My 3.5T, 10yr old Trane HP gasped its last. I'm considering a 4T Goodman replacement b/c (1) Trane did not cool enough on the hottest August days, & (2) two farthest rooms are generally not quite cool or warm (winter) enough. Those 2 back rooms are on each side of house w/ patio in middle so both rooms have 3 exposed walls.

The 4T has smaller current rating and same breaker rating as old 3.5T Trane & the line set requirements are same. So that is all ok.

The existing Trane 3.5T AH is set for Med Speed fan. The 4T Goodman AH at Low Speed over 0.2-0.5sp range is about same as a new 3.5T Goodman AH at Med Speed and both are about 100CFM greater than existing Trane.

Is there any reason not to move up to 4T?

 

We cant answer this question. Your contractor will have to do a load on the house. and then make sure the duct work is correct. Hot and cold rooms are not a unit size issue but a duct issue.

 

Thanks, airman; however, the 4T should certainly cool better in August than old 3.5T and that + blowing slightly more air should help, if not solve, the 2 back rooms. The duct work is inaccessible b/c it is between ground & 2nd floor and tearing any of that up is not worth fixing it.

The duct work cannot tell who is blowing the air, so w/ a 4T unit blowing only a little more at Low Speed than the existing one at Mid Speed, seems to me ought to be an improvement, but sometimes guys with more knowledge come up w/ something I did not consider.

 

The problem you could gain cooling at the expense of humidity control, so you have cold damp air instead of warmer dry air.

I'm in AZ and I had almost the exact situation as you. I had a 3.5T unit about 15 years old, and it only cooled the center of my house. The ends got very little air and temps could easily be 10 degrees higher than the center.

I got 4 estimates for a new system and all recommended a 4T for this side of my house, which is what I got. I was also planning to add zoning, but that was done a few days later.

It wasn't extremely hot (for AZ) when I got the new unit, but it was in the 90's. I used the unit for a few days, and I think the temp uniformity was even worse. This units cools MUCH more than the old one (both Trane) so the center of the house would cool fast, but the ends could never catch up.

Also, if you upgrade, I'd highly recommend a two stage unit, with variable speed blower. If you get zoning, its mandatory.

I had the zoning installed with three zones. The ducts were not changed but were changed around a bit because of the zoning. Anyway it works GREAT. It probably added an extra $1800 but its fantastic. I would never go without this again in a house.

Before all the AC people here start yelling, I will add that I have no doubt if I replaced all my ducts, spent lots of money having it all balanced, and had an AC guy living in my house to always adjust it, that I could get good temps without zoning.

I have a home automation system control all the thermostats, do it raises the temps of rooms I'm not using, and makes sure where I am is cool, so I expect it to save some on cooling costs as well.

If your ducts are impossible to get to zoning might not work for you, but it works great here (after some tweaking.)

Also, you want a two stage compressor with a variable speed fan. If you get zoning, its basically mandatory.

 

A load must be done on your home. Just upsizing the equipment will typically lead to worse conditioned space woes.

 

Since 3 walls face the outside, I'd do what I could to reduce the heat-gain heat-loss of those rooms.

Then a longer runtime will do more for equalizing room temps; increasing the Btuh tonnage merely satisfies the room TH too soon & usually increases room temp differences.

If the air coming from those ducts is losing too much of its initial temp; then that problem needs to be addressed.

Adjustable Dampers on the closer duct takeoffs from the main might help push a little more air to those distant rooms...that will increase the static pressure on those long ducts which should increase the airflow velocity & thus CFM to those rooms. It depends on what the air handler can handle pressure-wise...

Do a whole house load-calc & one for those 2 distant rooms.
IMPORTANT; I'd use 0.7-ACH infiltration rate & put the CFM in the air make-up blank. Print & follow the easy instructions!

 

One other addition I forgot to mention was air returns. Around here the builder puts in one per system. We added one to each of the trouble areas and it really helped. Unfortunately its more air filters to change, but maybe I can change them less often too.

 

Installing more Return Air Filtering Area in those rooms is very important & will help pull more airflow into those rooms, it also lowers the total static pressure on the system helping it to deliver more air.

The extra filter area helps reduce the velocity through those room filters to closer to 300-fpm, as that is what manual D calls for when using the throw-away media filters; use a very low pressure drop filter in those rooms.