Indirect water heaters explained for boiler homes
An indirect water heater is an insulated storage tank with a heat exchanger coil inside, and the home's boiler supplies the heat instead of a burner or element on the tank. It suits houses that already have a hydronic boiler, and it gives fast recovery with low standby loss.
How does an indirect water heater work?
Hot boiler water circulates through a coil inside the tank, passing heat to the domestic water around it without ever mixing with it. When the tank temperature falls below its setting, a sensor calls the boiler and a circulator pump sends heat to the coil. The tank has no flame, no flue and no heating element of its own, so there are fewer parts to fail and no combustion in the tank.
Most installations give the tank priority. The boiler stops sending heat to the radiators or floors for a short time while the tank recovers, then switches back. Because boilers are far more powerful than a standard tank burner, recovery is quick. The design also pairs well with low-temperature hydronic heating.
What are the benefits and limits?
Benefits come from consolidation.
- One heat source. One boiler serves heating and hot water, so there is one flue and one service call.
- Fast recovery. A large boiler reheats the tank in a short time, which suits busy mornings.
- Low standby loss. Indirect tanks are usually well insulated.
- Long tank life. With no combustion or element in the tank, many are built to last a long time, though water quality matters.
The limits are just as real. The boiler has to run in summer to make hot water, which can be wasteful if it is large or old. A boiler that cycles at low load loses efficiency, and the upfront cost is higher than a basic tank. If the boiler fails, both heat and hot water go with it. See boiler versus furnace in Canada for how boilers compare more broadly.
When does an indirect tank make sense?
It fits best where a modern, efficient boiler is already in place or being installed. A condensing boiler running at a low return temperature works well because the tank coil takes heat at a modest water temperature for part of the cycle. An old, oversized boiler tends to short cycle in summer, so an indirect tank is a weaker choice there.
It is also a good answer for homes with radiant floor heating, as covered in the radiant floor heating guide, since the same hydronic plant already exists. In a house with a furnace and no boiler, a separate tank or a heat pump water heater is usually simpler. The broader set of options is in water heating system options.
How should you size and install one?
Tank size follows peak-hour demand, as with any storage heater, and hot water tank sizing shows how to estimate it. The boiler must be able to supply both the space heating load and the tank without leaving the house cold in midwinter, so an installer should check the boiler output and the piping. A thermostatic mixing valve at the tank outlet is standard practice, since stored water can be hotter than the tap setting.
Gas and fuel work needs a licensed trade, and the local authority may require a permit. Request written quotes that spell out the tank, the controls, the piping changes and the warranty. Solar can be added later through a second coil in some tanks, a route covered in solar hot water cost in Canada.
Quick answers
Is an indirect water heater more efficient than a regular tank?
It can be, because it has low standby loss and uses the boiler's efficiency. The result depends on how efficient and how well sized the boiler is.
Can I use an indirect water heater with a furnace?
No, it needs a boiler or another hydronic heat source. Homes with furnaces normally use a standalone tank, a tankless unit or a heat pump water heater.
How long does an indirect tank last?
Many are built for long service lives because there is no burner or element in the tank. Water quality, maintenance and the warranty terms are worth checking.