What if one appliance could heat your home and your water? That’s the idea behind a combi system. Instead of a separate furnace and water heater, a single gas appliance handles both. Some setups also run in-floor radiant heating. Here’s how combi systems work, what can go wrong, and how to choose the right setup for your home.

A combi system, short for combination system, uses one gas appliance to provide both space heating and domestic hot water for your taps, showers, and appliances.

These are often called hydronic systems, because they use heated water to move heat around your home. That water can heat your air through your ductwork, warm your floors, or both.

Combi systems are common in townhomes, condos, and homes where space in the mechanical room is tight. They’re also a popular choice when replacing an older furnace and water heater at the same time.

The appliance heats water. That hot water does two jobs:

• Hot water for your home. When you open a tap, the system sends hot water to that fixture.
• Heat for your home. When the thermostat calls for heat, hot water is pumped through a coil in an air handler. A fan blows air across the coil and through your ducts. In radiant systems, the hot water flows through tubing in your floors instead.

Most combi systems give domestic hot water priority. If someone starts a shower while the heat is running, the system briefly focuses on the shower and then returns to heating. With proper sizing, you usually won’t notice.

Combi tank (combination water heater)
A gas water heater with a large storage tank that also supplies hot water to an air handler for space heating. It’s a familiar, straightforward setup that works well in many homes.

Tankless water heater with an air handler
A high-efficiency tankless water heater paired with a hydronic air handler. The tankless unit heats water on demand for both your taps and your heat.

Combi boiler
A high-efficiency boiler built to handle both space heating and domestic hot water, with a separate heat exchanger for each. Combi boilers offer higher heating capacity and can run air handlers, radiant floors, or baseboard systems.

Benefits

• One appliance instead of two, which saves space
• One gas line and one vent instead of two
• High-efficiency options available, especially with tankless and combi boiler setups
• Can support radiant floor heating
• Quiet, even heat

Trade-offs

• If the appliance needs service, both your heat and hot water can be affected
• Sizing is critical. An undersized system may struggle on the coldest days or during heavy hot water use
• Regular maintenance is important, especially with hard water
• Fewer technicians are familiar with hydronic systems than with standard furnaces

Here are some of the issues we see most often, and what can cause them.

Hot water but no heat
Possible causes include a thermostat issue, a failed circulating pump, air trapped in the lines, a faulty air handler fan or control board, or a stuck zone valve.

Heat but no hot water, or water turns lukewarm
This can point to a mixing valve problem, scale buildup in the heat exchanger, a system that’s too small for your hot water demand, or a heating call that’s running at the same time as heavy hot water use.

Hot water runs out quickly
Common with an undersized combi tank, or when the tank’s setting is too low to keep up with both heating and hot water.

Low pressure or frequent pressure drops (boilers)
Often caused by a leak somewhere in the system, a failing expansion tank, or a relief valve that’s letting water out.

Banging, gurgling, or kettling noises
Usually a sign of trapped air, scale buildup, or low water flow through the heat exchanger.

Error codes on the display
Most tankless units and combi boilers show a code when something’s wrong. Note the code before calling, as it helps us diagnose the problem faster.

Cold rooms or uneven heat
Can be caused by airflow problems, a clogged air handler filter, air in the lines, or a system that isn’t balanced properly.

A few things you can safely check first:

• Make sure the thermostat is set to heat and has working batteries
• Check that the power switch and breaker for the appliance and air handler are on
• Replace a dirty air handler filter
• Note any error code on the display

If none of these solve the problem, it’s time to call a technician. Don’t try to open the appliance, adjust gas components, or bleed a pressurized system yourself.

A combi system only performs as well as it’s designed and installed. Before recommending equipment, we look at:

• Heat loss. We calculate how much heat your home loses on the coldest days, based on its size, insulation, windows, and layout. This tells us how much heating capacity you need.
• Hot water demand. Number of people, bathrooms, and fixtures that might run at once. The system needs to handle both heating and hot water during a cold morning rush.
• Heat distribution. Whether you’ll heat through ductwork, radiant floors, or both. Existing ductwork is checked to make sure it can deliver the airflow the air handler needs.
• Air handler sizing. The air handler’s coil and fan are matched to the heating load and the water temperature the system supplies.
• Gas supply. High-efficiency tankless units and combi boilers often need a larger gas line than a standard furnace or tank.
• Venting. All gas appliances must be vented to Ontario’s gas code, CSA B149.1, and the manufacturer’s instructions, with proper clearances at the outside termination.
• Piping and controls. Correct pump sizing, valves, expansion tank, and controls so heat and hot water work together smoothly.
• Anti-scald protection. A mixing valve keeps tap water at a safe temperature, even when the system runs hotter for space heating.
• Condensate drain and electrical. High-efficiency units need a condensate drain and a power supply.
• Service access. Enough room around the appliance and air handler for maintenance and future repairs.

Every home is different, so we offer three main combi setups.

Bradford White Combi Tank: Simple and reliable
Bradford White’s Combi2 combination water heaters pair a storage tank with a built-in heat exchanger coil for space heating. The 50-gallon model offers up to 55,000 BTU per hour of space heating, and higher-capacity models are available for homes that need more.

• Familiar tank design with a large reserve of stored hot water
• Straightforward installation and service
• A good fit for smaller homes, townhomes, and homes replacing an existing combi tank

Navien tankless + H2Air kit: Efficient and compact
Navien’s NPE-A2 condensing tankless water heater connects to a hydronic air handler through Navien’s H2Air kit. The tankless unit heats water on demand for both your taps and your heating.

• High-efficiency condensing operation
• Endless hot water within the unit’s flow rate
• Wall mounted to save space
• Built-in recirculation available on NPE-A2 models
• A good fit for homes with moderate heating needs that want efficiency and compact equipment

Qualifying Navien NPE-2 series tankless water heaters are currently eligible for a $400 manufacturer rebate on units purchased by December 31, 2026. See Navien’s 2026 water heater rebate page for full details.

Navien combi boiler: Higher capacity
Navien’s combi boilers are built to handle larger heating loads and hot water demand from one wall-mounted appliance. They use a stainless steel heat exchanger and a separate heat exchanger for domestic hot water, and come in a range of sizes.

• Higher heating capacity for larger homes
• Can run air handlers, radiant floors, baseboards, or a combination
• High-efficiency condensing operation
• A good fit for larger homes, radiant floor systems, and multi-zone heating

A Bradford White Combi Tank may be the right fit if:

• You prefer a tank with a stored reserve of hot water
• You’re replacing an existing combi tank
• Your home has modest heating needs

A Navien tankless + H2Air setup may be the right fit if:

• You want an efficient setup at a lower up front cost
• You want high efficiency and endless hot water
• You want to free up floor space
• Your heating load fits within the tankless unit’s capacity

A Navien combi boiler may be the right fit if:

• You have a larger home or higher heating load
• You want radiant floor heating or multiple heating zones
• You want one appliance that can grow with more complex heating needs

We’ll walk you through your options and recommend the setup that fits your home and budget.

Whether you’re replacing an older combi tank, troubleshooting a system that isn’t keeping up, or planning a new installation, our technicians can help. We serve homes across York Region, including Markham, Richmond Hill, Stouffville, Unionville, and Thornhill.

Learn more about our hydronic air handler services

Learn more about our water heater and combi boiler installation services

Learn more about Markham Heating’s Protection Plans