Signs you need a new gas furnace?


Having never owned a home before, and certainly never having had to service a furnace before, what signs should I look for that my furnace is past the point of serviceability and simply needs replacement? That giant cloud of gas that forms in the basement shortly before detonation is an obvious one - what others?
Some facts about my situation, but I guess I'm more interested about the general case.
My particular gas furnace is approx 25 years old. It seems to heat just fine, however its startup behavior seems weird to me.
basically, tick, tick, tick, tick, .......(about 10 seconds later) then ignition, burners doing their stuff for about 10 seconds, blower comes on and air starts moving. Burners shut down blower shuts down 10 to 15 seconds later.
All of this seems normal, but I'm wondering why it takes so long for the gas to ignite (I assume the tick, tick, tick is the sound of the igniter sparking) and whether or not this is one of those things where you just close you eyes and walk away.
I keep my filters clean and change it regularly.
At what point would I KNOW that this furnace is DOA and shouldn't be repaired? At what point should I just call in the HVAC dude and have him/her/it repair it?
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061014 1903 - Eigenvector posted:

You don't need a new furnace until the heat chamber is cracked and emits carbon monoxide fumes into the air stream. It sounds like the ignition is trying several times to fire the gas before it finally does. Maybe you could take the cover off the ignition chamber and watch it while it is working. The gas control first turns on the gas valve and ignition. If the ignitor does not fire the gas, the gas valve will shut down. This is a safety device. Some systems have a purge fan the starts before all else does, and this has an air flow switch to insure that there is air flowing through the firing stream before the gas valve will open and ignition begins. If you have a purge fan, make sure that this is running when it is supposed to. Also, check to make sure that whatever is supposed to "see" the flame does so. Maybe an electric eye needs cleaning, or a thermocouple needs to be readjusted.
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That sounds exactly the same as my Lennox of the same era (1980). Upon startup, it makes about 8 different very loud clicks. On my particular model, there is no standing pilot -- instead a pilot is lit electrically, then lights the furnace. One of the clicks is the gas for the pilot turning on. If you're close and watching, you'll also hear a 'zzzz' sound of the lighter. Once the unit senses the pilot is on, it fires up the main burners (click!). There are several other clicks in this sequence that I have no idea what's going on, but it's been doing that for the 3+ years I've owned the place and seems to be normal (at least it hasn't broken down yet).
Pretty much all gas furnaces run the burners for awhile, then when it gets up-to-temp, the blower kicks in. If it didn't, you'd get cold air blowing out your vents. Conversely, when the thermostat quits calling for heat, this kills the burners, but the blower keeps going for awhile longer (either a time delay, or until the internal temp falls below a threshold, depending on the model).
So yours sounds normal to me. Everntually I want to replace ours due to effiiciency (it's about 75% efficient internally, probably much worse overall) and we're in MN where it get's a good amount of hours running every year so kicking up the efficiency would pay off sooner or later. But for now I'm making do with the old one...
Like you, the clicks disturbed me when I first moved here. Our old house had a furnace with a standing pilot and it was all but silent other than the blower motor...
-Tim
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tick, tick, I keep waiting to hear the <KABOOM> as the gas builds up to a critical level. So I guess as long as my furnace isn't venting CO to the air I should be okay. If it does break down, I think that will be the point at which I give in and go for an efficient modern one.
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It always a good idea to have your furnace serviced for preventive maintence BEFORE it breaks on a cold nite!
way cheaper and a clean serviced furnace is more efficent and safer.
Waiting till something breaks just isnt a good idea:(
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An annual service call to inspect the unit is well worth the money. You are paying for a recommendation and you should ask any questions that you have before they leave regarding noises that you are hearing or life expectancy. Rust in the heat exchanger is something to be expected.
wrote:

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Eigenvector wrote:

tick,tick,tick...then whoosh
Gas furnaces need ANNUAL inspections to determine when the heat exchanger fails.
Heat Exchangers will rust for a number of years. Rust, by itself,is not bad. The bad part comes when the rust goes all the way thru the heat exchanger walls and now you are distributing combustion by-products in the air you breathe
This rust is a common HVAC ploy to get you to replace the furnace LONG before you need to.
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Not true -- use one every time we cook on the stove. My furnace's sound is much different -- like I said in the post it's a loud click that seems to be coming from the gas valve (as some of the clicks correspond with the pilot or burners coming on, etc). Perhaps the OP's sound is different than mine, if he really means a 'tick'.

Probably great advice, but I don't personally heed it. Spending $100-$150 to have someone vacuum my furnace (I did it once) seems a bit much I do agree that it's a good idea every few years. I just wish the person who would be telling me it's time to replace the unit wouldn't be the same person who has a vested interest in me purchasing one.
We do have multiple CO alarms in the house. Worst-case scenareo -- the unit goes in the middle of a cold snap in February and I'll be without heat for a few days and have to make a split-second purchasing decision. But we have several backup heaters we can use and get buy if this is the case...
-Tim
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I do preventive maintence on machines that arent furnaces. To a observer it might appear I JUST clean it.
but what you really get for your bucks is a experienced tech who knows what fails and checks for it.
sometimes that knowledge prevents a much larger and expensive failure at a later perhaps really inconvenient time.
like 1am on christmas eve, or worse spoot a small gas leak before its a big one.
I dont have my furnace serviced yearly either, but every few years. its more than looking for a bad heat exchanger
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Where they hell do you live that annual servicing costs over $100? I'm in Chicagland and $80 is the norm.
It's a lot more than vacuuming out the furnace if you've got an outfit that knows what they're doing.
-- Todd H. http://www.toddh.net /
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Minneapolis.
I was being a bit facetious when I said "vacuuming".
-Tim
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Not true, I have used one, not mine of course but it certainly didn't do that. I do apologize, I was mixing in some dry humor in with my comments. My water heater behaves exactly the same way and I understand that it's perfectly normal. It does sound ominious to the untrained ears though.

I purchased this house in March, just long enough for it to use the furnce in the early spring a few times. The real estate agent insisted on an inspection and cleaning of the unit before the sale, although had I caught this before sale I would have raised hell. The "maintenance" guy cleaned the furnace by flipping the filter around so that you couldn't see the dust. So obviously the seller was a trifle dishonest about the furnace condition. Judging by the quality of the electrical work in the basement, also totally inept when it comes to home maintanence.
So I'm gonna have it inspected thoroughly in the coming months, and will also install the CO detectors as soon as I get some money - dumped everything I had to pay off card debt.

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That's pretty darned old. Well past the age for which you should be seeking annual cleaning/inspection/servicing. They should check for CO emission, clean the burners, do a visual inspction of the heat exchanger, and perhaps other tests on the HX if visual and CO tests warrant further investigation.

Now. :-)
Just ask the question about the long duration of ignitor ticks prior to turnon during the annual servicing you should already be doing.
And get yourself some Kiddie Nighthawk plug in CO detectors for your bedrooms if you don't have em already. No way I'd be without em with a furnace of that age.
Best Regards, -- Todd H. http://www.toddh.net /
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At 25 years your furnace is probably beyond it's design life but you can probably get more years with proper maintenance. There are lots of rules of thumb such as if it will cost more than $600 to repair it then replace, etc.
IMHO, One thing to look at -- if you plan to live there for awhile -- is if it would make good economic sense to replace your furnace with a high efficinecy furnace system. With gas and electricity prices going only up the pay back period is going to be pretty short in most locals and the high efficiency system will help moderate your future bill totals. Even if you don't plan to stay in that house for long a high efficiency system will likely help your resale of the house.
Just my 2 cents -- good luck.

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Jay Stootzmann wrote:

For me, the incremental cost difference between a 60% efficient gas furnace (likely figure for 25 yr old model) and a 80% efficient model is INFINITY.
With 60% I pay a grand total of $250 for the YEAR. So 80% reduces that to $220. $30 into $800 is longer than the expected life of the furnace!!!!!
Oh bring back 60% for those of us who almost don't need heat!!!!

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10. Your ice cube trays freeze faster on the kitchen counter. 9. The banging noises from the cellar keep you awake. 8. Mice wear sweatters and mittens when they run across your living room. 7. Your children go outside to warm up. 6. Your utility provider sends you gold plated thank you letters at Christmas. 5. Your utility meter vibrates so much for use that it falls off the wall. 4. Flocks of migrating birds gather and fly in circles in the warm air over the top of your chimney. 3. Your house smells like furnace exhaust. 2. Santa Claus leaves a nasty note that he can't drink the frozen milk next to the rock hard cookies. 1. You wife insists.
--

Christopher A. Young
You can\'t shout down a troll.
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