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Primary/secondary pump sizing

Guy_6Guy_6 Member Posts: 450
The primary pump on the boiler is simple- size it as 1gpm per 10,000 BTUs of boiler. An 80M needs 8 GPM flow, a 140M needs 14GPM.
The secondary pump is sized for the SYSTEM itself. If you need 10 GPM, then simply choose the correct pump. Just be certain that there isn't any excessive head (restriction kinda) in the system. Normally, the pump for the system is rather straightforward.

The indirect will need the correct pipe sizing and pump to unload the heat from the boiler. If you use too small of either, the boiler will short cycle, and efficiency will be lost. The Munchkin manual outlines the piping and pump if you are using a Super-Stor, if not, check with the tank manufacturer for the correct sizing.


  • Joe D.Joe D. Member Posts: 19
    Primary/secondary pump sizing

    I have a question about primary/secondary operations. Does your primary pump volume depend on the total volume in gpm of the secondary pumps in the system?
    I am a do-it-yourselfer, and am going to go the pri-sec route heating my house. All I will be feeding is a DHW tank, and a single zone baseboard for the house.
    So, in general terms, if I pump 10 gallons a minute thru the house, and the tank has 3 gallons a minute, does the primary pump need to be capable of 13 gpm?
    Hold it, I just realized it after I wrote this. The DHW pump will not be running at the same time as the house heating (Munchkin boiler)........
    But, I still would like that type of question answered. In theory, What if I had 3 zones of baseboard, at say 5 gpm each. Would the primary need to be 15 gpm capable.
    I am curious about that. I have Seigenthalers "Modern Hydronic Heating" book, and I can't recall reading that particular question being explained.
  • Empire_2Empire_2 Member Posts: 2,343
    Another great book to read

    "Pumping Away", By Dan Holohan. Great tips in this book and can clear up all of your questions. Waite a minute....I think it's available on this very sight. Why yes it is. Really,...A great read.

    Mike T.
  • WaterHeaterGuyWaterHeaterGuy Member Posts: 80

    Size your secondary pump in accordance to the flows needed in the secondary loops. In your home with 1 zone of baseboard, I'm guessing you're looking for about 1-4 gpm, depending on pipe size.

    In regards to your theoretical installation, no. The boiler requires what ever flow the manufacturer asks for. Stick to that first and formost... especially with a Munchkin boiler. If you have 15 gpm moving across your closely spaced tees, 5 gpm would bypass the boiler until a zone was satisfied. What primary/secondary piping does for you is allow you to meet your Munchkin boilers flow requirements, independant of your house flows.
  • Joe D.Joe D. Member Posts: 19

    Thanks, guys, for the information. That is just what I wanted to know. Joe
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