We've been without heat for the past two days with nighttime temperatures hovering around 0°C.
So I learned a few lessons on how to fix a broken furnace.
1) I measured the resistance across the gas valve and measured 4MΩ. Actually, there are two coils in the valve. They both measured 4MΩ to common while they measured 11MΩ across the two. I thought to myself that that is odd. I was expecting a low resistance across a solenoid valve and open circuit if the coil was blown. Why did two 4MΩ together add up to 11MΩ? Had I gone to the internet in the first place I would have been given the correct answer but of course I didn't. More on this later. Lesson #1.
2) I called in HVAC service and the young technician said he thinks the gas valve is ok because he can hear hissing and can smell gas. I can assure you that his hearing and sense of smell is a lot better than mine. So he checks the igniter and reports that there is only 80V going to the igniter and it should be 120V. He thinks that there is a problem on the controller board and needs to be replaced at a tune in excess of $800 and 3-5 days to get a new board. Checking the internet was Lesson #2.
3) So I said, if there is a problem on the control board I am willing to have a look at it instead of waiting 3-5 days for heat to be restored.
Result: Success today and heat is restored.
Here are the lessons learned (after going to the internet).
Lesson #1. While I was expecting to read low resistance across a solenoid, apparently this is not so. There is a bridge rectifier in the solenoid circuit that shows 4MΩ on an ohmmeter. This part I have not figured out as yet. In any case you cannot do a resistance test to check a newer gas valve.
Lesson #2. In our previous furnace the igniter was a HV spark gap. Not anymore in newer furnaces. They use a hot surface igniter (HSI) which is a silicon nitride high temperature resistor. This should measure about 10-20Ω on an ohmmeter. I can only assume that the technician verified this and placed the fault on the controller board quoted in excess of $800. The knowledge I gained was that there are 80V HSI and 120V HSI. My device was the 80V kind.
The resistance of the HSI was about 4kΩ, yet when I applied 120VAC directly to it, it did glow, but only one time. I assume that applying 120V on the 80V device killed it but this is purely speculation. I can only assume that the HSI was faulty and by applying 120V it gave its last breath of life and then died.
The old HSI part is White Rodgers 768A-15 which has been replaced with 768A-815. Delivery was quoted as 1-4 weeks.
I ordered a 768A-843 which arrived in two days. I figured that the only difference was the style of the connector that has to mate with the furnace. The only problem is one needs the proper tool to extract the molex-type contacts. Fortunately for me I do have that tool. So I just swapped out the new contact housing and replaced it with the old one.
The bottom line here would be to check the HSI first and it should read 10-20Ω. Comments on the internet state that the HSI has a finite lifetime and should be replaced every 5 years. Our furnace is 13 years old.
All in all, I paid for a service call and the cost of a new HSI from the internet. The cost of the service call was my lesson on how a house furnace works. I saved over $1000 by doing the repair myself. My fingers and toes are now warm and cozy as I wrap up this post.
Edit: Here is a photo of the HSI.
So I learned a few lessons on how to fix a broken furnace.
1) I measured the resistance across the gas valve and measured 4MΩ. Actually, there are two coils in the valve. They both measured 4MΩ to common while they measured 11MΩ across the two. I thought to myself that that is odd. I was expecting a low resistance across a solenoid valve and open circuit if the coil was blown. Why did two 4MΩ together add up to 11MΩ? Had I gone to the internet in the first place I would have been given the correct answer but of course I didn't. More on this later. Lesson #1.
2) I called in HVAC service and the young technician said he thinks the gas valve is ok because he can hear hissing and can smell gas. I can assure you that his hearing and sense of smell is a lot better than mine. So he checks the igniter and reports that there is only 80V going to the igniter and it should be 120V. He thinks that there is a problem on the controller board and needs to be replaced at a tune in excess of $800 and 3-5 days to get a new board. Checking the internet was Lesson #2.
3) So I said, if there is a problem on the control board I am willing to have a look at it instead of waiting 3-5 days for heat to be restored.
Result: Success today and heat is restored.
Here are the lessons learned (after going to the internet).
Lesson #1. While I was expecting to read low resistance across a solenoid, apparently this is not so. There is a bridge rectifier in the solenoid circuit that shows 4MΩ on an ohmmeter. This part I have not figured out as yet. In any case you cannot do a resistance test to check a newer gas valve.
Lesson #2. In our previous furnace the igniter was a HV spark gap. Not anymore in newer furnaces. They use a hot surface igniter (HSI) which is a silicon nitride high temperature resistor. This should measure about 10-20Ω on an ohmmeter. I can only assume that the technician verified this and placed the fault on the controller board quoted in excess of $800. The knowledge I gained was that there are 80V HSI and 120V HSI. My device was the 80V kind.
The resistance of the HSI was about 4kΩ, yet when I applied 120VAC directly to it, it did glow, but only one time. I assume that applying 120V on the 80V device killed it but this is purely speculation. I can only assume that the HSI was faulty and by applying 120V it gave its last breath of life and then died.
The old HSI part is White Rodgers 768A-15 which has been replaced with 768A-815. Delivery was quoted as 1-4 weeks.
I ordered a 768A-843 which arrived in two days. I figured that the only difference was the style of the connector that has to mate with the furnace. The only problem is one needs the proper tool to extract the molex-type contacts. Fortunately for me I do have that tool. So I just swapped out the new contact housing and replaced it with the old one.
The bottom line here would be to check the HSI first and it should read 10-20Ω. Comments on the internet state that the HSI has a finite lifetime and should be replaced every 5 years. Our furnace is 13 years old.
All in all, I paid for a service call and the cost of a new HSI from the internet. The cost of the service call was my lesson on how a house furnace works. I saved over $1000 by doing the repair myself. My fingers and toes are now warm and cozy as I wrap up this post.
Edit: Here is a photo of the HSI.
Last edited:
