Boiler No-Heat Diagnosis: A Step-by-Step Guide for Hydronic Heating Systems

The Diagnostic Mindset for Hydronic Systems

Hydronic systems have more failure points than forced-air systems — the boiler itself, the zone valves, the circulator pumps, the expansion tank, and the distribution piping all contribute to system operation. A no-heat complaint might be a boiler problem, or it might be a stuck zone valve, a failed circulator, or an open low-water cutoff.

Start at the boiler, but don't assume it's the problem until you've ruled out the distribution system.

Step 1: Is the Boiler Getting a Call for Heat?

Jump the thermostat terminals at the boiler's terminal strip — usually R and W or TT terminals. If the boiler fires on a direct jump, the problem is in the control circuit upstream: thermostat, zone valve end switch, relay, or wiring.

If it doesn't fire on a direct jump, the problem is at the boiler or its safety controls.

Step 2: Check the Safety Controls

Low Water Cutoff (LWCO): The LWCO opens to prevent the boiler from firing without adequate water. Check the sight glass on steam boilers — water level should be at the center of the glass. On hot water systems, check system pressure (should be 12–20 PSI cold). A probe-type LWCO should be cleaned and tested annually.

High Limit: If the boiler overheated during a previous call, the high limit may have tripped. Manual-reset limits require pressing the reset button after addressing the cause.

Pressure Relief Valve (steam systems): If the PRV has been weeping, system pressure may be insufficient for proper operation.

Step 3: Zone Valves and Circulators

If the boiler fires but only some zones aren't getting heat, the problem is in the distribution system. Zone valves can fail mechanically (stuck closed) or electrically (end switch not completing the circuit). With the zone calling for heat, verify 24V at the valve motor terminals. If voltage is present but the valve doesn't open, the motor has failed.

Circulators fail most often after sitting idle over summer — the impeller shaft seizes. A circulator that's hot to the touch but not moving water has likely seized and needs replacement.