Furnace Advice Needed

[Krazzykitt]

I have read just about every thread regarding issues with Furnaces and i still dont understand it, here's my problem...I have a Atwood 8531-IV DCLP 30,000 BTU Furnace, it was replaced by the previous owner about 8 years ago. The prior furnace that was installed from the factory was a Hydro Flame FA-7832, are these the same furnace?

Here's whats going on...when the furnace is first turned on the fan will run for about 5-10 seconds before burner kicks on, burner and fan will run for approx 8 minutes before burner kicks off, fan will continue to run for approx 30-45 seconds before burner kicks back on, this time the burner will only run for about 4 or 5 minutes before cycling back off, this cycling will continue till the set temperature is reached at the thermostat.
According to the Atwood Installation Manual the furnace should have a minimum discharge rate of 36in² now each 4 inch vent gives you 12in² of air flow and the 2 inch vents dont count towards the minimum discharge requirements. Taken from the manual...

Required Minimum Discharge - (

also see STATIC PRESSURE TEST)

MODELS REQUIRED DISCHARGE

8516-IV, 8520-IV .................................................. ..........24in

2

8525-IV & 8531-IV .................................................. ......36in

2

8535-IV .................................................. ........................48in

2

ALL MODELS

- TOP OR BOTTOM DISCHARGE SYSTEMS..................48in2

8525-IV, 8531-IV, 8535-IV

VERTICAL INSTALLATION ..............48in2

(

EXCEPT WHEN USING THE THREE REAR DUCT OPENINGS OR MODELS WITHOUT DOOR)

•

See MINIMUM CLEARANCE TO FLOORBOARDS, WALLS & SIMILAR COMBUSTIBLE
BUILDING MATERIALS.

•

Each 4-inch duct opening provides 12 in2 of discharge area. Provide
an extra 12 in2 of non closeable duct discharge area for each
closeable register used.

•

Use of 2˝ ducts does not count toward achieving minimum discharge
requirements. Ducting in dead air space with no return air, such as
holding tank areas, does not count toward achieving minimum discharge
requirements.

•

Adjust ducting installation to obtain air rise of 100°F-130°F.

Flexible Ducting Systems

When designing Flexible Duct Systems:

•

avoid sharp bends or crushed ducts

•

stretch all ducts and run them directly to outlets, keeping quantity
and angles of bends to a minimum
1. Remove knockout plates from desired outlets.
2. Attach a duct adapter to each opening, by inserting flange over casing,
locking the tab into casing slot and turning adapter 90°.
3. Attach and secure FOUR-INCH flexible ducts to adapters.
4. Run ducts to desired location within RV, secure to registers.

5. Additional ducting may be needed to maintain correct static pressure.

My rig only uses 2 of these 4 inch ducts and 2 of the 2 inch ducts. So from what ive read i need to add at least 2 more 4 inch ducts to allow the furnace to have proper static pressure? hopefully someone can shed some light on this, out of all the threads ive read there was never a mention of Static Pressure.

[HotMech]

It sounds like you have an overheat in the furnace.
What mean is, the replaced furnace is probably of a higher BTU output than the coach was originally ducted for.
If it can't move the air needed to keep the heat exchanger cool enough, the over heat switch trips, then when it cools enough, it will relight the burner.
You probably just need to increase the airflow through, the furnace. if it has another knockout, maybe add another hose and vent. I don't know what kind of rig you have, but I have a 30k furnace in my 32ft, class A.
K.

[MartyB]

Hi, I have been in the heating industry for 35 years, and you may want to consider enlarging you return air as well as adding additional heat supply.
What you you are experiencing is called shot cycling , And in fact the high limit cut off is opening when the heat exchanger can't get enough air across it. It is a safety device, keeps your heat exchanger from over heating. Quite often it is a return air issue, the furnace is starving for air to move across the heat exchanger. Make sure upon installation of your new furnace there is no blockage at the return air inlet, or a plugged filter if it has a filter? Another option is if you have a blower speed adjustment? To raise the CFM on the blower.
Wish you success.

Marty B.
Alltradesmechanicall.net

[Krazzykitt]

This furnace is in a 36' Class A. I was thinking that the furnace might be bigger than the original. The original furnace was a Hydro Flame FA-7832, anyone have a idea what the BTU output of that furnace is.
How do i find out if the blower has a adjustment? And is the return air the vent in front of the furnace?

[MartyB]

Hi, there should a name plate on the old furnace that has a stamped or printed BTU input on it near the MFG name. The return air is the grill you are asking about on the inside of your coach. The fan speed if there is one will be on the outside access to the furnace.
Hope this helps?
MartyB

[MartyB]

FA- 7832? Sounds like a 32000 BTU?

[MartyB]

One other thing comes to mind! The Thermostat, may need to be upgraded as well? The old T-stat, may not have the correct anticipatory rating.
For a propane furnace it should be .4 mili amps. The new thermostats now have built in anticipator. So there is need to adjust them. A inexpensive home depot or Lowes ( honeywell ) thermostat will do!
That's really all I can think of that may be causing any issues.
MartyB

[oldbeek]

I am experienceing the exact same problem. I have a suburbane sf-30 The static pressure talked about is .2 inch water column. Measured with a water filled manometer. It measures the force of your fan. Close all ducts but one and attach the inlet of the manometer to a piece of cardboard. cover the last duct with the cardboard and it will cause the manometer to register. I have 50 sq in of output duct all replaced with full 4 inch aluminum ducting and 45 in of return. You are surely under ducted. can you remove all your ducts and run it? Does it still cycle? My installation was perfect in all standards ie:static press, fan speed, voltage, amp draw, pressure to jet, jet size, limit sw ect, but it still cycled with all ducts removed. I am now putting in the next smaller jet. My Dinasour board only lets the limit switch cycle 6 times and then it shuts the furnace down for one hour. After one hour it starts the cycle over. To short cycle the board and start the furnace over manually, pull the 12 volt fuse and put it back in. Remember when troubleshooting that, rvs have a propane regulator that is set 11 to 11.5 in. Another regulator is built into the gas valve at furnace and hot water heater that regulates down to 10.2 to 10.5 in