eddie

I have what I have found by searching other forums is a fairly common problem with a Trane XE90 furnace, I get a code when it dose'nt ignite (3flashes of a led) that shows the pressure switch is stuck open. Some times the furnace lites fine with no problem. Other times it will run the exhaust blower and start the igniter and drop out before the gas lites. Sometimes the exhaust blower will run and run and finally it will ignite after many minutes. There seems to be no pattern to it. Suggestions in other forums have been possible water in the system, clogged vent, locked up rotor in the exhaust blower. I have checked all of these with no luck. Since it happens sporatically I am sure it could be hard to determine the cause. I have a friend in the HVAC bussiness and it has him stumped.

Aceinstaller

Air switch troubleshooting checlist......

Plugged flue (bird nests, etc.)

improperly sized flue (see installation specs)

plugged pressure switch tube (remove from air switch, and inducer assembly to clear. then take paper clip and clear connection for tube to inducer assembly. replace tube and call for heat.)

lack of combustion air causing negative pressure in utility room (ensure utility room has proper access to cumbustion air required for make and mark of unit.)

improper low voltage readings for proper operation of air switch. (check electric diagram for proper votage, and see if the voltage at the air switch matches.)

improper voltage at inducer motor. (check diagram for proper voltage, check voltage at motor to see if matching.)

dirty inducer blower wheel. (rare occurance, but worth looking into last) Once I found a mouse that made his way all of the way down the flue into the inducer housing.

If all of these checks are passed, then you have a rare occurance of an air switch going bad.(replace it)

eddie

Thanks for all your information. I am sure the flue is clear as the exhaust fan puts out a lot of air. I don't know what Trane specifies but the vent I.D. is 2 inchs which matches the exhaust outlet. It is about 35 feet long.The blower motor run freely. and the tube is not plugged up. You can gently blow or suck on the hose and hear the switch click. .I will try to test voltages as you suggest. As I said sometimes it works with no problem and other times it clicks on and off many times before it will finally light. I only recently realized that this was occuring but as I think back I believe it has been happening for several years. Thanks again.
Also the furnavce is in a large basement so combustion air should not be a problem.

Aceinstaller

if you blew down the tube into the switch, you might have ruined it. hence these diaphrams inside the switch are extremely thin and easily broken with too much pressure.

My recommendation was to clear the connection at the inducer assembly with a paperclip. But I guess I should have mentioned WARNING DO NOT BLOW INTO OR INSERT FOREIGN OBJECTS INTO PRESSURE SWITCH AS IT IS VERY FRAGILE.

Also, 2" vent piping should be increased to 3" after a specific length which is determined in a vent table that comes with the installation instructions of the furnace. I can't stand the attitude that a few bad apples in this industry have.("who needs instructions?") It seems to me that every year there is something different recommended by our manufacturers to perform on a unit for proper operation and efficiency that is overlooked by some installers.

For a distance of 35 feet, you should replace that 2" flue with a 3". Having it installed with a 2" pipe for a long period of time put quite a bit of sress on your inducer motor as well as the pressure switch. After replacing the flue, don't expect these parts to last forever.

zander

It sounds a lot like two problems to me.

It could almost anything associated with the drain, flue, inducer motor, pressure switch.

Well step one at this point would be to replace the pressure switch now that you have subject it to pressures it is not designed for. I would do this mostly for safty reasons.

Has any one used a magnehilic or monometer or electronic monometer to figure out what the pressures in the system are? This is nessessary because without this information you are guessing away at what the problem may be. Mabey an educated guess but none the less.

How old is this furnace? Age tends to point to the most likely part to fail.
Im not super familiar with tranes but some times the pressure switch circuit is tied into the high limit circuit and the fault code covers both issues.
I would check out that flame sensor too. Easy to do and eliminates it as an issue. I don't know if that train circuit board has a specific fault code for it.
I would also think about temperaly making it a singe pipe unit.

Worst case senerio is that you have a hole, crack, seperation in your heat exchanger and the switch won't make because the inducer motor can't work against that. Secondary heat exchanger could also be plugged up.
Someone familiar with condensing furnaces needs to show up with the correct tools.

I can't spell but i can think clearly.

Aceinstaller

Yes,

using a monometer would be the most proffesional way to determine if the inducer assembly is pulling enough pressures.

zander, Have you ever heard of any products vent tables carry 2" pipe for 35'?

I know that that is a undersized flue. period.
and an undersized flue is the biggest reason for this problem. Just to bring the installation up to specs for it's intended use, I would replace the 2" with 3" and then troubleshoot.

zander

Yup, seen plenty of vents that where per manufactures specs 2" and well over 35 feet. Brand has much to do with it. A 40,000 btu furnace could have a vent pipe 2" dia and 60 feet equivelant length.
A 120,000 btu furnace probably needs a 3" pipe to be corect per manufactures guidelines regaurdless of the length.
You could spend alot of money and time changing out that flue and it turns out to be nothing to do with the flue. AND, even though, it may be under sized it could have nothing to do with the issue at hand.
Your going to have to pay someone to trouble shoot it most likely.
I would seek someone out who is willing to stand behind their work. ie, not charge you for everycall when they can't figure it out.
Let us know what it turns out to be Eddie.
Ive heard Kentucky is high on the DIY hvac and low on the professional.
Could be a conection there. Good luck

eddie

I'm not sure but I think the problem is solved. It has worked OK since Saturday
Feb. 25. I noticed a sag in the vent pipe of about 2 and a half inches in the last 15 feet from where it exited the house. It is run thru a side wall. I mounted the pipe solidly in line with the pitch of the rest of the pipe and the furnace has come on correctly every time. Keeping my fingers crossed. Thanks again to all who gave advice.

dave_borchert

I temporarily solve this problem by removing the tube connecting to pressure switch to the combustion chamber and poking a wire through to clear out accumulated mineral ash that is obstructing the narrow nipple. This appears to be a design problem with the furnace, since the HVAC company that installed the furnace is unable to solve it.

Aceinstaller

do not stick any foriegn objects into the pressure switch or blow into it either. only clear the inducer assembly with paper clip.