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Gravity system conversion

This is my first gravity Ive done ALOT of reading if anyone has pointers on setup im all ears. had a ATCO coal boiler that was converted to oil decades ago Im guessing orig boiler was 1930s. its a ranch w dual mains and dual returns 2.5" The piping is overhead in the whole house and drops to 2 pipe rads i each room, the biggest room has 2 rads. There are standard Ts no monoflow or anything special the heat was terrible w the old boiler its a rental and tenants complained of huge fuel bills, house is uninsulated and original crap windows.
Anyway modern boiler going in heat loss says around 80k btus needed they want cheap so something around 100k input. I did all the pipe surgery yesterday and had my guys cut the old boiler apart and remove. got bushings and reducers and am ready to pipe 1" at boiler connects. Will be piping with a bypass loop with ball valves to adjust flow and ball vavles on each supply main in case balance is needed one loop is longer more rads. From what Ive seen and read here I want to pump away from boiler on supply I will be using the right circ I read the links on pumps and also the educational section on gravity. My question is really is where to bring in supply water the air scoop xpansion tank and if needed check vavle.
I was going to come up off boiler 2 ft elbow into straight run over 18" for air scoop and between that run do water feed and valve/drain for purging. after scoop/xpansion tank go vertical to circ then check valve then T in for bypass loop then ball valve before splitting into 2 runs to tie the 1"into the mains. On return I'm simply tieing in the mains to 1" and T into the bypass loop w a ball valve into boiler. budget job for extended family member no funds for mixing valves or anything fancy they wanted to use a used boiler its like that lol.


  • GBart
    GBart Member Posts: 746
    Honestly I'd rip all the pipe out of the basement, old large diameter pipe adds to the load especially if it isn't insulated.
  • delta T
    delta T Member Posts: 860
    I recently tore out an old gravity system (rads were fried from house being 'winterized' incorrectly). I did a heat loss on the house, and came up with a substantial heat loss for the basement which had un-insulated concrete walls, no under slab insulation and about 1/2 of the outside wall of the basement was exposed (walk out on the east side). Existing system had no heat downstairs, but all the water lines were exposed and not split and cracked from freezing, so I started to wonder how much heat the old exposed steel lines gave off. I added up the total length of the mains and all the run-outs and calculated the heat loss from the piping using 170 average temp. The heat loss from the pipes was so much that it was actually matching the heating load of the basement fully. To compensate for taking out this heat emitter, the outside walls were framed and insulated, and I still needed 30' of baseboard to heat the basement.

    moral is as Gbart says, that old large diameter piping sucks heat out of your system to a degree which is kind of astounding, the old timers knew this and took advantage of it, but if you assume there will be minimal loss, you are in for some trouble without insulation or elimination of that piping.

    That being said, if you are leaving the existing piping, maybe draw us a diagram of the piping to the boiler and the system that you intend to use and we can give you some pointers. If it were me, I would definitely pipe P/S because you want to be able to optimize the flow on the boiler and keep your efficiency up as much as possible while still being able heat the huge mass of that system up at a flow rate that makes sense for it. It is highly unlikely that those two flow rates (boiler and system) are the same. A bypass loop will help, but it just means that a single flow rate is being divided between the boiler and the system, and you will likely either overpump the boiler, or underpump the system if you just have one pump. P/S will give you a LOT more lee-way to dial in the flow rates correctly.

    Always pump away from the expansion tank, not necessarily the boiler....
  • jimmythegreek
    jimmythegreek Member Posts: 52
    Thanks guys for the responses. Gbart if I could I would. It's a weird shed roof thats kinda flat no attic and no basement the pipes are under a crawl space u cant crawl into so pulling up the floors to recipe and do copper fin isnt an option. There is old asbestos on most of the piping I can see in space below joists and I have taken into account the huge pipes into the heat loss calc. I read on primary secondary piping and I'm weary of using 2 pumps and hoping for the best. I figured a the bypass I could adjust the valves and make sure I'm not condensing the boiler and keep it up above 140/150 coming back in. I saw somewhere on here pics of a guys conversion and the commenta on his piping was to pipe away on supply side and to put a check valve above the circ and to bring water in b4 circ next to expansion tank. Cant find that post now but he had his pump on return like a typical hot water baseboard install. How does the P/S piping work w boiler cycling I would think it would cycle the burner alot compared to a bypass where she would burn longer to make temp in whole system before starting to cycle on high limit. I was gonna make restrictor plates if I needed to to balance roads there are valves on the supplies the piping is all exposed on supply side up high it drops into the rads from above
  • Gordy
    Gordy Member Posts: 9,514
    If the pipe is insulated that makes a nice built in buffer.. would have been nice to just home run the rads with pex to a manifold though. Sounds like that would have been a monumental task too. Careful on your pump selection there isn't much head to overcome in those big pipes. Easy to over pump. 8- 10 gpm should be the goal. 20- 30 degree delta on the rads.
  • jimmythegreek
    jimmythegreek Member Posts: 52
    Yes 9gpm is what I came up w after figuring rads and puping using steamheads chart on here. Grundfos 10gpm at 3.5 head is what I'm planning on using.
    Can anyone give me the basics of P/S setup using 2 circs? How do u setup the pump wiring I'm guessing a switching relay having tstat control primary on boiler and then what controls the secondary loop circ I've never done one. We do steam alot of 1 pipe and basic HWBB gas and oil around here. Thanks for any info guys
  • hot_rod
    hot_rod Member Posts: 15,029
    I would also suggest a hydro sep to get all the functions in one box, air, dirt, magnetic and hydraulic separation.

    An ECM circulator and maybe add some TRVs?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gordy
    Gordy Member Posts: 9,514
    The hydro sep is definitely a time saver. Well worth the dolllars.

    Trvs are a nice bonus, room zoning at its best.