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Heating · Boilers & Hot WaterIndirect Water Heater Installation
Indirect water heater installation adds an insulated storage tank that is heated by your boiler through an internal coil, so the tank has no burner, no gas line and no vent of its own. It is the right choice for a boiler-heated home with two or more bathrooms, where it recovers far faster than a standalone tank. Pricing is by written quote, since the tank size and the piping decide it.
Inside the tank is a coil of stainless or enamelled steel tubing. Boiler water runs through the coil, the domestic water around it picks up the heat, and the two never mix. Because the boiler is the only heat source, there is nothing in the tank to light, vent or service on the gas side, and with thick insulation and no flue up the middle it loses little heat on standby. A sensor or aquastat in the tank tells the boiler when to reheat it.
It is piped and controlled as a priority zone. When the tank calls, the boiler control pauses the space-heating circulators and sends the boiler's full output to the coil until the tank is satisfied, then returns to heating the house. The tank gets its own circulator or zone valve, a temperature and pressure relief valve, and a tempering valve on the hot outlet so water can be stored at 60°C and delivered cooler at the taps. Where the cold supply has a check valve or backflow preventer, a thermal expansion tank goes on the domestic side too.
Recovery is the reason to choose one. A typical standalone gas tank has a burner of about 40,000 BTU per hour, and a residential boiler has two to four times that to put into the coil, so a 40-gallon indirect keeps up with back-to-back showers and a filling tub in a way the same size of conventional tank cannot. Against a combi boiler, the difference is flow: a combi heats water as it passes and is limited to what its exchanger can do at that instant, which suits a one- or two-bathroom house. A larger home with several showers running at once is better served by stored hot water, and combi boiler versus boiler and water heater compares the layouts.
Two things decide whether it suits your house. The boiler has to fire in summer to make hot water: a modulating condensing boiler does that efficiently in short runs, while an old cast-iron boiler kept hot from June to September wastes fuel, so on those we usually pair the tank with a boiler replacement. And scale from Halton and Hamilton's hard water builds on the outside of the coil and slows recovery, so the tank needs a drain you can reach and a periodic flush. Sizing follows the number of bathrooms and the largest tub, with the boiler's output confirmed against both loads; see what size boiler do I need.
An indirect tank is the right call when
- The house is heated by a boiler and the standalone water heater is due for replacement
- Two or more showers run at the same time, or a large tub drains the current tank
- You are replacing the boiler anyway and want one burner and one vent instead of two
- The existing water heater vents into a chimney that would otherwise need a liner
- A combi boiler was quoted and its hot-water flow rate looks tight for the household
How it works
Bathrooms, tub volume and peak-hour use set the tank size; the boiler's output and its control are checked for a domestic hot water priority function.
Tank set and levelled, coil piped to the boiler with its own circulator or zone valve, tempering valve, relief valve and expansion tank fitted, and the tank sensor wired to the boiler control.
Air purged from the coil loop, priority tested by calling for heat and hot water together, outlet temperature set at the tempering valve, and the old water heater disconnected and removed. Request an indirect tank quote.
Common questions
Does an indirect water heater work with my existing boiler?
With most hot-water boilers, yes, as long as the boiler has spare capacity and a control that can give the tank priority, or one can be added. It does not work with a forced-air furnace, which has no hot water to send through the coil. We check the boiler's rating plate and its control before quoting.
Will my radiators go cold while the tank reheats?
Not noticeably. Priority pauses space heating only while the tank recovers, and cast-iron radiators and warm floors hold their heat far longer than a recovery takes. Most boiler controls also include a priority time limit, so if a tank call runs unusually long the house heating resumes and the tank finishes afterward.
How much does an indirect water heater cost to install?
More up front than a conventional tank, which typically runs $1,200–$2,500 installed as a straight swap. The indirect tank itself costs more, and the job adds a circulator or zone valve, a tempering valve and boiler-side piping. In return there is no second burner or vent to maintain. You get a written quote after a site visit.
Indirect tank or combi boiler: which is better?
For a one- or two-bathroom house with limited space, a combi is simpler and frees the floor. For three or more bathrooms, simultaneous showers or a deep tub, an indirect tank on a heating boiler delivers more hot water at once. If the first description fits your house, see combi boiler installation.
What maintenance does an indirect tank need?
Very little, because there is no burner and no flue. The relief valve is tested and the tempering valve checked at the annual boiler service, and in hard-water areas the tank is flushed through its drain to move scale off the coil. Glass-lined models also have an anode rod to inspect; stainless tanks do not.