A/C air flow question


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Old 09-18-13, 06:50 AM
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A/C air flow question

I have a single unit that controls two zones, upstairs and downstairs. When the downstairs is on, the upstairs will also be blowing cool air. I have just noticed this recently as it is getting very cool upstairs but staying the correct temp downstairs.

The upstairs seems to be getting 60-70% of the air flow while down gets the other 30-40%. So if I set it on 76 on the 1st level, the 2nd level will stay around 73.

The strange thing is when I use the fan only setting, the air flow is normal. However, when I set it to cool it gets "stuck" blowing cool air to both zones at once.

Any ideas what the problem could be?
 
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Old 09-18-13, 04:10 PM
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Actually, what you described is pretty normal operation for a two-story house running a single A/C unit. Since hot air rises, without cooling, the upstairs would be noticeably warmer than the downstairs. So, to adequately cool the upstairs, more cooling (air) needs to be sent to the upstairs.

Normally, there are dampers in the ducts feeding the rooms. The dampers are located near where the duct branches off the main feed. If you check them, I suspect you'll find that the downstairs dampers are closed considerably, while the upstairs are wide open. To balance the airflow between upstairs and downstairs, open the dampers a little in the ducts feeding the downstairs rooms. This will send less cool air upstairs and more downstairs. It's a trial-and-error thing as to the proper setting. Open the dampers a little bit and try it for a day or two. If not enough, open them a little more.

Note, that (assuming you heat the house using the same ducts), you may have to readjust the dampers come heating season, since you need less heat upstairs than you do downstairs. This is the opposite of what is needed for cooling.
 
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Old 09-18-13, 04:30 PM
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Hello Bob, thanks for the reply!

The part that concerns me is this only happened over the weekend and we have lived here for years.

I do have a 2 story home running on a single unit, but I have 2 independent zones controlled by the one unit. Zone 1 being ground level and Zone 2 being the 2nd floor (or maybe the other way around). So when Zone 1 is on and the downstairs is cooling, the upstairs should not get any of the cool air.

I had always assumed there was some type of damper or diverter near the blower that would shut air off to the Zone that is off and open it to the Zone that is one. Now, my problem is that it almost seems like this thing is stuck in a "both open" position.

I apologize for any confusion on my part due to lack of understanding of how these things work. Typically, the upstairs would be warmer and Zone 2 (upstairs) would run more often than Zone 1 (downstairs), but now my problem is when Zone 1 runs, both zones are getting air flow.
 
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Old 09-18-13, 05:01 PM
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OK, I think I understand now how your system is setup. I didn't realize that the system was suddenly behaving differently that it normally behaved. While I'm not familiar with a zoned forced air system, I suspect that it works the way you described. There must be a thermostatically controlled set of "valves" that control airflow to both upstairs and downstairs. When the thermostat in a particular zone calls for heat or cooling, the "valve" for that zone opens allowing air to get sent to that zone. When the thermostat is satisfied, it closes the valve, thus shutting off the airflow to that zone.

It would appear that the thermostatically controlled zone "valves" are malfunctioning. I suspect that your diagnosis is correct, the upstairs valve is stuck in the open position. I don't know where this set of zone valves is located, however I would suspect it's close to the furnace. I suspect that it shouldn't be too hard to locate.
 
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Old 09-22-13, 08:35 AM
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Maybe the upstairs T-stat is not shutting off at its setpoint...?

Switch it to the off position on the stat or, turn the T-stat setting way up & see if the airflow shuts off.

I take it the downstairs airflow is switching off when it should according to the T-stat; if not, then the zoning components appear to be stuck.

When both T-stats are switched off or, are satisfied, then of course, the A/C or heat pump should turn off. Troubleshoot the problem...
 
 

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