Trane / AmStd Problem with Blower Off Delay in Cooling

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Old 06-16-15, 06:54 AM
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Trane / AmStd Problem with Blower Off Delay in Cooling

I had two new American Standard systems installed last year, 5 ton downstairs and 3 ton upstairs, both with 2-step Scroll compressors and variable speed air handlers. The downstairs air handler is a communicating model (though it's set up for normal 24v control), while the upstairs air handler is not. Incidentally, after noticing very low airflow upstairs when on stage 1, I did some research here on this forum and discovered that it had not been wired properly (was using Y LO and Y instead of Y and BK). The installer came back and fixed it.

After that was fixed, we did have some problems with humidity upstairs, which the installer improved by setting a slower fan speed and also by turning off Comfort-R (the early part of the Comfort-R profile was helpful, but the fan-off delay at the end of the cooling cycle was putting a lot of the humidity right back into the space).

However, I've just discovered that there is still an fan-off delay. After the call for cooling is satisfied, the blower continues to run (at the same speed) for exactly 60 seconds. That alone seems pretty odd, as the documentation doesn't even list that as an option (1.5min/100% and 3min/50%, along with Comfort-R). I've confirmed that S4-1 and S4-2 are off, which should result in no cooling off delay, right?

I will have the installer come back out and have a look, but before I waste his time, I just wanted to make sure that this wasn't normal behavior, even if it's contrary to what the manual seems to indicate.

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Old 06-16-15, 07:16 AM
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I don't know if it's normal or not, however (personally) I like to let the fan run for a short time (60 seconds sounds about right) to get the residual "cold" out of the evaporator coil after the compressor/condenser shuts down. I feel it's slightly more efficient that way, since the evaporator coil will still be cold shortly after the compressor turns off. I don't think that the extra 60 seconds of the blower running after the compressor turns off will introduce much humidity back into the house.
 
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Old 06-16-15, 08:35 AM
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Hi Bob,

It's tough to say whether or not it's hurting the humidity upstairs. Downstairs, the humidity (during cooling season) is consistently in the upper 40% range, while upstairs it's usually about 55% (certainly tolerable, but a bit higher than I'd like).

But I don't know how much of that is due to the downstairs not having this cooling-off delay, vs. the fact that upstairs has the frequently mentioned issue of the low stage of the two-step compressor being around 70%, while the blower in low stage runs at 80% (the net effect of which is that low stage gets 400cfm/ton instead of 350cfm/ton).

Additionally, if this presumed anomaly (cooling-off delay existing despite this function being turned off) is the result of a bad control board, I'd like to have it replaced just for peace of mind.
 
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