Home Furnace Diagnose Problem

[joeqsmith]

As we approach winter and are heating the house, the furnace decides to fail.  Being a bit of a electro mechanical buff, I thought I would have a look.   The furnace is fairly new and uses the PVC for venting and the air intake.   The fault code was the pressure switch had failed.  Read a few threads and watched some videos to get some idea what to look for.  Interesting enough, from what I read, the switches rarely go bad.   

I cap off the lines when the furnace is not in use but with the shop vac attached they appeared clear.   I then checked all the water drain lines which were clear.    Next I pulled the blower to have a look.  Powered it up on the bench and everything appears in very good shape. 

After going over the rest of the furnace and not finding anything wrong, I measured the switch.    Pulling a vacuum to close the contacts, it read between 20 and 300 ohms.  It almost appears somewhat linear with pressure.  It will take a few days to get a replacement.   The switch is ultrasonically welded shut so there was no way to inspect the contacts without damaging the part.   

I connected it to my bench supply and limited the current to about 1mA at 5V.  With it applied across the contacts and cycling the switch, it wouldn't current limit.    I cranked up the voltage to 12 and increased the current to 10mA.   After a few cycles, the switch starts to work as expected.   Reinstalled and the system is working fine. 

I wonder if the designers were not aware that the switches require a small amount of current to keep the contacts clean.  It could just be a poorly made part.  It certainly looks it.  Designed obsolescence to make money on service calls.   The replacement part uses a separate micro switch that is mounted to the diaphragms housing.   

[duak]

The designers should know about minimum current thru switches, but they don't.

To change the subject slightly, have you washed out the inducer blower?  This blower moves the combustion air.  Exhaust gases often condense in it and leave solid deposits that can build up and block it.  I have a Carrier furnace that has a rubber cap above the impeller that is used to pour warm water in to wash out whatever collects inside.  The original installer pointed this out and said it should be done annually at the end of the major heating season.  I've usually been good about this and the furnace has worked flawlessly for 14 years now.

[floobydust]

At least it died before the really cold weather hits. Is this a Lennox? I curse those now.
I would expect the switch contacts to have self-wiping action, but wonder if it's a myth because signal relays rely on special contact plating despite the high force.

[joeqsmith]

When I pulled the inducer blower, I went so far as to split the cases to see if there were any signs of rubbing.  For the most part, everything still appeared in new condition.   On this particular furnace, there is a drain tube coming out of the blowers housing for the water.   This is tied in with the five other water drains.  All of this was also inspected before I finally looked at the switch.   

After running a bit of current through the contacts, the furnace has been working fine.   

I'll cut the old switch open once I have the new part installed and see what the contacts look like.   Looks like they spent the least amount possible on the safety switch. 

[joeqsmith]

The new pressure switch arrived.  It's like what was shown in the picture, designed around a micro-switch.   I have not pulled out the bad switch yet as it's been flawless after running that 10mA through it. 

This person takes the one I have apart but for some reason, fails to show the switch contacts.   I'm guessing this cheap safety switch fails a lot. 
https://www.diychatroom.com/f17/pressure-switch-theory-333337/

Not being a furnace designer,  if the cheap safety switch were to fail short and say the exhaust were plugged, it would still seem safe.  The furnace's combustion chamber, air intake and exhaust are all sealed.   The blower has a temperature sensor to shut it down if it over heats.  Looking at it with my totally ignorant eyes, it seems like they thought it threw from a safety standpoint.   Then they were told to cut and much cost out of it as possible. 

Link to wiki micro switches.
https://en.wikipedia.org/wiki/Miniature_snap-action_switch

[Brutte]

No hijacking attempt, I am repairing Philips HD4680/55 electric kettle (2400W at 230V) that is about 5 years old.

This thing has an Otter Z5 microswitch.
Z5 is mechanically bistable, there is a bronze flat spring inside (part of electrical contact). It is actuated by both a lever (this bar at the top) and by a bimetallic temperature actuator (the peculiar washer with tab). So there are two opposing forces counteracting. Once the sum of forces pushes the bronze spring mechanism pass its tipping point, it filps. Just like in OP pressure switch.

The tipping point is on the OFF side.
So when you switch it OFF->ON it first passes tripping point and then it flips and afterwards it electrically connects. No arcing.
But in the reverse direction ON->OFF it first electrically disconnects and then there is arcing and you need to push it further to get to a tipping point whre it mechanically flips.

The only way to switch it ON->OFF without arcing is to whack it either with lever or with bimetallic actuator instantaneously. 

The bimetallic actuator is also bistable so in theory this should work fine when steam heats the actuator, as long as no force is acting on lever. However, for the lever push mechanism Philips decided to use a plastic that swells in moist conditions. In the whole kettle there is only one component that does that! 

The effect - Z5 is no longer a snap action switch, contacts are are burned down and the swelled plastic presses on lever preventing disconnection. Z5 is available online but the plastic thingy is not.

[floobydust]

[...] if the cheap safety switch were to fail short and say the exhaust were plugged, it would still seem safe. [...]

The furnace's firmware does a self-check. The sequence is:
read the switch (contacts should be open)
start the draft inducer blower
wait a bit
read the switch (contacts should be closed)
If OK then proceed and do trial for ignition

A stuck open or shorted switch, or bad input on the controller board, or failed blower, blocked tubing, jammed diaphragm etc -  would get detected because the firmware is looking for the change. This gives functional-safety coverage for many pieces of the hardware, which is what you need to prove for regulatory.

The switch contacts are likely low spring tension so you can use smaller diaphragms (cheaper), I'm glad Microswitches are being used instead of homebrew switches.

[joeqsmith]

[...] if the cheap safety switch were to fail short and say the exhaust were plugged, it would still seem safe. [...]

The furnace's firmware does a self-check. The sequence is:
read the switch (contacts should be open)
start the draft inducer blower
wait a bit
read the switch (contacts should be closed)
If OK then proceed and do trial for ignition

A stuck open or shorted switch, or bad input on the controller board, or failed blower, blocked tubing, jammed diaphragm etc -  would get detected because the firmware is looking for the change. This gives functional-safety coverage for many pieces of the hardware, which is what you need to prove for regulatory.

The switch contacts are likely low spring tension so you can use smaller diaphragms (cheaper), I'm glad Microswitches are being used instead of homebrew switches.

I would have expected it to work something like this.  When I was sorting out the problem and it seemed to not care about the initial state of the switch, only that it had closed some time after the blower was active and that it remained closed.  I was thinking, that seems a bit unsettling.

If the switch was opened while the igniter was active, it would disable.  If I closed the switch fast enough, it would immediately enable the igniter.   Once the gas was turned on, if the switch was opened, it would turn off the gas valve and latch it off.   Now, that said when I get ready to change that switch, I will have another look to make sure but it seemed like jumping that switch was enough to get it running.   

One thing I noticed is if the switch remains open, it appears they will continue to cycle the blower, forever.   When the furnace first stopped working, I went to have a look an could hear the blower cycling but the furnace wouldn't start.  I grabbed the blower motor and it was very hot to the touch.   I'm not sure why they wouldn't just set a code and lock it out after several minutes of retrying.   

I'll post once I retest some of these conditions. 

********************

Just installed the new part.   First I shorted the circuit with the power off, then turned on the furnace.  Sure enough, it came up with a different error code.  So it does check the initial state.    I let it try and start up with the circuit open and indeed it does seem to try to restart, I'm guessing forever.   

You can see the difference between the new and old designs.   The new switch required the larger spade.   

I checked the contact resistance of the old part and it still seems fine.  Once I am certain the new switch is isn't going to have a problem (sad that I have no trust in new products), I will cut the old part open and get some good pictures of their design.