Hydronic Baseboard Heating: terrible knocking but no air issues (AR&SS Radiantrim)

GPatterson

Sorry if the main wall isn't the best place to post this, but there was no hydronic heating category.

I moved into a 60s house a couple years ago and am having terrible trouble with noise on my baseboard water heaters. At night, one of the bedrooms on the top floor knocks something terrible. The knocking occurs maybe every 5 minutes or so when the boiler is on and it knocks every 1-2 seconds for approximately 30 seconds, slowly tapering off until it stops.

I bleed the suckers all the time. I bleed them when they're cold, I bleed them when they're hot, I bleed them when they're knocking. Never do I get even a fart of air coming out. I've had people come in to look at it, but they havent even the foggiest of ideas and the best they can recommend is tearing the wall out and investigating.

I believe they are AR&SS Radiantrim heaters, but I can't 100% confirm. The bones of an AR&SS are in the basement, but I can't be certain that that's the same design being used in the house right now. As such, below is a picture of a current heater.


What else could be causing this knocking if there is no air in there? Is it possible there is air in there and I just can't get it out? When I do bleed it, I do so for about 1-2 minutes. Maybe that's not long enough. I drained the entire house last winter to replace my boiler. Obviously that took a lot of bleeding over weeks to get rid of all that air, but the knocking still continues and only in one of 3 upstairs bedrooms. I wouldnt be surprised if it's the highest bedroom though.

GPatterson

Also, I want to describe the knocking a little bit more. It sounds more like someone is knocking on wood. Not especially loud (3yo sleeps like 2' away from the heater). I can't pinpoint where the knocking is coming from. I try to do a little sound testing, but it's so decentralized that I can't lock it down. It might not even be from the heater board at all. There are also occasional one-off louder noises that seem of an entirely different origin, but I'm less worried about those.

This only happens at night and that's probably just because we havent had terribly cold days in Pittsburgh in the last couple winters that get the heater working so hard.

Jamie Hall

It may be expansion of the baseboard heater when the heat comes on. If it tapers off and stops after the heat has been on for a while, that is almost certainly what it is. Chek to see what pipes might be rubbing on the floor boards, or for places where the heater can't freely expand..

bburd

This is almost certainly expansion noise. If not from the baseboard itself, then from the heating pipes moving against the framing, flooring or other parts of the house. If you can find where it's happening, try slipping some cut sections of plastic milk jug in between.

you can stop bleeding, it's not an air problem.

hot_rod

Adding an outdoor reset control can also help by lowering water temperature and keeping the system running with less on/ off cycles.

Oversized boilers, which were common in that era also add to short cycling.