Indirect water heaters don't generate their own heat — they share heat with the home's boiler. A heat exchanger coil inside the storage tank receives hot water from the boiler loop and transfers that heat to the domestic hot water. The boiler does the work; the indirect tank just stores. Common in homes with hydronic space heating (cast-iron radiators, baseboards, radiant floor) where a high-efficiency boiler is already in place.

Where indirect makes sense

Homes with existing hydronic heating (boiler + radiators or baseboards): the boiler is already heating water for space heating; adding domestic hot water via indirect is the most efficient option
High-efficiency modulating-condensing boilers: these run at 95%+ efficiency, which makes the indirect tank's domestic hot water cheaper than a dedicated gas tank water heater
Properties without natural gas service for a standalone DHW unit: if the boiler runs on oil or propane, an indirect tank piggybacks on that same fuel
Light-commercial residential (B&Bs, group homes) with central boiler: single boiler serves heating + DHW efficiently

Indirect vs combi boiler

The alternative for hydronic-heated homes is a combi boiler — a single unit that handles both space heating and tankless domestic hot water in one wall-mounted device. The Navien NCB-240E and Noritz NRCB199 are the volume residential combi units.

Indirect vs combi trade-offs:

Indirect (boiler + indirect tank): better DHW recovery (the tank handles peaks), can outlast a combi by 10+ years, easier to service. Higher upfront install cost.
Combi boiler: single-unit simplicity, less floor space, lower upfront. DHW priority can briefly dip space heating during high simultaneous demand. Combi typically lasts 15 years; an indirect-equipped boiler can last 25+.

Major indirect water heater brands

Indirect water heaters are typically sold through plumbing supply houses rather than big-box. Major brands:

HTP — Phase 2 and Phoenix line. Plumber-channel volume brand.
Triangle Tube — Smart and Phase 3 line. Strong stainless-steel indirect tanks.
Amtrol — BoilerMate line. The volume residential indirect brand for 40+ years.
Burnham / U.S. Boiler — Alliance line, paired with their hydronic boilers.
Bradford White — manufactures indirect variants of their conventional tank line.
Bock — premium specialty indirect, often paired with oil-fired boilers in the Northeast.

Indirect tank sizing

Indirect tank sizing depends on boiler BTU output (the larger the boiler, the more rapidly the indirect tank can recover) and household DHW demand. Typical residential sizing:

2–3 person households: 30–40 gallon indirect
3–4 person households: 40–50 gallon indirect (most common)
5–7 person households: 60–80 gallon indirect
Light-commercial: 80–120 gallon indirect

Indirect tanks have very high effective recovery — a 50-gallon indirect with a 100,000 BTU/h boiler can recover faster than a 75-gallon gas atmospheric tank with 76,000 BTU/h dedicated burner. The boiler is doing the heavy lifting.

Install requirements

Existing boiler with capacity: verify boiler BTU output is sufficient for combined space-heating + DHW load. Undersized boilers create issues.
Boiler loop tap-in: indirect tank requires supply and return lines from the boiler loop, plus a circulator pump dedicated to the indirect
Boiler-side aquastat or controller: tells the boiler when the indirect tank needs heating
Standard DHW connections: cold inlet, hot outlet, T&P, drain — same as a conventional tank
Permit and licensed installer: indirect installs touch the boiler system; not a typical DIY job

Indirect water heater efficiency

The efficiency of an indirect water heater is the efficiency of the boiler heating it. A 95% AFUE modulating-condensing boiler delivers domestic hot water at effectively 95% efficiency — significantly better than the 60–67% UEF of a standalone atmospheric gas tank. For homes with mid-efficiency (80% AFUE) cast-iron boilers, indirect is less compelling — close to standalone tank efficiency.

Bottom line on indirect water heaters

Indirect water heaters are the right pick for homes with high-efficiency modulating-condensing boilers and hydronic space heating. The combined-system approach extracts maximum efficiency from a single fuel input. For homes with forced-air heating and no boiler, indirect doesn't apply — you'll want a standalone gas tank, gas tankless, or heat pump instead. For hydronic-heated homes upgrading both boiler and water heater simultaneously, the combi-boiler alternative (NCB-240E, NRCB199) is often simpler and uses less floor space.