20 year old Lennox heat pump - outside temps been 30's at night, 40's daytime

UPDATE: I went outside and the unit is covered in frost on 3 sides! I've been reading on Google about frosted heat pump units and can't determine how to manually defrost it. Apparently, the auto defrost is not working. Can I turn it on A/C to defrost quickly? How about turning it off via inside thermostat?

Thanks for your time.

In cold weather it is not smart to run the A/C.
You need to replace the defrost cycle control.

hvac1000 - Thank you for your answer. This helps me decide that we will call the local repair man to come and fix it. We had a little trouble with the A/C this summer and, as luck would have it, the company we called was the one who installed the original unit 20 years ago! The man who owns the company came out and told me about when he put it in.

20 year old Lennox heat pump - outside temps been 30's at nite, 40's daytime

UPDATE: I went outside and the unit is covered in frost on 3 sides! I've been reading on Google about frosted heat pump units and can't determine how to manually defrost it. Apparently, the auto defrost is not working. Can I turn it on A/C to defrost quickly? How about turning it off via inside thermostat?

Thanks for your time.
Broken sensor wire, repairman says. Replaced it. Works now. Maybe this will help someone...

I am glad it was only the sensor for the defrost cycle control and not the entire control. That would have been more expensive.

20 year old Lennox heat pump - outside temps been 30's at nite, 40's daytime

UPDATE: I went outside and the unit is covered in frost on 3 sides! I've been reading on Google about frosted heat pump units and can't determine how to manually defrost it. Apparently, the auto defrost is not working. Can I turn it on A/C to defrost quickly? How about turning it off via inside thermostat?

Thanks for your time.
You would be doing that all the time but you would have to shut off the OD fan every time you wanted to defrost the OD coil. On most heatpump defrost boards you can manually test. But on yours you have a timer. There is a little window to see if gears are turning if not replace but you can also manually turn knob it should go into defrost. Let me know