I originally posted this on alt.hvac, but received a courteous suggestion to post on this group.
I can either replace a heat exchanger for about $400 (part under warranty, labor based upon actual time), or buy a new furnace installed for a net price of $2,600.
Old furnace: 90% Carrier with a clogged secondary heat exchanger (still works for about 15 minutes at a time until the limit switch trips, so can maintain temp in current 30 degree weather). 88,000 BTU. Manufacturing date of 1989, but some evidence suggests it was installed in 1992. I replaced the motherboard a few years ago (did it myself for about $125). The igniter was replaced a few weeks ago (free because the service guy broke it during the tune & clean). Other than that, I think all the parts are original. I think it has a lifetime heat exchanger warranty (part only). Clogged secondary was diagnosed by: 1) Tripping of Limit cutoff switch; and 2) Soot in exhaust pipe.
New furnace: 94-96% Carrier Infinity 96. 80,000 BTU. Net cost of $2,600, which is $3,550 less: $150 Carrier rebate, $170 for paying cash (rather than credit), $150 utility company rebate, $400 for replacing a Carrier (bad heat exchanger program?), and $80 for tune and clean performed 10 days ago. 10-year parts and labor warranty. Lifetime heat exchanger warranty (part only). Because it is replacing a 90% Carrier, it is a pretty direct swap-out. The new unit is a little shorter, so a short collar to bridge the gap. PVC air supply and exhaust pipes are fine.
Situation: South East Wisconsin. I'm guessing the new furance will save $200/yr, but also make the house more comfortable because of the multi-stage, and hopefully a little more quiet.
Because of rebates, a single stage 90% Carrier is only $300 cheaper, and the other 2 stage Carrier is about $150 cheaper. The comparable Lennox is about $500 more expensive (primarily due to the $400 Carrier rebate). I have not priced a Trane or Rheem.
I currently have 2 furnaces, and the other Carrier is identical, except for being a smaller BTU. If I replace the furnace, I would keep the old one for spare parts.
Questions:
1) Any thoughts on replacing the heat exchanger, or the whole furnace (seems like the age of the furnace is on the edge of being "old")? 2) Does the price seem reasonable? 3) How can I determine if they will install it right? 4) Did they cause the problem when they tuned the furnace (dislodged rust that clogged the secondary)? Service man did say the heat exchanger was rusting, but should last a few more years. 5) If they didn't cause the problem, should they have been able to see the problem during the tune and clean (the problem arose when I set back the thermostat 5 degrees when we were away for the weekend, and the furnace needed extended run time to warm up the house)?Thanks in advance for your help.