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Old 07-27-2018, 10:26 AM   #1
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How does the gas furnace degrades over its lifetime?


When it comes to HVAC A/C unit, if nothing breaks, it's the refrigerant that needs refill once in a while, or the coil needs to be clean once in a while. Long term wise, the compressor just become less and less efficient at doing its job, and eventually not capable of converting hot air into cold air.

--

What about the gas furnace, given that nothing cracks or breaks, is there a component that gets less and less efficient over time and then eventually not capable to producing heat?
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Old 07-27-2018, 10:46 AM   #2
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Re: How does the gas furnace degrades over its lifetime?


No.....

Eventually the heat exchanger cracks or rusts thru and then it is unsafe.

Google: " cracked heat exchanger > images " to see what happens.

Once the furnace gets old enough that the cost of repairs starts getting too high most people invest in a new one.
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Old 07-27-2018, 10:55 AM   #3
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Re: How does the gas furnace degrades over its lifetime?


you should never need refrigerant added to an AC system - if you need to add it, that means some leaked out.

assuming you have always run your air handler with a properly fitted filter, the need for indoor coil cleaning should be minimal. a scope camera is the easiest way to look at it - make sure you don't drill a hole into your coil in the process.

the outdoor coil would probably be best served by seasonal cleaning - removing leaves/detritus, washing the coils down to remove dirt/dust buildup.

AC works until (insert component here) breaks. assuming the brazing was done well, and there are no corrosion issues, the things i think are most likely to fail are electrical components/exposed parts - e.g. contactors, capacitors, condenser fan bearings.
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Old 07-27-2018, 01:04 PM   #4
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Re: How does the gas furnace degrades over its lifetime?


The heat exchanger's metal rapidly expands and contracts every heating cycle.

Over time it weakens and eventually cracks or develops holes.

Also, many heat exchangers are made up of two stamped pieces held together by crimp rings. The expansion puts pressure on the rings and if the heat exchanger is overheated too many times they can pop off or crack the metal being held.

Generally, if you're getting major corrosion, it's caused by water condensing where it shouldn't be. If the furnace is set up right - fan speeds and fuel input, you shouldn't have this issue.

You can get a unit with a stainless steel heat exchanger which is far less likely to corrode. Not all manufacturers will offer this, most are aluminized steel.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
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Old 07-27-2018, 03:22 PM   #5
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Re: How does the gas furnace degrades over its lifetime?


Quote:
Originally Posted by user_12345a View Post
...

"Over time it weakens and eventually cracks or develops holes."

There's absolutely no way a typical homeowner can somehow open the furnace up and inspect the heatexchanger, then put everything back together without messing anything up right?


"...overheated too many times they can pop off or crack the metal being held."

What causes overheating issues? Fan speed not high enough to extract the air from the heat exchanger and blow it to the rest of home? What can the owner do to reduce/eliminate the likelyhood of overheating?

---

Follow up question, are "life time warranty" on heat exchanger pointless when shopping for a new gas furnace? I'm only asking this because after speaking with you and a few others online, it's very consistent that when the heat exchanger cracks, it's best to replace the entire gas furnace because the labor fee is very high. So after hearing that, what's the point to even have ANY warranty on the heat exchanger.
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Old 07-27-2018, 03:32 PM   #6
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Re: How does the gas furnace degrades over its lifetime?


The labor is high.

$600 - $1000.

You need a warranty as sometimes manufacturers have factory defects that they find out 10-15 yrs later.

Google: Carrier Class Action Lawsuit for more info.

If it goes in under 15 yrs it may be worth replacing but usually after that the next expensive repair added to it like a circuit board or exhaust fan will cost so much it is not worth having done both.
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Old 07-27-2018, 03:38 PM   #7
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Re: How does the gas furnace degrades over its lifetime?


Quote:
There's absolutely no way a typical homeowner can somehow open the furnace up and inspect the heatexchanger, then put everything back together without messing anything up right?
They aren't taken apart like that for inspection.

The mids can be inspected by removing the blower assembly, crawling in and looking up.

The high efficiency, they can put a spacer between the indoor coil and furnace and an access hole can be made, covered with metal.

They can inspect by removing the limit and using a snake camera.

They can do combustion analysis tests and see if the excess air goes up when the blower comes on.

Quote:
What causes overheating issues? Fan speed not high enough to extract the air from the heat exchanger and blow it to the rest of home? What can the owner do to reduce/eliminate the likelyhood of overheating?
Make sure it's set up properly, your contractor needs to set fans speeds and fuel input right. Needs to be installed properly - ducts, return drop, filter need to be sized large enough for furnace.

The thermostat should be set up to not rapidly cycle it. Honeywell uses cph setting, and they come from the factory set to 5 or 6 while 3 works better.

The thermal cycling is what causes the metal to fatigue and crack. Overheating makes this worse.

Quote:
Follow up question, are "life time warranty" on heat exchanger pointless when shopping for a new gas furnace? I'm only asking this because after speaking with you and a few others online, it's very consistent that when the heat exchanger cracks, it's best to replace the entire gas furnace because the labor fee is very high. So after hearing that, what's the point to even have ANY warranty on the heat exchanger.
I think if it's under 17 years old but cracked (middle aged), it makes sense to pay the labour and use the warranty. Still cheaper than new furnace.

Life may be 20 to 25 years. Old furnace, no point of putting a new heat exchanger when the other parts will fail.

Lifetime warranty is a marketing gimmick. 20 years should cover you.
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Old 07-29-2018, 10:09 PM   #8
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Re: How does the gas furnace degrades over its lifetime?


Depends how much of the arrest of the furnace has deteriorated. It also depends what climate you live in. The climates were you have a lot of humidity a lot of equipment so I get to go 15 years without problems from just deterioration. If your system is 17 years old changing out the heat exchanger I'm assuming it's only an 80% efficient furnace just depends how bad everything else is. If it's an 80% might not be too bad but if it's a condensating furnace I seen where they are just getting destroyed. Things Fall Apart. You're going to have to pay for labor on heat exchanger and it's going to be an 8 hour job at least. So you're going to be pushing a thousand bucks when you're done assuming that the heat exchanger is warrantied

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Old 07-30-2018, 07:11 AM   #9
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Re: How does the gas furnace degrades over its lifetime?


Also, the bearings in the blower motor can wear out. Particularly if they are bronze bushings rather than ball bearings.
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Old 07-30-2018, 08:14 AM   #10
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Re: How does the gas furnace degrades over its lifetime?


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... it's going to be an 8 hour job at least. ....
No more then 6hrs except for special cases. Should be able to knock it off in 4hrs, including picking up a new HX. Some models can be changed in under 40 min, start to finish.

Cheers!

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Old 07-30-2018, 08:37 AM   #11
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Re: How does the gas furnace degrades over its lifetime?


Quote:
Originally Posted by SPS-1 View Post
Also, the bearings in the blower motor can wear out. Particularly if they are bronze bushings rather than ball bearings.
Does this apply to PSC blower motor or ECM blower motor?
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Old 07-30-2018, 08:38 AM   #12
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Re: How does the gas furnace degrades over its lifetime?


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Does this apply to PSC blower motor or ECM blower motor?
Both. ECMs are generally ball bearing though, and are built a bit better for now.

Cheers!
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Old 07-30-2018, 10:11 AM   #13
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Re: How does the gas furnace degrades over its lifetime?


If you can do a heat exchanger in 4 hours on a residential furnace even 80% you are the man! I've never been able to do it that fast. There's always a few rusted up screws that you got in a fight in my experience doing it. If I had to bid the job I'd still put in 8 I couldn't do it at 4!
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No more then 6hrs except for special cases. Should be able to knock it off in 4hrs, including picking up a new HX. Some models can be changed in under 40 min, start to finish.

Cheers!
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Old 07-30-2018, 10:33 AM   #14
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Re: How does the gas furnace degrades over its lifetime?


4 is OK if you do the same unit over and over and over again like a Lennox Whisperheat and it is a small BTU unit. Once you get the bigger ones it may take 2 guys and that costs more.

6 would be average IMO. Some high efficiency ones take longer as you may have to deal with the venting depending on how it was installed.
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Old 07-30-2018, 10:41 AM   #15
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Re: How does the gas furnace degrades over its lifetime?


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Originally Posted by yuri View Post
4 is OK if you do the same unit over and over and over again like a Lennox Whisperheat and it is a small BTU unit. Once you get the bigger ones it may take 2 guys and that costs more.

6 would be average IMO. Some high efficiency ones take longer as you may have to deal with the venting depending on how it was installed.
Practice always makes perfect. I'd expect someone to charge 6 hrs, but that includes driving and picking things up. I change both RTU and resi HXs regularly. A 200k btu RTU is easier and faster then an 80k resi.

Cheers!
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