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Gravity system

joe amadio
joe amadio Member Posts: 41
I went to see a customer interested in an oil to gas conversion. There was an oil boiler there that was a replacement of the original boiler. The original system was gravity hot water. When this boiler was installed a B&G circular was also installed on the system(cant see model #). There is approx 130 ft of 2" black pipe running around the basement feeding 10 very large 4 tube cast iron free standing radiators. The house is approx 1600 sq ft. I would like to use a Burnham series 2 gas boiler. My main concern is the amount of water that the system holds. Should I be worried about thermal shock? Should the return water be tempered in any way? A bypass maybe? Also will the amount of water being heated affect normal sizing procedure ? Do I need to up-size to accommodate so much water?


  • clammy
    clammy Member Posts: 3,094
    recent job

    Joe i recently did a similar job a original gravity system with 2  2 inch supply mains and 2 returns all weil mclean raidant recessed rads from the 20"s i believe all the rads where 3/4 supply and return ,what we did was pipe bth supply together and both returns together we installed ball valves on the 2 returns to balance the 2 mains  ,We piped the boiler primary secondary with it's own circ and the system side has it's own pump also .We reduced the each main from 2 inch to 1 and tied them together in doing so we increased them to 1 1/4 ,the boiler piping was 1 1/4 also .the original was a  coal to gas weil if i remenber right a 64 series original .I think the gas burner rating was like 225mbtu we installed a 175 mbtu input  i believe it was a crown boiler ,it worked out great from a cold start she heated to above 140 pretty quick ,HO love it she said the rads used to take a long time to heat up and never got this hot ,we used 2 taco 007 pumps one for the system and one for the boiler this was a budget job and we had no velocity noises due to using a hi  head pump but we did use proper piping techinics and did not bull head the system mains together .If you can do a heat lose on the home so not to oversize the boiler and have to carry such a large one down also .To me  this system was not a true gravity none of the main pipes where large as i normally see nor did they reduce as they went around the basement which is odd for a early gravity these types of systems probalbly worked  better at a higher temp ,the system piping also did not have really any notiable pitch to it either but all in all a simple p/s piping worked out great if you are really worryed about low water temp you can easly a thermostatic return valve made by danfoss on your boiler piping loop with a by pass ,we did straight P/S  and had no promblem peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • kcopp
    kcopp Member Posts: 4,396
    Gravity systems....

    can kick your but if you try to use conventional wisdom.... Dan has a great book "how Come" that explains a lot of the hidden tricks. Don't over pump the system.... you are trying to mimick gravity flow. Size the boiler piping by reducing by 1/2 and then the next size down. So 2" pipe Mains  would only require 3/4" supply piping. If you go w/ a traditioal cast iron boiler you will need a system bypass and I would highly suggest a boiler protection control (ie Tekmar control) But then again....Why use a 82% boiler when Mod/Con is a great match ? Embassy Oxex, Triangle tube, etc.... kpc

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    At a minimum...

    If you decide not to go modcon, protect your customers investment by installing a thermostatic valve on the return line (Danfoss/ESBE termomix). That will guarantee that your boiler will not long term condense, and it will bleed only what is available for heat to the system.

    I have never over compensated a boiler size based on water volume, but then again, I haven't dealt with anything but condensing for the last 12 or 13 years....

    One of these days, and it might be sooner than we think, it will be illegal to use anything but a mod con boiler. That's the law of the land on the other side of the pond (UK, EU.)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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