Hi,
I am replacing Central heating AC system in the house and getting very different estimates for HVAC unit sizes from different contractors.
I live in Massachusetts close to the ocean (could be very humid)
Unit goes to two stores 27x25 totally 1350 sf addition built in 1974.
Addition opens to the old house ans adds about 250 sf to cool/heat. 1600 sf total.
It is normally insulated with smaller windows (still from 1974). Currently I have 85000 BTU heater and 49000 BTU AC all carrier still working - obviously not with full efficiency but still keeping us comfortable.
For Heater I am getting proposals from 56000 to 100000 BTU (92%)
for AC 36000 BTU 13 SEER from all.
All of the six contractors use their best guesstimates.
Any suggestions greatly appreciated

Have one of the better contractors do a manual J. It is the only sure way to know what size equipment to use. Like you said the contractors were all guessing and the manual J will put a stop to that.

Info on manual J.

https://www.acca.org/Files/?id=128 Microsoft Excel work sheet example


ACCA System Design (http://www.acca.org/design/)