I have paid for and received a Manual J (loads by room), Manual S (equipment selection) and Manual D (duct design) design for a heating-only residential project in the San Francisco area. The calculations were done using the ACCA approved WrightSoft software package by a consulting firm in the Midwest who appears to be credible.

So far, I have talked in person or by phone to 9 residential HVAC contractors in the San Francisco area, and each has proposed a system that is 3-4 times larger than the system indicated by the Manual J. Believe it or not, non of them have ever heard of Manual J. Some have refused the job or refused to guarantee it if I insist on the sizing indicated in the Manual J report.

A couple more data points:

My house is 33% the size of the project building and has a furnace with twice the capacity of that proposed by Manual J for the project building.

I have an engineer doing an electrical load calculation for the project. I spoke with his partner, also a licensed PE in California, who does the mechanical engineering work for their firm. He also had never heard of Manual J, but gave me some "rough" rules of thumb for sizing systems in my area (ie: typical BTU per foot, etc).

I am totally confused. If I go with the Manual J calculations and they are wrong, then the duct system will be significantly undersized and it will be very expensive to fix.

Is there any way to validate whether my Manual J calculation is correct (without hiring five engineers and averaging the results)?

Can anyone tell me why I shouldn't just go with the bigger system and play it safe? The additional cost is peanuts compared to the size of the project, we don't have any LEEDS, Energy Star, or regulatory compliance requirements, and the hassle that this is causing doesn't seem to be worth the effort to try and do it the 'technically correct' way.

Thanks in advance for your guidance.