Carrier code 31 on 58MXA (WeatherMaker 9000?)

SteveAnnArbor

Hi! I have a problem w/my Carrier Weathermaker 9000? (it's a 58MXA)
not heating.

Though the "component test" checks out OK, this is the ONLY time that
I see that I see the HSI energized (the ignitor). Also, the test of
the pressure switch in the op's manual (i.e., disconnect the inducer
motor, call for heat, and make sure that the HSI is NOT glowing after
about a minute) was fine.

I see a steady ~26VAC signal to the pressure switch.

Other checks:

- air intake/exhaust not clogged (good flow when inducer runs)
- inducer motor supply voltage fine (>120VAC)
- inducer cap fine (3.0 uF and resistance huge @ DC)
- install was years ago so PVC sizing must be right
- no backup of water in the drain system
- no clogging of the other rubber/plastic tubing (from the pressure
switch to the burner box)

So am I at a point where the board is in question or might it still be
the pressure switch?

I've tried shorting across the terminals, but an HVAC tech has
informed me that not doing it at the proper time or with the
connections previously ok (i.e., apparently you can't just disconnect
the leads and short the terminals) won't show the proper behavior.

???
HELP!

Thanks

HVACDave

Measure your voltage accross the pressure switch. If you have 26 volts accross the switch then it is open and it is what is holding out the rest of your start up procedure. This is assuming of course that your inducer fan is running. When any of your safeties is open you will measure the voltage accross the switch, if they are closed you will read "0" volts accross the switch but 24volts to ground on each side of the switch (in your case 26 volts).

SteveAnnArbor

Thanks for the reply!

I see a solid 26 VAC across the leads to the pressure switch (should've said this earlier). I tried shorting across these (with the leads removed) but I've been told that not doing it the right way and at the right time leads to a false neg...that the computer is actually a bit clever about what it's looking for...!

So are you thinking that this is a problem with the board rather than the switch?

Thanks!

#CARRIERMAN

Hi SteveAnnArbor

You definately have a draft safeguard problem. Lets start by removing the wire on the common terminal of the pressure switch. Start furnace up by using the thermostat. You only have 30 seconds to put the wire back on after the inducer starts, but you must keep the wire off the terminal for at least 2 seconds after the inducer starts. If the furnace fires up and runs ok, you will need to replace the pressure switch. If the furnace fires up and surges badly and seems to be running poorly. You will need to clean the secondary heat exchanger. This is a very long process and for the most part requires a proffessional to do this. It seems as though you understand the furnace sequence of operation. Let me know the outcome of the test I just outlined, we will go deeper into it if need be. But lets go one step at a time for now.

Good luck
Rusty

__________________
Character is doing the right thing even when no one is looking. Intelligence is understanding and practicing the meaning of character.

#CARRIERMAN

Hi again SteveAnnArbor

I just read your second response. When the inducer is running and all the wires are in their proper location. You should show 0 volts across the pressure switch. If you are showing 26 volts across the pressure switch with the inducer running, the switch is not closing. If this is the case, start the furnace up with the thermostat. You will want the common wire off of the pressure switch to do this test. When the inducer starts, press the terminal of the common wire against the wire going to the n/o terminal and hold it there. If the furnace fires up you know the switch is not closing. Could still point to same problem as I listed before.

Good luck
Rusty

__________________
Character is doing the right thing even when no one is looking. Intelligence is understanding and practicing the meaning of character.

SteveAnnArbor

Rusty/CarrierMan:

Thanks for the tip!

I did this last night and found that the furnace NEVER came on...I tried each of the wires and made sure that it was between about 10/15 and 20 seconds that it was off, after the inducer motor started.

So does THAT mean that the pressure switch is hosed and stuck in some bad position or that the microswitch on it is dead?

THanks for the help; I really REALLY appreciate it (and so does my wife! LOL).

SteveAnnArbor

Rusty/Carrier Man:

Well, thanks for the tip! I did this and--voila`!--HEAT. The furnace ran normally. I was getting set to go buy the pressure switch and I decided to recheck the outside intake (my wife had felt around and not found anything); we'd had a lot of wasp/bee activity this summer, and I was a little concerned.

We have one of those combined intake/exhaust ports which cover the pipe-inside-a-pipe PVC configuration (exhaust on the inside, intake on the outside; I presume that part of this idea--besides only having ONE hole through the house's "skin"--is that the heat from the exhaust air could be partially recaptured to heat the incoming air...so that you aren't wasting heat from the burning gas to heat the air, as well as cooling the exiting air; this is actually exactly what is done with liquid O2, say, for rockets, which you can see at most rocket park-style displays and at various museums). There was nothing except some condensate inside the exhaust pipe, and just a little dust and a cobweb in the intake space; I cleaned it up using a refrigerator coil brush and a vacuum (crevice tool) and...well, wouldn't you know it...HEAT with no problems.

I had no idea just HOW sensitive the overall flow was to what appeared to be relatively minor restriction of just a portion of the intake area...what an eye-opener! What a wild goose chase from an incomplete investigation of a fundamental; if I'd been thorough at the beginning, I'd have not had a few weeks of wondering if I could get this, myself. [ What was more misleading is that the exhaust flow was strong; it wasn't weak and so didn't seem as though anything was blocked... ]

[ FYI: I was also misapprehending the action of the pressure switch. I thought that the switch opened due to a signal from the board; clearly, it is a mechanical switch which operates directly in response to an (adequate) suction due to the airflow created by the induction motor, and this then is used to create a signal read by the board to say, "Hey, it's OK to proceed w/gas and ignition, because we've got enough air flow"... Your comments were part of what helped to get my head corrected in this respect; thanks!]

#CARRIERMAN

Hi SteveAnnArbor

I was very intrigued by your passion for the repair of your problem. I wanted to give you a little more information to help you understand as it seems you have a good grasp on the situation. The pressure switch on your particular furnace is looking for a pressure differential between the burner box and the exhaust of the inducer assy. This furnace is designed to run between a static pressure of 1.80" hg and 2.25" hg. If the pressure differential is less than 1.80" it will not allow the combustion process. I.E. restriction in the air intake or exhaust. The piece you see coming through the wall is called a concentric. It combines the air intake with the exhaust in a neat and clean looking package. This process can also be done with two pipes terminated properly. But does not look near as nice. Just happy that you were able to find and repair problem. Have a safe and happy winter.

Good luck
Rusty

__________________
Character is doing the right thing even when no one is looking. Intelligence is understanding and practicing the meaning of character.