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Water Hammer

Hi Mark,

1) The pressure control is not as accurate as a vaporstat, different kind of switch. You can and should operate at the lower pressure anyway, so have the contractor install a vaporstat.

2) The main vent is very important and needs to be operating properly, as well as sized properly. Sometimes two vents have to be used because the capacity of some newer vents is less than the old ones. Have it replaced, oversize the vent for better operation.

3) The purpose of the differential loop is to get the water back into the boiler as steam pressure rises in the boiler. Leave it alone, there are no moving parts inside and once you install the vaporstat, it will control the pressure so that the differntial loop won't come into play.

When water hammer occurs in the middle of the heating cycle, chances are that the returns need to be checked. Water hammer is not normal and is an indication that there are problems. Your contractor is not right about this. Check the "Find a Professional" feature at the top left for someone with more knowledge of steam systems.

4) The radiator traps are as old as the system, and were part of the Hoffman system. You can read about it in the "Library" feature. They are thermostatic traps, not buckets, and should be checked for operation. They keep steam in the radiator and out of the returns, where it can cause water hammer. So have them checked and repaired when necessary. The contractor should be able to purchase repair kits at the local supply house, no need to take the body out.

Best regards, Pat


  • Mark Riley
    Mark Riley Member Posts: 3
    Water Hammer & Hoffman Differential Loop & Pressuretrol

    I am a new homeowner of a home with a two-pipe steam system with a 2yr old boiler. I have found a good steam heat service provider but have a few problems/questions with the system that I want to get a second opinion on.

    1. Can't keep the pressure from going up to 3-3.5 psi, even with the differential pressuretrol set to shut off at 1.5psi. The suggested solution is to install a vaporstat with a backup as they are more accurate. Is this a reasonable solution?

    2. The main vent looks to be at least 40 years old. It hisses, and sputters and lets out air periodically throughout the heating cycle. Should it be replaced?

    3. About one-third to one-half of the way through the heating cycle, we get a pretty loud water hammer/rattling sound for about 1-2 minutes (an effective alarm clock at 1am); the source seems to be the Hoffman Differential Loop in the boiler room and it happens at the same time I hear air hissing out of the main vent just above it. What is the purpose of this differential loop? Should we replace it? Clean it? Clean out the returns? My service provider thinks this might be "normal" but he isn't awakened at night by it!

    4. We have a little bit of water hammer at the end of the cycle near a couple of radiators. We suspect the traps as the culprits. The steam traps are "HSC" bucket traps (I think). These traps get as hot as the supply pipe at the end of the cycle. Is this normal? The servicer thinks they are at least 25 years old, probably much older and they could be stuck in the open position. Does it make sense to figure out which ones are bad (service provider suggestion), or should we just 'bite the bullet' and repair/replace them all? And which? Repair or replace?


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