Fixing noisy Suburban furnace - Fiberglass RV

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Old 10-23-2013, 07:13 AM   #1
Senior Member
Name: Jim
Trailer: Scamp
Posts: 200
Fixing noisy Suburban furnace

In a different thread in modifications, I too was hoping to learn some tips here on how to quiet my Suburban furnace, but the messages took a different tack and never quite got there. I am re-posting my prior message here on a new thread so others donít have to read nine pages of comments before getting to it.

The noises coming from my furnace when it ran were driving me nuts. This is what I did recently to fix them and ended up very happy with the results.

I bought my 1983 Scamp about 6 years ago and the first time I used the furnace it made noises. Later, I bought a used replacement unit since something had to be better than that one. It was, but still had some noises. Earlier this year I took it out and wanted to fix the noises. After removing the heating unit from the housing and tested it using the pickup battery. Funny thing, the motor sounded fine, that was not the cause of the noises. I found the noise was not caused by a bad motor, bent fins, dry bearings, etc, even though it sure sounded like it.

With further investigation, I found there are no screws or anything securing the heating unit frame on the backside of the heating unit to the housing. This allows it to vibrate inside of the housing when running, thus producing the sounds, among other things. I reduced the noise a lot by bending the sides of the housing away from the heating unit by inserting a screwdriver and twist it on each side of the housing to hold the housing away from the rear frame of the heating unit. That worked for a while, but by the end of summer, some noises returned and were irritating me again.

I decided this time I would track down every shiny spot on it from rubbing against other metal, and isolate the source of the screeching sounds if it meant taking it apart down to its last nut and bolt.

I have kept the old original furnace as a "parts" furnace in case I needed some parts from it in the future. Before taking the current one out, I decided to check every point on the old one for signs of rubbing that was causing the screeching noise when it ran. I wanted to see if I could get some clues there also to help when examining the current one.

After removing the old heating unit from the outside housing I wanted to isolate the specific cause so I checked everything that moved one at a time. What I found was numerous contact points that were causing the noises, and a real surprise regarding the exhaust tube. The photos show the areas of metal I found that were bright and shiny and rubbing against other sheet metal.

I checked the closeness of the exhaust tube to the hole it passes through in a support piece. I found not only shiny spots on the tube when I wiggled it, but a crack in the tube where it connects to the heating chamber!! Now just what is that doing there?? After taking the photo, I twisted the tube and it didnít take much for it to fully break off. (see photos. Put cursor over photo for a description of what it is showing)

The guy I bought it from said his dad used it for a while. I didnít ask what his dad died from. Hope it wasnít asphyxiation.

So then I was ready to remove the current furnace again. Below are the steps taken to remove it and isolate the squeaks and screeches and eliminate them. It is not a difficult, scary or dirty process. Just take your time and it will turn out fine. It will be some of the best hour or so you have spent with your camper when done.
To remove the furnace from the camper:

Disconnect the battery. 12 volt power is going to the furnace and to the thermostat. Make sure gas is turned off at the tank.

Remove the two screws and front panel on the furnace

Remove the screw (s) along the bottom front of the furnace that hold the furnace into the cabinet space

Reach back along the side and disconnect the 4 wires leading into the side of the furnace. If you didnít already disconnect the battery, you will find out why when you disconnect the wires. They are still hot and those sparks tend to really surprise the snot out of you if you touch something or they touch the circuit board

Use ĺ" wrench and Channel Lok pliers to loosen the gas connection. Then should be able to remove the rest of the way with your fingers

Pull the furnace forward and it will come right out of the space.
To identify the cause of the noises:

Remove the two screws on the front side and bottom that have a label next to them that says "Donítí remove these screws except to service unit". You can then pull and push the heating unit out from the housing.

When pulling it out, be sure to feed the wires on the side into the hole so they donít get caught. You now will have a very good view of things.

Check every inch of the heating unit to find any shiny spots resulting from metal rubbing against metal. I have included some photos of those areas where I found some of the problem areas.

The piece of metal that holds the sail switch had shiny areas that showed it was rubbing against the side of the housing and the side of the heat chamber on both the old one and the current furnace. (See photo) That piece is just too close to the housing and causes noises. Somebody actually got paid to design it that way?? Also, bend the two tabs at the front of the metal plate away from the heater. Check that the sail moves freely.

Do the same for any other metal that vibrates against other pieces of metal. (see other photos). And no, the exhaust pipe showed no damage or cracking at all on the current furnace.

I wanted to specifically check the motor, so I opened the pickup hood, set the heating unit on the motor and connected it to the battery. I twisted the two blue wires that go to the thermostat together, and then connected the hot and ground to the pickup battery. It takes a few seconds for the motor to start as there must be some kind of delay switch or something. Funny thing, the motor sounded fairly fine, but there was a little whistling.

I removed the back intake housing (see photo) and found a mess of sand that fell out and more when I stood the heater up and shook it some more. The intake fan sucks in anything that is in the air, including sand since there is no air filter. No wonder it is so dirty. That gives you access to the squirrel fan on the back of the motor, as well as the blades on the front of the motor.

I ran the motor again and thought the sound was coming from the squirrel fan. I removed the squirrel fan (you will need two different sizes of small, long shank alan wrenches to get them off) and cleaned the dirt and varnish from old oil off it with a soaking in a strong cleaner, using an old tooth brush, and Q-tip, including between the individual fins.
I also removed the front fan blade as well as the motor and cleaned them also, and put some oil in each bearing even though it says it is permanently sealed. Like chicken soup for a cold, it may not help, but it canít hurt

When both fan blades were still off, I ran the motor again and it was super quiet. That showed that the problem was not bad or dry bearings in the motor. After assembling the blades and testing it again, it sounded just like it should producing a strong force of air.

When done checking everything, slide the heater back into the housing. Feed the other four wires back out of the side hole when inserting the heating unit back into the housing.

Be sure that the metallic tape on the front side that covers a large hole is good. ( see photo) If it is not, hot air from the furnace will escape and heat the area under the sink instead of directing it out the front of the furnace.

Replace the two "Do not remove screws" in the front and you are ready to place the furnace back into the camper.

However, before that, I drilled and put a sheet metal screw on the top, bottom and right side of the housing that screwed into the rear heating unit square frame. (see photo) There isnít much there to screw into, but just keep drilling a few holes until you hit it. There isnít anything around it to damage. I didnít put one on the left side as I didnít want to accidentally interfere with the movement of the sail switch.

To re-install in the camper:

Place a piece of old carpeting on top of the wheel well to keep the furnace from vibrating against it.

Line up the furnace and start to slide it back in. Keep the two wires that go to the thermostat to the left side so they donít get hooked by the furnace when sliding it back.

When it hits the back pipes, you need to reach back and line up the top pipe at the back of the furnace with the top tube coming from the outside of the Scamp. You just need to line up the top one as the other will line up automatically. When lined up, finish pushing the furnace back as far as it will go.

Reconnect the four wires and the gas line. Make sure the gas connections are tightened very hard.

Turn the gas back on at the tank. Check for gas leaks at the connection by using some soapy water to see if any bubbles are produced. If not, then the connection is tight. If you see bubbles, tighten the connection some more.

Light a burner on the stove to remove some of the air that is in the gas line and makes it easier to light the furnace.

Turn on the furnace at the thermostat and notice the huge difference in what it sounds like when running. No more screeching, just the strong force of air from the fan!

Now I know what it sounded like when it was new. If you do not hear the electrodes click, turn the thermostat off and wait for 15 minutes for the furnace to cycle and be ready to fire. Then try again. It should click and ignite.

Replace the screw securing the furnace to the cabinet and replace the front panel. You are done. I hope this helped solve the mystery sounds coming from your furnace.

I believe the biggest cause of the screeching was the dirty squirrel fan, the metal plate holding the sail switch rubbing against the heat chamber, and the rear part of the heater vibrating inside the housing.
Attached Thumbnails
1 - Shiny area on exhaust pipe.jpg   2 - Shiny spot on outside of sail switch plate that rubs against housing.jpg  

3 - Shiny area on rear heater frame.jpg   4 - Shiny sopt caused by sail switch against inside of plate.jpg  

5 - Crack in exhaust pipe.jpg   6 - Broken exhaust pipe.jpg  

7 - Rail inside housing. I bent it at the rear to keep it from vibrating against the heater.jpg   8 - Sheet metal screws I put into rear heater frame.jpg  

9 - Keep hole covered with the metalic tape.jpg   10 - Air intake pipe. Remove screws and see what all falls out.jpg  


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Old 10-25-2013, 08:59 AM   #2
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Myron Leski's Avatar
Name: Myron
Trailer: 19' Escape
Posts: 651
You, sir, are a detective that would make any Sherlock proud. And, great photos illustrating the issue problems.
Myron Leski is offline   Reply With Quote


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