Who among us tracks the accuracy of Manual J?

Who among us tracks the accuracy of Manual J?

I have hour meters on maybe 20 systems now.    I like to see the run time when it's "4" degrees in Kansas City.  99% rate in KC is +4' degrees.  My goal at 4' is 100% run time.   I have a super sealed home 2,100 square feet  with a 2 ton ground type heat pump R-30 walls, R-60 attic, R-8 windows & doors with 98% run time @ "+ 4' ".  When the strip heat turned on, it smelled up the home with the dirt  off the 5 KW strip, heat burned off the dust.   If you fine tune ACCA MJ  with a blower door number and infrared camera  you can get closer.   If  I find a real drafty building - I am more off.   Or building owner just turn up the heat more.  

4/5 of heat loss is at night. The wind tends to die mid-night and MJ tends to keep the wind up.  If you just run the number with Manual J I tend to get larger system sizes than needed - do not add more fudge factor. 

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Comment by Eric Kjelshus on March 7, 2016 at 6:53pm

Most of the Thermostat controls I use Look at RH - humidity also  but still its just longer run time.   I am mixed very humid to dry and cold.   I have been using outside air with a damper to outside to make the building positive.  Could the outside air make 20% -30% higher fan use and higher power use?

Comment by John Proctor on March 7, 2016 at 11:25am

Eric  It is doubtful that the fan motor is using more energy, the electrical use of PSC motors is nearly constant regardless of speed. What climate are you referring to? If it is a wet climate, then you need the latent removal and should go back to 350 CFM per ton. However the lack of latent capacity should not increase the electrical usage since the thermostat controls only to temperature, not humidity.

If the duct system is not restrictive, an ECM/BPM motor would deliver the right airflow at a lower watt draw.

Comment by Eric Kjelshus on March 6, 2016 at 2:11pm

Do you see the new AC with low sensible and very large coils needing more run time?  That last 2 ton this year was just 91% Sensible and 9% Latent.   the one I took out a AC built in 1990 was 70%/30% with very small very cold coil but 350 CFM/ton now razing the CFM to 400 per ton and not the Latent removal.   I am seeing power bill go up with higher run time and more power to run the fan?   Older air handler had drives with smaller motors.   

Comment by John Proctor on March 3, 2016 at 11:06am

Proctor Engineering has done many studies on Manual J vs. true load. The short answer is the sensible load is actually 2/3 of the Manual J 7 (note 7) estimated sensible load and Manual J8 calculates an even higher sensible load. The latent load estimate varies compared to the actual latent loads. Sometimes it is too high (a tight house in a wet climate). Sometimes it is too low ( a leaky house in a wet climate).

There is a Home Energy article on all this on their website or here:


Here is a letter from Trane opposing the acceptance of Manual J 8.


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