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MFTHW100NA2XN Laars boiler

chief Member Posts: 27
Hello, I am installing a new laars heating only boiler to replace an existing condensing unit. The laars is just a little less in btu output than the old unit  but meets the heat loss of the house. 93,000 but output. It is piped primary/secondary, my question is should 1” or 1 1/4” pipe be used for the primary loop. The manual indicates 1” or 1 1/4” depending on size of boiler, but there is nothing saying when the 1” can be used. 

The old unit primary loop was 1”. My secondary loop is only 1” and the old system ran with out any issues.  Obviously I want to do it right the first time. 

Thank you for your input. 


  • GroundUp
    GroundUp Member Posts: 1,862
    That depends on the piping. The ports on that boiler are 1-1/4" so it'd be simple enough to run 1-1/4" piping, but with a 93k load (you're cutting it REALLY close, output of that boiler will never be 93k unless it's perfectly tuned and using low temp radiant) a 1" primary will carry that just fine with about a 25 degree delta. If your piping is anything more than a couple feet to tees and back, 1-1/4" is cheap insurance.
  • chief
    chief Member Posts: 27
    Thank you 1 1/4” it is
    EBEBRATT-Ed Member Posts: 15,296
    10,000 btu/gpm you are just over the limit for 1" which is 8 gpm but on a short run with few fittings you would be fine. Either 1" or 1 1/4" would be ok