What it is

"No heat" on a boiler call breaks cleanly into three questions, and answering them in order keeps you from chasing your tail:

  1. Is the boiler getting a call? (thermostat → controls → the boiler's "go" signal)
  2. Is the boiler making heat? (it fires, the water gets hot)
  3. Is the boiler moving heat? (the circulator pushes hot water out to the emitters)

A boiler can fail at any one of those and the customer just says "no heat." Figure out which branch is dead first, then drill into it. Don't start swapping parts before you know whether the thing is even trying to fire.

How it works

A simple hot-water boiler runs like this: the thermostat closes on a call → the boiler control (an aquastat on older units, a fancier control board on mod-cons) sees the call → safeties prove OK (low-water cutoff, high-limit, pressure/flow) → the burner fires through its ignition sequence → the water heats to setpoint → and a circulator (energized on the call, often through a relay or the aquastat) moves that hot water out to the zones. When the thermostat satisfies, the call drops and it all stands down.

The high-limit (aquastat) caps water temperature so the boiler doesn't overheat. The low-water cutoff (LWCO) kills the burner if water level/flow is lost — a critical safety that will absolutely stop a boiler from firing if it trips or fails.

In the field

Branch 1 — Is it getting a call?

  • Confirm the thermostat is calling: 24V across the call at the boiler control. No call → it's a thermostat/wiring/transformer problem upstream, not the boiler.
  • On zone systems, confirm the zone control passes the call through (end switch on a zone valve, or the zone relay) so the boiler actually gets told to run. A zone valve that opens but whose end switch doesn't make = call never reaches the boiler.
  • Confirm the boiler has power and its switch (often a red service switch on the wall/boiler) is on. Sounds dumb; it's a real call.

Branch 2 — Is it making heat?

  • Watch the ignition sequence. Gas units: inducer/draft proves (on units that have one), igniter heats or sparks, gas valve opens, flame proves. A stall at any step parks it.
  • Check the safeties that block firing: high-limit tripped (overheated — often a flow/circulator problem feeding back), low-water cutoff open (low water/flow), blocked-vent or rollout switch tripped (combustion/venting problem), pressure switch open.
  • Mod-cons throw fault codes — read the display. It'll point you at ignition lockout, flame fault, low water, condensate, sensor faults, etc. Use the code; don't guess.
  • If the burner fires but locks out after a few seconds, it's usually a flame-proving problem (dirty flame sensor/electrode, bad ground, gas supply) — same logic as a furnace flame-sense fault.

Branch 3 — Is it moving heat?

  • If the boiler makes hot water but the house stays cold, the heat isn't getting out. Confirm the circulator runs on the call and that flow develops (the return pipe warms up after a few minutes).
  • No flow with the pump running → air-bound loop, seized impeller, stuck zone valve, or closed flow check. Purge the air; verify the pump and valves.
  • One zone cold, others fine → that zone's valve/pump/loop (see the zoning article). Whole house cold with a hot boiler → the shared pump or a system-wide air/flow problem.

Normal values & targets

  • Control voltage: 24V on the call/control circuit. Confirm the transformer is putting out ~24–28V.
  • Aquastat high-limit: commonly set around ~180°F for fin-tube baseboard systems (with a differential); the boiler cycles the burner to hold water temp. Mod-cons use a target/reset instead.
  • System pressure: cold fill near ~12 psi (residential); low/no pressure can trip flow/low-water safeties and air-bind the loop. (See the expansion-tank article.)
  • Flame signal (gas): like a furnace, a healthy flame-sense microamp reading (often a few µA up; below ~1 µA typically drops out). Verify against the control's spec.

These are representative; confirm against the specific equipment.

Common faults & what they mean

  • No call at the boiler — thermostat, low-voltage wiring, blown fuse/transformer, or a zone end switch/relay not passing the call. The boiler is innocent.
  • Burner won't fire, locks out on ignition — no/weak spark or hot-surface igniter, gas-supply issue, or flame not proving (dirty sensor, bad ground).
  • Boiler trips on high-limit / short-cycles on temp — often a flow problem in disguise: dead circulator, air-bound loop, or stuck valve means the heat can't leave the boiler, so the water overheats and the high-limit cuts it. Don't just raise the limit — find why heat isn't moving.
  • Low-water cutoff stopping it — genuine low water/pressure (leak, dead expansion situation), air, or a failed LWCO probe/float. Verify real water level/flow before bypassing anything (and never permanently bypass an LWCO).
  • Mod-con condensate/fault lockout — plugged or frozen condensate trap/line, or a specific sensor fault — read the code.
  • Boiler hot, house cold — circulation problem: pump, air, zone valve, or flow check.

Tech tips & gotchas

  • Split it into the three branches before touching a part. "Is it getting a call, making heat, or moving heat?" turns a vague no-heat into a directed hunt.
  • High-limit trips are often flow problems. Techs replace aquastats when the real issue is a dead circulator or an air-bound loop cooking the boiler. Confirm heat is actually leaving before blaming the limit.
  • Read the mod-con fault code first. It exists to save you time — ignition lockout, flame fault, low water, and condensate faults each send you down a different path.
  • Air is a top cause of "boiler runs but no heat." A loop with an air pocket won't circulate even with a perfect pump. Always have purging in your toolkit.
  • Respect the low-water cutoff. It's there because a dry-fired boiler is a serious hazard. Find the reason it tripped; do not jumper it out as a "fix."
  • The service switch and the thermostat. Embarrassing-but-real no-calls: someone bumped the red emergency switch off, a dead thermostat battery, or a tripped breaker. Rule out the dumb stuff fast before you go deep.

Safety / code notes

  • Never bypass or defeat the low-water cutoff or the relief valve. Both are primary safeties on a pressure vessel. Replace, don't jumper.
  • Combustion appliances: verify venting and combustion-air per the applicable mechanical/fuel-gas code, and test for safe combustion/CO before leaving — especially after any burner or venting work.
  • Line-voltage controls and pumps are a shock hazard — lock out/verify dead before servicing.
  • A boiler that overheated and tripped its limit may have a real flow failure — don't just reset and walk; confirm circulation is restored or you'll be back, possibly to a damaged boiler.