HVAC question

[sucka]

Help!! I'm so confused with AC tonnage for my new home.  I'm loathe to replace the furnace which is new and rated for a 3.5 ton AC.  A couple contractors recommend a 4 ton AC unit but have concerns about the blower capacity.  Will a 3.5 ton effectively cool a 3700 sq home?  Especially the second floor?  I know it depends on alot of factors...but generally speaking,  is that sufficient?

 

[LittleHockeyFan]

I'm wondering, cause I just don't know how it works with forced air furnace (we have electric heat).... can you not set a thermostat in each room so that the heat/cool isn't wasted on unoccupied space? For example, we have an empty bedroom right now, so I leave its door shut and keep it out of the a/c or heat loop.

We have the bedrooms unheated (or uncooled) during the day when they're empty and at night reverse the process (when kitchen/living and diningroom are empty) so that at any given point really only half the house is heated/cooled. Is that even possible with your set up? Because obviously, it saves some of the cost too.
just trying to help.....

 

[Bates]

You can close off a vent in a room but there is normally only one thermostat for the whole house with a furnace and AC.  One thing that helps a bit is to close off the ducts in the upper level while heating(winter) and close off the lower level ducts while cooling(summer).  As we all know heat rises so when you heat the bottom level that heat will get to the upper level and visa versa for cooling.

 

[Bullfrog]

It really depends on the level of insulation in your house, the air tightness of the building envelope, orientation of windows, etc.

I know it's not the answer you want to hear, but the cooling load needs to be properly estimated to pick the proper size unit.

Purchasing a unit that's too large can be a bad decision both energy and comfort wise. Too large and it'll cool the air too quickly and not dehumidify the house. If the house isn't properly dehumidified, then your tendency will be to lower the thermostat's setpoint leading to wasted energy. This is because even though the temperature is lower, you'll still feel uncomfortable because of the humidity.

For a house of this size (and 3700 sq.ft. is massive!) taking the time to calculate it properly is very important.

 

[Bullfrog]

I'm wondering, cause I just don't know how it works with forced air furnace (we have electric heat).... can you not set a thermostat in each room so that the heat/cool isn't wasted on unoccupied space? For example, we have an empty bedroom right now, so I leave its door shut and keep it out of the a/c or heat loop.

We have the bedrooms unheated (or uncooled) during the day when they're empty and at night reverse the process (when kitchen/living and diningroom are empty) so that at any given point really only half the house is heated/cooled. Is that even possible with your set up? Because obviously, it saves some of the cost too.
just trying to help.....

Unfortunately that's not possible with a basic, central forced air furnace unless separate heating and cooling coils are provided in the various spaces or other set-ups such as variable air volume boxes, etc. All of which are extremely rare in a house.