Replace Heat Exchanger or Furnace

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.
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Cant it be blown clean, what did Carrier say, its not leaking Co right, so it is still good, not cracked. Why is it sooted up , burning improperly. Im no pro, but those are a few things to think about. A new unit is only 94.5 max efficiency in Carriers larger than 38000, btu input models.
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Using just logic alone, I'd replace the heat exchanger, since it appears you're getting it under warranty. It will take a long time to recover the cost of a new furnace, even if you DO realize the savings you calculated. The last few % of effeciency is questionable in my mind - you already have an efficient furnace. Heat exchangers should last a lot longer than that - have someone you trust (if you can find such a furnace guy) evaluate this situation. Why did the first one get plugged up.
Other reason I would not send more moeny to the same furnace company, or use the same contralctor that cleaned your present furnace, is: many furnace heat exchangers last a lot longer than 10 years. You got some junk there and are thinking of buying more of the same.
It's a trap to build a crappy furnace, than offer you discounts and rebates to sell you more crap. I dispise that.
Rich
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Tokai wrote:

Hi, Soot? Rust? How come? Tony
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I recently bought the house, and this is the first time I have had the unit tuned and cleaned. So, the problems are not due to the company I called (unless he did something to clog up the secondary).
What I understand is: The primary heat exchanger has rust on it. He said the AC coils and the Humidifier could contribute to the problem. The smaller second furnace (same age and type) does not have a humidifier, and he said the heat exchanger looked good on that one. Other than that, I have no idea what is causing the rust. The furnaces are in a nice dry basement. Humidity use to be 50+% in the summer, but we installed a whole house dehumidifier that keeps it in the 40% range. If it was a prior humidity problem, I would have expected both furnaces to be affected equally.
He said soot in the exhaust pipe is a sign of a clogged secondary. My guess is that exhaust gas from the primary has to bypass the clogged secondary, and somehow creates a soot problem in the exhaust pipe. Maybe the soot is suppose to deposit in the secondary and be cleaned out by the condensate?
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Wrong! You either misunderstood or you are getting a line of crap. Primary flue gasses pass through the secondary. They dont bypass it! If the secondary is clogged, the furnace will shut down via the pressure switch or due to a roll out flame switch. One thing causes the condition of soot in the flue pipe. Improper or incomplete combustion. Several things could be wrong. As simple as too high or low gas pressure on the gas valve regulator or as bad as a cracked primary or secondary heat exchanger. Id check on a second opinion if you are concerned (which you should be). Also, ......rust in a primary is rather normal. Its metal and not always stainless so it will rust over time. Bubba
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After double-checking, it seems to be the Flame Rollout Switch.
The circuit board was giving the 33-error code, which is Limit Switch or Flame Rollout Switch. According to the wiring diagram, there are 3 sensors on the same circuit:
LS - Limit Switch, overtemp - auto reset ALS - Auxillary Limit Switch, overtemp - manual reset FRS - Flame Rollout Switch
According to Note #6, ALS is only on downflow models. On mine, the returns enter the bottom side of the cabinet, and the hot air goes out the top. I assume that it is an upflow model, and thus does not have an ALS switch.
The sensor that is tripping is level with the burners (primary heat exchanger), and on the left. It is a manual reset switch. The other sensor that is wired in parallel is about level where I would guess the secondary heat exchanger is. Since the sensor that is tripping is a manual reset, it does not appear to be LS. By process of elimination, I assume the switch that is tripping is FRS - Flame Rollout Switch.
The manual does say to check for adequate combustion air and exhaust as possible problems, as well as the blower.
I compared the intake and exhaust of both furnaces. The furnace having problems is the larger of the two. The good (smaller) furnace has a strong flow in the draw and exhaust, and the exhaust is humid and odorless. The problem furnace has larger pipes, but the flow is noticeably weaker (covered up the intake with my hand). The exhaust also has a slight odor, and gives me a headache pretty quickly. That same odor is slightly noticeable around the problem furnace (there is a CO detector in the room that has not gone off). I rodded about 5 feet into each pipe from the outside (about one half of the length), and I did not hit anything, and it did not make any difference.
Finally, a new problem is the furnace is starting to drip. It is new because it is just started dripping down the air filter.
Thanks for your help.
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I would probably repair the existing furnace, but then again, I have no idea how much damage a humidifier does to a furnace. Here in Maryland you would have to be on crack to want a humidifier, sicne it is already humid here year round. My furnce was built in 1982 and is working fine.
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Tokai wrote:

this is Turtle.
you want use to tell you about a problem here of furnases.
You have a 90% afue furnace that can be fixed for $400.00 + free parts.
You can get a new 94%-96% afue furnace for $2,600.00 which will save you $200.00 a year.
The saving between the old one and buying a new one is $2,600.00 - $400.00 = $2,200.00 to try to make up for with the savings of the new furnace with $200.00 a year savings. At this rate , it will be 11 years before you get your money back by way of savings of $200.00 a year. then if you figure in if you took this money and put it CD's it will double every 10 years. then this 11 years turns into 21 years.
how long are you going to be in this house ?
How Old are you ?
You will be looking for a new one about the time you get your money back.
You tell me what you want to do after reading this.
TURTLE
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After further analysis, I have finally accepted that once you are past the break-even point, it doesn't matter what happens. I use to think that having alternatives in different phases of their life would affect the analysis.
I talked a little more with the service department, and it is $350 to replace the secondary heat exchanger, or $800 for primary and secondary. I did not think there was that much more labor to replace both. In any case, for my breakeven analysis I will assume $800 since the primary is likely to fail before the break-even date. So given the options of $800, $2,600, 5% interest rate, and $200/yr savings, the break-even is 11 years. Less if the old furnace needs more parts replaced.
If the old furnace has to be replaced after 1 year, then it is self-evident that I should not have replaced the heat exchanger. If the old furnace lasts to 1 year before break-even, then I would have to spend $2,600 in year 10 to get a new furnace vs. $0 for a 10 year old furnace. Seems like I am still better off with the new furnace today. What about year 5? $800 today and $2,600 in year 5, or $2,600 today. I'm still better off with a new furnace because I have saved $200 per year in fuel costs.
The only time reparing the old furnace seems to make sense is if the new furnace has to be replaced before the break-even date. With a 10 year parts and labor warranty, that scenario would have a $0 cost.
I plan on living in the house for at least 8 years. Then the youngest will be in college, and we will think about downsizing. I'm guessing at least 10 years before we move, if ever.
The only remaining variable that could work against replacing the furnace is whether the fuel cost savings are realized. Assuming a marginal 2% increase in efficiency (vs. a possible 4%) and the DC blower, the $200/yr should be achievable.
Thanks for helping me to clarify my thinking.
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Study long, study wrong. replace the thing before you wake up dead.
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Round and round you go--with all the variables/scenerios you have there is no simple answer. If it requires so much thinking than it's probably best to replace the furnace. Your analysis makes reminds me of the following saying " Figures don't lie, but liars can figure". In the end, after all the figuring, the answer will come out the way you want it to. MLD
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responding to http://www.homeownershub.com/maintenance/replace-heat-exchanger-or-furnace-74114-.htm billyEM wrote: What you really want to remember about investing in your home:
Don't do it just because its right and you can afford it. Be a skinflint. Lowering pollution and fuel consumption is not your role here on earth.
A home is made to be flipped and maximally return money. Money!!! Money!!!
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Honey! Why are the dot foo people here, again? And for a five year old thread?
--

Christopher A. Young
Learn more about Jesus
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On Dec 28, 9:27pm, "Stormin Mormon"

It is interesting that this thread started back up exactly 5 years to the day after it originally began: December 28.
Could it be that someone's/some server's system date is screwed up?
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What's worse, is that I just guessed five years. I was going to say three.
Christopher A. Young Learn more about Jesus www.lds.org .

It is interesting that this thread started back up exactly 5 years to the day after it originally began: December 28.
Could it be that someone's/some server's system date is screwed up?
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On Dec 29, 9:19am, "Stormin Mormon"

Whoops...and I just simply trusted your math. Silly me.
It's actually 6 years later.
Man, does time fly!
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