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Gravity system

hydronx_3 Member Posts: 35
I came across this old gravity system today.  Does anyone know where the pipe going off the reservoir tank through the ceiling would typically go?  Does it just vent through the roof if the system were to get overfilled?  Another contractor had condemned the system saying that it had no pressure relief valve down at the boiler, but my thinking is that maybe these old systems are vented to the atmosphere?

I would ask the guys that installed it,.. but they are probably dead.


  • Jamie Hall
    Jamie Hall Member Posts: 18,924
    I was going to speculate...

    but I see what looks for all the world like a sight gauge on the side of that tank, so I won't.

    But two three thoughts come to mind.  First, to make a gravity system work, the whole thing -- except for the expansion tank -- had to be full of water.  Second, it was very common indeed to vent an expansion tank to the atmosphere, and the tank was commonly in the attic.  Has anyone checked the attic?  Is there a tank up there, in addition to the one that you see?

    Third, and perhaps most important -- there is no reason not to put a PRV on the piping in the basement somewhere, if you are doing some repiping anyway, if the AHJ has his knickers in a twist about no PRV.  They aren't that expensive, so just the fact that you know and we know that you don't need it, but he doesn't, may simply mean that you have to grit your teeth and put one in...  That is, assuming that you have checked and there is, indeed, an open vent somewhere up beyond that pipe...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • moneypitfeeder
    moneypitfeeder Member Posts: 248
    my guess is yes


    Scroll about halfway down the page it shows typically the expansion tank would then vent thru the roof, is there an attic you can gain access to to look?
    steam newbie
  • hydronx_3
    hydronx_3 Member Posts: 35
    attic access

    Thanks for the link to the article it was very helpful.

    I couldn't get into the attic, there is a pipe coming out of the roof pretty close to where it should be so that is probably it unless it is a plumbing vent, I didn't have my extension ladder.

    Does anybody have any experience with the fuel savings they could expect to convert this to forced circulation with a modcon boiler?
  • kcopp
    kcopp Member Posts: 3,990
    it is open to ....

    atmosphere.... No need for a PRV. Some would drain into the cast iron plumbing stack too. Others Had no expansion tank and only filled the rads 3/4 full. On the conservative side I would venture to guess 35% savings.... maybe 50%.
  • icesailor
    icesailor Member Posts: 7,265
    Gravity System:

    To me, it looks like a run of the mill, old gravity system. Completely intact and not ruined by any changes.

    So, In MY opinion, and it would be fine for someone to dispute my thoughts, I'd put an outdoor reset control on the beast and let that controller set the boiler temperature and it would run like it was designed to. Isn't that what you try to do with outdoor reset? Mimic what a gravity system did? All the old ones I ever worked on only got as hot as needed and when the thermostat was satisfied, it shut down.

    As far as the AHJ guy, I have a POV. Those that can, do. And those that can't, become inspectors. To harass those that can, do, and know what they are doing. They also like to make up rules that only they know and they put them on us. Not ALL of them, but enough of them to make my life uncomfortable.

    If there are regulations saying you can't have open gravity systems, I haven't heard of them. Massachusetts would have them if they were around. I'm teachable. Show me the regs.
  • bill_105
    bill_105 Member Posts: 429
    How poetic

    I don't know anything about the guy's heating thing.

    But, Mr. sailor's last two paragraphs belongs in a poetry contest. Shakespearean at its best!

    I love the shot at MA. They deserve it.
  • icesailor
    icesailor Member Posts: 7,265
    MA Gravity:

    You missed what I said. I'm from Massachusetts. I consider Massachusetts regs the best around. Massachusetts had the first water heater safety regulations in the country. Because of blowing up water heaters with uncontrolled side arm gas heaters. We have a uniform plumbing code. The code book where I work is the same one that Charlie Form W Mass uses. And if an inspector decides they "Like to see it this way" and it is contrary to the code, the board will set them straight. As far as plumbing, Massachusetts probably has the best trained technicians in the country because of our requirement of 6 hours of CEU on code compliance on gas and plumbing. Journeyman Plumbers have had 300 hours of applied schooling in plumbing theory during the three years they were apprentices  and Master Plumbers have an additional 100 hours. The pass rate is extremely high leading to a very professional group of individuals. We're not talking about handyman plumbers and heaters. We're talking really trained and smart installers. A whole lot better than when I started out almost 50 years ago.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,216
    Ice I almost agree

    Just way out west I still see lots of handymen working on the heat. They even use a washing machine hose to fill systems so they do not need a plumber to attach the feed water. I do not want to pay for another license but I really would like one for hydronic installations. I mean I have one for tin knocking but nothing for steam or hot water? As I travel looking at systems I am glad my father landed in MA since we are plumbers. I think the regulations here have a pretty good balance to them as a whole.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • scott markle_2
    scott markle_2 Member Posts: 611
    head pressure

    If I understand head pressure correctly you will need 30ft of elevation from the boiler to the top of your open system to achieve 12.9 psi at the boiler. 30X.434=12.9

    From my experience testing drain lines by filling them with water I would have thought this would be less, somebody let me know if my understanding of static head is flawed.

    With this in mind be aware that some boilers a not very tolerant of lower operating pressures. The sensors "trip" and cause a shutdown. The budarus GB142 comes to mind at requiring more than 12-13 psi to operate. I also believe that TT recommends a few psi above this standard pressure to ensure there are no nuisance shutdowns. Viesmmans are more tolerant, my own boiler which is on the second floor of my house operates at 10 and I'v seen it go to around 6-7 before shutting down. Looks like the new Vito's are using a flow switch as opposed to a pressure sensor so this might be a reason to look at these boilers if your attached to the idea of keeping things in the 19'th century.

    Why not loose the old expansion tank and put in a pressure relief valve and diaphragm tank? It's remarkable that an open system like this can last as long as it has without rusting out. Since this is and open system and there is likely to be lots of rust floating around I'd recomend a good flush with Romar or Fernox and filling with a inhibitor, Also consider a dirt separator, spirotherm makes a very nice combo air dirt and hydraulic separation device, although I suspect the system will take so little energy to pump that you could pipe it direct (no primary secondary).

    Also keep in mind that gravity systems when converted to forced circulation can become imbalanced, TRV's are a great way to deal with this. The old vito's also incorporated pump logic that would slow circulation when loads were down, I don't know of a way to do ODR pump regulation but I notice that there are coding addresses for it on the new vitos even though the built-in pump that could do this is no longer part of the package.
  • icesailor
    icesailor Member Posts: 7,265
    Gravity Systems:

    Some gauges have the pressure also marked in feet of head. The true old gravity systems had not a pressure gauge but an "Altitude" gauge. Altitude in feet. Altitude of the water level above the gauge. These were "open systems". With pressureized systems, the PSI pressure is only equal to the altitude on the gauge.

    Or as I learned, 1# PSIG supports a column of water 2.31' high or if 10# PSIG, the top of the water would be 23.1 feet high.

    As far as that tank, the boiler ssystem water goes in the bottom. It is on the second floor. It looks to be located in  bathroom soit is easy to lookat the fill line in the gauge glass. The vent comes out the top and may go out the roof. The lateral pipe connected above the tank is the overflow that probably goes down to a place near where directly below it is the boiler where the overvlow pipe ends. So, someone that is not paying attention when adding water to the system, will suddenly, hear a rush of water from the overfilled expansion tank.

    It gets your attention.  
  • scott markle_2
    scott markle_2 Member Posts: 611
    head conversion

    So head times 2.31 equals psi and psi times .434 equals head.

    Any compelling reason to keep an open system open?
  • hydronx_3
    hydronx_3 Member Posts: 35
    altitude gauge

    You are right on with the gauge here is a picture.

    The gauge glass has been replaced with vinyl tubing and you can't see through the crud built up on it.  The overflow pipe runs across the wall and down near the bathroom floor with boiler drain on it.  I checked that and the pipe was empty.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    No Relief Needed

    I dont belive that this system would require a relief on the existing boiler. Ill bet that the man who designed this system took into account the vertical stacking height, and that any increase in pressure beyond the design pressure, would be spilled either out the roof, or into a plumbing stack. An open system.  As long as the "altitude" stayed within its "designed" limits, everything would be fine. My bet is that even if you installed a relief on it, it would be there only for show, because providing that no changes were made in the stack, that would end up as the relief anyway. These old timers were pretty sharp. The boiler was probably hand filled, hence the sight glass in the tank, as were a lot of other boilers from that era. The system should run like a clock as long as nothing is changed, which leads me to my second point. Effiency. Even though this may work great, and look real cool,  there is litterally no efficiency to this system. Put in a good cast iron cold start boiler, such as a Biasi, do away with the tank, run the system as a closed loop. Then, install a bypass between supply & return, using a thermic valve with the pump on the boiler side of it. Then, use the new Honeywell AQ251 boiler control w/outdoor reset, and let the system use modulated water temp.
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