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 > Furnace runs but blows cold

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wa8yxm

Wherever I happen to park

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Posted: 02/06/21 03:54pm Link  |  Quote  |  Print  |  Notify Moderator

The sequence
T-state says "I'm chilly, Warm me please"
Control board starts blower blowing
Sail switch closes when blower is "up to speed"
Solenoid valve opens (Propane flows in theory) and igniter sparks.
Next step is an A/B Most Atwood and Suburbans are B
A: Thermocouple (Flame sensor) sends about 0.480 volts to the control board this causes current to flow and board says "OH. Flame is on" and heat continues till T-stat says "Ah. Nice and toasty, you can stop now" or Over temp cutout cutouts.

B: many Furnaces have a single wire ignition/sense system. in this system the wire carries roughly 1000 VAC during sparking and then switches to Sense lookin for the 0.480 volt DC thermocouple out. IF the TC (Thermocouple) is carboned up it may not get hot enough to generate the needed voltage and current.

Or if the change from Spark to Sense flubs Pop goes the sensor chip
(My theory of what killed mine.. And when I inspected the replacement which came from Dinosaur boards it appears it was their theory as well)

There is a LED on the furnace control board that will flash in most cases if it fails.. Whta is the pattern.

Page two... Even if it is working normally so much air flows it won't feel all that warm


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Dusty R

Charlotte Michigan 48813

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Posted: 02/06/21 05:48pm Link  |  Quote  |  Print  |  Notify Moderator

You can remove the thermal couple and check it with a propane torch and DC millimeter.

Dusty

enblethen

Moses Lake, WA

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Posted: 02/06/21 05:52pm Link  |  Quote  |  Print  |  Notify Moderator

OPs furnace should have a flame sensor and ignitor, not a thermal couple.


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Teleman

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Posted: 02/06/21 07:15pm Link  |  Quote  |  Print  |  Notify Moderator

enblethen wrote:

Can of air with the tube works pretty good!

I got into the blower with compressed air and cleaned all the contacts but it didn't help. I watched this video which explains the procedure for taking the furnace apart to replace the motor, igniter and burner. My motor is good but I'd have to go through all of this to access the sail switch.


RV Furnace Motor Squealing - Remove & Replace ... - YouTube

www.youtube.com › watch › v=oaTW0oNf6Qw



From what I can tell the blue and white wires of the multi pin connector on the board go to and from the sail switch and high level thermostat. Using tap connectors and shorting those wires would get the furnace working IF either the sail switch or high level thermostat were the problem right? If so I know I have to pull the motor to get to them. If not then it could be the igniter or the board. I guess checking the igniter would be the next step.

* This post was edited 02/07/21 09:08am by Teleman *

RobWNY

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Posted: 02/06/21 08:49pm Link  |  Quote  |  Print  |  Notify Moderator

Teleman wrote:

RobWNY wrote:

80% of the time, it's the sail switch

I was afraid of that. Are you familiar with this unit and what is involved in replacing it?

Unfortunately no but I have a friend that has had a lot of trouble with his furnace and when the tech came to repair it, I happened to be there. He told me about the sail switch being the issue 80% of the time. I just relayed what I was told. There are RV repair techs that have youtube channels and they have done videos on furnaces. You might try looking them up.


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Teleman

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Posted: 02/06/21 10:43pm Link  |  Quote  |  Print  |  Notify Moderator

RobWNY wrote:

Teleman wrote:

RobWNY wrote:

80% of the time, it's the sail switch

I was afraid of that. Are you familiar with this unit and what is involved in replacing it?

Unfortunately no but I have a friend that has had a lot of trouble with his furnace and when the tech came to repair it, I happened to be there. He told me about the sail switch being the issue 80% of the time. I just relayed what I was told. There are RV repair techs that have youtube channels and they have done videos on furnaces. You might try looking them up.

I linked to a youtube in the comment above.

wa8yxm

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Posted: 02/07/21 05:45am Link  |  Quote  |  Print  |  Notify Moderator

enblethen wrote:

OPs furnace should have a flame sensor and ignitor, not a thermal couple.
Says you? The flame sensor as you call it IS a thermocouple.. I know. I tested mine. I am aware they are often called "Flame Sensors" but there are two or three types of flame sensors. ONE of them, the one Atwood and Suburan both use, as well as my water heater. Is a Thermocouple

Dutch_12078

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Posted: 02/07/21 07:31am Link  |  Quote  |  Print  |  Notify Moderator

wa8yxm wrote:

enblethen wrote:

OPs furnace should have a flame sensor and ignitor, not a thermal couple.
Says you? The flame sensor as you call it IS a thermocouple.. I know. I tested mine. I am aware they are often called "Flame Sensors" but there are two or three types of flame sensors. ONE of them, the one Atwood and Suburan both use, as well as my water heater. Is a Thermocouple


Thermocouples use two dissimilar metals to generate a small voltage when heated that can be used to maintain the fuel supply for the flame. If it cools down, the fuel supply stops. Atwood and Suburban use thermocouples in their standing pilot water heaters, but not in their DSI water heaters, refrigerators, or furnaces.

Atwood and Suburban's DSI flame sensors work by sending a small current to the ignitor probes that is passed through the conductive flame to be sensed by the ignitor board where it's used to keep the fuel supply valve open. No flame, no current passes back to the board and the valve closes. A common flame sensor failure mode is that the burner lights and then quickly shuts off when the flame is not sensed due to misadjustment, dirt, corrosion, or board failure.


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dougrainer

Carrolton, Texas

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Posted: 02/07/21 08:22am Link  |  Quote  |  Print  |  Notify Moderator

wa8yxm wrote:

enblethen wrote:

OPs furnace should have a flame sensor and ignitor, not a thermal couple.
Says you? The flame sensor as you call it IS a thermocouple.. I know. I tested mine. I am aware they are often called "Flame Sensors" but there are two or three types of flame sensors. ONE of them, the one Atwood and Suburan both use, as well as my water heater. Is a Thermocouple


You just have to show your lack of knowledge. Doug

PS. 56,000 posts in 15 years and probably 95% are wrong.

dougrainer

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Posted: 02/07/21 08:26am Link  |  Quote  |  Print  |  Notify Moderator

Teleman wrote:

enblethen wrote:

Can of air with the tube works pretty good!

I got into the blower with compressed air and cleaned all the contacts but it didn't help. I watched this video which explains the procedure for taking the furnace apart to replace the motor, igniter and burner. My motor is good but I'd have to go through all of this to access the sail switch.

https://m.youtube.com/watch?v=oaTW0oNf6Qw

From what I can tell the blue and white wires of the multi pin connector on the board go to and from the sail switch and high level thermostat. Using tap connectors and shorting those wires would get the furnace working IF either the sail switch or high level thermostat were the problem right? If so I know I have to pull the motor to get to them. If not then it could be the igniter or the board. I guess checking the igniter would be the next step.


You do not have to remove the motor to check replace the sail switch. But on a 22 year old Furnace, I would replace BOTH the Sail Switch and the Hi limit switch since they can be intermittant and just messing with a sail switch may make it start working. For a few times. They are cheap so replace both. The Hi Limit can be the cause of your problem. Doug

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