Mark Gagle, owner of a local Salem, Oregon heating/air and plumbing company; Gagle’s, answered some questions I had on AC. Specifically I asked about size of the outside unit in relation to the sqft of the home and fenestration (windows and general heat loss) and one-story compared to two-story homes:
“If it is a newer home the windows, doors, insulation and direction facing are
not as critical on homes built from the mid 90’s to date. Anything prior to around
1994, the homes were not as well insulated and the windows and doors are more
of a factor. The most important factors for A.C. are the size of the return air coming back to the furnace/air handler and the location of the supply
registers for the second story of a home. If you have a single story home;
650ft. per ton is great. If you have a two story, main floor and upstairs, go
down to 550 to 600 ft. per ton.
What I’m finding out there is a lot of homes
were designed for heating only, and not for adding A.C. The supply registers for
the upstairs need to be in the ceiling, not the floor! Warm air rises, but cold
air drops. In heating mode, the upstairs works fine because the main floor is
helping to heat the upstairs. But in cooling, unless the registers are in the
ceiling, you won’t cool the upstairs! On most two story homes, the bedrooms are
all upstairs, and that is were you want cooling.
Another factor is the
thermostat location. It is usually located in a hallway, under or near a return
air grill. That is perfect for a single story home. By drawing air across the
stat going into the R/A [return air] grill you are always getting a accurate reading of the
indoor air temp. But the stat is also the systems sensor, and that is why the
upstairs is always warmer with a two story home. Once again warm air rises, so
all the heat from the main floor ends up upstairs! The same principal is true
for a main floor/basement home. Most two story homes have a 5 to 10 degree
temp. difference between floors.
As for return air duct sizing back to the furnace
for adding A.C.: a 14″ round duct is good for a 2 ton system. A 16″
round is good for a 2 1/2 to 3 ton system. A 18″ round is good for a 3 1/2
to 4 ton system. A 20″ round is good for a 5 ton system. These numbers are
for the max. size for adding A.C. If you don’t move enough air across a A.C.
I.D. coil you will turn it into a block of ice, and have to put the system into
heating to defrost the coil! The round sizes can be converted into rectangular
by multiplying by 3.14 (Pie) That will give you the sq. inches needed for
Of course this information is above and beyond what a home inspection is about (general visual inspection of systems) but it is still a good thing to know about as it may help me educate my clients more thoroughly.
If you have specific questions about duct sizing and AC/Heating system function refer to a Salem, Oregon Heating and Air contractor, but for questions about general systems, including structure, plumbing, electrical, kitchen appliances, etc….ask me, your Salem Oregon Home Inspector.
Jim Allhiser President/Inspector