Knocking sound in oil boiler

cloudy_christineMarch 8, 2008

Whenever the heat is on, there's an intermittent dull knock.

It started a couple of weeks ago. This week we called the oil company to arrange a service call, and the noise stopped, so we told them not to come. Of course it started up again that night!

Searching on the web, I found a sound file that pretty much duplicates the noise, except for the pitch. The (British) site calls it a "knocking, kettling noise" and says it is due to lime and sledge deposits. They sell products to remove the deposits.

I am reassured that it does not sound like the "pipes being hit with a hammer" sound that is dangerous. The sound is not as loud as the ordinary noise of the boiler, which is awful; sitting in our living room you might think you are in a rocket ship that's about to take off. That's how it always is.

The temperature of the boiler is where it should be, and we are of course keeping an eye on it.

If we need a new boiler, we'd been thinking it might be better to switch to a gas system, perhaps with a tankless hot water heater. In fact, the last guy who cleaned the burner advised us to switch to gas! But after reading on this forum, I realize we'd probably spend more in new gas piping, venting, etc., than we are likely to recover. (We have gas now, but only for cooking.) I also learned that a summer-winter hookup that uses the oil boiler to heat hot water is not necessarily a bad thing. Almost every contractor who ever worked in the house said we should get a water heater instead.

Here's the big problem: I have very little confidence in the people who come here to work on this stuff. Too many mistakes have been made. That applies to the plumbing companies and to the heating people. Who should I call for the correct diagnosis?

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What is the boiler water temperature when this knocking is happening? Your boiler could be low on water and the water is actually boiling or starting to turn to steam.

    Bookmark   March 8, 2008 at 3:05PM
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The knocking starts as soon as the boiler turns on.
It's set for a range of 170° to 180° but it can get as hot as 190°. (My husband says the boiler has always overshot its range, both up and down. Service people seem okay with this, I guess.) We've been watching it, and unless the gauge is wrong, it's not getting anywhere near boiling.

I forgot to ask: if the problem is lime buildup, would it make sense that this noise started pretty suddenly? Going from no knocking to constant knocking in a couple of weeks?

One more thing: the thermostat doesn't shut off the heat when it should. If it's set at 70°, the room may get to 74° (as measured by the thermometer on the thermostat). That made me worry that the boiler might be too hot, but it doesn't seem to be. Still, when two things change at about the same time, I wonder if they are related.

    Bookmark   March 8, 2008 at 3:53PM
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Is it a good idea to try adding water to the boiler?

    Bookmark   March 8, 2008 at 4:30PM
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You made a mistake when you cancelled the service call. He probably could have made the system do it again. I'd suggest you call them back and explain its an intermittant problem. And no don't add water to a hot boiler because if by chance the boiler water is really low you can crack the boiler. That would certainly take care of the noise issue though as they replace the boiler.

    Bookmark   March 8, 2008 at 5:46PM
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I really don't know why it would start knocking out of nowhere. Maybe the circulator is developing a bad bearing. Usually when the boiler gets low on water the first thing to happen is that you won't have domestic hot water. Then maybe it won't circulate anymore and your house will get cold.

When they start making steam you can hear the tea kettle sound just before it boils.

    Bookmark   March 8, 2008 at 6:51PM
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Here is my "possible guess": The auto fill system has had to add some replacement water (with air), recently .. maybe more than usual. There could be air trapped in the system, either due to the "quantity" of water added or due to a faulty auto bleed valve in the system. ?? The boiler shouldn't really knock, at any time, unless it has air in the system.

    Bookmark   March 10, 2008 at 1:48PM
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