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Why would this happen

working together against you.

1. The radiator- might that be cast iron? Or aluminum fins on copper tubes?

2. The fan coil unit- probably aluminum fins on copper tubes?

3. Is the building one-pipe steam (one pipe in and out of the radiators?) or two-pipe steam (one pipe in and the other out via a trap/device?)

What I am thinking is multi-pronged and all may not be inter-related:

One is that both heaters have different masses and condensing rates. The condensing rate of the fan coil unit is obviously faster when the fan is running than when it is not. Your fan coil sounds like a more recent add-on to the system.

Steam systems tend to be older hence they are less likely to use fan coils and more likely to use cast iron. Being a probable add-on and the building being indirectly controlled (whole building average versus individual apartments). I am supposing that at some level the apartment building as a whole has an imbalance with emitters. Some fans running and condensing but not all at the same time. This imbalance will cause complaints. The steam system may cycle -probably does- and not be always available at all times to each radiator or fan coil unit. The lower mass emitters (fan coils) cool off faster than their cast iron counterparts, so you feel colder in a fan coil room. The superintendent finds that heat complaints are less when steam is available more often, so he cranks up the pressure to keep the boiler running longer.

You can heat most buildings with much less pressure than you realize. Often ounces not pounds. You want to run the steam pressure as low as possible. More than that wastes fuel but also may cause water hammer especially when it encounters water pockets in the system.

The other aspect is that when the fan runs, the condensing rate goes up by a factor of several times. The sudden surge of condensate being hit with new incoming steam is one part, the other is the sudden collapse of the steam to condensate both of which may cause water hammer. That it coincides with higher steam pressures is not surprising.

Any of the above factors sound familiar or is there something else? My view from here.... and

My $0.02



  • Why would this happen

    In my apartment the heating units consist of a radiator and a fan unit/AC unit underneath it. When the fan is turned on I get water hammer. When the fan is turned off, no water hammer but limited heat in the apartment. This seems to happen when the building engineer turns up the steam pressure in the building. At other times when the fan is turned on there is not water hammer.

  • Thanks Brad.

    Great explanation. Heating unit is a aluminum fin/copper tubing system. My building,30 stories tall, is supplied by Con Ed steam. The building uses two pipes; steam trap at each radiator as well as a vacuum system. I appreciate your comment about steam systems cycling. That explains why the water hammer happens in a cycle. Building engineer suggested the heating unit might not be pitched correctly and hence the water hammer. Steam trap discs were replaced in October. If that doesn't work then I'll have to hire a pro.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    You are quite welcome...

    I take it you are an "owner" rather than renter hence you taking on the expense yourself. Some items on vacuum and others not? There are pro's abounding in the NYC area from the Find a Professional feature on this site.

    I do have to mention that if you have an issue, chances are others do too. In other words, the solution might be building-wide, not just your unit.

    It may be as simple as unit pitch, you are correct. Typically if a steam coil unit, if they are installed level, the pitch is internal to them. Check that yourself first.

    If Con-Ed steam the cycling may or may not be happening. Boilers (in-house steam generation) cycle as a function of their operation. Utility steam can be made to maintain a constant pressure at a given point in the system all the time, and let local control do it's thing. It can also cycle just as a boiler would by modulating a valve at the building entry. Many things to consider, many of which might be factors.

    Good luck!

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