a/c cycle times

[mrb8828]

After earlier discussions I lowered my blower speed from "high" setting to "medium/low" setting and getting longer run times 15-20 mins versus 8-10 mins. I was able to find specs on furnace blower speeds via Carrier website. According to their site the "high" blower setting is 2400 CFM, "med/low" 2200 CFM, and "low" 1700 CFM (5 ton/12 seer unit). My question is if 400 CFM per ton is the ideal fan setting then why in the world would they default set the cooling fan speed on "high" which is 2400 CFM (6x what you need)!
Does this sound like accurate information or is there much more to it?

Thanks for all the help.

[klyons20]

you have to consider more than fan speed

duct size supply and return
type of filter
type of A coil

everything that put into the air supply cause restriction.

Put it back to high leave it alone..

At least with more cfm's you sure that the liquid is boiling off the a coil.

regards,
kelvin

[beenthere]

Thats 2400 cfm max for the unit.

Theres awhole lot more to it.
Doubtfull you were getting 2400 cfm.

[mrb8828]

I was initially having humidity problem which I could only attribute to short cycles. I've had unit checked twice in 6 weeks and everything is to spec. I was hoping that setting back to "medium" speed would increase humidity removal by extending run times. Do you guys think I'm harming the system by lowering to "medium" speed ? According to Carrier's chart thats only 200 CFM less than max speed. If I do need to switch back to "high" setting do you guys have any suggestions to remedy short cycles/humidity problems?

Thanks,
Mike

[beenthere]

A 5 ton unit can have a cfm as low as 1750 on a psc motor, and not harm the unit.

[mrb8828]

I definitely do not want to harm the compressor but the high setting is just not allowing the evaporator to pull the moisture out of the air. I think the unit is oversized anyways (5 tons for 2400 sq ft, 2 story). The "high" speed is blasting air out of the registers and satifying the thermostat quickly. I have even partially closed off all of the downstairs registers to avoid that problem. If lowering the blower to medium setting is going to harm the compressor then I guess I will have to live with the humidity for now. The humidity consistently runs in the low 60's regardless of how low I set the thermostat.

Thanks for the replies!

[beenthere]

Try it on low, go out and listen to the outdoor unit, if it changes its sound, then go back to med.

How old is your system again.

[mrb8828]

Less than two years old

[beenthere]

Short of changing out the entire system, or having the air handler replaced with a VS blower.

You can have an installer do this.

Run a 8" bypass with motorized damper, power open, spring close.

Wire it to open on a cooling call, and wire it through a freeze stat, so that if the coil starts to get too cold the damper closes.

I've done this to a couple over sized systems, and it helps.

Might get you a 6 to 8% drop in RH.

[mrb8828]

That seems to be a better option than installing damper/baffle system throughout. Got a bid last year that was $2300.00. I will try to "low" setting tomorrow. Do I just listen for louder humming coming from compressor?

[hoozdis]

....make sure that compressor isnt sweating. What is your temp difference on supply vs return air?

[mrb8828]

temp difference is 15 degrees....I'm pretty much a novice at this so I'm assuming that a sweating compressor is bad. Last summer the compressor was sweating so badly that a stream of water was coming out from underneath the condensing unit. It was under warranty at the time so I had install company come out to check. They told me that it was normal for compressor to sweat on very hot/humid day. Was I sold a bill of goods?

[dan sw fl]

Originally posted by mrb8828

"high" blower setting is 2400 CFM
"med/low" 2200 CFM
"low" 1700 CFM (5 ton/12 seer unit).

My question is if 400 CFM per ton is the ideal fan setting then why in the world would they default set the cooling fan speed on "high" which is 2400 CFM (6x what you need)!

2,400 High / 2,000 CFM Nominal = 1.2
..OR.. only 20% over Nominal

WHO stated that 400 CFM / ton is Ideal setting?
400 CFM is a Nominal flow rate for measuring Air Handler performance in 'test' conditions or Average so to speak.

15' F delta T, you should have ~20'F for improved humidity control ( 50% R.H.).
So Medium setting will likely increase dT to ~17' or 18'F. RESULT ~ 55% R.H.

AND You never mentioned your Location ...
_ Maine or Texas _

5-ton is just about right size ..
but depends on Location, House / Window Orientation,
Insulation, Tightness, amount of duct in the Attic and WINDOWS .. Glass Tinting / # / Area.

[klyons20]

If you are using a 5 ton unit for 2400. there is nothing you can do but change the piston in the a coil to 3 1/2 ton and go purchase a 3.5 condenser.

regards,
Kelvin

[mrb8828]

I live in South Texas where it is already in the low 90's with 70-80% humidity and will only get hotter from here. Anyone know about this "sweating compressor" stuff? I thought that was normal on a humid day. I have never had 50% RH since I bought the home new 2 years ago (Always 60-65% regardless of temp setting). Unit has been serviced twice this year and tech says everything to spec. Short cycles will not allow dehumidification. If I'm going to fry the compressor on "medium" blower speed then I'll have to live with the humidity and figure something else out.

[fat bob]

My Carrier furnace and air handler are a single unit. The cooling coil is a separate unit and different sized coils can be attached to the furnace unit. There are separate pin settings for heating and cooling. If you adjust the blower speed for the furnace it will not affect the AC and vice versa. For AC, one wire is to match the tonnage of the outside condenser, in half ton increments, and a second wire gives three settings--315, 350, and 400--cfm per ton once you set the correct tonnage. It's almost impossible to figure out without studying the manual for your furnace or prior training.

[mrb8828]

My Carrier is the same way (furnace with attached a/c on top)..On my circuit board there are three color coded wires (black-high, blue-med, red-low) and three corresponding pins that say "cool", "heat" and "spare". Right now the black wire is hooked up to the "cool" pin and the red wire is hooked up to the "heat" pin, with the blue wire hooked up to the "spare". So you are saying that changing the wires around on those pins will not affect airflow to the coil?

Thanks!

[casturbo]

Originally posted by mrb8828
My Carrier is the same way (furnace with attached a/c on top)..On my circuit board there are three color coded wires (black-high, blue-med, red-low) and three corresponding pins that say "cool", "heat" and "spare".

Those pins are the output terminals to the fan. The board runs the "heat" pin in the winter for heating and the "cool" pin in the summer for A/C. What you hook to those pins is up to you (fan speed-wise). If you hook the blue wire to the cool pin it will run the fan on medium speed for cooling and whatever color is on the heat pin will run that speed. Almost universally, red is low, blue is medium, and black is high speed.

[fat bob]

I really don't know. I just know on mine, the yellow wire, for example, will adjust the blower to match to condenser tonnage. There are also green, purple, blue, and maybe a red one that all affect the blower. For some you have to power down and others you don't. It is really not intuitive unless you're a mechanical genius or you have a manual or prior training.