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Trusting Load Calc from a Seller

13 years ago

I need to replace my original AM STD 2.5t 10SEER G/E central air (roof-mounted cabinet) for a 1989 track home in hot central California (model#/SN worn off since placard was mounted externally). A local seller performed a free load calc and I helped with the measurements and attempted to verify figures being entered. But alas I couldn't hawk him enough and don't know where in the software they might fudge, to oversell me. At the end, he said it was bordering on suggesting a 4ton unit! But when I called BS, he said a 3.5ton should do. Before this, I didn't specifically distrust the seller, but neither did I have trustworthy history other than a BBB rating of B- (6 resolved complaints over last 36 months). They are categorized as construction and plumbing.

My questions:

Does the below quote seem reasonable for a 3.5ton?

How much can I trust the Load Calc results, done by a seller not by "my" contractor, to truly require 3.5ton. Should I just get quotes for 2.5?



I have a 6 month old quote for $8,000 to install:

AM STD 2YCC3042A1064A "PKG G/E 3.5T 51K 13SR 1PH R22"

AM STD 1F97-1271 "W/R STAT BLUE 1H/1C"

They pushed hard on their 2nd quote for $10.5k to upgrade to the 4YCY4042B1096A "76K 14SR 1PH 410". That ain't happening.

Interpreting Load Calc


I am dumbfounded examining how the load calc suggests 4ton to replace a 2.5ton that has worked flawlessly to cool and heat a single level 1063 ft2 living space (~1400 ttl). Nothing in the home is unusual per say, rooms are std 8' ceilings except the main living room is vaulted. It's a cookie cutter small 3BR/2B home, geeze.

Target Smr/Wtr temps seem appropriate:

101/32 outdoor, 75/50 indoor, with 3 relative humidity factor.

Summary Results:




I guess the fudge areas may reside along the estimations for the lines labeled (with BTU smr/wtr values in parenthesis):

Infiltration (1422/0),

Misc. Int Loss/Gain (6000/4455),

Duct Gain 30%/Loss 20% at values (7239/3479).

This was before I had a leak test done which proved there was very little loss of conditioned air.

Thanks in Advance!

If you want background info, here it is.



I've done everything a basic DIY'er can do to improve the living envelope. Including attic insulation to min R-20 everywhere and up to R35 above rooms, a quality radiant barrier above the insulation on attic floor and against interior walls, RB wrap along the entirety of all flex ducting and plenum, sealing every joint/connection along the ducting, and sealing gaps around exhaust fans, registers, outlets, etc. Every year I walk the home with a laser temp gun to find hot/cold spots along wall and ceilings. Not to mention the easy stuff (weather stripping, reflective window films, solar screen treatments, blah blah). This all started once my summer bill hit almost $500. I have appropriate expectations, understanding we use more than the avg home (computers and TV in every room and we work from home) and CA rates are very high also. I monitor my electricity closely with a meter in the panel and logging software to my computer and verify against PG&E Smart-meter software. Only the radiant barrier made any significant difference in reducing my utility bills.

Leak Test and Energy Audit:


I had the city test the envelope with the blower/door test and thermal imaging. The contractor was shocked and stated this was by far the most well sealed home he has ever seen in the 15 yrs he has been in the industry. His final report only included 4 leaks, 2 were restroom exhaust dampers not closing fully, one edge of attic door stripping, and a very small wear in weather stripping on back door. He didn't check the A/C unit, but the overall recommendation was to install Solar PV @ 2.0kw ac rating and install a whole house fan to "sync up" the temps in the morn/eve (b/c I reported the radiant barrier certainly keeps internally created warmth IN, as well as out, and it's noticeable at those two times). He added other than a solar attic fan when I replace my roof, I have already gone above and beyond optimizing the home, therefore going Solar.

On a side note, I know the home is sealed well when I have trouble igniting wood in my fireplace (pre-fab zero-clearance) until I crack a window.

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