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Are rule of thumb heating loads completely disconnected from Manual J calcs?

willib | Posted in General Questions on

I just joined this forum and started trying to figure out how to calculate heat and cooling loads.  I recently installed solar and want to get rid of an oil boiler with cold climate heat pumps.  

The house will likely end up with 5 zones, reusing duct on the 1st floor for 2 zones, and 3 mini-splits for the upstairs rooms.  When I use guidance like 20 BTU/sq ft for cooling and 40 BTU/sq ft for heating (Zone 5, MA)

Rule of thumb: 40 BTU sq/ft
Zone 1: 700 sq ft – 28K BTU ducted air handler
Zone 2: 2500 sq ft (includes some finished basement rooms) 100K BTU
Zone 3,4,5 – 870 sq ft 34K BTU – minisplits

I started learning to use the CoolCalc software and after manually entering all the rooms, dimensions, and being somewhat conservative:

Using CoolCalc
Zone1: 17K BTU heat, 14.3K cool
Zone 2:  41K heat, 18K cool (this part includes some of the basement) 
Zone 3,4,5: 18K heat, 13.2K cool (2nd floor, above zone 2)

Is this difference typical or am I doing something weird with CoolCalc?


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  1. walta100 | | #1

    The rule of thumb tends to work if your house has leaky single pane windows, no wall insulating and R10 in the attic and the equipment is still oversized.

    This article may be helpful.


  2. willib | | #2

    That's exactly my boiler!

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