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Old 11-25-2004, 07:40 PM   #1
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Furnace replacement...again

Ok, yes I read what I can find through Forum searches. I have a 64 22’ Safari. Furnace fired up once then died. Pilot burns but blows out as soon as valve is opened. Lots of rust through of furnace box, inside and out. 40 years old, It’s dead.



So, Several questions.



1) What to replace it with and how? Original is 13.5”x14.5”x22”. Sounds like a new Suburban NT24SP is “supposed” to fit needing raising and blocking. Is this the right model to go with for proper fit, heating and battery power conservation (3.5 amp draw)? Should I consider higher BTU model (7.5 amp draw)? I’m headed for Alaska…in winter



2) Mine has 115vAC and 12vDC lines with switch. What is 115vAC used for? Shore power? Don’t new ones only use 12vDC?



3) Can I make my existing original exterior vent cover work to keep original look?

Any help appreciated, Walter
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Old 11-25-2004, 10:44 PM   #2
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Walter, the Suburban NT series is the right furnace. You can position a new one so that the vent cover is in the same place, but you absolutely should not use the original cover. The new ones look very similar anyway, and you don't want to fool around with carbon monoxide.

As to size, I posted a while back about the way furnaces are rated now vs. the way they used to be. The older ones were rated by BTU output, while now they're rated by BTU input. See recent Suburban threads for the details. This is a critical point, especially if you're going to camp in really cold places. Replacing an old furnace with a new one of the "same" rating may be unsatisfactory. Check BTU input and output on the new furnaces against your unit's data plate to make sure you get the right one.

I think you can buy 115 VAC versions of the NT furnaces, but not dual-voltage models.

Bob
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Old 11-26-2004, 04:09 AM   #3
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Quote:
Originally Posted by walter1
2) Mine has 115vAC and 12vDC lines with switch. What is 115vAC used for? Shore power? Don’t new ones only use 12vDC?
On my furnace, the thermostat wiring looks just like 120 vac wiring.

Tom
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Old 11-26-2004, 10:20 AM   #4
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Originally Posted by tcwilliams
On my furnace, the thermostat wiring looks just like 120 vac wiring.

Tom
All furnaces that operate on 120 VAC use an internal transformer to provide low voltage (12-24 VAC) for the control circuit. The wires run to the thermostat are usually quite small, and the low voltage doesn't pose a safety risk if you touch the bare terminals.

Whoever wired your trailer may have used Romex, the same wire that's typically run to outlets, to connect the thermostat. That's very much overkill for thermostat wiring. For comparison, look at the wire run to your house furnace thermostat - it's usually about 20-24 gauge (very small), and there's not much to keep your fingers away from the wire terminals.

If you're not really clear on what's there and/or not comfortable with electrical work (even low voltage stuff), I'd suggest getting an electrician to sort things out. It's easy to damage circuitry if you hook something up wrong.

Bob
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Old 11-26-2004, 10:23 AM   #5
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Bob,

The 120 romex is what the factory used for the thermostats in the 67 trailers.

Tedd
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Old 11-26-2004, 11:17 AM   #6
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Walter1.

You can use a Suburban NT-30SP as a replacement.

A word of CAUTION about your original furnace.

They have a great tendency to explode, for a number of reasons.

Take it out of the trailer and deep six it.

Andy
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Old 11-26-2004, 11:37 AM   #7
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Bob, Thanks for the help. I’m looking at purchasing a new Suburban NT30SP for ~$400.

Am I correct using your 76% efficiency figure that this should have input of 30K BTU and output of 22.8K? This should give me approximate BTU output value of old unit with 5.5 amp draw. I am going to Alaska…cold.



Old unit must be dual voltage you spoke of. It has 120v and 12v lines with big transformer inside.

Remember I’m new at this: 64 Safari

Old unit slides OUT to outside of trailer and has large 13.5”x21” vent cover. It looks like new units have 2 small round intake/exhaust vents. Is this correct? Do new units install from OUTSIDE or INSIDE? Am I going to have to build large patch panel to cover exposed area? Any photos of Exterior with new unit installed available?
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Old 11-26-2004, 11:40 AM   #8
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Thanks Andy, I am going to trash old unit, NOW.

Do you have experience with 64 model installation? I'm concerned about exterior appearance/fit.
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Old 11-26-2004, 11:57 AM   #9
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After romaval of the old furnace, you will have to plate the exterior and then cut new holes for the newer style furnace.

Andy
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Old 11-26-2004, 12:06 PM   #10
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Andy, Thanks, That's what I didn't want to hear but expected I would. I'm OK fine with gas, electrical, furnace mechanical install but completely lost when it comes to exterior aluminum panel fabrication. I want it to look as original as possible but be safe/correct.
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Old 11-26-2004, 12:57 PM   #11
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Quote:
Originally Posted by walter1
Am I correct using your 76% efficiency figure that this should have input of 30K BTU and output of 22.8K? This should give me approximate BTU output value of old unit with 5.5 amp draw. I am going to Alaska…cold.
Yes, the NT30 is the right choice. One important note: although you could buy a higher-BTU NT furnace in the same physical size, that's not a good idea. Your original ductwork was designed to provide proper airflow for the original furnace. If you substitute a larger furnace but don't change the ducting, you run the risk of too-high duct temps and a possible fire or erratic operation.

Thanks for chiming in, Andy. My original '74 Suburban installed from the inside and had a small vent outlet very much like the modern furnaces. Obviously the even-older furnaces were very different in construction. My bad.

Bob
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Old 11-26-2004, 01:09 PM   #12
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Quote:
Originally Posted by walter1
Old unit must be dual voltage you spoke of. It has 120v and 12v lines with big transformer inside.
Have you identified the thermostat wires yet?

Off hand, I wonder why a furnace would be designed to run on 120 vac or 12 vdc since most people have a charging device of some sort (that runs off of 120 vac) for their battery(s).

No big deal, just curious.

Tom
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Old 11-26-2004, 01:45 PM   #13
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Tom, I'm also curious. No, I'll crawl in today and chase thermostat wiring. Moving thermostat does engage transformer. It does have terminals on furnace unit marked 120vAC and 12vDC and I confirmed those powers at both. Also has 120vAC circuit breaker labled "Furnace" and swith on furnace fan housing labled 120V/off/12v.

Curious, but now that I've confirmed rust through, it's going to the dump.
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Old 11-26-2004, 02:15 PM   #14
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Quote:
Originally Posted by walter1
Tom, I'm also curious. No, I'll crawl in today and chase thermostat wiring. Moving thermostat does engage transformer. It does have terminals on furnace unit marked 120vAC and 12vDC and I confirmed those powers at both. Also has 120vAC circuit breaker labled "Furnace" and swith on furnace fan housing labled 120V/off/12v.

Curious, but now that I've confirmed rust through, it's going to the dump.
Walter,
My 64 Overlander also had both 12V and 120vAC run to it with a switch to flip depending on if you were plugged into shore power or running off of your battery. The original owners manual described the operation of the furnace and the process you went through to switch it from 12v to 120vAC.

The original furnace was replaced and the 120vAC line was coiled up and capped off with wire nuts. You can use it as an additional outlet under the sink if you have the need, or leave it capped if you don't.

If you go to the photo section and pull up the pictures of my airstream after restoration, you will see a picture of the outside patch and vent installation for the new unit if that would be of any help.

Mark
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