Electronic Dampers on Return Air?

[S_Helton]

Yes, he does. He is also considering doing away with 2 systems, and going back with just 1 system with zoning.

I was just making points of why I would not recommend doing that.

[S_Helton]

Dash- I agree, you shouldn't plan for future owners. But I still stand by my opinion that it is not smart to replace 2 systems with 1 system. Then zone it, and hope that you never want the whole house to be comfortable.

Besides, if the load calcs he did are correct he has a total house heat gain of 83,606 BTUH. He wants to know if a 5 ton unit, which is nominally 60,000 BTUH depending on manufacturer, is enough. I again stand by my comments.

[beenthere]

If he was only putting in a unit sized to the largest zone of his 2 zones. I would agree it would be a bad idea.

[beenthere]

Besides, if the load calcs he did are correct he has a total house heat gain of 83,606 BTUH. He wants to know if a 5 ton unit, which is nominally 60,000 BTUH depending on manufacturer, is enough. I again stand by my comments.

That gain is maintaining the same temp on both floors.
By setting the sat 10° higher in the unoccupied zone.
That could knock off 12,000 to 24,000 BTUs. He needs to do a load calc at 10° higher ID design temps to find out.

[S_Helton]

That gain is maintaining the same temp on both floors.
By setting the sat 10° higher in the unoccupied zone.
That could knock off 12,000 to 24,000 BTUs. He needs to do a load calc at 10° higher ID design temps to find out.

Yes, but again I say, if his lifestyle changes and he wants both floors occupied, he will be sorry.

[beenthere]

Yes, but again I say, if his lifestyle changes and he wants both floors occupied, he will be sorry.

Yes, on OD design condition days he could end up sorry if he suddenly changed his mind and wanted both zones the same temp.

He's taking the time to check and double check things. I don't think he's going into this lightly.
So the odds of him changing his mind are not high.

Done right, systems sized like this work.

Done wrong, no system works right.

[Daltex]

By setting the sat 10° higher in the unoccupied zone. That could knock off 12,000 to 24,000 BTUs. He needs to do a load calc at 10° higher ID design temps to find out.

That was going to be my next step. To see what design temp I can get from a 5 ton.

Should I assume a 70% sens. capacity on a 5 ton @ OD design temp of whatever it is- say 100*?

[beenthere]

Since your not changing out your duct system. 70% might be ok to use.
Different brands, have different SHR's depending on coil match ups.
What is your indoor occupied area design going to be.

[Daltex]

I am changing out the upper ductwork. I'm guessing it can handle about 1000 CFM now and has a mix of flex with either thin or cracked insulation. No mastic to be seen on any connections. I'll do what I can to the downstairs as it's to small also but I may be limited as it's in between floors.

ID design at 75* in occupied areas. 83* in unoccupied. I haven't had the chance to run the numbers yet but if I can't get close to this w/ a single 5 tons then I'll stay w/ 2 seperate units. The downstairs is currently a package unit standard AC with electric heat that the previous owner installed 2 yrs ago.

I have a few changes I'm going to try (interior storm windows) but I'll post back what I can get with the 5 tons for both floors.

With proper ducting what should the sensible % range be?

(More rambling) The upstairs stat is in the landing at the top of the stairs in the middle of the house. There are bedrooms branching off in each direction N & S. With the OD temp @ 95* the stat setpoint is 78* and the bedrooms are 72* w/ 50%RH. As the OD temp drops around 3-4 am the temp in the rooms rises to 77*. All upstairs doors are open. Downstairs set and stays around 78*. Ugh...

[beenthere]

A fair amount of newer units run around 75% sensible.

The stat in the landing causes that in a lot of homes.

An 83° ID design temp will drop the load a lot.

[Daltex]

I started playing around with the ID design temp and to get the load down to the sensible capacity of a 5 ton @ 75% (45,000) I have to raise the ID design temp to 95*. I guess I'll stay with the two 4 ton units but it was an education seeing if it would have worked. Thanks again for all the advice!

[beenthere]

Your welcome.
It was worth checking into.