420sel AC and heat at the same time.
 

Last nite, it got cold. I turn on my heat and noticed that the A/C would hissed on intermittently causing a loss of heat. My A/C makes a slight hissing once it is on and recycling. What happened here? The A/c has been working erratically during the summer. I had replaced the blower motor and ETR and replaced the ac condensor over the summer.

John

The heat should be way overpowering the AC. Not sure on the hissing. Would tend to think it's a bad monowater valve for the heater if you have the heat turned up and no heat. Probably the monowater valve diaphragm is torn, can be replaced seperately.
Gilly