INDUCED DRAFT BLOWER RUNS CONTINUOUSLY

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Thomas G

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I have an Armstrong furnace installed when I built the house in 2000. Just recently I started to have problems. First issue was the induced draft blower would start, burners would come on but main blower would only run occassionally. Had the blower motor replaced by a tech. Main blower runs fine now but can't get heat. The Induced draft blower (IDB from now on) runs continuously and never seems to clear the sequence to let the burners fire. AC runs fine. Tech thought it would be the relays on the control board but wanted too much to replace it. I replaced it with the factory authorized upgrade and the problem still exists. I have checked the IDB flue discharge and plenty of air is coming out so there is no blockage. I've tried disconnecting the hoses to the diaphram on the furnace front, tried pinching them and tried disconnecting the wires from the switch on the diaphram (pressure switch?). IDB still runs. Looking for what to try next to narrow this down so the next replacement part is the right one.
 

Dana

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Replacing a 20 year old furnace one part at a time is expensive and frustrating. I feel your pain, but it's hard to offer specific advice without so much as a model number.

Given it's age and the fact that it's most likely to be ridiculously oversized it's probably time to start thinking about replacing the whole unit with something right-sized for the load. Since you probably have multiple heating season's worth of fuel use data (exact dates and amounts) it's possible to use the old furnace to measure the heat load to get a sense of how big a replacement furnace should (or more specifically shouldn't) be for maximum comfort. (Bigger is the opposite of better when it comes to hot air furnaces.) In my area it's far most common to see furnaces 3x or more oversized for the 99% load, and relatively rare to find one right-sized (= 1.2-1.4x oversize factor for the 99% load).

At least think about it. Most furnaces will make it to 25 years or more before the reliability, maintenance, and repair issues point toward outright replacement, but many hit that point before age 20. Yours may be one of the latter. Replacement time is an opportunity moment to right size and gain an improvement in comfort, not just reliability or efficiency. Even if you're not ready to make that step this year, running the fuel-use load measurements way ahead of time is worth it, since that day is eventually coming.
 

Thomas G

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Replacing a 20 year old furnace one part at a time is expensive and frustrating. I feel your pain, but it's hard to offer specific advice without so much as a model number.

Given it's age and the fact that it's most likely to be ridiculously oversized it's probably time to start thinking about replacing the whole unit with something right-sized for the load. Since you probably have multiple heating season's worth of fuel use data (exact dates and amounts) it's possible to use the old furnace to measure the heat load to get a sense of how big a replacement furnace should (or more specifically shouldn't) be for maximum comfort. (Bigger is the opposite of better when it comes to hot air furnaces.) In my area it's far most common to see furnaces 3x or more oversized for the 99% load, and relatively rare to find one right-sized (= 1.2-1.4x oversize factor for the 99% load).

At least think about it. Most furnaces will make it to 25 years or more before the reliability, maintenance, and repair issues point toward outright replacement, but many hit that point before age 20. Yours may be one of the latter. Replacement time is an opportunity moment to right size and gain an improvement in comfort, not just reliability or efficiency. Even if you're not ready to make that step this year, running the fuel-use load measurements way ahead of time is worth it, since that day is eventually coming.


Thanks for your note Dana. I have all the model info, as follows:

- Furnace - Armstrong GUK125D20-3A
- Serial No. - 8400E16109
- Main Blower Motor - A.O. Smith 3/4 hp, Model F48J38A45 ( replaced with something "universal" by the repairman)
- Control Board - Honeywell ST9120C2028, PN 40403-001, replaced with PN 40403-003 (current upgrade)

Had the repairman take a look when he did the blower and heat exchanger is in good condition. Considering furnace replacment is probably in the 5 grand range I think I'm still ahead of the curve on this one. That said, these things don't last forever so I'll be taking a look at the data as you suggest so I'm prepared when the time comes.
Back to the issue of the IDB running constantly, based on the wiring diagram on the furnace panel and that came with the control board I put that in correctly. The exception could possibly be the thermostat wires; for those I did not look at the diagram but installed them to the same terminals as they were originally. Tomorrow I'm going to connect the 2 wires that go to the diaphram and see if completing that circuit stops the IDB. Any other trouble shooting suggestions would be welcome.

Regards
Tom
 

Stuff

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To identify what is going on:
  1. Power off system
  2. Disconnect all thermostat wires from the control board.
  3. Power on system
Do indoor blower and inducer come on? If so indicates a roll-out or limit switch is tripped.

Most of the time the diaphragm/pressure switch is only looked at after a heat call from the thermostat.
 

Dana

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I couldn't find an online manual for the GUK125D20, but I suspect the "125" is the input BTU (125,000 BTU/hr), which means it's likely to have at least 100,000 BTU/hr of output.

For reasonably tight 2x6 /R19 type construction with clear glass (not low-E) double panes that would be right sized for a ~4500-5000' house at +5F (Akron's 99% outside design temp), enough oversize capacity to cover even Polar Vortex disturbance event cold snaps. Is that your house? 100,000 BTU/hr is enough furnace output to heat my 2x4 framed sub-code 2400' house + 1600' of insulated basement with clear glass storms over the 1923 vintage wood sashed single panes down to about -100F outside, a temperature not seen in my neighborhood (or Akron's) since before the last Ice Age.

For a typical 2000-3000' new-ish house in Akron it's likely to be on the order of 3-4x oversized. (Remember, ASHRAE draws the line at 1.4x oversizing for best comfort & efficinecy in residential heating.) If it's married to a central air conditioner using the same air handler the AC is probably similarly oversized. A pretty-good 30-40,000 BTU/hr condensing 2-stage can be had for under $1500, and installation isn't rocket science- often DIY-able, but it will be more complicated and expensive if the AC has to be replaced too. If that's the case it's a good idea to get a handle on both loads. Sometimes/often the best solution is a modulating heat pump to handle both heating & cooling, even if gas is currently somewhat cheaper from an operating cost point of view. If gas is a LOT cheaper a right-sized hydro-air solution with a cooling coil and hydronic heating coil in the air handler operating off a condensing water heater can work out well.

The AC oversizing factor can be inferred by measuring it's duty cycle on afternoons when the temperatures are near the 1% outside design temp, which in Akron is 85F. As with heating systems, oversizing the AC by more than ~1.5x is going to reduce rather than enhance comfort. To the untrained it might seem like it's "struggling to keep up" with very long on-cycles, but that will do a lot more for reducing the humidity than a 3x oversized AC in

So, go ahead and patch the furnace up- control components are relatively cheap, but start measuring your loads, think about what makes sense for a replacement.
 

Thomas G

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To identify what is going on:
  1. Power off system
  2. Disconnect all thermostat wires from the control board.
  3. Power on system
Do indoor blower and inducer come on? If so indicates a roll-out or limit switch is tripped.

Most of the time the diaphragm/pressure switch is only looked at after a heat call from the thermostat.

Hi Stuff,

Thanks for your suggestion. Sorry it took so long but I messed up my back and getting that low can only be done early in the day and I've had other demands on my back that prevented me from getting to this sooner.

I disconnected everything as you suggested. Induced Draft Blower comes on but main blower does not. I also jumpered the diaphram switch with thermostat connected and disconnected with no impact; IDB still comes on right away. The only things I see left are the ignition control that attaches to the gas valve and a sensor of some sort on the side of the igniter chamber. I have no idea how to check the ignition control and I looked at the sensor on the side of the ignition chamber and didn't see anything, but don't know if I would. I'm starting to think the brand new board I purchased is bad but I have to think the likelihood of that is remote. Ready for the next step.

Regards
Tom
 

Thomas G

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I couldn't find an online manual for the GUK125D20, but I suspect the "125" is the input BTU (125,000 BTU/hr), which means it's likely to have at least 100,000 BTU/hr of output.

For reasonably tight 2x6 /R19 type construction with clear glass (not low-E) double panes that would be right sized for a ~4500-5000' house at +5F (Akron's 99% outside design temp), enough oversize capacity to cover even Polar Vortex disturbance event cold snaps. Is that your house? 100,000 BTU/hr is enough furnace output to heat my 2x4 framed sub-code 2400' house + 1600' of insulated basement with clear glass storms over the 1923 vintage wood sashed single panes down to about -100F outside, a temperature not seen in my neighborhood (or Akron's) since before the last Ice Age.

For a typical 2000-3000' new-ish house in Akron it's likely to be on the order of 3-4x oversized. (Remember, ASHRAE draws the line at 1.4x oversizing for best comfort & efficinecy in residential heating.) If it's married to a central air conditioner using the same air handler the AC is probably similarly oversized. A pretty-good 30-40,000 BTU/hr condensing 2-stage can be had for under $1500, and installation isn't rocket science- often DIY-able, but it will be more complicated and expensive if the AC has to be replaced too. If that's the case it's a good idea to get a handle on both loads. Sometimes/often the best solution is a modulating heat pump to handle both heating & cooling, even if gas is currently somewhat cheaper from an operating cost point of view. If gas is a LOT cheaper a right-sized hydro-air solution with a cooling coil and hydronic heating coil in the air handler operating off a condensing water heater can work out well.

The AC oversizing factor can be inferred by measuring it's duty cycle on afternoons when the temperatures are near the 1% outside design temp, which in Akron is 85F. As with heating systems, oversizing the AC by more than ~1.5x is going to reduce rather than enhance comfort. To the untrained it might seem like it's "struggling to keep up" with very long on-cycles, but that will do a lot more for reducing the humidity than a 3x oversized AC in

So, go ahead and patch the furnace up- control components are relatively cheap, but start measuring your loads, think about what makes sense for a replacement.

Dana,

My home is 2x4 construction, 33oo sq ft plus about 1200 sq ft basement. Based on that I'd say my furnace is probably about right based on your note. AC is about 3 ton as I recall, maybe 3.5 so that should be about right too. Will keep your notes in my furnace file for future use though.

Regards
Tom
 

Dana

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Dana,

My home is 2x4 construction, 33oo sq ft plus about 1200 sq ft basement. Based on that I'd say my furnace is probably about right based on your note. AC is about 3 ton as I recall, maybe 3.5 so that should be about right too. Will keep your notes in my furnace file for future use though.

Regards
Tom


Depending on the particulars a reasonably tight house of that description will usually come in between 45,000-60,000 BTU/hr @ +5F (Akron's 99% temperature bin), which would make a 125K-in 80% efficiency furnace (~100K out) about 2x oversized, give or take. That's way better than being 3x oversized, but still sub-optimal for comfort. It's pretty easy to verify the oversize factor with a fuel-use heat load calc. Fuel use load calculations can also be used to measure the effectiveness of insulation & air sealing upgrades, etc.

Three tons AC sounds like the right ball park for most 3300' houses, maybe as much as 1.5 oversized, which is not a disaster. That too can be verified by measuring the duty cycle of the compressor on afternoons when it's around 85F outside (Akron's 1% design temp.)

Most houses in the 3000' range come in around a ton per 1300-1500', but there's a range. This is a plot of square feet per ton of real houses based on Manual-J's performed by Energy Vanguard, an energy & comfort consulting company based in Decatur Georgia, clipped from their blog site.:

square-feet-per-ton-air-conditioner-sizing.png


Most of those houses are in the Gulf states, with 1% outside design temps somewhat higher than yours. Note that the worst performing house in the 3000' range was about a ton per 900', which would call for a (3300'/900=) 3.5 ton AC on a house your size. If the ducts & air handler are (improperly but commonly) located in an attic above the insulation it can add a half-ton to a ton of parasitic load.

The best in class house in your size range in that graphic was the outlier with only a ton per 3300'. (A 1 ton air conditioner for a 3300' house!) That low load is most likely due to favorable shading factors and window placement/performance.
 

Thomas G

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Right now it sounds like a roll-out switch is tripped. Most have a reset button on them.
Get a multi-meter and measure voltage on pins 1 and 6 of the 6 pin molex style connector. Both should be 24vac or so. If not then limit switch is open or wiring not properly connected. https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-0781.pdf

Ok, with the male portion of the plug removed, measured across (female) terminals 1 and 6 and got about 25V and both blowers came on. Followed the white wire on your schematic to the rollout switch(?) which appears to be a small sensor that just sits flat on the ignition box (Picture Sensor 1). There is no hole in the box there so it does not "see" inside the box except for any heat transfer that might be taking place. Took it down and there is a button of sorts on the back side between the terminals. I get continuity across the terminals whether I'm pushing the button or not. Does not seem to want to reset to any position but the one it's in. The red wire from the block goes to a sensor (Picture Sensor 2) that has 2 reds on one terminal and a white that goes to the rollout switch (2 whites) on the other terminal. This was mounted so that the plate was upside down but the sensor faces up towards the flame.

Based on this information can you further pinpoint the problem? Rollout would seem to be it to me and a quick you tube search seems to indicate the same but not familiar enough with the device to say for sure.
 

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Dana

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Most limit switches normally closed (continuous), and continuity is broken when tripped- it's a circuit-breaker. If it hasn't tripped, resetting it has no effect.
 

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you're jumping around a bit. I meant for you to measure the voltage with things plugged in - should be zero volts between the two. Otherwise with the plug pulled measure the continuity on the plug between 1 and 6 to see if one of the sensors is open or not connected. Should be close to zero ohms.
 

Thomas G

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you're jumping around a bit. I meant for you to measure the voltage with things plugged in - should be zero volts between the two. Otherwise with the plug pulled measure the continuity on the plug between 1 and 6 to see if one of the sensors is open or not connected. Should be close to zero ohms.

I'd like to recap to make sure I understand correctly and I make the right checks.

1) With molex plug inserted into mate on the control board, check between molex connector pin 1 and ground. Should get approx 24 V

2) With molex plug inserted into mate on the control board, check between molex connector pin 6 and ground. Should get approx 24 V

3) With molex plug inserted into mate on the control board, check across molex pins 1 and 6. Should get 0 volts.

4) Remove molex plug and check across pins 1 and 6 of make plug for continuity/ohms. Should get continuity/0 ohms.

5) With molex plug removed, checked across pins 1 and 6 on female side on the control board with Simpson multimeter. Got a 24 V reading and both fans kicked on immediately.

6) No ground is marked on the schematics and furnace housing itself has been non responsive. Not an expert at these things, can I use one of the neutral terminals on the control board?

7) Based on Dana's comment, rollout switch is not the culprit.

Looking forward to my next assignment.

Tom
 

Thomas G

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Here are my readings for the numbered items above plus a few extra. Note that all of these checks are with no thermostat, humidifier, etc. wires connected to control board.

1) With molex plug inserted into mate on the control board, check between molex connector pin 1 and ground. Should get approx 24 V

Get reading of 24V. IDB comes on, main blower stays off.

2) With molex plug inserted into mate on the control board, check between molex connector pin 6 and ground. Should get approx 24 V
Get reading of 24V. IDB comes on, main blower stays off.

3) With molex plug inserted into mate on the control board, check across molex pins 1 and 6. Should get 0 volts.

Get reading of zero V. IDB comes on, main blower stays off.

4) Remove molex plug and check across pins 1 and 6 of male plug for continuity/ohms. Should get continuity/0 ohms.

Continuity between 1 and 6

5) With molex plug removed, checked across pins 1 and 6 on female side on the control board with Simpson multimeter. Got a 24 V reading and both fans kicked on immediately. - Checked again. Get reading of 24V. IDB comes on, main blower stays off.


Other items checked:

  • With roll out switch disconnected, both blowers come on (Sensor 1) . With roll out switch connected, only IDB comes on.
  • With Sensor 2 disconnected, both blowers come on. Connected, only IDB comes on.
  • With Diaphram switch either connected or disconnected, only IDB comes on.
Any insights would be appreciated.

Regards
Tom
 

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Tests show normal system. Put the old control board back and see what it does. Maybe you mis-wired when swapping it out.
 

Thomas G

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Thanks for getting back to me. Will check wiring against schematic. Must confess that I wired the board as it was and I can't be sure that one of the techs didn't move something and not put it back in the same place by accident. Will let you know what I find out.
 

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Since suspecting wiring then verify the draft inducer motor (your IDB) is wired to the "DI" and one of the neutral terminals on the board.
 

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Back to the basics. From the St9120 manual - order of operation after Terminal W is energized:
1. Combustion air blower is energized.
2. Air proving switch makes (air flow is established)
3. Ignition system is energized.
4. Gas valve opens and main burner lights.
5. Heat fan-on delay timing begins. When timing is complete, circulating fan is energized at heat speed.​

So next question - does inducer blower run without a heat call/thermostat off?
 

Thomas G

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Thanks for your replies. First, yes, IDB is connected black to D1 and white to a neutral terminal.

Second, IDB starts immediately when thermostat is in off, cool or heat mode, even when the temperature set point is lower than needed to activate for heat and higher than needed to activate for AC. It runs ALL the time.

Other things I checked yesterday and this morning because of your suggestion to recheck wiring:

  • There is a coil(?) with 3 wires coming off of it; a black and 2 yellows.
    • The black goes to a separate piece of romex in the compartment that exits the side and goes off somewhere.
    • One yellow goes to the EAC terminal.
    • The other yellow goes to a neutral terminal.
    • The white wire from the romex goes to a neutral terminal.
    • I think this romex is the carrier for the current to engage the AC breaker and the condensing unit since it disappears into the ceiling of the basement. However, this coil/transformer is not shown as a separate unit on the schematics as far as I can see. I have attached 2 pictures of the coil and 1 of the overall installation below. You can see the romex at the bottom.
  • [FONT=Georgia, Times New Roman, Times, serif]Assuming that EAC is for Electronic Air Cleaner, I do not have one of those. I do have a humidifier, assuming that is what HUM means.
  • [FONT=Georgia, Times New Roman, Times, serif]The system is supposed to be set up so that the humidifier turns on and off with the main blower. I did this to minimize excess moisture and potential rust in the system. I will check that later.[/FONT]
  • [FONT=Georgia, Times New Roman, Times, serif]There are 2 black wires coming from the safety switch. The first is fairly heavy gauge (10?) and is connected to the S1 terminal which matches the schematic. The second is lighter gauge (14?) and goes directly to the HUM terminal. This is not a new connection and looks like a wire that was added by the installer when the furnace was initially installed. When I disconnect this wire, the IDB stops.[/FONT]
Not sure of the significance of this last item but seems to be some kind of key. I also did what I could to check continuity across the 2 relays on the old board. Both tested exactly the same so I am reasonably sure that 1) the old board was fine and both techs who were here just wanted to do the expensive things first and 2) the new board is fine also and the relays are not the problem.
[/FONT]

Otherwise wiring appears to match the schematic.
 

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Stuff

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The "coil" is a relay that is not hooked up fully so isn't doing anything. Probably used to trigger a remote booster fan or similar. EAC (electronic air cleaner) is what the terminals were for and runs with the main blower. HUM is humidifier. That terminal turns on with the inducer motor so only runs on heat.

The last statement is the root of the problem. Your safety switch is rigged to connect power to HUM which is internally bridged to DI so runs inducer.

Do you have the schematic of the unit (should be a piece of paper glued somewhere)? If not should be like page 6 of https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-0781.pdf
You need to trace power wires coming from outside. Should go to safety switch and then to S1 - nothing should be on HUM except for a humidifier connection.
 
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