The resistance of the probe changes with temperature, and that's how it tells you the temperature.  My probe, in my return air line, appears to be giving the correct resistances at the right temperatures when bare wires dangling.  But when I wire it to my control board and touch the contacts there, my number goes up much higher, at those on-board contacts.  meaning the board is playing a roll now in how many ohms I'm reading, when it's wired to my 2008 Lennox zone control board.   Is that normal?

No.  A thermocouple generates a very small voltage that corresponds to temperature.  It's build by connecting two electrically-dissimilar metals together.

OP, you speak of a thermister, not a thermocouple.

FWIW, you test a thermistor unhooked and note the temperature when you take your read.

It’s called a Heat Anticpator.

It is adjustable but let a pro do it. And make sure he’s got some gray hairs.
HVAC Multimeters have a function to allow this.

This site gives a layman’s explanation.
Heat Anticipator

A modern stat can be programmed to do a much better job.

You’re just using thermocouple instead of thermistor.
Fairly common that  the words get inverted.

It sounds like you understand the thermistor changes resistance.

And if anyone is bored and wondering,

as ALKVA stated, thermocouples do generate a voltage. Pretty cool invention going back over 100 years.

Doubletap

I would assume that when you measure the resistance with the sensor hooked up to the board there is some circuitry on the board that you are measuring in parallel with the sensor that changes the resistance your meter sees.
I would expect that to be normal and it’s why you disconnect components when you electrically test them.

Thanks, this was the perfect reply.  It corrected my technical error on what tech probe is being used and answered my question:  that it's normal for the resistance to need to be measured while unhooked for the manual method of testing it to work.

Just trying to figure out my AC/Furnace.  It was in the high 60's yesterday and I tried turning it on for the first time to test it.  The install crew are good dudes, but made many error's I had to correct - from not hooking up the return sensors, to getting polarity wrong, adn so the AC zone system wasn't working right.  The most maddening was the Honeywell THM5320R1000, where the polarity of the 24 VAC power was hooked up backwards - BUT IT STILL LIGHTS UP AND "WORKS".   It just won't send out a signal, if hooked up that way.  I'd like to smack the engineer who designed it to work like that.  Just don't let it work at all, or give an error code, or something.  Took me a while to figure that one out.  Kind of judgy on my AC guy not able to figure it out all.  He brought in reinforcements the next day to get back at it, and when I showed them what it was and that I fixed it and the T probe issue, they actually shook my hand and said "we don't get customers who can do that", which I guess is kind of funny.  Good way to downplay a potentially pissed off customer if nothing else I suppose.

I'd be more grouchy, but I just had a 5ton installed with indoor/outdoor and furnace and fan and replaced about 30' of corroded out 4" flue gas double-wall pipe, and all that, for $9k.  Which is a big number, but in 2024, that's actually a good deal, and they are good dudes who come back immediately when I asked.  So.. well anyway, mad or cool - doesn't matter, here I am now.  

So anyway, now that we have a couple cool days, I'm trying to exercise the new furnace.  And it's been odd.  My control board (a 2008 Lennox Harmony III) is giving me the two lights that indicate it's not happy with the discharge air temperature still being too cold it thinks (where "discharge" is not the flue gas, but the temperature of the air from the duct system being sent from the unit, to the house - had to look that one up).  My flue gas gets quite hot, and I can touch and hold the pipe, but I doubt I could if it were any hotter and its little fan is working; so that seems normal.   But I get an error code on my AmeriStar rig of E3.2  and tP5 lights up.  Thought I saw an E2.2 as well.  And the whole system will run for a minute or so, then turn off, and the house-air-circ fan will stop blowing at full speed, yet the house is still cold - so shouldn't it still be running?  Not sure what all that means.  

So, as a starting point, I was just checking here if something was amiss with my Thermister set-up.  Since it goes from 15,000 ohm (depending on the T), to 400,000 ohm, once wired it, and didn't know if that was normal, or perhaps part of my problem.  The guys not only had messed up a number of things, including not wiring that back in, but they had cut those wires even, and I hand to extend them with copper wire.  Which, if it were a thermocouple, I think messes things up if you do that.  But in this case, I think maybe it's OK, because I measure the same resistance across it both with and without the copper wire attached, but again, i was just posting here to see if that was the problem.  I don't think it is.

For all I know, I may not have a problem.  It's just not operating as I was expecting, since the fan isn't running full-time during the heat cycle, and it's shutting itself off ever minute or so, then turning itself back on and running for a minute or so.  I'd probably say it's probably fine, but I'm getting those error codes.

What is the temp split between the heated air at the plenum [or as close to the furnace as possible] and the return air? [the air coming back to the furnace, measure at a return duct or the air filter] It should be 60F or so, if it's not, something is wrong. If the split is way off, the furnace will have a short life span and be inefficient and it will feel uncomfortable. You could have air flow issues, too much filter restriction, or the blower speeds or board is acting up. You should have an ECM type blower motor so blower speeds should ramp up properly to give you the proper split IF set up correctly.

I don't have that info, but will try to get.  I don't think that's the issue, in fact I've taken out the filter entirely and letting it draw cold air right there from the basement, and no change.

It's odd.   E3.2 means Open Pressure Switch.  With a factory note that "in most cases, the pressure switch is not the problem."  And to verify the wiring and tube routing are correct.

Pretty sure the tube and wiring are factory installed, but the tube goes to the fire-box area, and hooks to a nipple sticking out.  Ok, so it's pressure monitoring something in there - I'm guessing the negative pressure of the induction fan draw(?).   And the wiring looks factory installed to me.  But it's odd, because it works fine and starts up fine, and is happy when the induction fan kicks on.  I know this, because I purposely power-cycled the unit, unplugged the wire, and it's unhappy and immediately goes to E3.2 without even starting if I do that.   And if I unplug the the hose, it also quickly craps out, and won't even start the burners.  So, when I have it plugged in, it's happy, says the induction fan is working fine (I'm guessing that's what 3.2 is all about), implying it's reading negative pressure (I'm assuming that's the case?).   and starts up the flames.  Runs for a full minute or so happy, and then turns off the flame and gives me error 3.2.  

Does this mean I have too much flue-gas back-pressure when under flame, and this new system is more sensitive to measurement that than my old one was?  It's a long flue system, of 30' of 4" double wall horizontal (angled up) to a chimney that vents 2 stories up.  All that worked fine before, but maybe the sensitivity setting is higher on this unit?  

Options?

Ah, pressure switch failure.......I wonder if it's trying to go to high fire, there is a pressure switch issue, the gas valve fails to open and the second stage [or variable] never is reached and then it starts the entire cycle all over after it goes into a safety cool down mode using the blower which usually ramps up to high speed during that cycle. Does the furnace side have pressure switches made in Costa Rica? [yeah, that sounds stupid but they are known issues] Even resistance when they are closed will cause the board to go "WTF" and cause run issues. Sometimes it'll work, other times it won't, that is why they can be such problem children repair wise.

Other issues could be condensation in the line going to the pressure switch and improper drainage of the entire condensate system internally or externally.

And proper grounding is INCREDIBLY important in the proper running of a modern HVAC system, the old one didn't give a flying rats ass about it. The floating ground/improper installation issues I ran across with new furnaces were always a PITA because the customer almost never believed me.

yea I'm not a fan of the design, as any condensate in that tube is going right to the sensor.   Looks clear now, and is very new, so I don't know if any chance of build-up has happened.  There's a little shut-off switch for non-draining drainpan thing, that seems fine.  Can't comment on the grounding - I assuming wiring it to house ground is generally standard and sufficient?  Other options?

Seen a lot of installs replacing an older furnace that just used the metal electrical wiring sheath as the ground [pretty much a floating ground] and honestly, the electrical didn't give a damn which way it was wired. On new ones, you had better have a solid ground [using new appropriate rated wire with a ground wire] or it'll exhibit all sorts of issues. As far as pressure switches go, they aren't going anywhere, tend to be cheap, and everyone uses them. I always make sure the hoses want to drain back to the port and keep the pressure switches higher then the port they come from.

Thanks for the replies. My worry is the vent flue system is just a bit undersized when under full load, since it's rather long (30' of 4" pipe into a chimney that goes up 20'), and so the pressure switch might not actually be in error, when under load.  It's apparently good enough during start-up and there's just air blowing, but add to that the combustion product volume, and now is it too much?    So I'm wondering if I can get a pressure switch that's just a bit less sensitive; since the house has been running this way fine for over 50 years plus, up to now.    Or am I barking up the wrong tree on that?

4''? It's fine. They just need to take the read at the port and what the manufacturer says it should be. They are safeties so they should NEVER be replaced with one with a lower pressure rating. Factory specs only. Any other answer from an installer/repairman and they need to be booted off property. Make sure the ports are clear of debris including bugs, swarf from manufacturing and so on. An accurate manometer is what is needed to see what is going on while the furnace is running. T into the pressure switch line and watch what is happening and the reads. I have seen brand new furnaces with heat exchanger issues that had run issues from day one and some that were installed unevenly that never drained condensate and caused running issues after they ran for awhile.

I actually drilled and tapped a port in my exhaust vent to drain condensate before it came back to the heat exchanger and it's own drains. It taps off a LOT of it due to the run length. I keep a spare set of pressure switches in my spare parts box.

Ok thanks for that - that's encouraging.  They said 4" was fine, but kind of quickly.  And it is proper double-pipe tubing installed correctly (That I could tell).  I looked up what I could find on building code, which wasn't easy for a layman to figure out.   I think it said 4" was fine for 10m length (which is 30').  Replacing that, and the chimney(!) would be quite expensive, if that were the issue.  Sounds like your voice of experience is suggesting the issue is something related to the pressure switch or wiring nuances; which hopefully are a lot less involved to correct.  

Thanks for your help.

Ahhh, you have a 2 in 1 vent system? Bryant, carrier, payne furnace?

Double

I don't know what that is

Here's the pressure sensor.  I personally suspect if I get a lower deltaP rated one, they'll will do it.





But maybe not.   I pinched the connector hose and it INSTANTLY shut off the burner.   So if the switch were reading pressure issues, it wouldn't even run for that full minute, even, I don't think....

For the heck of it, I reversed red and yellow wires.  

No change.  Kind of surprised it ran at all reversed like that, but it's running the exact same.  Blower on for 5 sec, fire on, house air blower on after 5 sec of fire,  one minute later,  fire off.   A few moments (minute? Later house blower off.  E3.2

(So I reversed them back now)

YOU DO NOT F WITH SAFETIES AND CHANGE THEIR SPECS.  Immediate DQ of any and all warranties, installer will refuse to do anything, manufacturer will run away. The pressure switch is there to make sure the furnace is venting properly. The limit switches are there to make sure the funace isn't overheating. All safeties should never have their OEM specs changed unless the manufacturer OK's it. [coleman/Evcon did on their trailer/manf house furnaces years ago]

You need to use a manometer to see what is going on. If it's [vacuum, most pressure switches are - pressure rated] too low, that is on the installer to make right, not you.

And you have an 80% system, non condensing. Kind of a dinosaur nowadays.

Do me a favor and unhook the hose from the pressure switch port and blow thru it. If you can remove it from the furnace port, see if you can look into it and insert a small bit into it. ANY obstruction can cause issues. It may be nothing more then a defective from new part.

FWIW, my bad, I assumed a brand new install would be a modern 90% efficiency condensing furnace.

Sure.  Thanks again for the help.

I was going to go that, but for $16k in Texas, or $9k for 80% furnace - was an easy decision

I understand that, I'd do the same thing if I were you, my money would go to a higher SEER rated A/C system long before I'd spend it on the heat side.

OP, do me a favor and jump the thermostat red [h]eat wire and the [w]hite wire [NOT the common wire if white] and let the furnace fire and run and see what happens.

Will do.  Amd to be clear, you are talking the two wires going to the pressure sensor, yes?  One is yellow and the other reddish orange.  Those two, right?

Right now, I'm back at it, and ran an auxiliary ground to a steel drain pipe.

This house is old. And this thing was not wired back to supply ground.  Amd the steel condiout in contact, stops before making it back. I don't know if it it's grounded at all.  So, let's see how it runs with a wire hooked to to the body going to a steel ground drain..

The gas line I suppose is a round. But it has that yellow flex connection at the end, so that may not be conductive.  That, and relying on the explosive gas filled piping as the electrical ground gives me pause.

No, the thermostat control R and W wires for a call to heat.

I wondered about the ground, everytime I had a call for a new/er furnace with weird issues, I learned to check grounding after the first couple of head scratchers. The codes it will throw are basically more of a "WTF'' code from the board.

Nope, same behavior with the supplemental ground.  Could be an insufficient home grounding though, I didn't grind the steel.

I may revisit, but I'll do your short circuit test.

Oh, right
Well, I will say, hot wiring the sensor appears to be working



As you say, not an acceptable long term solution.   Heck. I Wouldn't even be doing it as a test if I didn't have a CO monitor right next to me.

But it's working and has been running for several minutes now nicely

So you could have a bad pressure switch or you really do have an obstruction at the port, in the heat exchanger [not likely on that furnace] or the vent.  A manometer would run that down to the correct repair/fix. If pressures are in spec then I'd replace the pressure switch.

Random follow up question - what happens to condensate in the 30' long flue pipe?   It's not clear to me how that is supposed to drain out.

There should be zero condensate in an 80% furnace flue, temps should reach hundreds of degrees and there isn't any condensate that drops out due to cooling.

I have pics somewhere of rotted flue pipe from furnaces that had moisture and venting issues. Some are quite scary in that previous techs never noticed it.

If there is condensation then you have a venting issue, possibly due to length, especially if it's sideways [vertical]

That makes sense.   The old one did, and had drips - but was old, and didn't have nearly as good a flue fan.  Also, it had a 5" expansion, which I imagine slowed down the flue flow and gave time to condense, which it did in that section and dripped.  New flue piping also feels hotter than I recalled, which also goes to your point of it should be hot enough to not condense.  4" continuous diameter keeps velocity up, and the flue fan keeps running for a bit even after the flames go out, to clear the line.   So That makes sense.

It hits the vertical 100 year old masonry chimney, and I guess that probably has some condensation inside there, but the chimney checked out OK to them, and other inspectors.  At one time, the house ran coal.

To close it out - the pressure probe was doing its job.  When they installed the new flue, they ran it all the way through chimney opening and into the other side wall of chimney, which was restricting flow.  They came back out, discovered and corrected by scooting it back a bit, and running fine.  Ran for 10 minutes all good.

Thanks for the help.

FWIW, it's a -.70 probe, which is kind of aggressive sounding,but working now.

-70 is middle of the road as far as pressure switches go.

Pressure switches are on furnaces for a good reason, dead people don't sue but their families do.

Glad you found the issue though I question why they didn't run an aluminum vent pipe up the entire chimney as that is almost always how it's done locally.  Keeps the velocity up and condensation of flue gasses down because the smaller diameter vent heats up quicker and keeps the moisture from collecting.

This