(Before I start, please know that relationship with my installer has been tense; hence posting the questions here.)

Over the weekend, I finally got the upstairs return that the installer poorly installed connected properly.  It was a job.  Whew. But now, there is a return in the upstairs closet (upstairs is about 500 sq feet total, closet at top of stairs, central, with 2 huge louvered doors).  When the unit is running, there is definitely a breeze going into the upstairs return.  The air travels straight down a 7x14 duct, through a 5x5 hole into the basement where it turns 90 degrees, runs in a joist pan about 15 feet, then turns 90 degrees directly into the main return plenum attached to the GeoMax heat pump.

However, the unit is still going into Stage 2 cooling lockout.  DATs is reading 46 degrees.  Inside house temp was 73 degrees...I set the thermo to 68 to force it on to measure the return duct air flow.

I know Stage 2 cooling lockout occurs because the coil temp gets too low and unit locks out stage 2 to prevent coil freeze up.

Two questions...
If the house is cooling adequately, does it matter about the lockout?  I mean, wouldn't it just mean that the unit can quite happily cool the house using only stage 1.  Or is it better to think that it's a problem because the thermostat is calling for stage 2 for some reason (delta not enough, etc.) and the unit is locking out?

The root question here is WHY is there a lockout...is there not enough air coming through?  Is it the wrong temperature?? 

The air flow from the downstairs returns (much shorter path) is much more than from the upstairs.  Ideally, I could pull mostly upstairs warm air into the returns, but I can't control that depending on the zone.

I measured the return ducts and this is what I found...

GeoMax unit is 4 ton - variable speed - max speed set to 1000 cfm (yeah, kinda on the low side)
Air filter measures 29x32 (trying finding replacements for that size) which is 928 sq inches. 
There are lots of rules of thumb for a 4 ton unit, but 1000 cfm seems to require 500 sq inches of return.  (formulas/rules of thumb)

The actual return plenum right by the unit is HUGE, but it's connected to 2 return ducts.  One is the upstairs return detailed above (joist pan, 7x14 = 98 sq inches).  The other is duct board connected to 4 downstairs return vents.  The duct board measures 8x22 = 176 sq inches cross section of return duct, plus the joist pan = 276 sq inches of return duct.

The actual return duct vents are;  upstairs - small - 5x5 = 25 sq inches (probably less because it has decorative metal bars on it)
downstairs - four 6x12 vents = 4x72 sq inches (again, less because of the decorative metal).

In my mind, that adds up to about 313 sq inches of return, less if you approximate 0.8 for the metal on the duct vent itself....so, maybe 250 sq inches of open return duct.   276 sq inches of actual duct cross section, unit trying to blow 1000 cfm.

Am I missing something here, or does that sound really out of whack?  My calculations seem to indicate my returns are HALF the size they need to be.

Posted By stuart.wyss on 24 May 2010 08:04 PM
GeoMax unit is 4 ton - variable speed - max speed set to 1000 cfm (yeah, kinda on the low side)

Yeah Why so low!

Noise. He dialed it down because it sounded like a airplane whooshing. The unit is right under the kitchen area and noticeable when it ramped up to 1500 cfm. He also installed it right before winter and was trying to keep the air flow lower in order to keep the DAT temperature up.

So he fixed the noise, not the problem

1000cfm is borderline for 1st stage, horrible for 2nd stage

Posted By stuart.wyss on 24 May 2010 08:04 PM
/>Am I missing something here, or does that sound really out of whack?  My calculations seem to indicate my returns are HALF the size they need to be.

Yes you need more air, Can you find an installer who knows what manual D is?

400 CFM per ton produces a temp split of 17-21, depending on a number of variables. Dropping airflow to 250 CFM per ton will drive efficiency way down and produce a temp split approaching 30 degrees. Such a high split likely explains the DATS lockout. You could end up with condensate and mold problems in humid weather as such a large split may overwhelm duct insulation

Your description of the return path includes a 5x5 hole into the basement. That's a problem. It doesn't matter how big the filter is or the upstream grill sizes - that 5x5 section acts like a kink in a garden hose.

Fixed! My HVAC guy came by a couple days ago and turned up the fan speed. The heat upstairs is well under control now, and the unit no longer into lockout. He's also promised to reconfigure the duct work to control the noise more. Lovin' it!

Seems we're in the habit of fixing symptoms here (vs problems). DATS is a control installed on zone systems to keep poor design from harming equipment or icing air coil.
Curiously if system didn't allow heat pump to employ stage 2 when only 2nd floor is calling (as we have suggested before) you probably wouldn't have lockouts, you'd have better design.
As Engineer points out your return is 25 sq" to second floor. Greatfully, returns aren't dampered or system wouldn't work at all with down stairs not calling.
Good Luck,
Joe