Knocking sound in furnace - http://www.hvacmechanic.com/ Forums

Original message

bgawlik

71.169.143.120

"Knocking sound in furnace" , posted Thu 13 Dec 21:14

I have a strange problem that has cropped up this year. When my gas hot water furnace runs I can hear a knocking sound when the boiler is on. The sound even transmits itself through the baseboard heaters that are close to the furnace itself. As soon as the boiler is off the sound stops. (Even when the circulator is still pumping hot water). I have made sure that the air traps have all been purged and there is no leaking water at all.

Replies:

smythers1968

66.232.220.95

"Re(1):Knocking sound in furnace" , posted Thu 13 Dec 22:00

What brand boiler is this?

On the boiler there should be a gage that shows water temp and pressure. What is the temp and pressure when this is happening?

Life lesson #12 Know when not to do it yourself.
WOW That Can Hurt!!

static

74.140.76.11

"Re(2):Knocking sound in furnace" , posted Thu 13 Dec 22:45

It's them dang old water hammer elves again

tom446

66.66.203.58

"Re(3):Knocking sound in furnace" , posted Fri 14 Dec 09:51

My first thoughts would be low on water or tempurature too high. What are the tempurture and pressure gauges reading?

If in doubt call a pro !

Freon

72.155.49.29

"Re(4):Knocking sound in furnace" , posted Fri 14 Dec 19:32

See if you can't find those pesky water hammer elves by checking the expansion tank. It may only need air as an exorcism remedy.

bgawlik

71.169.143.120

"Re(5):Knocking sound in furnace" , posted Fri 14 Dec 20:42

Wow, thank you all for the great suggestions!

In answer to some questions here are some particulars...

Slant/Fin Prodigy wall mount propane boiler

Running at 180 degrees and 12 lbs water pressure.

Sound is like a tea kettle that is boiling hard. The knocking sound comes from the boiler itself, not the fans or pumps and at times is loud enough to hear through the baseboard pipes that are close to the boiler.

bgawlik

71.169.143.120

"Re(6):Knocking sound in furnace" , posted Sat 15 Dec 16:49

Many thanks to all of you for guiding me to the expansion tank. It was low on pressure. A few minutes of attention and the knocking went away. Great call!