I have a single family rental in a low- to middle-income area. The furnace is about 22 years old and seems like it may be on it's last leg. When it turns on, it can take 2-3 tries for the burners to light - but it does always light and start, and then functions fine. Another quirk is the furnace won't light unless the front cover is put on upside down. I know that sounds odd but it's true. I previously lived in this house for about 3 years, it did this since I bought the house, but I never worried about it since it otherwise was fine.

I have the furnace serviced every year by various companies and each time the HVAC contractor has said it's safe and functions ok. But they can't explain the odd behavior, I had another that said he "thought" it was a problem that could be fixed for about $450, but would not guarantee that would fix the problem, or even if the part could be found nowadays anymore.

I'd not really want to put a lot of money into repairing an old unit. Still, would you replace it now (house is currently vacant) or just wait until it dies and then replace? I have been thinking it could last a year or another 10 years so just keep it as-is, but not sure if I am thinking about it correctly, like would the replacement bids cost me that much more if it goes out unexpectedly and they know I'm in a bind?

The other part of this is that return air ducts were never put in (it is a gas forced air furnace). The house is small and according to the local inspector, there is nothing against code in doing this - just makes it less efficient - but some HVAC contractors I called would not put a new one in without also adding the new return air ductwork, which obviously increases the cost quite a bit.