Slowing fan motor speed in air handler

Manuel6

I am currently in the process of correcting a botched HVAC system installation (mainly ducts that are too small or improperly connected). Among the many problems that I have to deal with is the fact the air handler is a 3.5 ton unit and the outdoor unit is a 3 ton (by the way, it's a straight cool system with 10k electric strip). The air handler fan has three motor speeds low, medium, and high. My question regards lowering the fan speed and its effect. The fan speed is currently on high, and consequently, along with the small ducts, the velocity of the air is much too high. It sounds like a rocket is about to blast off in some of the rooms when the a/c turns on. I plan on increasing the duct sizes and lowering the fan speed to low. Will lowering the fan speed have any unintended consequences that are not immediately apparent to me?

jmosh

in heat mode i bet it will overheat or possibly slowly melt wires or controls. electric heat can kill so be careful, and duct sizes are usualy sized to the specific room sizes , number of ducts per room and total, length of duct , return size, dampers and difuzers yes or no, and some other less important specs.
generaly 4" bathrooms, and closets, 6" for small rooms and small bedrooms, 8" for larger rooms and master bedrooms and rooms with no exterior walls require less or no duct at all ( depending)

jmosh

oh yea 3 ton condenser and a 3.5 ton air handler is not uncommon, but a little odd as it would be somewhat understandable if it were a heatpump.

Manuel6

So Jmosh....in other words keep the fan on high, right?

jmosh

well a little more info on the size of your house , ducts and return size would be helpful, but lowering the fan speed and increasing duct size and number of ducts would be a disaster in my opinion

Manuel6

I live in Central Florida. The house has 1500 sq. ft under a/c. The exterior walls have 3/4" of an inch of insulation. The main trunk line is 12" flexible duct, which needs to be increased. The return lines are two 10" flexible ducts- also too small. The return grill is 16x20...too small.

There will be no added ducts. A west facing addition near the back of the house with two french doors (one adjacent to the other, south and west walls) and one window has a 6" duct. This is the supply duct that needs to be increased. Also, a 100 sq.ft. remodeled kitchen with 3 west-facing windows and a west-angled skylight has a 6" inch line that needs to be increased. Other than that, the ducts need to be sealed properly. With respect to the fan, the manual that came with the unit discusses a fan setting for mismatched units, which is what peaked my interest. And I know that during the heating cycle there has to be a minimum amount of cfm's blowing past the elements, what I didn't know is to what degree the fan would be slowed down if put the fan in one of the other slower settings and how that would affect the heating cycles.

jmosh

increasing the 2 6inch ducts to 8 inches, i would say you can change the return box to a 18X24, 20X25 or 20X30 those are pretty common sizes but the 2 10 inch lines should suffice. the supply i guess you could change to a 14 inch , but i think with the 2 increased increased ducts and if you put a couple of turns in the flex suppling the noisy outlets they would be fine also.

Manuel6

Thanks Jmosh!