Radiator Question

glorifiedbFebruary 24, 2009

Hello all,

I recently purchased a 1916 bungalow with original radiator heat. They are the tall cast iron radiators in front of most windows. Here is a photo of one:

I think the system works fine because it heats very well. However, the radiators make a couple of loud bang noises when the heats kicks in or the system starts to cool down. I am guessing that this is due to expansion of the pipes. It's not a major issue and we've gotten used to it. However, is this normal or something that I should get checked out?

Any help would be greatly appreciated. Thanks!

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I think the folks over on the heating & air conditioning forum would be most helpful on this question.
First question would probably be if you have steam or hot water heat. I'm guessing from your post that you have steam heat. In that case, it is either simply pipe expansion, or could also be a sign of what they call 'water hammer' in steam heat systems which is a symptom of a larger problem. It's actually not that hard to fix if you have the proper information.
I have steam heat and have found help on the heating forum here, and at the website linked below.
Hope this helps!

Here is a link that might be useful: HeatingHelp.com

    Bookmark   February 24, 2009 at 2:44PM
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I'd suggest posting or researching the forum at old house web dot com. Radiators have been discussed alot over there.

Here is a link that might be useful: Old House Web

    Bookmark   February 24, 2009 at 3:04PM
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Sounds like it might be a water hammer. Sometimes air in lines can also mimic this sound. Newer systems have arrestors to help eliminate this. But, very old systems sometimes don't. If it is a water hammer, it can not only be annoying, it can actually damage the system because the noise is made from shock to the pipes when flow is abruptly ceased. I think it should not be hard for a plummer to retrofit a system with a solution, nor terribly expensive. You may want to ask a plumber to stop by to identify if it is.

    Bookmark   February 24, 2009 at 6:27PM
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Wow, thank you all for the replies. I guess I should have mentioned that we have hot water heat, not steam heat. I will check the other forums and see what I learn. If it's still not clear, then I might have a plumber stop by and check it out.

Thanks again!

    Bookmark   February 25, 2009 at 12:05AM
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A forced hot water system will expand if heated from a cold condition and the pipe movement will make noises. It might help to be sure the system has been bled of trapped air.

It is interesting to see that the radiator is painted with a metallic paint as if that might improve it's ability to radiate energy. Since it is probably painted with a low solids metallic paint it probably doesn't matter much but the worst possible surface for efficient radiation would be a polished metal and the best surface would be any other kind of paint in any color or finish.

    Bookmark   February 25, 2009 at 1:19PM
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glorifiedb, thanks for posting this question. I am experiencing the same situation in my house recently.

I have hot water heat, large cast iron radiators. I have bled the air from all of them except the one in the kitchen whose bleed valve had been painted over by the PO. The kitchen one heats up though, so I don't think it's an air in the pipes issue. The kitchen radiator isn't one that "bongs".

This is our third winter in this house, and the first winter where I've heard the most banging. The only thing we've changed this year is my husband put pipe insulation around the pipes leading to one radiator (the front hall's radiator). Here's an example of banging: heat goes on at 7am (we have programmable thermostats, two zones). It shuts off at 9am. The radiators do retain heat for a long time. Around 10:30am, the living room radiator (which is about 10 feet long) will give just a single "bong". It always comes from one end of the radiator, at the pipe when it goes down into the floor to the basement.

Another difference is that this is the first winter we've lived here where we've had spans of cold weather for a week at a time. (We're northeast of Boston). The previous winters seemed to have more variability from day to day.

mightyanvil, it sounds logical that the pipes are expanding and contracting going from cold to hot, and vice versa. But would this cause only a single "bong" at one point of the radiator? I can live with it, except I'm worried that it is a precursor to a serious (=expensive) problem developing. Is it something I should get a plumber in to nip in the bud?

Could it possibly from having sediment built up into the pipes from sediment-filled town water as I bleed the air out and new water comes into the system? Our city had a major water main break at the end of 2007 which then sent sediment-filled water through the city.

Should I be draining and flushing the system every year to clear it out?

Thanks in advance!

    Bookmark   February 28, 2009 at 1:22PM
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I would check with a heating company or an old house web site, like this:


Also, radiant heat is fabulous and you would be shocked to know what those radiators (or ones with the same styling) would cost new, so don't switch to forced air. You've got a great heating system.

    Bookmark   February 28, 2009 at 3:12PM
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If a plumber hasn't checked out the system by all means have one do it immediately. A forced hot water systems is usually simple and the cost of tuning it up would be well worth it. Then you have someone you can call in an emergency.

A 10 ft long cast iron radiator is pretty unusual and would expand in length quite a bit. If the feet are constrained it could force the pipe to move and make a noise.

    Bookmark   March 1, 2009 at 7:15AM
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Hi mightyanvil,

Thanks for your response. I got out the measuring tape and the radiator is just shy of 7 feet long, and not 10 like I had thought. (And I realize that I can't trust myself to eyeball lengths like that!)

Thanks for mentioning that part about the feet being constrained. Your comment made me realize that the two radiators that are doing this single "bong" are the ones that have those "radiator cabinets" around them (wooden sides, metal grille fronts). I'll take a look and see if the feet of the radiators are constrained somehow.

The forced hot water system is heating oil based, with oil burner and boiler. Every year we do have the oil company's guy out to do the steps on their annual tune up. What he does all involves the parts in the basement (oil burner, boiler, etc). Do you think we should also have a separate plumber in to assess the system beyond that?

Thanks again for your helpful comments!

    Bookmark   March 3, 2009 at 8:23AM
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From your description, here's what I bet is happening...

As the radiator and pipes cool after the heating cycle, they're being caught on something. Maybe the floor, maybe some aspect of the floor framing, maybe on the wall trim.

As they continue to contract, the area where they are "caught" finally releases and the radiator lets out a sympathetic banging noise. Sort of like plucking the string on a guitar once.

Get down on your hands and knees with a good flashlight and start examining things. First look at the radiator's feet. Do they show any indication of movement across the floor?

Then look at the pipes where they come up through the floor to see if they might be rubbing somewhere.

It could also simply be a case of the expanded metal in the radiator contracting, but normally you'd hear that as a successive series of pings and light knocks like the engine on a car cooling down.

    Bookmark   March 4, 2009 at 2:22PM
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Try removing the covers. They reduce the heat output of the radiators so unless the radiators are oversized you don't need them. I've never seen a cover that looked better than the radiator.

    Bookmark   March 4, 2009 at 2:39PM
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Hi mightanvil, I agree with your comment about the radiator covers reducing the heat output. All of our other radiators are uncovered and painted, and I love the details that show like that. For certain reasons, my spouse wants to keep those two covered. Especially the living room one: it is almost 7 feet long and the thermostat for the downstairs zone is in that room and it's possible that taking off that cover will have the thermostat turn off too quickly.

Hi kframe, thanks for your input! It is true that the sound is not like a car engine cooling down with multiple little knocks. It is usually a single "bong". I guess it is time to get on my hands and knees and examine the floor there.

Thanks again everyone for your input!

    Bookmark   March 7, 2009 at 12:09PM
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