Gadget Man, Furnace question.-Update_Final

[Gadget Man]

OK Guys, I have a question on my new Atwood furnace, or a question on the way it's controlled.

I've already experimented with 2 different types of thermostats, one is a basic Honeywell round stat, the kind I'm sure most of us grew up with in our homes.
Then I tried a fancy digital stat. Both are 24 volt stats.
I have read a lot of feed-back on how you can't use a 24 volt "house" stat in a camper with a 12 volt system.
So today I said to-heck with it and disconnected the stat altogether and just used an alligator jumper wire and jumped the red and white wires together, figuring it would run non-stop till it got to be 99 inside the trailer.
Well...something sort of strange was going on. The furnace fired up and ran just fine, but every 2-3 minutes I could hear the gas valve on the furnace close. The blower fan would continue to run for a minute then the gas valve would pop open again the heat would continue. The blower never shut down and even with the gas valve closed, it continued to blow warm air.
It was like the furnace (gas valve) was cycling on and off as if controlled by a heat anticipater. I'm sure it would have continued to run untill it satisfied the stat, but since there was no stat connected, I'm wondering why it was cycling.
I did just go on E-Bay and ordered an Official 12 volt Atwood RV thermostat. I'll wire that in once it gets here and see if it still cycles the gas valve like it's doing now.







Anyone got any ideas, or it this how they are supposed to run/cycle ?

Gadget Man

[msnglinc]

My guess would be that there is a temperature sensor on the burner box. It is cycling to prevent the box from over heating. The fan stays on to cool it, once it is cool enough it opens the gas valve again. I believe residential gas furnaces have this safety feature. They may not cyle as much due to size and volume of air moved.

[mskobier]

Gadgetman,
I have the Atwood 8012-II 12k furnace, and mine does not cycle as yours seems to be doing. I was looking at the install manual for mine, and it does have an over temp sensor, that appears to shut the gas off if it gets too hot. Do you have another opening for supply air? I know they are stated to be zero clearance, but the instructions also say that an additional 4"x4" opening must be provided for fan supply if mounted in a zero clearance enclosure. Is it possible that you are not getting enough return air, and the furnace is tripping the over temp sensor. Have you tried opening up the cabinet it is mounted in to see if additional air supply resolves the issue? That is the first thing I would try. Bypassing the thermostat as you have done should allow the furnace to run continuously until you break the electrical connection. The gas valve should stay open, and the furnace should stay lit as long as the wires are connected.

In studying the electrical diagrahm of my unit, it appears that the over temp switch if tripped would have the same effect as the thermostat opening and closing. It is wired in series with the sail switch and the thermostat. The sail switch will not let the gas turn on if it does not have a suitable amount of combustion air flowing past it. Again, from what the diagrahm shows, your overtemp switch is tripping on and off. Again, this appears to be either a faulty over temp switch (opening at too low of temp), or you have a restriction of the supply air such that it is not removing enough heat from the heat exchanger, and the over temp is tripping.

One other thing. On mine there is a side discharge opening from the heat exchanger chamber. This opening is a 4" circular hole and is located on the right side of the furnace near the front. There should be a metal blanking plate in that opening. If not, you are blowing heated air into your enclosure as well as out the front. This will cause the air to recirculate inside the enclosure instead of return cooler air from the room This would also cause the overtemp switch to cycle on and off.

A mechanical thermostat could care less if it is being supplied 24v or 12v. Its just a calibrated bimetal strip that opens and closes a set of mechanical contacts, depending upon the temp setting. The electronic thermostats are a different story. They need the 24v just to run the electronics, so they probably would not work.

Hopefully, this has given you some ideas on where to look for answers.

Mitch

[mskobier]

Gadgetman,
Ok, I just went and looked at your build pictures. It appears that you are using the side discharge as well as the front discharge of heated air. Is it possible that the flex duct from the furnace to the register came off during installation? I would still see if an additional opening for intake air would eliminate the limit switch cycling.

Am I correct in that you installed a 20k furnace in yout CT? Thats a lot of furnace for such a small insulated space. You will definately have no problems keeping it warm. In my 7x16, I installed the 12k unit. I discovered while working in my CT, that a Big Buddy Heater on high (18k) would run me out in a very short period of time. I could turn it down to the medium setting (9k) and it was still too much. turning it to the low setting (4.5k) was not quite enough to maintain the temp. Now these settings were before the CT was insulated and the outside temps were in the mid 20's. Now, the low seting is more than enough to warm the trailer up from cold (~30 degrees) and actually make it too hot inside. Now that the 12k is installed, which is about 9.12k heat output, I find it is more than adequate for my needs.

Mitch

[Gadget Man]

Mitch:

Yes, it makes perfect sense that it is is overheating and tripping out on the high temp cut-out. I did a bit of research on the Web and found a few things, all of them pointed to an overheated furnace. It said replace the "faulty" high temp switch as one of the cures.
I also went back and read through the installation manual for the unit...Who reads these things....hahahahaha
It said the unit needs 35 inches of supply air to the unit. Or a space 3 x 11.
Well, I have to say I have no supply air to my unit. None. Zero, Zip, Nada, Nothing.
It's actually built into a 3/4 inch plywood box. There is plenty of clearance all around the unit and the factory exhaust outlet is installed, but there is no incoming air into my cabinet.
I am using the side discharge opening and I'm confident that pipe is installed correctly.
I have over 5 inches of clearance on each side of the furnace between it and the inside of the cabinet.
I need to get my hole saw out and drill (3) 3 inch holes in each side and cover them with a nice grill.

After reading the above comment and reading a little on the Inter net. I'm very confident this will fix my problem.

I'll have time on Friday to work on this, since I'm off from work and there is no way you will ever catch me near a shopping mall on Black Friday.
I'll post pictures this next weekend.

Thanks for the help.

Gadget Man.

[Gadget Man]

It was killing me, so I had to jump on this repair right away.
I bought a 4 x 10 plastic wood grained register grill to use for my supply air for my furnace. That's 40 square inches. The owners manual says I need 35 in2
I assume that means 35 square inches.

I cut it in on the right side or in the bathroom, then fired up the furnace.
To my horror, it still cycled my gas valve. Not every minute like before, but it still cycled about every 3 minutes, it would be off for a minute and then pop back open.

So, I guess I need more supply air, I cut another 4 x 10 hole on the left side. I had not planned for this, so I only bought one register grill.
I fired up the furnace again and it still cycles the gas valve on and off.
The valve stays open longer now with both openings but still the furnace won't run 100% of the time. (remember I have the T-stat hard wired.)

So now I have (2) 4 x 10 openings, or a total of 80 square inches of supply air and still no luck.
You can see if the last two photos, that I have about 6 inches of clearance on both side of my cabinet.

Any more suggestions...?













Gadget Man

[BC Cargo]

Just as a test maybe point a fan at the value and see if it becomes happy to stay on with good ventalation.

[pete42]

Are you familiar with a volt-ohm meter and feel comfortable using one?

I guess a better question would be do you have a volt-ohm and feel ok using it?

if the answer is yes then take a voltage reading across the high-temp overload it should read zero volts when closed and 12 volts when open.

a reading across the gas valve would be 12 volts when open and zero volts when closed.

Easy way to think about most electrical problems is to visualize a light bulb circuit. [hot wire->light switch->wire to bulb->bulb->wire to neutral]

the high-temp overload would be like the switch if it were closed and the gas valve would be like reading across a lightbulb.

last resort would be to remove the furnace and run it while it sits ouside on a stand or the ground.

pete

[Engineer Guy]

I've got 2 Atwood Furnaces. The '89 one is a 12k BTU pretty identical in Form Factor to the one pictured. I've worked pretty extensively on the older '83 one; a 16k BTU or so. I don't have the Schematic here, but know it like the proverbial back of my Hand.

I don't believe you have any problem, nor need to make further Mods. Rigorous testing will tell...

There's 2 'Snap' Mechanical Sensors on mine; each the size of a Quarter with 2 mounting Screw Holes on them. My Reverse Engineering, to understand the functions, concluded this... One Snap Sensor is Normally Closed. So, the Furnace comes on and runs until the Temp Setpoint is reached/exceeded. So long as the Thermostat is calling for Heat, this Sensor opens, interrupting Control Voltage to the Gas Valve, which discontinues the Heating. The Fan runs, and cools down the Plenum. This Sensor closes again, and Gas/Ignitor Spark comes back on, exactly as you describe. This continues until the Thermostat no longer calls for Heat. However, the Fan continues to run; the major difference between this Sensor function and the next one I describe.

The other, similar-looking Sensor is the 'Fail Safe' Sensor. If all else fails, or some weird, unforeseen situation arises, this Sensor heats up; opens; and ensures the Furnace operation ceases so as to not burn down the Trailer. The Fan stops, etc.. You can hear the Plenum Metal 'ticking' as it cools off, since there's no Fan to force cool it. But, Mission Accomplished. Fire avoided. It has a higher internal Setpoint cutoff temp than the other Sensor.

I removed these 2 Sensors during Troubleshooting and attached my Ohmmeter. I heated each one with a Match to see how they were supposed to operate. I replaced a bad one after a local RV Shop 'Gomer' wanted to argue with me that I didn't know what I was doing. Turns out I did. I voted with my Feet [and Wallet] and no longer patronize that Shop.

This sort of simple, Mechanical Control is cheap, and works. These Replacement Sensors are $7- or so. Whether a Sensor is N.O. or N.C. is represented differently on any Schematic. You can hear them 'click' and operate, like a little 'Frog Clicker' Toy of Olden Days.

Trailer Thermostats need to not have a Mercury-type Switch in them. Those old Models are sensitive to level, and suitable only for fixed Installations. The nice thing about using a Clip Lead to short together the Thermostat Wires is that it takes any Thermostat out of the equation and tells you how the Furnace ought to work.

If there's some Air Output/Air Distribution problem, then the confined Furnace won't raise the heated volume temp way up as it should. Confinement would cause 'premature' heating and opening of the cycling Temp Control Sensor. Barring that scenario, things sound OK. As an aside, our ~80% efficient House Furnace, circa '86, operates identically to the Atwoods. Heat up; run the Fan; cool down. Repeat as necessary...

Conventional Wisdom is to use only a Thermostat where an internal Battery powers the Electronics, including Programming Setpoints memory. The Furnace Leads do not [and should not] supply power in that scenario. Last time I checked, Wally World had one such Thermostat, and Home Despot had one or two... Both my 'Stats are Old School Mechanical types.

I'm a big fan of Ye Olde Scientific Method. When the Environment is the Variable under question, change it/make it irrelevant. Remove the Furnace - IF it's not a huge hassle - and run it in Free Air. I did this for my Troubleshooting. You'll hear/see the Sensor click on/off, and cycle the Gas Valve as 'normal' operation in an unconfined Environment. I short the Thermostat Wires together; power the Furnace off an ancient +12 VDC Battery Charger; and supply the correct pressure from a 20 lb Propane Tank plumbed in safely and temporarily outside [to ensure heavier-than-Air Propane dissipation]. This sort of Test Config takes less time to set up, actually, than cutting out new Vents, etc., on speculation. For my Brain, establishing the 'baseline' of proper Furnace operation is huge! You then know what not to question, and chasing one's Tail is minimized. As is the case with my '89 Atwood, I would cover up any 'extra' Venting temporarily to force all warm Air out the Front Panel. This would ensure Plenum cooling by Fan in a known manner, and would help establish Furnace operation baseline.

Disclaimer: I Speed Read this Thread, and might have missed something...

P.S. - Thermostats are designated '24 Volt' because that's the standard House Furnace AC Control Voltage. They're 'dry' Contacts, as noted above, and can also handle +12 VDC in a Trailer. In the Olden Days, the round Honeywells had a small, visible Mercury Switch in them. Today, most all 'Stats are 'dry' Bimetallic Spring-driven Switches.

The other primary Home Thermostat convention is called 'Line Voltage'. It can typically handle either the 115 or 230 VAC used to directly control Electric Baseboard Heaters.

[BC Cargo]

Not sure what wire you are running to the thermostat but if is broken or has a loose contact at the furnace screw terminal, the vibration of the fan maybe making and breaking the circuit. That is if you have shorted it at the thermostat. Shorting it at the furnace terminal screws would find this problem...
Also for others...I would install the thermostat in reach of where I sleep so when I get hot in the night I can turn the thing down. In the morning I don't have to leave the covers to turn the furnace onto toasty.

[pete42]

engineer guy wrote"There's 2 'Snap' Mechanical Sensors on mine; each the size of a Quarter with 2 mounting Screw Holes on them. My Reverse Engineering, to understand the functions, concluded this... One Snap Sensor is Normally Closed. So, the Furnace comes on and runs until the Temp Set point is reached/exceeded. So long as the Thermostat is calling for Heat, this Sensor opens, interrupting Control Voltage to the Gas Valve, which discontinues the Heating. The Fan runs, and cools down the Plenum. This Sensor closes again, and Gas/Igniter Spark comes back on, exactly as you describe. This continues until the Thermostat no longer calls for Heat. However, the Fan continues to run; the major difference between this Sensor function and the next one I describe."

just so I understand are you saying your furnace shuts off every time the plenum reaches a certain temperature?
then plenum cools furnace restarts and continues to run until the snap switch opens again thus preventing the furnace plenum from becoming to hot.
it sort of creeps up on the thermostat set temperature in stages.
sounds like a good safety
it does makes me wonder if it causes the furnace to cycle more times reaching set temperature but not anymore than others once the set temperature is reached.
But it also raises the question if your furnace just doesn't have two over temperature "snap switches" one for plenum one for firebox?
Once the furnace reaches the set temperature and shuts off the temperature will rise some if it rises higher than the plenum snap switch set point I can see "they" would not want the furnace to come back on until it cools.

pete

[pete42]

Gadget man
looking at a wiring diagram of your furnace Atwood 7920 II

when the thermostat calls for heat the blower motor starts to run fed from the DSI control board
it should stay running for a set time determined by the DSI circuit if the furnace does not light.

once the blower is running and thermostat is calling for heat
the holding circuit is thus thermostat, over temperature snap switch, sail switch, into the DSI control board.

DSI control board opens the gas valve and starts the igniter

the gas valve is held open if the thermocouple senses the flame and the thermostat is still calling for heat.

The igniter also shuts off once the flame is sensed or it times out.

I hope this helps

Pete

[Engineer Guy]

On my older Furnaces, if the 'Stat is closed and calling for Heat, the Furnace runs in the manner Pete42 describes, and the Fan does not shut off at any point in the Run Cycle. Meanwhile, the 'more important' of the 2 Snap Sensors on the Plenum opens-and-closes. Because it's in the Signal Path to the Gas Valve, it opens-and-closes the Gas Valve. The Thermocouple [or a newer Electronic equivalent like a Thermistor] confirms throughout all this that the Ignitor - repeatedly sparking and igniting the Gas - is working. The Fan, running all the time, cools down the Plenum, finally cooling the Snap Sensor, which signals the Gas Valve to open again. The Plenum heats up; and so on. I recall that both Snap Sensors are on the Plenum, since that's where the Heat action is...

In a quiet environment, you can hear the Spark Plug-style Ignitor 'ticking' repeatedly to ignite the Gas.

When the Thermostat is satisfied and the internal Contacts open, the Fan continues to run for several minutes, cooling the hot Plenum. Once the Snap Sensor opens and shuts off the Fan 'one last time' in any given Heat Cycle, only then does the Furnace fully turn off for that Heat Cycle. I didn't mean to convey that the entire Furnace shuts off prior to the 'Stat Contacts opening [when the 'Stat Setpoint has been reached].

The other Sensor isn't involved in the normal Heat Cycle, at least in my Furnaces. It only does anything in a Thermal Runaway scenario; it shuts off the Furnace totally until cooled. It resets itself. IF something abnormal, like overheating due to confinement, is happening here, only then would this 2nd 'Fail Safe' Sensor be involved at all, and shutting down the Furnace fully [including stopping the blowing Fan 'just like that'].

Control Boards have taken over some of these Control Functions. However, I wanted to convey the Logic of how these Furnaces work; not much has changed. Also, the fundamental Safety overrides are still included; our Litigious Society sees to that...

[bdosborn]

One thing that helped our furnace to function a lot more efficiently is a small fan to pull air down from the roof. The hot air in our trailer would rise to the ceiling but it would still be cold at the bed. It doesn't take much of a fan, a muffin fan is enough to de-stratify the air in our trailer. The furnace ran a lot less often after we put the fan in. I have two fans, one pulls air from the roof down and one pushes air up from under the bed.





Bruce

[pete42]

Bruce has a good point just like ceiling fans in ones home can be made to either push the air hot air down (winter) or pull the coll air up (summer).
a fan moving the air around in a trailer does wonders.

Engineer guy I take it you have the DSI control on your heater.
it is I think important to have a working knowledge of all units in your camper
so that when something goes wrong you have somewhere to start or at least describe whats going on.