I recently purchased a townhouse with an air-source heat-pump and have 3 main questions. The unit is 22 yrs old (same age as the house).

1) Now that the cold weather has set in (since +/- November...I live in Montreal), it seems to go through the "defrost" cycle frequently. The first time I heard the valve release "hissing" noise and saw steam come off the outdoor unit, I thought it was blown but now realize it's normal. But how often is "normal" for the unit to go through that cycle each day? Every time the unit turns on/off?

2) The house also has an electric furnace, which is the "aux" source on the thermostat. During winter months, should I switch it to run off of the furnace instead of the heat-pump? (Note that in Montreal/Quebec, electricity is the main energy source because it is inexpensive).

3) Is there a limit to the coldest outdoor temperatures where a heat-pump will still be effective? I have noticed since the weather got colder that the air coming out of the registers is never really warm. Is this a physical limit of a heat-pump or just that my unit is old and dying? (I also recently suspect a duct issue in one particular spot but to be investigated)

4) What are the main regular maintenance items that I need to plan for? (I am a capable handyman but interested to know what I can do myself or if pros are needed)

Thanks in advance. I appreciate any insights as I am new to heat-pumps.