Boiler Not Working: What to Check First, From the Manuals

By Ray Bennett · Updated 3 October 2026 · 9 min read · Checked against the sources at the end

Boiler Not Working: What to Check First, From the Manuals

Short answer

Check that the thermostat and zones are calling for heat, the boiler's switch and breaker are on, and the gas or oil is on. On a hot water boiler the gauge should read about 12 psi cold and no more than 24 or 25 hot. On a condensing boiler, look for ice or a nest in the vents outside. Then read the lockout, and reset it once, the maker's way.

A boiler not working in the first cold week is often something simple you can check yourself. The makers’ own manuals list much the same checks, and most of them cost nothing.

This page covers hot water boilers, the kind that heat radiators or baseboard with water, with a short part on steam and on the combi boilers that also make your hot water.

Before you start

What to carry: a torch, and your phone to photograph the display and the gauge. Fifteen minutes.

Gas, oil or electric? A gas boiler has a gas pipe and valve. An oil boiler has an oil line and a burner with a red reset button on the front. An electric boiler has neither, only a heavy power cable, so it skips the fuel check. All hot water boilers follow the same pressure checks. Steam is different, see below.

Hot water or steam? A hot water boiler has a round pressure and temperature gauge and pipes full of water. A steam boiler has a glass tube on its side showing the water level.

Four things are not a reset job. A smell of gas. Weil-McLain and Burnham both say do not try to light any appliance, do not touch any electrical switch or use any phone in the building, and call the gas supplier from a phone outside it. If you cannot reach them, call the fire department. On propane, Weil-McLain says to smell near the floor, because the gas collects there. Every other smell is in a boiler or radiator that smells.

A carbon monoxide alarm, or soot. CPSC says when a CO alarm sounds, get outside to fresh air and call 911. Burnham says black soot in or near the burner area is a sign of a possible carbon monoxide hazard.

Overheating, or gas that will not shut off. Weil-McLain, Burnham and Lochinvar all say do not cut the power to the pump. Shut the gas off at a valve outside the boiler instead.

A boiler that has been under water. Burnham says it must not be run in any way until a qualified technician has inspected or replaced it, and recommends replacing it. Navien and Lochinvar say theirs must be replaced.

The checks that cost nothing

Five checks from the makers' manuals, in the order to do them. Each one costs nothing.

Check 1Free, a minute

Is anything calling for heat?

The thermostat. Set it to heat and a few degrees above the room. Burnham’s Series 2E manual says that if the boiler sits in standby, check that the thermostat and the zone controls are actually calling for heat.

A combi boiler. Navien and Lochinvar both say a combi does not heat the radiators while it is making hot water for a tap. The heat comes back once the tap is off.

If the thermostat itself is blank or dead: a thermostat not working.

Check 2Free, a minute

The switch and the breaker

There is usually a service switch on the boiler or on the wall nearby, and it looks like a light switch. Check it is on, then check the breaker. One of Navien’s checks for a heating fault is that the boiler has power at the right voltage.

After a power cut, Navien says its NCB-E combi restarts by itself with no reset needed. If yours did not: not working after a power outage.

Check 3Free, unless the tank is empty

The gas or the oil

Oil. Weil-McLain’s checks for an oil boiler that will not fire include fuel valves turned off and not enough oil in the tank. Burnham lists an empty tank and a closed shut off valve on the oil line too.

Gas. Check the gas valve on the pipe to the boiler is open, and that other gas appliances in the house are working.

Check 4Free, ten seconds

Read the pressure gauge, on a hot water boiler

Cold. Weil-McLain and Navien give 12 psi as the typical cold fill pressure for a home system, and Burnham fills its Series 2E to 12 psi. Some need more: Burnham’s Alpine and Peerless’s PureFire need at least 14.5 psi.

Hot. Weil-McLain and Lochinvar say the gauge should not read above 24 psi, and Triangle Tube 25. Higher may mean an expansion tank problem.

Too low. Many boilers stop on low pressure to protect themselves. Burnham says a low water shutdown is serious, a qualified service technician must inspect the system before the boiler runs again, and water must never be added to a hot boiler. Let it cool fully first. Triangle Tube says cold water in a hot boiler can make the heat exchanger fail early.

Check 5

The vents outside, on a condensing boiler

A condensing boiler has plastic pipes out through the wall. Burnham says that if one stops and its vent comes out of a wall you can reach from the ground, check the terminal for a bee’s nest or ice, and the burner should light about 30 seconds after the blockage is gone. If you find nothing, or cannot reach it from the ground, call a gas service technician.

Read the gauge properly

The numbers the makers give for a home hot water system. The relief valve on many home hot water boilers is set at 30 psi; the number is on the valve's tag.

The gauge rises as the water heats, Weil-McLain says, so a reading that climbs from about 12 cold to below 24 hot is normal. Burnham’s Series 2E ships with a 30 psi relief valve, and Weil-McLain says the ECO Tec’s hot pressure should stay at 24 or below so its own 30 psi relief valve does not weep.

A reading close to 30 means the relief valve is about to open. If it opens or drips, Watts says to call a plumber at once, and Navien says never to plug or remove the valve. Why the pressure drops or climbs: boiler pressure too low or too high.

Steam is different. A steam boiler runs at only a few psi: Peerless says its steam limit is usually set under 5 psi, and Weil-McLain says never above 10. Its relief valve opens at 15, so Weil-McLain says a reading above 15 psi means a gauge or limit control problem.

If it has locked out

Check 6Free, once

Read the display, then reset it once, the maker's way

Gas boilers. Many modern boilers show a code and lock out after failed tries to light. On propane, Weil-McLain’s CGa locks out after 4 failed tries to light the pilot; once the fault is cleared it resets with the power off for more than 2 seconds, or restarts by itself an hour later. Burnham’s K2WT locks out for an hour after six failed tries.

Once, not over and over. Reset a gas boiler once. If it locks out again, call a technician: a lockout is a safety control reporting a fault. Triangle Tube’s Prestige stops taking resets after 5 in 15 minutes and shows a reset limit code. Its Prestige Solo and Excellence boilers made from December 2011 to April 2019, with a serial starting AE, PT, PA, PTE or PEA, were recalled because a delayed ignition can release carbon monoxide. If yours is one of those, do not keep resetting it.

Oil boilers. Weil-McLain says press the burner’s reset button only once, because repeated presses put oil in the combustion chamber. Burnham says if it trips again, call the heating contractor before pressing it again.

Switches that are never just reset. Burnham says that if the flame rollout switch opens, the cause must be found and fixed before the boiler restarts, and the switch is replaced, not reset. A blocked vent switch should only be reset after a professional has fixed the cause. Peerless tells PureFire owners to call a qualified service person if any safety control trips.

The same oil burner controls are covered here: Honeywell R7184 reset and Beckett GeniSys reset.

Some rooms warm, others cold

Air in the system. Weil-McLain’s troubleshooting table lists air in the system as the first cause of radiators or baseboard that do not heat, or are noisy: bleed it through the vents on the radiators or baseboard. Low pressure is its second cause. One radiator at a time: a radiator not heating up.

A zone valve. Honeywell says a normally closed V8043 zone valve can be opened without power by pushing the lever over the stop to MAN. OPEN. Honeywell says its end switch, which turns on the circulator, is not switched on when the valve is opened by hand, so that zone only heats while another zone has the boiler and pump running.

The pump. Taco says never to run a circulator dry, as it can be permanently damaged. When a Taco 007e shows it is stuck, Taco says to switch its power off and on, and it then tries to restart 100 times over about 15 minutes.

Condensing boilers, and steam

The condensate drain. A condensing boiler makes acidic water that drains through a trap. Weil-McLain says a frozen or blocked condensate line can spill water out of the boiler, and Peerless says a float switch stops the PureFire with a blocked drain message. Weil-McLain’s check is to pour water slowly into the top of the drain tee and see it run out of the end of the line.

Flue gas at the drain. Weil-McLain says flue gas at the condensate tubing means the trap has dried out. Triangle Tube’s 2019 manual says to turn the boiler off and call a technician.

Steam boilers. Weil-McLain says the normal water line is halfway up the gauge glass. Burnham says to call a qualified service agency if the water is bouncing or not visible while the boiler is making steam, and water is never added to a hot boiler. It also says a float type low water cutoff is blown down weekly to flush sediment, following the card on the boiler, and that water comes out scalding.

What each one costs

The fix	Cost
Thermostat, switch, breaker, a vent cleared	Nothing
Bleeding the radiators or baseboard	Nothing, a radiator key
Yearly inspection the makers ask for	A service visit, every heating season

Questions people ask

What should my boiler pressure be?

About 12 psi cold on a home hot water system, Weil-McLain and Navien say, rising as it heats. Weil-McLain, Lochinvar and Triangle Tube say not above 24 or 25 psi hot. A steam boiler runs at only a few psi.

My boiler has no heat but hot water works.

On a combi, check no tap is running, because Navien and Lochinvar say it heats the tap water first. Then check the thermostat and zones are calling for heat.

The reset button keeps tripping on my oil boiler.

Stop pressing it. Weil-McLain says repeated presses put oil in the combustion chamber, and Burnham says to call the heating contractor if it trips again.

How often should a boiler be serviced?

Every year. Weil-McLain and Lochinvar require a qualified technician to inspect it at the start of each heating season, and Navien every year, preferably then.

Do I need a carbon monoxide alarm with a boiler?

Yes. CPSC says to put battery or battery backup alarms on every level of the home and outside sleeping areas: what to do when one goes off.

Can pipes freeze if the boiler stops?

Yes. Weil-McLain warns that a power cut or a boiler failure in cold weather can freeze the plumbing within hours: frozen pipes.

Sources: Weil-McLain gas boilers user manual 550-100-226, CGa Series 3, Ultra Series 4, ECO Tec Series 2, EG Series 7 and WGO Series 4 manuals, U.S. Boiler Company (Burnham) cast iron and condensing user manuals, Series 2E, K2WT, V8H, MPO-IQ, Alpine and Independence instructions and its flood damaged boilers statement, Navien NCB-E user and installation manuals, Lochinvar Knight user manual, Peerless PureFire and 63/64 manuals, Triangle Tube Prestige user manuals (2011 and 2019) and 2025 recall notice, Watts 174A relief valve tag, Taco 007e and Honeywell V8043 instructions, and CPSC "Carbon Monoxide Questions and Answers"