Indirect Water Heater vs Tankless Coil

For a New Hampshire home with boiler heat, domestic hot water may come from a tankless coil built into the boiler, a separate indirect water heater, a combi boiler, or another system. An indirect water heater often provides more stored hot-water capacity and lets a cold-start boiler remain off between calls. A tankless coil can still fit a low-demand home or an existing system where space and project scope favor keeping it. A.J. LeBlanc Heating compares the boiler, controls, demand, piping, water quality, and project scope before recommending a change.

Tankless coil: hot water generated on demand inside the boiler

A tankless coil is a domestic-water heat exchanger built into a boiler. Do not use "side-arm" as a synonym here because that term can describe a separate external heat exchanger or water-heating arrangement.

Tankless coil characteristics:

No separate hot water tank. Saves floor space.
Boiler must stay hot year-round. Even in summer when there is no space heating demand, the boiler must maintain water temperature to feed the coil. This is the largest hidden cost.
Capacity is model- and condition-specific. Available hot water depends on boiler output, coil condition, incoming-water temperature, controls, and simultaneous demand.
Inconsistent water temperature. Temperature can drop noticeably as demand changes.
Lower upfront cost. Coils are factory-built into the boiler.
Higher operating cost. Year-round boiler operation for hot water alone is inefficient, especially in summer.

Indirect water heater: separate storage tank, heated by the boiler

An indirect water heater is a separate insulated storage tank (typically 30 to 80 gallons) connected to the boiler through dedicated supply and return piping. The boiler heats water in the tank when needed; the tank stores hot water for use. There is no separate burner, no separate flue, and no separate gas or oil consumption beyond what the boiler uses.

Indirect water heater characteristics:

Substantial storage capacity. A 40 or 50 gallon tank handles peak demand (multiple showers, dishwasher, washing machine) without temperature drop.
Boiler can shut down between calls. In summer when there is no space heating demand, the boiler only fires occasionally to maintain the tank. Much more efficient than tankless coil operation.
Consistent water temperature. The tank stays at setpoint regardless of demand.
Higher upfront cost. The tank, piping, controls, and indirect zone valve add to the installation.
Lower operating cost. Especially in summer and shoulder seasons.
Long lifespan. Quality indirect tanks (HTP SuperStor, Bradford White, Amtrol, Phase III) often last 20+ years.

Compare performance and operating cost

Performance and operating cost depend on boiler controls, standby losses, coil or tank condition, household demand, storage size, water temperature, and current fuel rates. We size the option to simultaneous demand and inspect how the existing boiler is controlled.

Combi boilers as a third option

A combi boiler (combination boiler) is a wall-mounted boiler that includes a built-in tankless on-demand heat exchanger for domestic hot water. Unlike a tankless coil in a traditional boiler, a combi unit is designed from the start to be efficient at producing hot water:

Variable-speed circulator and modulating burner adjust to demand
High-efficiency condensing operation
Compact wall-mounted design
Common in smaller NH homes, condos, and tight mechanical rooms
Still has the simultaneous-demand limit of any tankless: very high peak demand (multiple showers + dishwasher) can outpace the unit

Navien is the most common combi boiler we install in NH; Viessmann, Bosch, and Lochinvar also make strong combi units.

Heat pump water heater (HPWH) as a fourth option

For homes with hydronic heat but limited hot water demand, or homes that want to start reducing fuel use, a heat pump water heater installed alongside the boiler is increasingly attractive:

Operates at very low cost (up to roughly one-third the electricity of a standard electric tank)
The federal 25C tax credit for heat pump water heaters ended December 31, 2025; NHSaves rebates and financing are the live incentives in 2026
Decouples hot water from the boiler entirely, allowing summer boiler shutdown

An HPWH does require adequate basement or mechanical room space and ambient temperature above 50°F. Most NH basements meet these requirements.

When each one fits

Tankless coil makes sense when:

The home has very low hot water demand (small household, vacation home)
An existing tankless coil boiler is still working and replacement is not in the budget
Mechanical room space is extremely tight

Indirect water heater makes sense when:

The home is in normal residential use with multiple bathrooms or active hot water demand
The boiler is being replaced (good time to add an indirect zone)
You want consistent hot water temperature and adequate peak capacity
You want the boiler to cycle off in summer

Combi boiler makes sense when:

Mechanical room space is limited
Hot water demand is moderate
You want a single wall-mounted unit serving both space heat and domestic hot water

HPWH makes sense when:

You want to decouple DHW from the boiler for summer savings
You have adequate basement space
You want to maximize NHSaves and utility rebate eligibility

Schedule a consultation

If you are evaluating hot water options for your NH boiler home, or your existing system is not meeting your hot water demand, contact A.J. LeBlanc Heating or call 603-623-0412. Serving New Hampshire families since 1928.

Frequently Asked Questions

Is an indirect water heater more efficient than a tankless coil?

It can reduce year-round boiler standby operation in the right design, especially when replacing a warm-start arrangement. The actual difference depends on the boiler, controls, tank, demand, and piping.

How long does an indirect water heater last?

Service life depends on the tank, water quality, installation, operating conditions, maintenance, and repair history. We evaluate condition and parts support rather than promising a fixed lifespan.

Can I switch from a tankless coil to an indirect water heater?

Yes. Adding an indirect tank is straightforward when the boiler is being serviced or replaced. The boiler needs a dedicated zone for the indirect (zone valve, circulator, and an aquastat or smart control).

What size indirect tank do I need?

Storage size is selected from simultaneous demand, boiler output, recovery, fixtures, household use, and the selected tank's performance rather than one default gallon range.

Do indirect water heaters work with all boilers?

Many boilers can serve a properly sized indirect zone, but available output, controls, piping, venting, and the home's combined space-heating and hot-water demand must be checked.