Short definition
An indirect-fired water heater is a storage tank with an internal heat-exchange coil that runs hot water (or steam) from a separate boiler through it. The boiler does the heating; the tank stores the hot water. Common in older WA homes that have hydronic central heating — the same boiler does space heat in winter and DHW year-round.
What it is
Inside an indirect tank, a coiled copper or stainless tube carries the boiler’s primary loop water. The tank’s potable water surrounds the coil and absorbs heat through the coil wall. A control thermostat on the tank calls the boiler when stored water drops below setpoint.
Two main variants:
- Single-coil. Boiler-only heating.
- Twin-coil. Two separate coils, typically for boiler + solar thermal or boiler + electric backup.
A code-relevant detail: the heat exchanger is either single-wall or double-wall depending on what’s in the boiler loop. UPC requires a double-wall heat exchanger when the boiler-side fluid is non-potable — which includes any system with glycol antifreeze or treated boiler water containing corrosion inhibitor (Sentinel or Fernox additives, common on modern boilers). Single-wall is allowed only when both sides are potable, which is rare in real installs.
Why it matters to a homeowner
If your home has cast-iron radiators or hydronic baseboard, you likely have an indirect tank — and it stays connected when you replace the boiler. The boiler swap drives the new install; the indirect tank usually has another decade of life.
When a contractor’s quote on a boiler replacement says “re-use existing indirect,” they mean the tank itself is still good and only needs a new control thermostat and possibly new coil-side connections. When they say “replace indirect with HPWH,” they’re proposing to disconnect the tank from the boiler and install a standalone HPWH — a bigger change with bigger upside if you’re also moving toward space-heat electrification.
The Legionella note matters here: indirect tanks coupled to a modern condensing boiler are sometimes set to lower storage temperature for efficiency, which can edge into the bacterial growth zone. A thermostatic mixing valve at the heater output lets you store at 140°F and deliver at 120°F.
When you’ll encounter this term
- A pre-purchase inspection of an older WA Craftsman or mid-century rambler identifies a hydronic boiler with an indirect tank.
- A boiler quote includes “re-use existing indirect” or “replace indirect with HPWH.”
- A no-hot-water complaint in a hydronic-heated home — diagnose the boiler side, not the tank itself.
- An HVAC contractor proposes adding a solar thermal twin-coil for the lower coil while the boiler tops up via the upper coil.
Common variants and what an indirect water heater is not
- Indirect vs. direct (electric or gas). Direct means the heating element or burner is in the tank. Indirect means heat comes from an external boiler via coil.
- Indirect vs. combi boiler. A combi boiler does instantaneous DHW with no storage. An indirect uses a separate stored tank.
- Single-coil vs. twin-coil. Twin-coil for solar+boiler or boiler+electric backup.
Common failure modes
- Coil fouling. Boiler-side scale on the coil reduces heat transfer; flush the loop or descale the coil.
- Tank corrosion. Same anode-rod story as a standard tank; some indirect tanks are stainless and don’t need an anode.
- Boiler short-cycling. If the indirect’s setpoint differential is too narrow, the boiler fires too often.
- Cold-spot DHW. Weak boiler delivery or undersized coil — lukewarm hot water at the tap.
Washington note
Indirect tanks were common in Seattle, Tacoma, and Spokane homes built between roughly 1920 and 1980 with hydronic heating — cast-iron radiators, hydronic baseboard, sometimes radiant floors. Today they’re getting less common as those boilers fail and homeowners shift to electric: ductless heat pumps for space heat plus standalone HPWH or tankless for DHW.
If you have one and the boiler is going, the decision tree is:
- Boiler still working but DHW thermostat failed. Replace the thermostat ($60–$150 part); the tank is fine.
- Boiler failing, hydronic system staying. Replace the boiler with a modern condensing unit; keep the indirect tank if it’s under ~15 years old.
- Whole-home electrification. Replace boiler with ductless heat pumps and the indirect with a standalone HPWH or tankless. Eligible for stacked WA utility rebates.
WA-amended UPC follows the standard double-wall heat-exchanger rule when boiler-side fluid contains glycol or corrosion inhibitor.