Hi Guys,
I have a 6 yr old Ducane MPGA125B5A Furnace that is driving me nuts. The smart valve led flashes three times and it was getting cold in the house before it would reset and repeat the cycle. three flashes indicates open pressure switch so I rodded out the vac port and retried. Same situation. Replaced pressure switch, Same issue. Swapped control board, same issue.
Smart valve or draft inducer is all that's left.
Anyone have any Ideas???? Those are expensive parts to be trying out.
Ron

 

I'd start by checking to see if the pressure switch is actually opening. You want to connect a multimeter bewteen the furnace sheet metal and the pressure switch contact to see if you get 24 VAC switched on when the inducer motor comes up to speed and that it stays on. If it does, it's not the problem.

If it is opening, it's signaling a problem, but there's only a 5% chance that the pressure switch itself is bad.

 

No it's not:

Furnace blockage, or venting pipe blockage, or combustion chamber air intake screen blocakge (certain furnace models only)or accumulation of condensate stuck in the secondary or sloshing around in the inducer due to drainline backup(easy enough to cure), can cause a pressure switch to either not close, or close but then open back up prematurely. Test pressure switch as stated.

 

Are you thinking the control voltage may be not getting to the switch? The pressure switch( Normally open) is brand new. I did test the original before removal for continuity to see if it was making or not and it was closing @ startup of the inducer blower. I will check for voltage at the pressure switch and see what kind of readings I get.Do you think it may just be the inducer is not creating enough vacuum to hold pressure switch?

 

Yes? If you have a problem with drafting, the new pressure switch is not going to cure that. And if the pressure switch does close, make sure it stays closed, for as long as the inducer runs for. Obviously this test only means anything for when the flame goes out. If the flame does not go out, well then we know that for that cycle, the possible marginal nature of the drafting problem(if there is one...I'm just saying...) has allowed it to work on occasion.

 

15.74 vac @pressure switch while running, Switch is closing . Voltage stays steady @ 15.74 Unfortunately right now it won't flash the led 3 times. 24 VAC when unit is not running
Any Ideas? Do I need to replace the transformer?

 

venting pipe is not blocked, I removed t near furnace and Hotwater heater and I can see all the way up, also the leg to furnace is clear. I did pull vac line off pressure switch while furnace was firing and flames went out immediately as they should have

 

15.74?

The biggest issue here actually is in finding out that if the flame goes out or does not happen, if the outgoing voltage goes away while still retaining the incoming pressure switch voltage, regardless of if 15.74 or 24. Maybe your furnace has somethign on the board or ? that causes enough resistance that does not allow excess current to as readily push through the resistance of a test meter.

I've ran into this scenartio myself. Just the other day I got no voltage at either wire nuts (to sheetmetal ground or boiler copper piping) coming and going from a thermostat that was a working thermostat, nor at these wires where hooked to the zone valve. And I have ran into that scenario on some gas valves also (for WORKING gas valves). And no, it is not the meter's fault, nor a bad ground connection as I can pick up 24 volts elsewhere to the same sheetmetal or boiler piping ground

Furd - you have the answer on that one? ?????...the case of the missing (24 VAC voltage)?

 

Naturally. What we want to know is the voltage at the pressure switch when the flame does not occur or goes out, during the normal operation of the furnace (not with you removing the vacuum line to make the flame go out).

 

Right now we are trying to get the pressure switch issue behind us first. No use moving on til we do.

But your furnace troubles could be other things. Maybe your flash code does not get real specific as to what is failing along the 24 volt pathway. If you have some other low voltage switch tripping out, that could maybe be the cause also. Maybe there is 24 volts higher upstream in that circuitry, but the voiltage is dimming down by the time it gets to the pressure switch, which may be the last item in that circuit, due to a problem in the wiring or part upstream. ???

 

Allright I'll check the magic diagram and look for the other things in that particular circuit. I believe there are two other limits and just an interconnect. Um, should I be getting 24 volts at all time on the pressure switch? I tried a second control board, which the pressure switch connects to, and other than the connection with the smart valve, there are no other parts really.

 

Magic diagram says two auxillary limits are in the curcuit, however, I have no idea where they are.

 

Let's jsut say you should get voltage on both terminals of the pressure switch that are the same voltage when the inducer fan runs. Although it be best to erase all doubt if both were closer to 24. For the fun of it could yo do that test i said where you put one test lead to the one pressure switch terminal and the other test lead to say the common (neutral) wire for the transformer?

At the gas valve terminals does the 24 volt?, or does the 15.74 volt reading show up there?

 

Um, 24 volts on one side of the pressure switch, 16.76 on the other side. 24 volts at the gas valve. The 24 volts side of the pressure switch drops to 16.76 when the switch is closed.I'm gonna guess that maybe one of the Auxillary limits are bad?

 

try checking the voltage going to the inducer motor.see if the voltage going to inducer is steady,especially when the furnace blower kicks on. i had a situation where the flame would drop out when blower kicked on the inducer motor would slow slightly causing flame to go out. .how long does it run before it kicks out? in extremely cold weather,you could be having chimney issues

 

What kind of chimney issues would you be talking about?I'll check the inducer motor voltages later on Today.
Thanks for all of your help and Ideas.

 

Anything else on that line could not cause that behavior, since the line side also would be that same low reading. It is either a problem with the switch not making good contact or some acting force in the pressure system is not allowing the contacts to close good enough. And maybe if this has gone on long enough, has caused the contacts to go bad from arcing. That is - if you are certain in your testing that you were making good metal contact.

You could ohms test those contacts by sucking on the vacuum port and test across the spades of the switch (with the furnace turned off) to see if 100% continuity. Anything less means the issue is in the pressure switch itself. And if you DO get 100% continuity (0 ohms), that then the issue is in the inability of the vacuum to draw the contacts inside tight enough to make good contact. IOW, problems in the drafting of the furnace.

A chimney issue would mean..... as in partially 'plugged'.

 

Chimney is clear, Visually I can See to the sky with no restrictions when I removed the Connector near furnace.Pressure switch is good Readings taken when I pulled vac on it were good on the new one and the old one as well.I'll get to the line voltages later on.

 

So based on that, it sounds to me that the pressure switch cannot close fully during normal furnace operation. So, if that is the reason, if you joined the two pressure switch wires together (of course for testing purposes only!, for your safety), the furnace should run correctly every time.

 

The furnace also has checks to make sure that the pressure switch is open before turning inducer motor on and then checks that the pressure switch has closed before it will start the ignition sequence so jumpering will not work .
Any other ideas???

 

just because it is clear does not mean that you cannot have problems.you said that the water heater vents into the same chimney?this is an 80 percent furnace right.some manufacturers want you to use a chimney liner with an 80 percent.could be downdraft issues,especially since it has been exrtemely cold. did the problem start when it got extremely cold recently.you need to put a tee and hook up a manometer on the line going to the pressure switch to determine if you are getting flucuation in pressure when running.pressure switches are very reliable,and very sensitive. on top of that,I have installed many ducane brand furnaces,and i have had very few problems.in fact you hardly ever see anbody on a heating help site that has problems with ducane