Carrier 58MVP and 3T AC...Blower Speed?

[edk]

Hi Folks,
First timer here! Quick homeowner question, if you need more info I'll provide it.
I had 2 Carrier 58MVPs installed, one in basement for first floor, one in attic for 2nd floor and attic. Seperate ductwork as well. The upstairs unit also has a Carrier 3T AC Unit. On winter heat cycle the blowers operates normally, mostly low stage and lower blower speed, occasionally high stage and high blower speed on coldest nights. Seems OK.
My question is rather naive. On summertime cooling (upstairs unit) should the blower speed vary as well? I checked the installation manual and I guess there are a couple of DIP switches that control CFM based on tonnage. Does that mean on cooling their is only one blower speed. In fact that's what I get. There's only one speed "high" when the AC is running. Is this right? Does variable speed blowing only apply to heating and not cooling? As I said if you need more detail I'll comply.
Thanks for the great site as well.

[fat bob]

Depends on your outdoor condenser unit and your thermistat. If you have a two-stage compressor on the outside unit, like this http://www.simpsonsheetmetal.com/PDFs/38-tdb.pdf
it should operate on a low and high blower speed. If you also have a humidity control on the thermostat, like this thermidistat,
http://www.xpedio.carrier.com/idc/gr...lit/019113.pdf, it will slow down the blower on both high and low compressor if the humidity is too high, giving you four blower speeds. (sorry if the links don't work)

[edk]

Thanks Bob
No I don't think it's a 2-stage compressor (I'll check)and the thermostat does not have humidity control.
So if it's single stage w/ no humidity control would a fixed blower speed based on tonnage be normal for the 58MVP?

[jrbenny]

Yes, fixed in the fact that it will always attempt to deliver the programmed AC airflow. The variable speed motor can adjust its output for wet coils and dirty filters to deliver the programmed CFM.

[edk]

Thanks jrbenny
The AC unit is a 38TXA. I believe that's a single stage compressor.
My problem is that when I had the system installed I assumed the AC blower would operate in the mostly low speed mode like the heat cycle. The installer (big local third generation family firm) didn't tell me that it wouldn't or that I would need a 2-stage cooling unit. We agreed to place the 58MVP in the attic directly above the master bedroom. So it's loud on cooling (I think 1200CFM?) at bedtime in the Summer. Again the variable blower on low heat is almost inaudible in the Winter so I'm not faulting the 58MVP.
So what could be done? Would a humidistat on a single stage compressor with a variable blower set to low low humidity slow down the blower on the MVP to increase the on time and quiet things a bit...or what? Any ideas?
Really I don't plan on doing the work myself but I had huge huge problems with the installation contractor and need some education. An example ...the 16" return air duct in the upstairs hallway ceiling lead to nowhere when they finished. Essentially blanked off. The return air duct to the MVP was getting its air from unheated air space in the attic. The attic guy and 2nd floor guy missed each other by 5 feet. And I'm the one who discovered it by checking their work. And there was more.
Oh yeah, I saw a reference to a high pressure and low pressure switch in the 38TXA istallation pdf file from Carrier. Is that anything that might be useful? Can the 2 DIP swithches be set to a lower CFM in the MVP without freezing things up. Any info would really be helpful.
Again guys, thanks..

[jrbenny]

A thermidistat would allow it to run 200 or 300 CFM lower when the humidity is above setpoint.

A two-speed outdoor unit would run at half the CFM most of the time.

Properly sized duct would also solve the problem. You shouldn't hear air noise if the duct is sized properly.

[edk]

Thanks again jrbenny

The noise I referred to is the sound from the 58MVP blower not necessarily duct noise...if there is a difference. The unit sits on the attic floor (on blocks in a pan) directly above the bed really only 6 feet above my bed. Remember it's bedtime and noise perception is really greater than it would be during the day.
Would a 2 stage AC unit have cost much more (3T) the TDA or TDB over a TXA? Any idea how much? I presume the installation cost would be about the same except for a slightly different field wiring.
I'm a little ticked that I wasn't made aware of this by the installation contractor. I paid a lot for top shelf new gas service, two 58MVPs, a 3T TXA, all the ductwork, 2 programmable thermostats, and hot water heater...and it took them 7 months(!!!) to finish the job!

Thanks again

[Short Circuit]

Since you have a single-speed outdoor unit @ 3T, the max speed for your motor should be 350cfm x 3 = 1050. There are dip switches on the furnace board which you can change for the AC (furnace) speed in AC mode. I'd set it for 1050cfm max. See if this helps. For heat, the motherboard has it's own motor speeds based on the BTU output of the furnace, so don't worry about messing up the furnace in heat mode. Just make sure you know what you're doing, but I'm fairly positive that this is your problem. If these switches on the furnace board were never modified since install, then the switches default to their maximum speeds, which is probably way more than your guessed-at 1200 cfm. Sounds like your install contractor didn't do the "complete" job needed. You need to get your hands on the installation & maintenance manual for that furnace to get to the page describing the exact positioning of the dip switches...