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Capacity of 3/4” outlet boilers

So maybe I’m overthinking things but I have 4 zones (apartments) each with a 3/4” run to 4 panel radiators. The old oil boiler had 2” pipes on both ends which went down to 1” before splitting to the 3/4” feeds. These new boilers I have been looking at all seem to have 3/4 outlets. I can’t imagine that could flow enough for the system demands. Or if I get one of those variable speed pumps and use zone valves would the pump just work harder but get the required flow? Haven’t found much information online and this whole pipe size thing has my head running in circles.

Any input is much appreciated.

Comments

  • Steamhead
    Steamhead Member Posts: 16,796
    What model is the new boiler?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
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  • Jamie Hall
    Jamie Hall Member Posts: 23,170
    Make and model would help... but if you are dealing with flowing water, you will have a definite head loss, but not that much, going through a short 3/4 inch section. How much flow (gpm) are we talking here?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • nickmg555
    nickmg555 Member Posts: 16
    I haven’t decided on a boiler yet. The entire system has been sitting out of order for a few years. The boiler is junk originally it just needed a control board but it ended up freezing last winter and blew out the pumps. I was looking at the Rinnai boilers m and l series the wall mount ones.

    BUT. I just found a boiler for sale locally a laars mascot 2. I’m aware they are discontinued but have heard good things about laars and the connections appear to be 1” so I’m thinking this boiler may work for me. It’s 125,000 btu so should work just fine (small apartments) the highest panel radiators is about 20-22 ft. Would I be better off with separate pumps per zone so that upper zone can have a more powerful pump? Or can I still get away with a variable pump and valves?
  • Jamie Hall
    Jamie Hall Member Posts: 23,170
    Remember that your head loss in a heating loop has nothing whatever to do with how high it has to go. The height to the upper apartment determines your static fill pressure, but the head loss the pump sees is determined by the pipe and miscellaneous and heating units and it doesn't matter what elevation they are at. Variable pump and valves should work fine -- if they are piped right.

    Is that bargain boiler the right size for the load? Do have a heat loss calculation for the building? And does it match? An oversize boiler at any price is not a bargain.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 7,561
    You need to size the boiler based on your homes BTU loss rather than pipe sizing. How big is the house? How many lineal feet of baseboard?
    Don't assume there is anything correct about your existing system.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    STEAM DOCTOR
  • nickmg555
    nickmg555 Member Posts: 16
    > @Jamie Hall said:
    > Remember that your head loss in a heating loop has nothing whatever to do with how high it has to go. The height to the upper apartment determines your static fill pressure, but the head loss the pump sees is determined by the pipe and miscellaneous and heating units and it doesn't matter what elevation they are at. Variable pump and valves should work fine -- if they are piped right.
    >
    > Is that bargain boiler the right size for the load? Do have a heat loss calculation for the building? And does it match? An oversize boiler at any price is not a bargain.


    It’s been a while since I got the numbers but I ran a couple different ideas from getting larger panels and running low temp 140 to running standard and 180. Also ran numbers for current condition at time vs future condition as I change windows and tighten up the building. I got btu requirements from 100,000 to 160,000 max

    I’m actively improving the building so I’m going based off the lower models which average me around 110-125,000 btu. This boiler I’m looking at is 125,000 modulating and has optional domestic hot water as well. I certainly don’t want to oversize but I don’t want to be “just enough” especially when temps in this area can average 10-30 below freezing during the heart of winter.
  • nickmg555
    nickmg555 Member Posts: 16
    edited September 2019
    > @Zman said:
    > You need to size the boiler based on your homes BTU loss rather than pipe sizing. How big is the house? How many lineal feet of baseboard?
    > Don't assume there is anything correct about your existing system.


    3 stories and a mostly sub grade apartment with a total of approx. 2900 Sf. I’m planning to gut and start fresh as one apartment has cast iron radiators and one has panels and one has a mix it’s just an absolute mess. Different pipe sizes all over mix of copper and black iron. One thing I can’t stand is disorganized plumbing and I’ll happily drain it and sawzall everything out and build a plumbing cabinet up the building and start fresh. I like neat and easy to follow pipes. This building drives my ocd insane every time I go in the basement.

    I’m thinking boiler and components to an ecm variable pump and 4 zone valves leading 3/4 pex A with expansion fittings for less loss and each unit will have 4 panel radiators plumbed in parallel and oversized to run lower boiler temp. Although I might consider baseboard heat throughout the basement apartment to have that curtain of warm air against the cold exterior walls.
  • Zman
    Zman Member Posts: 7,561
    Your heat loss is likely in the 60k-80k range.
    I don't know of a boiler that size with 3/4" piping. What models have you been looking at?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein