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View Full Version : Need help with size of new HVAC in SoCal



SunnyDee
01-23-2007, 07:22 PM
I am looking to replace the original floor furnacea with a central heating&cooling system. The home was built in 1953, 1700 sqare ft. located in Southern California (about 5 miles from the coast). I've had 7 estimates, most of them very similar, but one (who performed the heat-load calc.) about 50% higher than the others. The calculated numbers are:
BTU Heating: 31,540
Heating CFM: 1,351
BTU Cooling: 34,618
Sensible Cooling: 28,553
Latent Cooling: 6,364
Cooling CFM: 1,353
To satisfy these, that contractor bid Trane equipment that was 3 ton cooling and a two stage, 63,000 BTU furnace. That bid is too expensive.

I am leaning towards the contractor who is bidding Carrier equimpent. However, he recommends 3.5 ton 38HDR air conditioner and the Infinity96 58MVB080-20 (80K BTU, 2-stage, variable speed) furnace.

I think he is oversizing the equipment. He didn't do a heatload calculation, he's been in the business locally for 24 years and says he knows these homes very well. I want to ask him to bid the 3 ton unit instead and the MVB060-14 (60K BTU).

Is he oversizing the equipment? Would my proposed equimpent be sufficient based on the heatcalc numbers?

dash
01-24-2007, 10:17 AM
Depends on the accuracy of the load calculaton,and at what indoor temperature was used in the calc.


If the carrier guy didn't do a calc.,then I'd be inclined to go with the one you have,If it's accurate.

You could do your own ,on this site,see hvac-calc above,for a small fee.

gonekuku
01-24-2007, 01:10 PM
I know you can do the math, but here it is for you:

Calculated required Heat BTU 31540
Calculated required Heat Tons 2.63

Trane
Furnace BTU 63000
Furnace Tons 5.25
Furnace oversize 99.7%

Carrier
Furnace BTU 80000
Furnace Tons 6.67
Furnace oversize 153.6%

If the calculations are correct and they factored in all losses, I doubt you would ever need stage 2 on either furnace. Did the calculator guy use the correct R Value for the exterior walls? Did he know what the window U (R) value truly was? If you guess poorly on both, you could be off by 50% real fast.

These are just my numerical opinions.