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Primary loop sizing on small retro fit.

Hey Fellas
I have used this forum as a resource but this is my first post here. Ill be the first to admit that most of my experience is to do with new modcon boilers so looking for some help from the old school guys.

I am doing a retrofit for a small residential system and I want to make sure I am sizing the primary loop correctly. I will be replacing a Allied MiniGas 75kbtu boiler with the 20 year newer model. The current system does not have a primary/ secondary loop piping but I will be installing it as I believe these are not actually high mass boilers like a cast iron one and I am concerned with condensation as this is not a modcon boiler (steel heat exchanger).

The system is a three zone valve system all the same temperature suppling copper fin tube baseboards via 1/2 pex lines. According to my charts if I am going for a 20DT the most I am going to be using is 3x12000btus=36000btus (3x 1/2 pex zones), so a 1" copper secondary should be more than enough for my secondary loop. For the primary loop however I feel like the 1" copper would be too small.

If the primary and secondary loop piping is the same size could that leave me with any problems or should I upsize my boiler loop to 1-1/4 copper? The outlets on this boiler are 2" and the accessories it comes with are 1-1/4", so this is throwing me off a bit.

If I do up size to 1-1/4 boiler loop could that leave me with a small DT and cause the boiler to short cycle?

Im looking for the right way forward here, thank you for the help.

1/2" pex @ 20DT (1.2GPM) = 12000btu
1" copper @ 20DT (7.5GPM) = 75000btu
1-1/4 copper @ 20DT (12GPM)= 120000btu


    EBEBRATT-Ed Member Posts: 11,087
    If the boiler output is 75,000 use 1" copper is fine.

    75000 btu/10000 btu/gpm=7.5 gpm. 1" will do 8 gpm with no issue especially for a short loop with little resistance.

    My concern would be a 75000bty boiler for a 36000 btu load you more than 2x oversized. You may need a buffer tank to keep from short cycling but looks like you tight on space
  • Dentville
    Dentville Member Posts: 2
    I agree that the boiler is over size for the current set up. The home owner wanted me to upsize the boiler because they found that there was always a lack of heat. Im expecting to find some sort of issue with the distribution system when this is done. If I size the boiler for this house it comes to 60000btus but if I size it for the baseboards its like 35000btus...
    You don't think ill have any problems with return temps too low causing condensation if I go with 1" primary and 1" secondary?
    EBEBRATT-Ed Member Posts: 11,087
    Think about it.

    If the boiler make 75,000 and the baseboard is using 36000 that means (75000-36000) or39000 is crossing the bridge and going back into the boiler return. No way you will get low return water temp but you will get short cycling.

    I would do a heat loss and size the boiler right. Then you will know if they need more baseboard or not. Putting a boiler in with a rating more that the baseboard can produce make little sense.

    If you use the 75K boiler you will end up setting the limit with a 30 or 40 degree diff to minimize short cycling and even that won't work .

    Buffer tank and size the boiler right
  • Zman
    Zman Member Posts: 7,114
    edited November 2021
    It is my understanding that the reason for primary secondary on low mass fin-tubes is to avoid thermal stress and micro flashing in the tubes. Ed's advice above is "Spot-On"
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein