
• Explosive ignition: a reduced pilot flame (clean or replace the pilot injector) or a faulty slow ignition valve.
• Noise: scale in the heat exchanger (descale and service), a high gas rate or a low water flow.
• Wrong water temperature: an incorrect gas rate, or a sticking water section.
• Pilot will not light or stay lit: no gas, a closed isolation valve, air in the line, a blocked pilot injector or no spark.

Installation and care

• Air and compartment space: the Main BF and FF take combustion air from outside and their instructions permit an unvented compartment when the specified clearances and air circulation are maintained. This is model-specific: check any other heater's compartment cooling and ventilation requirements. For an airing cupboard, the Main instructions require BS 6798 and BS 5440-2 to be followed and a perforated non-combustible partition, such as metal mesh with openings under 13 mm, to keep clothes away.
• Shower control: Main specifies a thermostatic or pressure-balanced control for a shower supplied from these heaters. Check the shower valve's suitability; a thermostatic valve gives the best comfort and protection against excessive delivery temperature.
• Hard water: the Rinnai KM instructions require suitable water conditioning against limescale in hard water areas. Follow the actual heater's water-quality limits and maintenance instructions.
• Frost: Main's instructions advise isolating and draining a heater left unused in an unheated place during cold weather; include the electricity on the FF. Powered frost protection, where fitted, needs its specified supplies available. Follow the model's procedure if those supplies will be switched off.
• Overheat protection: the Rinnai KM range has a bi-metal switch that opens at 97 °C and a separate thermal fuse that melts at its set temperature. A melted fuse is replaced with the specified part, with the cause investigated before reuse; it is never bridged out.