43 years old, pilot out this morning. Probably a hint. When I look. 2 stage, 9 speed
80%, 9?% ,. Presently 80000 btu. No AC

Bi level, I added an intake on the basement floor years ago to make it warmer downstairs

Downsize to 60000? Present easily keeps up even at -25.

IME, most older systems were oversized. Putting too large (BTU-wise) a furnace on undersized ductwork is a recipe for inefficiency and short heat exchanger lifespan.

Get it sized properly.

How big is the place?

If it were my house, and an 80k unit survived for 43 years, I'd put the same size back in. For a property I was going to rent or sell I would go with 60k if it wouldn't be considered undersized.

Also if it were my place I would go with something 90+% AFUE, because I don't like to waste things. Saving for going with 90% over 80% is entirely dependent on how much gas you burn and its price, but typically it would take 10 years for the gas savings to cover the higher up front cost.

We have a Lenox Pulse, I'm told they are obsolete.

What is this "furnace" you speak of?

Obsolete and dangerous as hell. Make sure you have the heat exchanger inspected!

Old furnaces were very inefficient, which is why they were so oversized in most cases. Very doubtful it was anywhere close to 80% efficient- probably more like 40%-50%. Have a heating outfit do a calculation before replacing it to be sure you are getting the most efficient unit for your home... it will be worth the money in the long run...
Depending on if you want to have PVC exhaust duct run to the outside or not will determine if you want to go with a 90% or better unit. Under that rating the existing metal exhaust duct will work fine. If it is a short distance to the outside the PVC duct would be a worthwhile investment, as long as you have a route for it to be hidden and installed with some grade so it will drain any condensation.

Seems like furnaces have a lot in common with cell phones - the one you bought yesterday is obsolete today.

1500 sq ft, 2 floors. I do have a path for pvc to outside, blank wall. 75, so 10 years I may not be here, but plan on living forever. Resale is aconsideration at some time for heirs.
I like the idea of a slow constant circulationwith a 2 speed, or 9? Colorado is in lockstep with California so availability of new gas appliances is going to get hard. Already you cant replace a catalyic converter with OEM, so suppliers will not ship anything except CARB
Nephew works sheetmetal for hvac, so will pick his brain.

Good

Lennox pulse furnace safety notices:

This document describes carbon monoxide gas (CO) leak safety hazards on certain models of Lennox hot air heating furnaces and announces a Lennox pulse furnace safety inspection and recall/repair program for these systems.

Lennox Industries Inc., the maker of Lennox Pulse furnaces, announced it has set up a free program to inspect Lennox furnaces installed from 1982 to 1989 to check for carbon monoxide leaks or as the company stated: "because of "increased instances of corrosion in some Pulse furnace heat exchangers in units installed before 1990."

The old furnaces had carbon steel heat exchangers. The new ones are SS. When they get holes in them , they are very dangerous. The gas flame allows you to be exposed to carbon monoxide.
I’m making the assumption you understand how heat exchange works. Hasbeen

But you have me on Ignore

so good luck with your 1989 unit

Here in Central Texas a lot of our hvac gas furnaces where over sized not because they were inefficient. They were over sized because we were after a larger fan for the cooling season in summer.

Some new ones are aluminumized steel.

Two speed with a smart thermostat is the way to go IMO.... get a good warranty with the unit. Most of the new furnaces have a lot of electronics in them, including the computer that runs all the systems and tells you what is wrong when something fails. This has saved me a ton of money several times because it helps me diagnose the system and replace parts on my Day & Night Plus 80 many times. Had to have the computer replaced right after we got it, but they took care of it on their dime and haven't had a problem since then...

I’m thankful I don’t have to change boilers,furnaces, water heaters, chillers, or heat exchangers any more. It’s a nasty job.

Two stage, communicating, 90 plus, and make sure it has a surge protector.