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Running an old gravity hot water setup?

griffithg Member Posts: 2
Hi all,
Recently bought a house in central IL built in 1910 with a gravity hot water system. First time running something like this and ended up with a few questions.

The boiler is original, Giblin and Co No 24 from the markings. Converted to gas at some point after the 50's, the original attic tank was decommissioned and it currently has a 18 gal compression tank in the basement ceiling. Manual fill with an strap on aquastat and a pressure relief. That's set at 180F.

17 rads over 2 floors about 15' or so from low to high.

Had the folks out that have been maintaining it for a few decades out a couple weeks ago. They gave it a once over and said it was good to go and that was that.

Started to get a cold spell over the weekend so fired it up. Didn't have any heat on the second floor. Tried to bleed them and got only a little air out and noticed the compression tank seemed heavy so I drained that out, brought the pressure back to 12 psi and was able to bleed the second floor. So now I've got nice heat from all the radiators.

I've got 11-12psi cold, and see 19-22 psi when up to temp. Seems like that's about right? Seems pretty close to the 30psi OPV limit.

Should there be some kind of LWCO on this or is that not an issue with this style of setup?

Anything I should be doing if I need to go away for a couple days?

Are there any "smart" thermostats that work well with these systems? Just looking to be able to keep an eye on it if I'm out. I know the learning stuff in Nest type devices is not a good choice here.



  • jumper
    jumper Member Posts: 2,307
    Gravity works better with expansion tank above system than in boiler room. Pressure increase means your using heat to produce work to compress air.
  • neilc
    neilc Member Posts: 2,712
    i would question whether you only bled pressure from the compression tank,
    did you re establish the air cushion by introducing air to the tank while draining,
    it could still be water logged,
    known to beat dead horses
  • griffithg
    griffithg Member Posts: 2
    neilc said:

    i would question whether you only bled pressure from the compression tank,
    did you re establish the air cushion by introducing air to the tank while draining,
    it could still be water logged,

    I believe it was the latter. I closed the isolation valve and drained roughly 15gal (3 5gal buckets) out.
    Air was going in between, for lack of a better word, glugs of water much like an old water cooler.
    Some water did travel back when I opened the isolation valve.
    Tank has an ATF-12 airtrol on it.
    EBEBRATT-Ed Member Posts: 15,767
    Sounds like you are ok. It should have a low water cutoff. They were not used on hot water back in the day but are required now in most areas. Your pressures sound fine
  • mattmia2
    mattmia2 Member Posts: 9,850
    Keep in mind this system has a large volume of water so it expands more than a modern pumped system would so unless it has an unusually large compression tank the pressure change between hot and cold will be a bit more than you're used to seeing.
  • leonz
    leonz Member Posts: 1,168
    edited October 2022
    If it is a gravity system it should have an open to air expansion tank in the attic with a vent through the roof or a basement drain pipe dropping into a laundry sink or a floor drain.

    A bottom fed gravity hot water system requires that the radiators be bled if and only if they were drained as a full intact system would have no air in it.

    A top fed gravity hot water system requires no bleeding as its fed from the top and if is drained it will simply be refilled and the air in the system rises to the top and out with no need to bleed the radiators.

    It sounds like the system was butchered sometime in the past and needs to be fixed the right way .

    I hope you have time to send more pictures of the system and especially the plumbing in the attic as something is definitely amiss.