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South Florida - FAN SPEED

2K views 19 replies 5 participants last post by  Gwing21 
#1 ·
Recently moved into a 1500 sqft home with a 3 Trane AC from 2016. AC seems to be perfectly sized and runs about 12-16 hours a day. Set to 72 during the day and 68 at night.

Humidity has been an issue for us, but not what you’d expect. It’s too low. At night we are at around 35%, and with a newborn it’s just too low. We wake up with dry noses.

Another thing I noticed, when outside temp is 87, it can’t seem to manage to cool lower than 74 during the day, since it’s set to 72 it’ll run non stop from noon to like 6-7pm and hover around 73-74. Today I checked the intake temp, ~73 and output ~49. Seemed like a large delta.

I checked fan speed and it was set to 2 out of 5. Looking at the manual it seems 4 is default. Maybe it was lowered in the past due to previous owners wanting lower humidity?

I cranked it up to 5 and got a delta from 72 to 51. My question is, why is the delta t so high? Is raising fan speed going to give me a bit more humidity and hopefully get more out of the coils and lower temp more during the day? Only lowering temp from outside by 13 seems a bit low.

Any feedback would be appreciated befo
re I go and break something lol
 
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#2 ·
Raising the fan speed should increase the humidity as well as overall capacity. Run times should be shorter due to the shift from dehumidification to actual cooling as well as slightly increased capacity.

What's the complete model number of the air handler?

If it's a 3 ton air handler, it needs to be on high.

A wetbulb return air temp measurement is required to know what the supply should be and the measurements need to be taken near the air handler.
 
#3 ·
Raising the fan speed should increase the humidity as well as overall capacity. Run times should be shorter due to the shift from dehumidification to actual cooling as well as slightly increased capacity.

What's the complete model number of the air handler?

If it's a 3 ton air handler, it needs to be on high.

A wetbulb return air temp measurement is
required to know what the supply should be and the measurements need to be taken near the
air handler.
Here is the model number. tmm5a0b36m31saa

When I bought the house, home inspector said it looks like they replaced the compressor in 2016 but left the older air handler. Not sure what year it is or if that’s true.

Either way, it was set to 2 and now it’s at max of 5. I’ll see how that works out. Feels much better but it’s also later so we’ll see
 
#6 ·
The delta-t depends on the indoor humidity and airflow, not the refrigerant.



Raising the fan speed improves efficiency and capacity. Current draw is little higher but capacity increase exceeds current draw and run time is reduced.
 
#10 ·
A newer 3 ton AC working correctly should be plenty for a 1500 sq ft home. Note that depends on factors other than just size - insulation, exposure, operational efficiency....... The fact that your delta is 20+ degrees on the higher speed setting suggest the equipment is operating correctly but check your filter is clean and that you have an appropriately sized return/filtering area (600 sq in for 3 tons).

If you still can't get down to 72 when its 87 outside, I'd look at your insulation and weatherproofing.
 
#11 ·
600 SQ inches? I think that might be the problem. The house is old and was retrofired with an AC in a small closet. The handler basically is the size of closet. Closet door is I believe either 20 or 24 by 80 high, louvered. I'm not how many SQ inches the lovers let through but their quite tight. The lovers run from top of the door to the bottom with about 4 inches of frame on the door.

Even with the fan speed now on high, the door doesn't seem to slam shut or have a big vacuum when trying to open. There is some but not crazy amounts. I also don't hear a noticeable difference in engine noise when open or close.

The only way to take advantage of more would be to cut a hole into the adjacent closet that uses sliding glass doors. They aren't flush and have about a 2 inch gap all around so they would have plenty of flow. I would obviously like to not do that.
 
#12 ·
Without capacity measurements (measuring airflow and enthalpy via wetbulb readings), it's a guess as to how well it's working.

Low airflow can cover up reduced capacity and temp measurements can look normal.

If you measured your temps at vents and not air handler, the delta-t is higher than you think. You could have a lot of duct loss into the attic if your ducts were run through it.
 
#19 ·
the filter can be at the air handler inlet or at the return grill.

there's always a way to put a larger filter.

fiberglass ones aren't restrictive to airflow so i don't think the filter is the problem.
 
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