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Heating · Boiler RepairCombi Boiler Repair
Combi boiler repair usually comes down to one of three faults: a diverter valve that will not switch between heating and hot water, a scaled plate heat exchanger, or a low-pressure or ignition lockout. A diagnostic visit is $90–$150 and most repairs fall in the $150–$800 range. If the unit gives heat but no hot water, or hot water but no heat, suspect the diverter valve first.
A combi makes heat and hot water with one burner, and the diverter valve decides where that heat goes. It is a motorised three-way valve: at rest it sends boiler water to the radiators, and when a tap opens it swings to the plate heat exchanger. Stuck on the heating side, you get warm radiators and cold taps. Stuck on the hot-water side, the taps run hot and the house goes cold; half-stuck, the radiators warm up every time someone showers.
Temperature swings at the tap point to the hot-water side. Tap water is heated inside a brazed plate heat exchanger with narrow passages, and the hard water in Halton and Hamilton leaves scale there every day the unit runs. A scaled plate transfers heat poorly, so the boiler overshoots, cuts the burner and relights: the shower goes hot, cold, hot. A sticking flow sensor does something similar by losing track of the tap at low flow, and a clogged inlet filter starves both.
Low-pressure lockouts are the third family. The heating loop should sit around 12–15 psi cold, and the boiler stops firing when its sensor sees the pressure fall too far. That water went somewhere: a weeping relief valve, a flat expansion tank, a leaking radiator valve or an air vent. Opening the fill valve gets the heat back for a night, but the leak still has to be found, and why a boiler loses pressure explains the usual suspects.
Ignition codes are read, not guessed at. On a Navien combi E003 is an ignition failure and E012 a loss of flame, and the causes run from a fouled igniter or flame rod to a blocked condensate trap, a snowed-in intake or gas pressure that sags when the burner ramps up. Codes differ by brand and model, and the label inside the panel wins; our Navien combi error code guide lists the common ones. For boilers that are not combis, see boiler repair.
Combi boiler faults we are called for most
- Radiators hot but taps cold, or taps hot and the house cold: the diverter valve
- Shower temperature that cycles hot and cold, or hot water that fades as the tap is opened further: scale on the plate heat exchanger
- Hot water that will not start unless the tap is wide open: a sticking flow sensor or a clogged inlet filter
- A pressure gauge near zero and a low-pressure code on the display
- Repeated ignition lockouts, often on the coldest mornings when the condensate line or the intake has iced up
How it works
Tell us the code on the display, the pressure reading and which half is missing: heat, hot water or both. Those three facts usually narrow the fault before the cover comes off, and we will say whether topping up the pressure is safe to try.
We drive the diverter valve and watch it travel, measure temperature rise across the plate heat exchanger, check the flow sensor signal, test the expansion tank charge and measure gas pressure with the burner at high fire. The fault and the price are written down before a part is ordered.
After the part is fitted the unit is run in both modes, heating and hot water, and the combustion is checked with an analyser. If scale caused the fault we flush the plate and tell you how often your water needs it done.
Brands: We repair Navien, IBC, NTI and Viessmann combi and condensing boilers, and other residential brands as parts allow.
Common questions
Why does my combi have hot water but no heat?
Most often the diverter valve is stuck on the hot-water side, so boiler water never reaches the radiators. Before calling, check that the thermostat is calling, the heating mode is switched on at the boiler and the pressure gauge is in range. The full checklist is in why a combi has hot water but no heat.
Can I top up the pressure myself?
Yes, once. With the boiler off and cool, open the fill valve slowly until the gauge reads 12–15 psi, then close it fully. If the pressure is back down within days, stop refilling and call: water is leaving through a relief valve, a leak or a failed expansion tank, and each refill adds oxygen that corrodes the system. If water is dripping from the relief pipe, do not refill at all.
How often does a combi need descaling?
On Halton and Hamilton water, plan on a flush of the hot-water side about once a year, the same interval as a tankless water heater; softened water stretches that. The job needs service valves on the hot and cold connections. If your installer left them off, we add them on the first visit so later flushes are quicker and cheaper to do.
When is a combi boiler not worth repairing?
Replace when the primary heat exchanger is leaking and out of warranty, or when the unit is past about 15 years and the repair is a major part such as the heat exchanger, fan or control board. Diverter valves, sensors, igniters and expansion tanks are worth fixing at almost any age. If replacement is the answer, combi boiler installation covers sizing and what the job involves.
Is a combi lockout an emergency?
No heat in freezing weather is, and the 24/7 line treats it like a furnace no-heat call with no after-hours, weekend or holiday surcharge. No hot water with the heating still working can usually wait for a booked visit. If you smell gas or a carbon monoxide alarm sounds, leave the house first and call from outside. Otherwise, request a repair visit.