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Best Indirect Water Heater

3 expert-curated picks ranked by performance, value, and long-term reliability

Indirect water heaters are storage tanks heated by a hydronic boiler rather than by integral combustion or electric elements. Common in homes with hydronic space-heating systems (radiators, baseboard, radiant floor) where the existing boiler can also heat domestic hot water through a separate heat exchanger.

Top indirect water heater picks

Best mainstream: Amtrol BoilerMate WH-7L

40-gallon indirect tank. Stainless steel internal coil heat exchanger. Compatible with most residential hydronic boilers. 7-year tank warranty. About $1,300 unit.

Best larger capacity: Amtrol BoilerMate WH-7Z (80-gallon)

80-gallon indirect for larger households. Same engineering as the smaller WH-7L. About $1,800 unit.

Best stainless: HTP SuperStor 60-gallon Indirect

Fully stainless steel construction. Lifetime tank warranty. Premium positioning. About $2,400 unit.

Best compact: Buderus ST300 Indirect (40-gallon)

German-engineered. Compatible with Buderus boilers and other major brands. About $1,700 unit.

How indirect water heaters work

An indirect heater is a storage tank with an internal heat exchanger coil. The boiler circulates hot water (typically 160-180°F) through the coil; the coil transfers heat to the domestic water in the surrounding tank.

Advantages:

  • No separate burner or electric element — uses the existing boiler's capacity
  • Higher efficiency than standalone water heaters when paired with a modern condensing boiler (AFUE 95%+ on the boiler translates to similar effective efficiency for DHW)
  • Longer service life — no combustion components in the DHW tank to corrode or fail
  • Single heating system for both space heat and DHW reduces equipment complexity

When indirect is the right choice

  • Existing hydronic space-heating system with a boiler already in place
  • Replacing aging boiler — opportunity to add indirect tank simultaneously
  • Cold-climate homes where the boiler runs much of the year anyway (shared utilization)
  • Homes converting from oil-fired boiler to gas (good time to add indirect)

When indirect doesn't fit

  • Homes without existing hydronic boiler (combi boiler or standalone water heater is the right pick)
  • Forced-air heated homes (no boiler exists)
  • Smaller homes where boiler usage is low (DHW load alone doesn't justify boiler cycling)

Indirect vs combination boiler

Indirect (boiler + tank)Combi (single unit)
Equipment count2 units (boiler + tank)1 unit (combi)
DHW storageYes — buffer capacityNo — instantaneous only
Single failure modeNo — boiler can fail while DHW continues if tank is hotYes — both functions lost together
Install cost$5,000-$8,000 (boiler + tank)$7,000-$11,000 (combi)
Footprint2 units1 unit (wall-mount)
3 products
$949 – $1,795
Updated May 2026

Quick Comparison

# Product Brand Rating Price
1 Rinnai RU199iN Sensei Tankless Water Heater Rinnai 4.8 Check current price Amazon
2 Rheem Performance Platinum 50-Gallon Gas Water Heater Rheem 4.6 Check current price Amazon
3 AO Smith Signature Premier 50-Gallon Gas Water Heater AO Smith 4.5 Check current price Amazon