# Furnace Troubleshooting



## SaveFerris (Sep 19, 2009)

I have been fighting with my furnace since last summer. It has been working intermittently, showing many of the signs of a faulty sail switch, similar to this post from last October. For me, it only happens when the outside temperature is below about 50 degrees and I'm running on batteries.



I checked the batteries first, and they seem to be okay. 



Yesterday, I pulled the furnace and here is what I found, running on batteries with the gas line disconnected:



- Voltage coming into the furnace was 12.58 V with the furnace off

- When the blower came on, the voltage coming into the sail switch was 11.30

- When the next stage started (ignition), the voltage dropped down to 10.50

- The sail switch appeared to be operating correctly. The switch was thrown as soon as the blower started running, and the voltage drop across the switch was minimal (about 0.06 V)



So while I was testing it, it was getting to the ignition stage just fine. I have only been able to recreate the problem once while testing, and that was with all the lights and fans turned on to artificially lower the voltage as much as possible.



Does that amount of voltage drop when the blower comes on seem reasonable? I considered that maybe the blower motor is going bad, so it is drawing more current than it should. The air being pushed out the vents farthest from the furnace has always been very weak as long as we have had it.



Does anyone have any other ideas?



Nathan


----------



## CdnOutback (Apr 16, 2010)

That seems to me to be a very high voltage drop... How old are your batteries? That is lower voltage than where it would drop when drawing to turn a starter motor. JMO...


----------



## SaveFerris (Sep 19, 2009)

CdnOutback said:


> That seems to me to be a very high voltage drop... How old are your batteries? That is lower voltage than where it would drop when drawing to turn a starter motor. JMO...


The batteries are only 2 years old, but they went through a cold spell a year and a half ago and were at least partially frozen.

Right after I bought the Outback, I left it parked outside without shore power for a couple of weeks. I wasn't paying attention, and the propane detector drained the batteries just in time for the temperature to drop to 15 below zero for a while.

I popped the access caps and found some ice at the top of each cell, so I thought they would be useless after that. Still, I used them all last season and they seemed to have plenty of life left in them.

I had them tested at an auto parts store and an RV dealer Friday, and they both said the batteries were still good. They each did a load test, and the voltage looked great. I remain suspicious that the problem might be the batteries after all, but I just couldn't justify buying two new batteries after having two "experts" tell me they are still good.


----------



## dirtengineer (Jun 6, 2010)

Sounds like a battery problem.


----------



## SaveFerris (Sep 19, 2009)

dirtengineer said:


> Sounds like a battery problem.


I haven't ruled out the batteries yet, but I also have not ruled out a bad blower motor.

Running on shore power just now, I turned the furnace on and it tripped the 15 amp 110V breaker on the house circuit. That, combined with the fact I can hardly feel any air flow through the vent registers, seems to indicate the blower motor might be the problem.


----------



## dirtengineer (Jun 6, 2010)

Tripping the 110V circuit also points to bad batteries, not a bad blower. The blower is 12V. Likely, your converter is pulling lots of juice trying to feed bad batteries. If it were me, my next step in troubleshooting would be to get a hydrometer and check for bad cells.


----------



## CamperAndy (Aug 26, 2004)

Disconnect the batteries and try again on shore power. With only the blower load the converter will pull a much lower load. If it then trips the 110 you have a converter issue to go along with the furnace issue.


----------



## SaveFerris (Sep 19, 2009)

CamperAndy said:


> Disconnect the batteries and try again on shore power. With only the blower load the converter will pull a much lower load. If it then trips the 110 you have a converter issue to go along with the furnace issue.


I disconnected the batteries and ran the furnace last night. It did not trip the 110 breaker.

Before I had the batteries load tested, I checked the cells with a floating ball type tester. Battery 1 had consistent readings with all 4 balls floating. Battery 2 had 5 cells with 2 balls floating (50% charge), and 1 cell with 1 ball floating.

That is why I was surprised by the results of the load testing. I thought they would tell me at least one of the batteries was toast. I have always taken the floating ball tester with a grain of salt, so I figured it might just be completely off this time. I will be getting an actual hydrometer next to recheck those cells.

I was also checking the air flow through the vents again last night, and I realized it is not the vent closest to the furnace that has the only decent flow. The one up front has good flow, but the flow from the mid and rear vents is barely perceptible.

Then I read this post last night, and it occurred to me I might have a couple of crushed air ducts. The mid and rear ducts are probably right above the water tank, which I have filled up too fast many times. I imagine restricted air flow in the ducts could create backpressure in the blower, which would slow the air flow, which would affect the sail switch. That would also explain why I could not recreate the problem once I had the furnace pulled.

So it is possible I am dealing with a couple of compounding issues here. I will use a real hydrometer to recheck the batteries, and also see if I can open up those ducts. I like the air mattress idea, Andy - I may give that a shot.


----------



## CamperAndy (Aug 26, 2004)

SaveFerris said:


> Then I read this post last night, and it occurred to me I might have a couple of crushed air ducts. The mid and rear ducts are probably right above the water tank, which I have filled up too fast many times. I imagine restricted air flow in the ducts could create backpressure in the blower, which would slow the air flow, which would affect the sail switch. That would also explain why I could not recreate the problem once I had the furnace pulled.
> 
> So it is possible I am dealing with a couple of compounding issues here. I will use a real hydrometer to recheck the batteries, and also see if I can open up those ducts. I like the air mattress idea, Andy - I may give that a shot.


The sail switch is in the combustion air flow circuit and not the heated air circuit. I think you were dealing with a low voltage issue that caused the ignition problem. That said you could still have crushed ducts impeding your heated air flow. Also a common issues is the duct that connects the furnace to the main trunk duct comes apart and the air is getting dumped into the under belly.


----------



## SaveFerris (Sep 19, 2009)

I picked up a hydrometer and tested the batteries again. After charging, then sitting for a few hours, the SG was 1.250 for all cells on Battery 1, and 1.225 for all cells on Battery 2. Voltage was good from both batteries at about 12.8.

The furnace worked this weekend, but the temperature only got down to about 50 overnight. After one night of use, SG was basically unchanged the next morning.

Does a SG of 1.225 sound low? According to the chart on this website, it should be 1.265 with a full charge.


----------



## dirtengineer (Jun 6, 2010)

This does appear to be a mystery. Perhaps someone smarter than me will chime in. That SG is low and doesn't match the voltage that you are reading. Perhaps there is a surface charge that is giving erroneous voltage readings. Also, make sure you isolate each battery before checking voltage or they will always be the same.

One other thing you can do is try one battery at a time and see if the performance is different. Do something like run the same number of lights and check the voltage for each. I would use lights because the load is easy to repeat and it takes the furnace out of the equation. Four or five fixtures is a pretty good load and would make a bad battery sag. That would at least give you a comparison of your two batteries.


----------



## SaveFerris (Sep 19, 2009)

dirtengineer said:


> This does appear to be a mystery. Perhaps someone smarter than me will chime in. That SG is low and doesn't match the voltage that you are reading. Perhaps there is a surface charge that is giving erroneous voltage readings. Also, make sure you isolate each battery before checking voltage or they will always be the same.
> 
> One other thing you can do is try one battery at a time and see if the performance is different. Do something like run the same number of lights and check the voltage for each. I would use lights because the load is easy to repeat and it takes the furnace out of the equation. Four or five fixtures is a pretty good load and would make a bad battery sag. That would at least give you a comparison of your two batteries.


Yep, it's got me scratching my head. I did isolate each battery when testing the voltage. I haven't tried running on just one battery at a time yet - I will give that a try.

I'm about ready to get new batteries just so I can take that off the list.


----------

