I read another post on here similar to mine. My furnace is on and everything, but cold air is coming out of the ducts. The post said to try tapping the gas valve (in case it is stuck). That didn't work. Also, I read that there is supposed to be 24 VDC on the valve prior to lighting off the machine. My question is: where do I take these readings? Thanks for the help.

24 VAC. Not DC

Depends on the valve.

Post exact model and brand.

This is what I saw on the sticker:

SV9501M2528
Reg 3.5 Adj 3-5 range
24V 50/60HZ
0706D3C12 19025475

I know my problem is somewhere in the ignition area. The rotary pump at the suction is working and air is sucked through the filter. The outlet pumps are working, but cold air is coming out of the ducts. I don't know if I was supposed to see a flame at the ignition area, but I didn't.

What you posted looks like it came from a Honeywell Smart Valve control, not the actual model of the furnace.

Okay. I don't know what that tells me.

Post exact model and brand.We need more info. It's like telling us your car has Champion spark plugs; okay, but what kind of car is it?

The label on the case of the furnace says:

Excel Comfort Systems, Inc.
Model #: MPGA050B3
Serial #: 6239940024
20251801B

It's a forced air furnace with natural gas

There we go... okay, start here: do you have a voltmeter? Locate the pressure switch (typically a small canister, two or three wires and a small hose connected to it), and check for voltage between terminals after inducer starts.

I see the small hose, but two sets of wires. One set was for control, the other for the ignitor. Also, I don't know where the terminals are. I'm so confused.

Confused? Good, that's the first lesson, recognize your disposition-you're well on your way here. Success is one step closer... you are looking at the Smart Valve. Remember this for later. Keep looking for the pressure switch, the hose will most likely be rubber, approx. 3/8" diameter.

Okay, but I still don't know where the terminals are.

Where the wires are connected. If they are recessed into switch body you may have to back the wires off a little. Or, try this: remove the wires and use your multimeter to check for continuity between terminals with inducer running.

I found a troubleshooting guide for the valve that helped locate the test terminals. It had me check the 24V common to hot, which gave me about 27 VAC. It also had me test 24V common to 24V thermostat/pressure switch. When I turned the P/S to ON, the reading jumped to around 15VAC but dropped back to 0VAC. Is the voltage supposed to stay high like the other? Or is it supposed to drop again?

Voltage should be all or nothing. Partial voltage indicates some resistance in the circuit that shouldn't be there. Did you check the pressure switch yet?

Voltage should be all or nothing. Partial voltage indicates some resistance in the circuit that shouldn't be there. Did you check the pressure switch yet?


Take a look at this to troubleshoot.

That is the same troubleshooting guide I was using. The common to pressure switch voltage is what was giving my the spike and 0 reading. It tells me a bunch of stuff to check, but I'm not entirely sure how to check all that stuff.

At some point you may have to just bite the bullet and call in a pro. Not all problems can be solved over the internet in spite of the massive conglomeration of infinite genuis available here.:(

At some point you may have to just bite the bullet and call in a pro. Not all problems can be solved over the internet in spite of the massive conglomeration of infinite genuis available here.:(

Agree

I called the pros. I told them what I've done so far and they said to see if the pressure switch is clogged. I took off the pressure switch and blew in it and I heard the bellows move, so I know it isn't clogged. I hooked it back up and tried to check for a voltage while the furnace was on, but I got nothing. I am supposed to see a voltage, right?

Did you follow the troubleshooting guide posted by hvac1000 yesterday? This is what a pro would do...

Some people never listen or learn..

Like I said earlier, I was already using the same troubleshooting guide. After a series of resistance checks, I am pretty sure the rollout is the problem. Another problem is that I don't know what a rollout is.

A rollout switch is typically installed near the main burners to sense over-temperature if the flames are straying away from target area. They are almost always of the type that requires manual reset or replacement. Look for a small device with 2 wires and a reset button in the center.

Would the button pop out if it tripped? I found two of them, but the middle button won't push in (see question above), but it'll rotate. Does this mean it isn't tripped?

If they don't click when pressed, they didn't pop open. Internal oxidation is another possibility. Do any look like they have ever been wet?

I don't know. Where can I buy new ones at?

Don't start changing parts to find your problem. That can get needlessly expensive fast, and there goes the money you thought you were saving by doing it yourself.

What led you to speculate that a rollout switch may be the problem? I reviewed the troubleshooting chart linked by hvac1000 and I see no suggestions that specify this part. No offense, but if you have a voltmeter, eyes, and the ability to understand English, this flow chart is a snap to follow. Perhaps a pro should be called in.

It was the limit switch. I was having trouble finding it, since I didn't know what it looked like. Now I know, and it's only going to cost me ~20 bucks. Thanks for your help everybody. Peace.

If the limit switch tripped that usually means there is a problem that caused it to trip. Keep an eye on it after the replacement.

If a resettable limit switch trips, is doesn't necessarily mean the switch is bad, it means something occurred to cause it to trip. As hvac1000 said, keep an eye on it.