Boiler repair: what to know

What the job involves, what changes the price, and what to ask a pro.

What boiler repair is

Boiler repair diagnoses a boiler that has no heat, leaks, makes noise or short cycles, then replaces the failed parts and removes air from the system. Hot water boilers heat the house through radiators, baseboards or in-floor loops, so one failed part can leave the whole house cold.

Signs you need it

  • The boiler runs but the radiators stay cold, or only the upstairs or one zone gets warm.
  • The pressure gauge reads below its normal range, or the pressure falls back down after you add water.
  • The boiler gurgles, knocks or bangs, or the radiators make banging sounds as they heat.
  • Water drips from the pressure relief valve, the circulator pump or the base of the boiler.
  • The boiler shuts off and restarts every few minutes, or the burner does not ignite.
  • A carbon monoxide alarm goes off, or you smell gas. Leave the house and call the gas utility from outside.

What the pro does

  • Read the boiler's fault code and check the aquastat, the high limit and the low water cutoff.
  • Check the circulator pump for power and movement, and test the zone valves or zone relays that send heat to each area.
  • Test the igniter and flame sensor on a gas boiler, and measure the gas valve signal and the combustion readings at the flue.
  • Check the expansion tank and the pressure relief valve, and replace the valve if it weeps or fails the test.
  • Bleed air from the system at the boiler and the high points of the radiators, then bring the pressure back to the set range.
  • Run the boiler through a full heating cycle to confirm each zone heats, the burner shuts off and the pressure holds.

What changes the price

Every job is different, so we do not publish prices. These are the things that move the quote up or down.

  • Which part failed. A sensor or igniter is a smaller job than a circulator pump or a heat exchanger.
  • Boiler type and fuel, such as gas, oil or electric, and whether it is a condensing or non-condensing model.
  • Whether the system has leaks, sludge or a corroded heat exchanger, which can change the repair from a part swap to a bigger fix.
  • Number of zones and whether zone valves or zone relays need replacement.
  • Age and parts availability for the model.
  • Access to the boiler and piping, such as a tight basement or a mechanical closet.
  • Whether the repair needs gas line, venting or electrical fixes before the boiler can run safely.

What a written quote should spell out

  • The diagnosed fault and the reason the part failed.
  • Each replacement part with its model number and warranty.
  • The pressure reading before and after the repair and the air bleeding steps.
  • Combustion readings and the carbon monoxide reading for gas boilers.
  • Any venting, gas or electrical fix needed, listed separately.

Questions to ask before you hire

  • Did you find out why the pressure dropped, or only add water?
  • Will you test the pressure relief valve and the low water cutoff during this visit?
  • Which zones did you test, and did each one heat after the repair?
  • Will you check the expansion tank and tell me if it needs replacing?
  • Do you service my boiler brand, and what parts do you carry on the truck?
  • Will you run a carbon monoxide test on a gas boiler before you leave?
  • Will you give me the pressure and temperature readings in writing?

Red flags

  • The pro adds water over and over without finding the leak or the cause of the pressure loss.
  • The pro bypasses or removes the pressure relief valve or the low water cutoff.
  • The pro says the boiler needs replacing without testing the igniter, the pump and the zone controls.
  • The pro will not show the readings or the failed part.

What you can do yourself

You can watch the pressure gauge, bleed air from a single radiator with its bleed valve when the system is cool, and keep the area around the boiler clear. Do not open the boiler's controls, gas valve or pressure relief valve, and do not add water to a boiler that keeps losing pressure. Pump, valve, igniter and heat exchanger work belong to a licensed technician.

Licenses and permits

Boiler repair is commonly done by licensed heating contractors, and gas or oil work may need a separate trade license. Check with your state or local licensing agency for the requirements that apply to your repair.

Boiler repair questions

Why does my boiler keep losing pressure?

Pressure loss usually points to a leak, a faulty expansion tank, a worn pressure relief valve or a fill valve that does not seal. A technician should find the source, since adding water only hides the problem.

How do I bleed air from radiators?

This applies to hot water systems. Turn the system off and let the radiator cool. Open the bleed valve with a radiator key until water sprays out, then close it. Watch the boiler pressure afterward and call for service if it keeps dropping. Steam radiators use air vents instead, so leave those to a pro.

Why do my radiators bang?

Banging usually means water is trapped or the radiator is not sloped right, or that water is moving through a pipe that is not sized or pitched for it. Ask the technician to check the air and the piping together.

Is it worth repairing an old boiler?

It depends on the failure. A single inexpensive part on a sound boiler is usually worth fixing. Frequent failures, a cracked heat exchanger or parts that are hard to find usually point toward replacement.

About this guide

This national guide is written and checked by infinibell. Rules and conditions differ by place, so local pros are credited on guides for the cities they serve, not on national ones.

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