HVAC replacement sizing

11 years ago

I posted last summer about my AC running a lot during the
hot, summer days. It’s not unusual for the 2nd
floor system to run for a couple of hours at a time; especially if the
temperature is lowered a couple of degrees at night. The house is 2 stories, full basement,
approx. 1600 sq ft per floor, almost 30 years old, all electric, and with
original Lennox inside and 12 year old Lennox condensers outside. The service guy last year added some
refrigerant but said everything was okay.
I called a different company for the tune up this year and they said the
2nd floor evaporative coil needs replaced but a new system is
recommended. They gave us a bid for an
Amana 2 ton 16 seer variable speed unit for approx. $4500 after rebates. We don’t anticipate staying in the house more
than a couple of years. He said the
attic insulation is good, the ductwork in the attic is insulated and looks good
also, and the house overall has good windows, etc. The unit is in a conditioned closet on the 2nd
floor. To my knowledge, he didn’t do a
formal load calculation. The current
unit maintains the temperature okay but takes a long time to adjust down a
couple of degrees. We are located in Missouri
with warm, humid summers and cold winters. The house is shaded fairly well with no west facing windows. We don’t use the 2nd floor heat
too often in the winter. The current 1st
floor system is a 2.5 ton and seems to do a good job with heating and cooling the 1st floor/basement with run times that are much shorter than the 2nd floor.

Is it reasonable to expect the new, matched unit to do a
better job cooling the 2nd floor with shorter run times?
Would it make sense to upgrade the size to 2.5 tons or ask for a load
calc to make sure? Are there other
things that should be checked before assuming another 2 ton unit is correct?

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Comments (6)

dovetonsils
11 years ago
You would definitely benefit by a load calculation. 1600 upper floor sq ft, ductwork in the attic, history of slow cooling response, 2 tons might be light.
mike_home
11 years ago
If both floors are 1600 sq. feet, and the duct work is in the attic then it does not make sense that the first floor has a 2.5 ton condenser while the second floor is only 2 tons.

Find someone who will do a load calculation. I would think you need at least 2.5 ton for the second floor and possibly more. Have the contractor go into the attic and do a thorough inspection of the duct work. All joints need to be well sealed. Make note of the insulation value of the duct work and how it is configured. You should invest in fixing any issues.
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udarrell
11 years ago
last modified: 11 years ago
" We are located in Missouri with warm, humid summers and cold winters."

1600-sf / 2-Ton is 800-sf per/ton; everything has to be design engineered well for 2-Ton to handle

the loads at your design conditions with high humidity latent load factors.

Always remember that the duct system & return-air filter area has to be sized right for any tonnage used!

Get all members of your family to help you do a load-calc which can then be used for sizing the A/C.; - print the instructions...DO IT...DO NOT CHEAT...Do the floors separately; 1st floor may be oversized at 2.5-Ton; when needs replacement you'll know how to size it.

http://www.loadcalc.net/

Since you live in a humid climate area, do NOT oversize the A/C.

In fact I would use a 78 or 80°F 50% indoor design setup (not 75°F); which would only occur on exceptionally hot & humid days. The A/C should run nearly all the time during the hottest hours of the hottest days; there is a delay in the heat load so it may run all the time the first half of the night.

Slightly Undersizing in a humid climate condition is a wise decision to get the humidity low enough for comfort; I'd consider an ECM 'variable speed' indoor blower motor to use its dehumid capabilities. . .
sktn77a
11 years ago
last modified: 11 years ago
"Slightly Undersizing in a humid climate condition is a wise decision to get the humidity low enough for comfort; "

Wise words. A properly sized AC will lower temps about 1-2 degree per hour and will run pretty much constantly on a very hot day. A "couple of hours" run time doesn't seem out of the ordinary.

And the price seems reasonable for a 16 SEER 2-stage system.
ionized_gw
11 years ago
IMHO, the analysis of how a properly-operating system is running trumps a load analysis. There is sometimes some guessing involved in the load analysis. Guesses will include infiltration rate unless a blower door test is done. Unless you have good knowledge of what is in the walls, insulation will be a guess too. As sktn77a already wrote, 2 hours runs on a hot day is not indicative of any deficit if the current system is running well. It is a good sign that it is sized well.

You have lots of good advice suggesting variable speed and avoiding oversizing to control humidity. The trouble is that variable speed is going to cost you more up front and you don't plan to stay.

Now for a provocative question. What reason did the service company give you for suggesting that the coil needs to be replaced? Maybe they want to do it just to line their pockets.
udarrell
11 years ago
A well performed load-calc, on the other hand can be more reliable than an A/C that has never had its "Delivered Performance checked."

Numerous tests have shown that the average delivered Btuh is only 59 to 63% of the A/Cs nominal Rating! Therefore, an A/C's actual performance usually becomes mere guesswork...