hillell - There was a great discussion about steam heat systems here about a year back, i think Kframe was the steam heat guru.

There is also a website called heatinghelp.com which has a whole bunhc of stuff on steam heat as well.

Venting is a big issue with steam heat and banging -- I changed the main vent on my system and learned how to use the vents at the radiator and the huge amount of banging went down considerably.

What about the renovation could've cause this? Sounds like there is air in the radiator. Can you bleed the line? Open each one up until it doesn't spit anymore, that should take care of air.

I grew up in a 1907 house. My dad had to bleed every radiator every year due to the air buildup. There was one that always knocked regardless, but it worked for all the others. Sounds like that's all you need to do.

OK, first off, we don't know what kind of heating system this is.

Obviously something with a boiler, since there are radiators, but the two systems that use boilers and radiators -- steam and hot water -- are VERY different.

And, in fact, there's more than one type of hot water system, and MANY types of steam systems.

However, judging by the comment "water collecting in a low spot," it sounds as if it is a steam heating system.

Hillell, can you describe your system? Be as specific as you possibly can.

The first question is, obviously, steam or hot water? If hot water, gravity hot water or forced hot water?

How many floors?

How many radiators? How many pipes on the radiators (1 or 2?)

What kind of renovations (major structural or minor face lifting)?

Where any radiators removed during renovation?

Were any structural members in the basement moved in proximity to the pipes?

Did you have someone out to inspect/adjust the boiler before you fired it off?

Those questions will get us started on possibly figuring out what the problem is.

You know, it would be a good idea regardless to spend the money (shouldn't be a huge amount) to have someone come in and do a maintenance check on your system. How long has it been since the boiler, etc. was looked at? First thing we do when we rehab houses is to do a heating system check. Then we try to encourage homeowners to have it down every year or so. It'd answer a lot of questions, offer you some peace of mind, and potentially safe you some $$$

Here's another vote for a maintenance check. We have our boiler serviced annually and I think it makes a big difference in how efficiently the system runs.

As for the knocking radiators, my first thought is that, as someone else said, you have air trapped inside of the radiator(s). Have the offending units bled, or do it yourself.

I assumed Steam as well because of the same water in low spots comment. With steam, its a whole differnt ball of wax than water.