By air flow control swotch, are you perchance refering to the speed control for the blower? If so, you probably have a burned out speed control resistor. The blower speed is controlled by inserting resistance in series with the motor. The speed control shorts out parts of the resistor progressively in steps and at maximum speed, the entire resistor is shorted (bypassed). By your description, the motor is good, there is electric power avalable to the heater motor circuit, therefore, fuses/breakers in the circuit are good. You'll need to consult a repair manual to locate the resistor assembly. Often, these are mounted on the air flow duct with the resistors in the air flow. The resistors are cooled by the induced air flow. If you are lucky, it is accessible from under the hood, but if unlucky, it'll be under the dash.

I just now came back to see some responses here. Hey, Coolmen CJ, congratulations on being NCI Certified. No it is not spam; I nor no one else makes a penny from the information I provide on my Web sites. However, if you are a good Tech, you will get a lot more work, because when folks get interested in how their HVAC system is performing & in knowing how to reduce energy use, they will call a contractor for Tech help. I think he is getting a little too hi tech. for the home owner to do all this checking. How many home owners are going to buy air testing meter? (They don't need it.) A lot of dealers don't have them. Later paulbm I don't actually disagree with you Paul. The Tech needs all the air testing instruments; the folks running my simple test only need a thermometer & either a humidity gage or do a wet bull temp check. They can respond on twitter or on a forum, or e-mail me, & I'll go much further with that seemingly insignificant information. Just using that little info data; I can tell them an amazing amount of performance & trouble shooting information, concerning their system's performance. If they provide supply air & rerurn air wet bulb, I can closely ballpark with psychrometric software the BTUH that their system is delivering within those operating conditions. Every Tech could do that, especially if they had worked in HVAC/R since before the mid-1970's. The mfg'ers ought to provide the condenser splits so I wouldn't have to do extra figuring to get the condenser temp split at differing load conditions. Challenge me with actual test data & I'll show what we can come up with I need to know the "SEER Rating of the condenser." Typing with eraser ends of pencils, cause my hands are messed-up is difficult so I'll quit, for now. - udarrell Here is a link that might be useful: Simple A/C Performance Checks Anyone Can Do

pdp at the very least I would add damper control for supply air between first floor, second floor and basement. I would look at an 80K 95+% eff furnace that would include the new full modulating furnaces like York/Luxaire and Carrier/Bryant Infinity systems. It appears you have a four ton AC condenser which on the surface sounds oversized. I would discuss with quoting dealers. Any hot/cold spots in your home should be pointed out to dealer for possible improvement. Do you have adequate return in your home's ductwork? Do you have a pleated media cabinet? if not, I would add one. IMO

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