Air Changes vs Cooling Load

[booksman]

Our super insulated, super tight home in central Indiana is finally ready for the installation of air conditioning and fresh air. I've done my homework (I think) and so I feel I can talk intelligently with the contractors who have come to look at what we've got. Yesterday, the fourth guy out (and by the way actually was familiar with our building materials) talked quite a bit about the importance of getting enough air circulation in a tight, well insulated home even to the point of superceding whatever the Manual J load calculations show. I told him that our home (1750 sf of ranch with full basement) had a calculated load of 12K BTUH cooling (and that came from two different guys who have been out before this one). He immediately said that he would probably need to put in a 3 ton air handler to get sufficient air circulation. Is that ok? My impression has been that the air handler needed to be matched to the condenser pretty closely. In fact, the other 3 guys all came up with a 1.5 ton compressor with a 2 ton AH. His argument is that I don't want to end up with "dead air" spots in the house, but I should have at least 9 ACH through the air handler. I'm stuck! I have no idea if this guy is right or not. Any thoughts? Like I said, he actually was somewhat familiar with our construction method and has put in Apriaire's HRVs in other homes. Help!

[BaldLoonie]

3 ton AH on 1.5 ton outdoor unit? Not going to do much dehumidifying. If anything, get a 2 ton, making sure that when the cooling is on, the blower drops to 600 CFM. If you want it at 800 CFM for constant fan, that could be rigged up.

[browntigerus]

> He immediately said that he would probably need to put in a 3 ton air handler to get sufficient air circulation. Is that ok?

Run away. 3ton AH on 1.5 ton compressor is just stupid.
You would not like the result.

Get the other guys.

[engineerguy]

My own super insulated, super tight 1320 SF home has a calculated cooling load of less than 9000 BTUH and heating load less than 19,000 BTUH. The air handler runs at 400 CFM for both heating and cooling. The one ton AC is definitely larger than necessary for 90 plus degrees outside and 77 degrees inside. With five registers, one for each bedroom, and two in the kitchen/living room, we have plenty of air circulation.

Do not oversize the AC. If it's slightly too small, the house temp will rise a degree or so in the late afternoon. A ceiling fan is nice for the living room in summer.

You will almost certainly need a whole house dehumidifier.

[docholiday]

I see where he's coming from but his solution is simply incorrect. What his concern is, is that the individual room loads are so small that he will need very little air fed to the room in order to satisfy the load. His concern is will it have enough velocity to throw it and evenly cool the entire room. He's trying to reinvent the high velocity system without any real engineering. He apparently doesnt know it will not work and it will be noisy as all get out.

For your home, you are likley not going to cool the basement. If you install a 1.5 ton unit it will require roughly 6 - 6" runs or equivelent (600 cfm). If you need less or 100 cfm is too much for a given room he could use 5" for those as long as the system has the airflow availble.

Personally I would install a 2 ton unit as a minimum. If your home is that well insulated and tight, you are not going to have the humidity infiltration so cycling the outdoor unit will have little effect. I would also install the HRV.

[bluetooth751]

Originally posted by engineerguy
My own super insulated, super tight 1320 SF home has a calculated cooling load of less than 9000 BTUH and heating load less than 19,000 BTUH. The air handler runs at 400 CFM for both heating and cooling. The one ton AC is definitely larger than necessary for 90 plus degrees outside and 77 degrees inside. With five registers, one for each bedroom, and two in the kitchen/living room, we have plenty of air circulation.

Do not oversize the AC. If it's slightly too small, the house temp will rise a degree or so in the late afternoon. A ceiling fan is nice for the living room in summer.

You will almost certainly need a whole house dehumidifier.

This could be the most intelligent comment I ever read a HO make. Then maybe your an engineer. Either way I wish every customer thought like you.