# Natural Gas Furnace - no heat



## madrussian

Got an Amana/Goodman natural gas furnace. Model CHPF1824A6AA, about 12 years old.

Has worked great until late last week. LED lights blink 3 times, indicating an induction air problem.

I checked the vacuum switch - infinite resistance when induction fan is off. When induction fan turns on goes to zero resistance. I think this is OK.

Checked limit switch, it being cold - is closed at zero resistance (not open) - i think this is OK. 

I need to check the . But what else do I need to check?


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## madrussian

I also tested the ignition element - giving me 50 ohms - so i am assuming this is OK?


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## Snoonyb

The #CHPF1824A6AA, is for the "A" coil, not the furnace.


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## buffalo

Does it fire at all? Check and make sure your exhaust and intake pipes are not blocked either by snow or pests .


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## madrussian

buffalo said:


> Does it fire at all? Check and make sure your exhaust and intake pipes are not blocked either by snow or pests .



Nope does not fire. The induction fan turns on, vacuum switch closes, but nothing after that. Vacuum switch seems to operate properly but the thing does not fire up.


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## madrussian

anyone know the order of operation? Should burner kick in next, then main blower turn on?

I am thinking main control board...


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## kok328

what's your voltage on the HSI after the vacuum/pressure switch closes?


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## madrussian

kok328 said:


> what's your voltage on the HSI after the vacuum/pressure switch closes?



HSI is the igniter? Should I unplug the igniter and measure voltage across those two leads?


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## nealtw

You didn't answer all of Buffalo's question.


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## kok328

madrussian said:


> HSI is the igniter? Should I unplug the igniter and measure voltage across those two leads?



Your trying to determine voltage TO the inginiter so test the leads that connect to the igniter, not the leads on the igniter.
and of course you know not to touch the igniter with your fingers.
Inspect the igniter for hot spots and hairline cracks but, i think you already ohmed it out at 50


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## madrussian

nealtw said:


> You didn't answer all of Buffalo's question.


Sorry - yeah replaced all filters. Didnt climb on the roof (not a ladder guy). However given the information that when induction fan comes on, the pressure switch closes, I think it is safe to assume there is no blockage on top of the roof.


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## madrussian

kok328 said:


> Your trying to determine voltage TO the inginiter so test the leads that connect to the igniter, not the leads on the igniter.
> and of course you know not to touch the igniter with your fingers.
> Inspect the igniter for hot spots and hairline cracks but, i think you already ohmed it out at 50



Sorry yeah that is what i meant - voltage to the igniter. I checked this and I now think the power supply is suspect. got 120 VAC going to the PS, but nothing coming out of the 24 V.


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## kok328

If your transformer was bad you would not have gotten that far in the ignition sequence.


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## DFBonnett

FWIW, some years back I had the same problem, but on a less complicated furnace. The gas valve had failed and did not open. A new gas valve was the fix.


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## WyrTwister

Can you visually verify if the HSI comes on or not ?

     Can you post a photo of the control wiring diagram ?

     Pretty sure the HSI operates on 120 VAC .

God bless
Wyr


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## madrussian

WyrTwister said:


> Can you visually verify if the HSI comes on or not ?
> 
> Can you post a photo of the control wiring diagram ?
> 
> Pretty sure the HSI operates on 120 VAC .
> 
> God bless
> Wyr



I assume the HSI should come on after the induction fan comes on:

Yes after looking at the schematic in the manual, the HSI is on the high voltage (120v) circuit.

I need to look at this a little closer. Should be able to check this voltage today - got a multimeter that works.


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## kok328

Your jumping all over. 
Use a systematic approach to troubleshooting. 
Start with line power and then check control power.


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## madrussian

kok328 said:


> Your jumping all over.
> Use a systematic approach to troubleshooting.
> Start with line power and then check control power.



Agreed. 

*Part of what I do not know is order of operation of a furnace.* No Idea. If anyone can provide this, would be a great help!

Part of my initial problem was the multi-meter I have was suspect, so some of the first troubleshooting was not accurate.

This is what I know/done:
1. First noticed an issue. Induction fan would run, nothing else.
2. The LED was flashing 3x. According to manual this is a vacuum switch/induction problem.
3. Replaced vacuum switch, no change.
4. Checked resistance of both switches. Both are OK. When induction fan OFF = infinite ohms. When induction fan ON = 0 ohms.
5. Checked resistance of HSI = 53.9 ohms.
6. Double checked all incoming voltages (with new multi-meter). Got 120 VAC at main on/off switch, junction box going into furnace and at the transformer.
7. Checked voltage going into the HSI. Getting ~1 VAC. *But again I do not know the order of operation.* So I do not know if this should be ON or OFF yet.


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## madrussian

Checked voltage coming off the control board to HSI - nothing there. (with induction fan ON)

Also checked primary limit and aux limit switched - both are closed (indicated normally closed on schematic).

When I was checking voltage on the control board a few of the relays were turning on/off, so possibly the control board has failed?


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## madrussian

With no other place to go I found a few pretty cool tools for the control board:
1. troubleshooting manual: https://www.manualslib.com/manual/556709/White-Rodgers-50a55.html?page=3#manual
2. Schematic for control board: https://arnoldservice.com/wp-content/uploads/2012/09/PCBBF112S-50A55-289-instructions.pdf

Using the troubleshooting flow chart, it has guided me back to the 120v/24v transformer. Got voltage going in, nothing coming out.


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## nealtw

madrussian said:


> With no other place to go I found a few pretty cool tools for the control board:
> 1. troubleshooting manual: https://www.manualslib.com/manual/556709/White-Rodgers-50a55.html?page=3#manual
> 2. Schematic for control board: https://arnoldservice.com/wp-content/uploads/2012/09/PCBBF112S-50A55-289-instructions.pdf
> 
> Using the troubleshooting flow chart, it has guided me back to the 120v/24v transformer. Got voltage going in, nothing coming out.



That could be a problem.


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## kok328

Check around on the transformer. Sometimes they have a manual overload reset button on them. 
Leave your meter on the hsi wires or at board during ignition and see if it ever gets that far. 
Are you still showing 3 blinks?


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## kok328

If you do replace the transformer put an inline fuse on the secondary. 
Short in the system may have burned it up.


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## Jeffh

Does the hsi glow red hot during the operation? That should happen next. Then, gas valve clicks open, flame sensor keeps gas valve open, low limit initializes, blower kicks on...


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## madrussian

It was the transformer. As soon as it was replaced, turned the system on, the HSI got red hot and gas flowed.


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## oldognewtrick

madrussian said:


> It was the transformer. As soon as it was replaced, turned the system on, the HSI got red hot and gas flowed.



We love happy endings.


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## madrussian

I actually stumbled across the fix. When schematic was reading 24V, I was measuring in DC, not AC. Doh.

the flow chart led me to replace the xformer but I dont think it was the 24v side, I think the 120V side wasn't completely operational.


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## nealtw

madrussian said:


> I actually stumbled across the fix. When schematic was reading 24V, I was measuring in DC, not AC. Doh.
> 
> the flow chart led me to replace the xformer but I dont think it was the 24v side, I think the 120V side wasn't completely operational.



You must of had 120 volts on that side and that is really all you can check on that side. After that anything that prevents you from getting 24 volts is the problem with the transformer.


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