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Old 11-28-2007, 11:03 AM   #1
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furnace blowing cold air

I recently purchased a '71 safari. In decent shape, though not pristine by any stretch. My furnace works in the sense that the pilot stays lit, and the blower works fine as well, but it only blows cold air. I thought maybe it would take a few minutes, but after 10 mintutes it was still cold air. I don't know the physics of furnaces, but obviously something that should be happening isn't. I've had problems with other trailer heaters, but it's usually a pilot not staying lit, or something like that. any ideas? I've tried to find other threads with this as it's subject but wasn't able to. Any help is appreciated, thanks!
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Old 11-28-2007, 02:06 PM   #2
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Ok, I'll make the same guess I've made before on this: $10 says it's your sail switch.

Remove the switch (no very easy job), blow it out really well with compressed air, and put it back in. I've done ours several times -- and have yet to use the backup switch that we purchased new several years ago.


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Old 11-28-2007, 02:24 PM   #3
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I really hate to advertise my profound ignorance, but what is a sail switch, and more importantly WHERE is it??!! I'll try to fix anything, but I'm more nervous about propane deals, so I want to be relatively confident about what I'm looking for. I have most of the original paperwork, the manuals for the individual appliances, that sort of thing, would it show this sail switch in the furnace literature? Thanks much for the tip!
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Old 11-28-2007, 03:03 PM   #4
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Quote:
Originally Posted by canine camp
I really hate to advertise my profound ignorance, but what is a sail switch, and more importantly WHERE is it??!! I'll try to fix anything, but I'm more nervous about propane deals, so I want to be relatively confident about what I'm looking for. I have most of the original paperwork, the manuals for the individual appliances, that sort of thing, would it show this sail switch in the furnace literature? Thanks much for the tip!
The sail switch or air prover switch is in the duct between the fan and the outer part of the combustion chamber.

You must pull the furnace to see it.

It's nothing more than a microswitch with a paddle on the end of the arm.

If not enough air is flowing from the fan to the outer part of the combustion chamber, it will not allow the LPG valve to open which would allow the LPG to flow to the burner.

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Old 11-28-2007, 03:09 PM   #5
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Originally Posted by canine camp
I recently purchased a '71 safari. In decent shape, though not pristine by any stretch. My furnace works in the sense that the pilot stays lit, and the blower works fine as well, but it only blows cold air. I thought maybe it would take a few minutes, but after 10 mintutes it was still cold air. I don't know the physics of furnaces, but obviously something that should be happening isn't. I've had problems with other trailer heaters, but it's usually a pilot not staying lit, or something like that. any ideas? I've tried to find other threads with this as it's subject but wasn't able to. Any help is appreciated, thanks!

I would also suggest that you purchase a 1973 "service manual."

For about 99 percent of the time, it's the same as your 71.

Over the short and long haul, it will save you money, since it goes into great detail on everything in your Airstream.

Tells you how to take things apart, test them, fix them and then reinstall them, in detail.

If your going to be your own mechanic, then the manual will help you many times over.

Andy
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Old 11-28-2007, 03:20 PM   #6
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Look at it this way: One thing you would NEVER want to happen is to have the main flame burning, but the fan inoperable. That would be a nice recipe for a fire. To prevent that, there's a little switch with a longish arm and a small sail on mounted to the arm. This switch is right in front of the fan where it blows into the heat-exchange chamber. The fan turns on, and it's "wind" blows over the arm on the sail switch, which, in turn, permits LP to flow into the fire chamber for burning. If the fan doesn't turn on, the sail switch doesn't trip, and no burn appears in the fire chamber. Presto! You're safe.

A sail switch has to trip pretty easily; after all, it's activated just by "wind." If it gets pretty dusty in there, the sail switch gets sticky and won't blow over. The result: The fan runs; igniter tries its best to light a fire; but nothing happens (except for cold, blowing air).

On the older units that I've worked on, the sail switch is reachable from the back of the unit. As a result, the whole unit has to be removed from the rig. From then on out, it's pretty straightforward (unless, like me, you have fat fingers and tend to drop teeny-tiny nuts). On the newer pancake units with a squirrel cage fan that I've worked on, the switch is located inside the curved plastic cover on the front of the unit. To access it, you have to undo a bunch of wiring and the like, and then remove the curved plastic cover. These are more of a job than one might hope for, but doable with some diligence.


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Old 11-28-2007, 03:50 PM   #7
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thanks very much to both of you, Lynn and Andy, I'm sure I'll be able to give it a shot. And I will buy a repair manual, sounds like an investment I'd be crazy to pass on. Not sure I'll be getting to this anytime soon, it just snowed for the first time, but as soon as I'm able I'll try to get to that sail switch to see if that's the problem. thanks again!
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Old 01-29-2008, 02:55 PM   #8
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Ok, finally had some time and snowmelt enough to get to my furnace...pulled it out (easier than I thought, and good thing I bought the 1973 repair manual as suggested, thanks a million)... the sail switch does work, in that the fan blows it so that it trips....BUT, it doesn't switch over the valve. The repair manual suggests that it needs to be replaced. Should this be my first option, or is there a more likely reason I should try first? Also, I couldn't tell from the manual and looking on the websites that sell sail switches what part number it is....I plan on taking it out, is the part number right on the switch itself? Any help is appreciated...I don't want to end up replacing ever single component, so I want to get the most likely problems first. Thanks!
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Old 01-29-2008, 06:01 PM   #9
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Quote:
Originally Posted by canine camp
Ok, finally had some time and snowmelt enough to get to my furnace...pulled it out (easier than I thought, and good thing I bought the 1973 repair manual as suggested, thanks a million)... the sail switch does work, in that the fan blows it so that it trips....BUT, it doesn't switch over the valve. The repair manual suggests that it needs to be replaced. Should this be my first option, or is there a more likely reason I should try first? Also, I couldn't tell from the manual and looking on the websites that sell sail switches what part number it is....I plan on taking it out, is the part number right on the switch itself? Any help is appreciated...I don't want to end up replacing ever single component, so I want to get the most likely problems first. Thanks!
If it is the sail switch, then you need to get one based on the manufacturer and model number of the furnace. I think I used Mark's RV parts (Internet) last time I bought one.


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Old 01-30-2008, 01:39 PM   #10
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Ok, I'll find the model info for the furnace somewhere... It was Mark's RV website that I had found the sail switches, so that's where I'll be ordering it once I get what I need for the part number. Thanks! Once I replace the sail switch, I'll post whether that makes the furnace work or not...
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Old 01-30-2008, 02:14 PM   #11
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Since you have the furnace out, this would be a good time to clean out the main burner chamber. Tip it up on end and see how much soot/rust falls out. Blow out with compressed air. The main gas orifice is inside the burner chamber. Look at your repair manual to see how to remove it and clean it. Little critters just love old propane and crawl into the smallest holes! Darol
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Old 01-30-2008, 03:13 PM   #12
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Funny thing about sail switches. When I ordered ours, it was going to be about a week. Soooo, with nothing else to do, I took the old one to the shop, gave it a really good blow-out with compressed air, and reinstalled it. That was a couple of years ago. And the new one is still in its box, never unpacked. Someplace.

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Old 01-30-2008, 03:36 PM   #13
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Sometimes low voltage will not let the fan get up to speed so the sail will move but not switch.

It is easy to check the sail switch with a meter the fan "closes" the switch.

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Old 01-30-2008, 05:10 PM   #14
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Without intent of hijacking a gentleperson's thread, I have a similar problem I would like to present to the group. In my 81 International with Suburban furnace, the unit works as normal when connected to shore power. The instant you unplug the shoreline and run from the battery the burner goes out. The battery is a brand new and fully charged (via new automated battery charger, not the univolt) group 27 Trojan AGM. The burner will re-ignite as soon as shore power is restored. The heater will also stay lit on battery only if the front cover is removed, and the warmed air is not directed through the duct work. Thanks for any ideas.
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Old 01-30-2008, 06:22 PM   #15
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When on shore power the converter kicks battery voltage up so I would suggest you have a corroded connection allowing a voltage drop that is overcome when on shore power.
IMO you need to check the voltage connections at the furnance and fuse many times just disconnecting and reconnecting is good enough but make sure they are tight and clean.

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Old 01-30-2008, 07:12 PM   #16
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jdalrymple, as Garry indicated, this doesn't sound at all like a sail switch problem. (Indeed, it sounds like the sail switch is doing exactly what it's designed to do, namely, shut off the propane flow under poor fan-blowing conditions.) Check to make sure that you're getting sufficient power to the heater. Corrosion could be an issue, as Garry pointed out.

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Old 02-13-2008, 11:24 AM   #17
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ok, I replaced my sail switch and that must not have been the problem, still blowing cold air. So, my repair manual indicates that I should test the valve (which I probably should have done prior to replacing the sail switch, which probably was fine).. I priced a valve and they're $100 and then some!! Is there any way to test and if faulty, repair a valve? could it just be gunked up to where it just can't move, where cleaning it would solve the problem, or if bad do these simply need to be replaced? If I have a bad valve and I need to replace it, I'm almost thinking of a new furnace entirely, though I don't want to spend that kind of money either! If I do, how do I find out what furnace would be best for replacement? Any help is appreciated, I'm getting frustrated!!
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Old 02-13-2008, 11:49 AM   #18
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Oh, boy, it was't the sail switch. I could have sworn on that one (and did).

I guess it would be time to test the valve itself. Unfortunately, I don't have a clue about how to do it, having never had the opportunity (yet) on ours.

Ok, just a little back to basics. If the fan is blowing, but no propane is flowing, are you very sure that the good sail switch is indeed blowing over? Low voltage to the unit can have the same effect, though different cause.

Sorry to hear that this is still a problem!!


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Old 02-13-2008, 12:29 PM   #19
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About furnace blowinf cold air. Free manuals. Go to google, type in "suburban furnace manuals". The 5th reply is from "Dudes". Enter there and you will find 2 downloads that should answer your questions. Jim K
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Old 02-13-2008, 02:25 PM   #20
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I will check that out to see what the free manual has to say. The sail switch was definitely blowing enough to click, both the old and this new one (I had the unit out and turned the fan on, and it easily moved the sail switch) I'm going to try switching the two wires on this new sail switch, it isn't an exact replacement, but Mark's RV indicated it is a substitute, but it didn't have the exact same electrode pattern, so I made a best guess.. It could be that they are on backwards, if so I hope I didn't mess something else up. If that fails, I'll check the valve to see if that's it and then I'll report what I find out. Thanks again, very helpful ideas and suggestions!
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