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Heating · Combo SystemsHydronic Air Handler (Combo System) Repair
Hydronic air handler repair on a combo system usually comes down to one of four parts: the circulator pump, a check valve, air trapped in the coil, or the aquastat or relay that starts the pump. The giveaway is that the taps still run hot while the vents blow cold. A diagnostic visit is $90–$150, and most of these repairs fall between $150 and $800.
A combo system heats the house with the same appliance that heats the water. A tank water heater, a tankless unit or a small boiler sends hot water through a coil inside a fan cabinet, the blower pushes house air across the coil, and the cooled water returns to be reheated. Builders across the GTA have fitted them in townhouses, stacked towns and condo suites for decades, because one gas appliance and one vent do both jobs and the cabinet fits in a closet.
When the heat quits and the hot water does not, the water heater is proven good and the fault is on the heating side of the loop. In order of likelihood: a circulator that has seized, so no water reaches the coil; a check valve stuck shut; an air pocket in the coil, usually after the water was shut off for plumbing work; or the aquastat, relay or board that should start the pump and fan when the thermostat calls. One test at the cabinet separates them: a hot supply pipe into the coil and a cold return means water is not moving. These are the same parts a boiler system uses, which is why the work sits with our boiler repair technicians.
The check valve also fails the other way. Stuck open, it lets hot water drift through the coil by gravity when nobody asked for heat, so the cabinet leaks warmth into the ducts in July and fights the air conditioner. That thermosiphon complaint, a warm closet and a higher cooling bill, is specific to combo systems and is fixed with a new spring check valve, not a thermostat.
Two design features confuse owners. Capacity is shared: a 40- or 50-gallon tank feeding the coil also feeds the showers, so back-to-back showers on a −15°C morning can leave the supply air lukewarm until the tank recovers, and storing the water at 60°C behind a mixing valve stretches it. The pump timer is the other: the control runs the circulator briefly at set intervals, summer included, so potable water never sits stagnant in the coil, and a pump you hear in August is doing its job. If the tank or tankless unit is undersized or failing, we say so; the options are in our guide to tankless and air handler combo systems.
Call for combo system repair when
- The fan runs and the vents blow room-temperature air while the hot taps work normally
- The supply pipe at the cabinet is hot and the return pipe beside it stays cold
- You hear gurgling or rushing water in the cabinet after plumbing work or a water shut-off
- Warm air drifts from the vents in summer with the thermostat set to cool
- Heat fades whenever someone showers and takes a long time to come back
How it works
Hot water at the taps, pipe temperatures at the coil, a call for heat at the thermostat, and whether the pump and fan actually receive the signal.
Circulator, check valve, relay or aquastat replaced, with isolation valves added if the loop had none so the next repair does not need a drain-down.
Air pushed out of the coil until the return pipe runs hot, supply-air temperature measured at a register, the pump timer confirmed, and the water heater's setting and mixing valve checked. Book a combo system diagnostic.
Brands: We repair hydronic air handlers fed by tank water heaters, Navien and Rinnai tankless units and wall-hung boilers.
Common questions
Why do I have hot water but no heat?
Because the water heater is working and the heating loop is not. Hot taps prove the burner, gas supply and venting are fine, which leaves the circulator, the check valve, an air-locked coil or the control that starts the pump. On a combi boiler the cause list is different: see hot water but no heat on a combi boiler.
Is a hydronic air handler the same as a condo fan coil?
No. A combo air handler gets its hot water from an appliance inside your own unit. A high-rise fan coil is fed by the building's central boiler and chiller through risers, and the split of responsibility between owner and corporation is set by the declaration. Who is responsible for a condo fan coil unit explains how to check.
Can I bleed the air out of the coil myself?
If the cabinet has a manual vent at the top of the coil and you can reach it, opening it with the pump off until water runs steadily is safe. Most combo loops are purged through a drain valve with an isolation valve closed, which is easy to get wrong and can flood a closet. Gurgling that returns means something is letting air in.
The water heater is a rental. Who repairs the combo system?
The rental company looks after the water heater itself under its contract. The air handler, circulator and piping are often the homeowner's, though some contracts cover both, so a no-heat fault on the loop can be your repair even with a rented tank. Read the agreement to see where its coverage stops, then call us for the heating side.
When should a combo system be replaced instead of repaired?
Replace the air handler when the coil itself leaks or the cabinet has rusted through from a long-term drip, since a new coil often costs most of a new unit. Replace the heat source when the tank is past 10 to 12 years or cannot keep up with both showers and heating. A pump, valve or relay is always a repair.