I have bids from four HVAC contractors to replace the 40-year old HVAC systems in a church. I have received estimates for cooling that vary from 7 tons to 10 tons.
For reference, we currently 7 tons of cooling, but performance is abysmal because there is no air flow. That's attributed to there being no return ducts. And, I mean no return ducts. Just a whole for the air to go into. For cooling, hot air from the upper level has to migrate down the stairs to the basement.
I haven't done the calculations myself and the answers from the estimators vary as you can see up to 30%.
Bid #1 - 8-ton (padded to 9-ton) bid estimator said he hired an engineer to do the estimate.
Bid #2 - 10-ton bid estimator was meticulous. Measured every window, every part of the building. From very big company with commercial and residential divisions.
Bid #3 Another 8-ton bid. Said it would take an additional 6 tons for fresh air blend.
Bid #4 The 7-ton bid is from a company I have used before and thought their work was top-notch. This bid is $5,000 less than the others. This is attractive unless he got the tonnage wrong. I swear he said his bid included fresh air blend.
Thanks for insights you have,