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Why Is There a Bang When My Furnace Turns On or Off?


Boilers are among the most robust systems out there for home heating. But as durable and dependable as they are, boilers sometimes act up. If you hear a bang when your boiler turns on or off, this can be very disconcerting. Learn what causes this noise and why it’s important to diagnose and repair your boiler rather than ignoring the problem.

Improper Radiator Slope

A one-pipe steam radiator heated by a boiler should be angled so the low end is on the same side as the steam pipe entering the radiator. This ensures that the condensate left behind when the steam cools down can flow back to the boiler. A radiator that slopes away from the steam pipe may trap water inside, causing a hammering sound once the steam builds up enough pressure to break through this blockage.

A simple fix could be to “shim up” one side of the radiator with washers or coins so it slopes slightly toward the steam pipe. If you’re not familiar with how to do this safely, or you add shims and it doesn’t seem to help, call Ehlers Heating & Air Conditioning for boiler repair services.

Radiator Valve Problems

If you recently closed a steam radiator valve partway to adjust the heat output, condensate may become trapped in the valve body. That’s why we recommend regulating the radiator’s temperature by using adjustable steam radiator vents rather than by partially closing the steam valve.

Try opening the valve completely to see if the banging sound stops. If the valve is already open all the way but you still hear banging noises, the valve may be too small or broken, or the valve seat may be worn out. Call a heating technician to repair or replace the valve with a new one so the banging stops.

Pipe Expansion

If the bang you hear at the start and end of a boiler cycle seems to come from behind the walls, this is most likely the result of pipes expanding and contracting in response to temperature changes. When hot water pipes pass through the walls and floors, they may bang against these narrow openings. This is completely natural and no cause for alarm. Still, the sound may be annoying and one you wish to eliminate.

The best way to minimize banging caused by expansion and contraction is to leave plenty of room around the pipes for them to move slightly. Also, make sure the pipes are mounted properly and feature a correctly sized zone valve.

Sludge in the Boiler

When banging noises come from the boiler itself, this indicates that the equipment has not been properly maintained. Sludge has most likely started to accumulate in the boiler or in the low-water cutoff switch. This critical safety control could kick in and lead to a total loss of heat if you don’t act soon enough.

To eliminate the banging, open the drain valve and empty hot, dirty water into a bucket. Repeat this process once a week to prevent sludge from building up again. If you aren’t familiar with flushing a steam boiler, ask a service technician to show you how to do this safely.

Contact Ehlers for Boiler Repair in Livonia, MI and the Surrounding Area

A banging sound when your boiler turns on or off may be completely harmless—or it could mean your heater is in desperate need of maintenance. To know for sure, you need a knowledgeable technician to diagnose the problem.

If your boiler makes alarming banging sounds, contact Ehlers at (248) 579-2362 to schedule a much-needed repair. Once we determine what’s wrong, we can perform the proper fix.

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Starting with our estimate with Eric to our service technicians Dave and Rob. They are ultimate professionals. We had a new Carrier furnace and AC unit installed and we’re very pleased with the work. We can tell that they take pride in their work down to the smallest detail. They were on time, courteous, know... -Horace