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Hot water gravity -- differing opinions

Jim_65 Member Posts: 184
If the piping branches serving the toe kicks are smaller in size than the larger main pipe size you may not be able to receive the full benefit due to the water traveling the path of least resistance.

We just completed a project where it was an original gravity system that someone took the old cast iron rads out and replaced them with either hot water baseboard or toekicks. Let's just say that they never performed as expected. We replaced the boiler with a high efficiency model and converted over to circulation with separate zoning for the cast rads, hwbb & toekicks.

My $.02


  • Mark Maremont
    Mark Maremont Member Posts: 6
    hot water gravity -- differing opinions

    I have an old oil-fired gravity hot water system in the Boston area. Want to convert to gas ASAP (leaking). Two questions:
    One guy is recommending one of two Burnham boilers -- the Series 2, or the SCG direct-vent model. The latter is more efficient, but I wonder about the safety. Is this a good boiler?

    Second question -- I've read about the primary-secondary piping here. But most plumbers I've contacted say it's not needed in our house, which is about 2000 sq. ft and has about 9 radiators. They say that's for larger houses. Thoughts?
  • Jim Erhardt_3
    Jim Erhardt_3 Member Posts: 80
    The beauty...

    ...of gravity systems is the comfort they can deliver.

    Since the thermostat simply fires the burner, the colder it gets outside the longer the thermostat calls for heat. The longer the thermostat calls for heat, the higher the average system temperature. In other words, gravity systems are "weather repsonsive." It could also be assumed that gravity systems are "variable flow rate" because of the same reason - the greater the difference in temperature between supply and return, the stronger the flow due to the difference in density of supply/return water.

    If I had a system like yours (not overly large with rediculous pick-up losses), and IF it was only one zone, and IF a separate water heater was used, I would be tempted to leave it gravity operation. A nice condensing gas boiler that is not sensitive to flow rate through its heat exchanger would be a good candidate. Good old fashioned gravity comfort with good efficiency (be sure to insulate the distribution piping).

    Sorry for the ramble and not (directly) answering your question...
  • Plumb Bob
    Plumb Bob Member Posts: 97

    P/S has nothing to do with the size of the house; you should not hire the person who told you that. It is needed when there are multiple zones and a high-head boiler. The boilers you are looking at are, I believe, low head, and if so, P/S gives you no particular benefit. But I don't have the full picture.
  • Mark Maremont
    Mark Maremont Member Posts: 6

    Thanks much for the help. There will be two zones -- one small one for a kitchen, the other one for the rest of the house.

    The gravity system works OK, but the rooms at the farthest corner of the top floor are a bit colder. Also, we need a separate circulator-valve zone for the toe-kick heaters in the kitchen. They don't work properly on the old hot-water system.

    Thoughts about the direct-vent gas boilers, vs the less-efficient one that vents up the chimney? I'm a bit nervous about the Burnham that vents out the side, because of carbon monoxide.

  • scrook_2
    scrook_2 Member Posts: 610

    Code requirements will limit how close to windows, doors, decks, the ground/max expected snowcover, etc. the vent and air intake can be, and a properly installed, adjusted and regularly serviced gas boiler should not generate CO, regardless of how it's vented. The installer can make or break a boiler, so try to get a good installer.

    See also: Resources, Find a Professional, at the top of the page here
  • Jim Erhardt_3
    Jim Erhardt_3 Member Posts: 80
    I agree...

    If you are now using any high(er) pressure drop radiation (such as toe kick heaters), pumping is the way to go. And as Jim stated, removing a radiator and replacing it with a stretch of baseboard on a one or two-pipe system usually doesn't work out well either.
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