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2 zones, 1/2" Copper (Existing) and 3/4" PEX (Proposed)

From a Burnham series 2 with a 37,500 BTU input rating I have a 700 square foot main house on a single series zone (about 100' of copper tubing) with total of 22 feet of 1/2" baseboards (not CI). I also have a 220 square foot guest addition that will have its own zone (about 80' of 3/4" PEX tubing) consisting of 3/4" baseboards with a total of 9 feet ( 4', 3', and 2' ) copper fin radiators.

The question: is the fact that there are two zones of different diameter pipe and radiators going to cause a problem? Will the 3/4" zone become a path of least resistance and cause the 1/2" zone to have low flow? Although I have the radiators and tubing already, nothing in the guest addition has been installed and I can switch direction if this all needs to be downsized to 1/2" tubing and radiators. Thanks very much for any expert advice that can be offered...


  • Jamie Hall
    Jamie Hall Member Posts: 16,331
    You will need some sort of balancing valve -- which you will need anyway. The best option would be individual zone valves, each with their own thermostat, primary/secondary piping, but you might be able to get away with a simple throttling valve (NOT a gate valve -- they aren't intended for throttling) on both zones, and fiddle until you get the balance the way you want it.

    You should also consider that with both zones flowing, you will have much greater flow than you do now (not double, but a good bit more).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Ironman
    Ironman Member Posts: 6,401
    Balance is the key. Why would you want to run 3/4' for nine feet of BB when you have 1/2" feeding 22' of BB?

    Nine feet of BB has a btu output of about 4500 @ 170* AVERAGE water temp. That translates to a flow rate of .45 gpm @ 20* deltaT. A 1/2" line will easily carry 1.5 gpm at a standard velocity of 4 feet per second.

    Use 1/2" with a balancing valve as Jamie suggested.

    Also: use one circulator with zone valves, not 3 circulators.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    Hatterasguydelta T
  • Paul48
    Paul48 Member Posts: 4,470
    Do a heat loss calculation, so you know exactly how much emitter you need. For such small spaces, you might consider upgrading the radiators to something that provides some residual heat after the boiler shuts down.
  • Shaneinseattle
    Shaneinseattle Member Posts: 3
    Jamie & Ironman -- thanks for your comments and advice; it's been a really big help. I've put in an order for 1/2" PEX but I'm having some issue finding the 1/2" baseboards I want. Are there issues using the 3/4" baseboards with the 1/2" supply/return tubing? Will flow be dictated by the 1/2" tubing and the radiators being 3/4" be an irrelevant factor?
  • bob_46
    bob_46 Member Posts: 813
    No issue using 3/4" baseboard with 1/2" tubing.
    IronmanSWEIdelta TRich_49
  • Shaneinseattle
    Shaneinseattle Member Posts: 3
    Thanks for the comments -- this is a great help. And very nice to know that I won't have to return the 3/4" baseboards even though I'm supplying via 1/2" tubing.