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I think a troubleshooting flowchart would be really useful and cut down on a lot of the initial "my furnace doesn't work, what could it be" posts. Make it sticky so it's easy to see.

It seems that all gas furnaces work pretty much the same way and follow the same sequence of operations and safety checks. I'm not qualified or I'd do it.
Good idea? Volunteers?

Thanks
 

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Well there are slight differences, and an 80/90% is not the same as an older furnace. But most 80/90% furnaces it goes something like this


Call for heat from stat--->Inducer motor starts--->Pressure switch closes--->15 sec purge--->ignitor lights--->Gas valve opens--->Burners light--->7 sec flame proof--->30-90 sec warm up--->Blower motor turns on.
 

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Discussion Starter #3
Thanks, JScotty. That's a good start.

It would be nice to have a short list of the things to check if a step fails. For example, from my limited knowledge, if the pressure switch fails to close, 1) check voltage at pressure switch, 2) make sure air filter not clogged, 3) etc.
It wouldn't need to include every possibility, but at least ~90%.

Also, I see a lot of threads with the furnace starting OK then going out. That should be included.

I'm willing to organize the thing into a doc or a Paint file with standard flowchart symbology if there's enough interest and response.
 

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But then that could lower the monthly hits of the sight. cut down on a lot of the member jocularity.

But it is a nice gesture.
 

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Perhaps knowing the purpose of the safeties would help. Note it is never a safe practice to jumper a safety, use your meter and see if it's open or closed and move on from there.

The pressure switch proves that the inducer is moving enough air through the heat exchanger and the flue is not blocked(at least not blocked before it ties in with a water heater). On a 90% or better AFUE a plugged condensate will back water up into the inducer where it will not spin fast enough to close a pressure switch even though the flue is clear. A plugged nipple on the hose going to the pressure switch can also cause a pressure switch not to close,even though the inducer is moving enough air through the exchanger it can't sense it.

Flame sensors take AC voltage from the control board and rectify it to a DC currant as the voltage jumps through the flame and returns to the control board via the burners and ground. Oxidation or sulfur from the gas put a fine coating on the flame sensor and act as an insulator for that < 1 microamp of currant. Rust on the face of the burners also act as a insulator for that currant.

Limit switches are safeties designed to trip if not enough air is moving across the outside of the heat exchanger. Dirty evap coils,dirty filters, motor failures,weak capacitors,control boards not sending voltage to the motors or not enough ducting are common causes of tripped limits.

Roll out switches trip when the flames or heat from the flames come out the front of the heat exchanger instead of passing through it. The most common cause is a serious breach in the exchanger. When a roll out switch trips it's time to put down the tools and call a pro in for a heat exchanger inspection.

On the subject of heat exchangers...They are steel and separate the byproducts of combustion from the air you breath. There are no safeties to shut down the furnace when it gets a crack, and they all crack or rust out at some point in time. The roll out will trip when the crack turns into a massive breach, or the exchanger gets full of soot from improper combustion, and flames belch out the front of the furnace. At that time you will get red tagged and have no heat until the furnace or heat exchanger is replaced so it's in your best interest to get the heat exchanger inspected every year(before the cold weather sets in).
 

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Thanks, JScotty. That's a good start.

It would be nice to have a short list of the things to check if a step fails. For example, from my limited knowledge, if the pressure switch fails to close, 1) check voltage at pressure switch, 2) make sure air filter not clogged, 3) etc.
It wouldn't need to include every possibility, but at least ~90%.

Also, I see a lot of threads with the furnace starting OK then going out. That should be included.

I'm willing to organize the thing into a doc or a Paint file with standard flowchart symbology if there's enough interest and response.
Wow you really are getting involved with this. :)

A true flow chart would be cool, with yes & no otions at each step like:



Do you recieve call from stat NO YES--->Does inducer motor run
|
|
V
Do you read 24v from W to R at thermostat

I'd be glad to help with the options etc but there's going to be a lot of info there so it will probably turn into a massive flow chart with all the possibillities that could need to be checked.
 

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OK well that looked a lot better before it reformatted. The down arrow is supposed to be under "NO" but I think you get the point.:thumbsup:
 

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I would anytime gladly ask the question anyway. maybe its me but i like to lurk here and read responses from pros and amats. the wealth of info (and jokes) is what makes this amazing.
 

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On that chart.

Do you have 24 volts between C and W. Yes, remove thermostat wires from board and jumper R to W. That makes no sense.
 

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Discussion Starter #13
The chart is messed up in that area. I think everything is there but some of the boxes need to be shifted around. I'll email them and see what they say.
 

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Hi guys,

I invested a couple of days researching articles about troubleshooting HVAC problems around the web and I sifted the best ones in my blog, hoping to help our customers deal with stressful situations and learn more about how these systems work, so here it is < Sorry, your link is not permitted here. >

I hope you find it useful too.
 
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