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Sizing Secondary Pump for Replacement Boiler

Mpls_Matt Member Posts: 2

Obviously I am an amateur for asking such a basic question, but I could really use some help with this.

I've basically settled on the Triangle Tube Prestige Solo 110 to replace my very outdated cast iron boiler.  The current system is a single series with cast iron radiators and a Bell & Gosset 3 piece Series HV circulator.  The PS110 comes with a Grundfos UPS 15-58F-FC primary pump.  How do I size the secondary pump to run the secondary circuit to the cast iron radiators?  I have no idea what the system pressure or flow rate of the current system is.

A second question is how do I size the expansion tank if I don't know what the system volume is?

If this helps, the house is roughly 1600 square ft.

thanks for any help,



  • Pump Sizing

    You don't need a secondary pump if you only have one zone since the large flow will safely pull all the heat out of the boiler's heat exchanger.  Use the Grundfos pump; remove the B & G.

    What is the size of your supply and return pipes on the boiler?  If not larger than 1", use a size 30 expansion tank which is one up from the smallest size, size 15.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Mpls_Matt
    Mpls_Matt Member Posts: 2
    Pipe sizes


    Thanks for the help, I appreciate it.  My single circuit is split into two pipe runs that run on each side of the house, prior to the split at the boiler the pipes are 1" 1/4"; but after the pipes split they are 1" from there.  So, I'm guessing you're correct with the size 30 tank.  The cost to go up to a size 60 tank isn't a lot more; is there a downside to just putting a really large tank in to make sure you have enough volume?  Can you have too large of a tank?

    Also, any issues with the freeze protection feature if I only use the one circulator?  It seems to reference this in the installation manual that you may have issues if you don't have a primary/secondary system.  Or is there nothing to worry about here?


  • HDE_2
    HDE_2 Member Posts: 140
    edited August 2010

    Are you sure its a true series system? Thats a bad design for CI rads and would be rare.

    I seriously doubt the PS110 boiler pump could handle the split, multi tap radiation system that required a HV circ previously

    You better get one sized for you, I would help but so much more info is required

    Tanks are sized on BTU and SYSTEM VOLUME, guessing the tank size isnt a smart move, but a # 30 would be a good guess.

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