An indirect water heater is a storage tank with no burner of its own: hot boiler water runs through a coil inside it and heats the domestic water. A combi boiler heats both the house and domestic water on demand, like a tankless heater with a heating circuit attached. Both let one appliance do two jobs, but they need careful sizing and professional design.
- The National Plumbing Code defines an indirect water heater as one heated by a medium such as hot water or steam, and requires pressure and temperature relief on every storage tank in the system.
- An indirect tank adds almost no standby burner losses of its own and recovers quickly from a well-sized boiler.
- A combi boiler has the same winter flow limits as a tankless heater, and usually pauses space heating while hot water runs.
- Natural Resources Canada says a water heater used for space heating must be labelled suitable for combined use, and the package should have a system rating.
- Some jurisdictions require a double-wall heat exchanger where boiler water and potable water meet.
01 /What is an indirect water heater?
An indirect water heater is an insulated storage tank heated by a boiler rather than by its own burner or elements. Hot boiler water circulates through a coil (heat exchanger) inside the tank, and heat passes through the coil wall into the domestic water. The boiler water and the drinking water never mix. When the tank's thermostat calls for heat, the boiler fires and a zone pump or valve sends its output to the tank coil.
The 2025 National Plumbing Code of Canada defines an indirect service water heater as one that derives its heat from a heating medium such as warm air, steam or hot water, and requires indirect service water heaters to have a pressure-relief valve and a temperature-relief valve on every storage tank that forms part of the system. The same discharge pipe rules apply as for any tank: see water heater permits and code.
- Fast recovery. A residential boiler often has far more output than a water heater burner, so a well-matched indirect tank reheats quickly.
- Low standby loss. The tank has no flue running through it, so the main standby loss is through the insulation.
- Long tank life. With no burner or flue, many indirect tanks use stainless steel or lined steel and avoid the combustion-side corrosion of gas tanks. Life still depends on water quality.
- One combustion appliance. Only the boiler burns fuel and needs venting.
The catch is that the boiler must run in summer to heat water, so its own efficiency at low loads matters. A modern modulating condensing boiler handles that well; an old oversized cast-iron boiler cycling on and off all summer does not.
02 /What is a combi boiler, and how is it different?
A combi (combination) boiler is a wall-hung boiler that provides space heating through radiators or in-floor tubing and also heats domestic water instantaneously as it flows, with no storage tank. When a hot tap opens, the boiler diverts its output from the heating loop to an internal plate heat exchanger for domestic water. Most combis give hot water priority, so space heating pauses briefly during long showers.
That means a combi has the same flow limit as a tankless water heater: its hot water output in litres per minute depends on its burner input and your winter inlet temperature. Two showers from 7 °C inlet water need about 44.5 kW, roughly 160,000 BTU/h of gas input at 95% efficiency, which is why combi boilers for Canadian homes are often sized by their hot water duty rather than by the heating load of the house.
03 /What is a combination space and water heating system?
A combination system uses a water heater, or a boiler, as the heat source for both domestic hot water and space heating. In one common Canadian design, a small pump circulates hot water from a water heater through a fan coil in the furnace ductwork, or through in-floor tubing, while the same heater serves the taps. Natural Resources Canada's Water Heater Guide describes these systems and suggests they can suit houses with lower heating requirements.
The same guide sets out the cautions:
- The water heater must be sized to meet the greater needs of the combined functions.
- It must be specifically labelled as suitable for combined space heating and water heating.
- Look for a performance rating for the combo package as a system, not just the water heater's rating, because its ratings as a water heater do not indicate its performance as a space heater. CSA published a test method and rating standard for combination systems, CSA P.9, in 2011.
- Where a hydronic boiler is used, some jurisdictions require a double-wall heat exchanger so that the boiler loops cannot contaminate potable water.
- A combination system requires professional design and sizing; the guide points to HRAI training based on the Unified Canadian Guideline for integrated (combination) heating systems.
04 /How do you size an indirect tank or combi boiler?
Size an indirect tank the way you would size any tank, by first-hour demand, then check that the boiler's output is enough to deliver the tank's rated first-hour performance. Manufacturers publish first-hour ratings for indirect tanks at a stated boiler supply temperature and flow; if your boiler runs cooler, as condensing boilers often do for efficiency, the tank delivers less. Use the what size water heater method to estimate demand.
Size a combi boiler by its domestic hot water flow at your winter temperature rise, the same arithmetic the Water Heater Size Calculator uses for tankless: kW = L/min × rise × 4.186 ÷ 60. Then check its heating output range suits the home's heat loss, especially its minimum output. A boiler that cannot turn down low enough will short-cycle in mild weather.
| Home | Often a good fit | Why |
|---|---|---|
| Condo or small home, one bathroom, hydronic heat | Combi boiler | Saves space; flow demand modest |
| Family home, two or more bathrooms, hydronic heat | Boiler plus indirect tank | Stored hot water handles simultaneous use |
| Low-load new home with ducted air | Combination system, professionally designed | One heat source for a small heating load |
| Forced-air furnace home, separate water heater | Keep them separate | Simplest; replace each on its own schedule |
05 /What maintenance do combi and indirect systems need?
These systems combine two sets of maintenance: the boiler's combustion and venting checks, and the water-side care of any hot water equipment.
- Annual boiler service by a qualified technician: combustion analysis, venting and condensate checks.
- Descaling the domestic heat exchanger of a combi in hard water, as for a tankless heater.
- Testing the T&P valve on an indirect tank and inspecting its anode if it has one; see anode rods.
- Checking the heating system's pressure and expansion tank, which is separate from any domestic expansion tank.
- Keeping a carbon monoxide alarm near sleeping areas, as with any fuel-burning appliance.
Because the boiler heats water year-round, a failure means no heat and no hot water at once. Knowing where the manual shut-offs and the boiler's reset are, and having a service contact before winter, is worth the few minutes it takes.
06 /What do these systems cost compared with a standalone tank?
A standalone tank is the cheapest to install, while combi boilers and boiler-plus-indirect systems cost more because they are also replacing or adding a heating appliance. The editorial planning ranges below assume a detached house with existing hydronic heat.
| Job | Typical range |
|---|---|
| Add an indirect tank to an existing modern boiler | $2,500–$5,000 |
| Replace a boiler with a condensing combi boiler | $7,000–$14,000 |
| Replace a boiler with a condensing boiler plus indirect tank | $9,000–$18,000 |
If you are weighing a combi or indirect system against a simple tank replacement, a quote request describing your heating system type and number of bathrooms helps installers propose comparable options.
FAQQuestions people ask
Is my hot water tank connected to my furnace?
Usually not. In most Canadian homes the furnace and the water heater are separate appliances that may share a chimney. In combination systems the water heater supplies heat to a fan coil in the ductwork, and in hydronic homes a boiler may heat an indirect tank.
Is a hot water tank a boiler?
No. A water heater heats domestic water for taps and fixtures. A boiler heats water for space heating, though it can also heat domestic water through an indirect tank or, in a combi, directly.
Is a combi boiler better than a tankless water heater?
They do similar hot water jobs. A combi also heats the house, so it replaces two appliances with one, which suits homes with hydronic heat. In a forced-air home, a combi is not the usual choice.
Can I add an indirect tank to my existing boiler?
Often, if the boiler has enough output and a spare zone or can be piped with a priority zone. Older, oversized boilers can run inefficiently in summer, so have a professional assess the boiler first.
Does an indirect water heater need a T&P valve?
Yes. The National Plumbing Code requires indirect service water heaters to have pressure relief and temperature relief on every storage tank in the system.