Powering Atwood Furnace with 12V power converter - Issues!

Hey all,

I'm trying to solve a problem with a brand new install of an Atwood AFMD35111 RV furnace. It's in a Tiny House, one certified as an RV. It was no 12v system, just AC so we purchased a 12v power converter rated at 20A to power the furnace. The furnace takes 11.1 amps according to the manufacturer.

When the thermostat trips, I can hear the blower try to start but then click off. It's like there's not enough power for it. If I trigger it multiple times, sometimes it will fire up then works perfectly. Basically, my theory is that hitting it a few times starts the blower spinning a little bit, so it needs less juice to get going when I trip it next. If I leave it a few minutes between tries, it won't fire up.

I've checked the sail switch, limit switch, talked to Atwood, talked to another tech, eliminated every other option. I've tested the thermostat, taken the thermostat out of the circuit and just bridged the wires, etc. Everything is fine except it not starting the blower. No blower = no ignition.

An interesting thing is that if i bypass the circuit board/breaker switch and apply 12v directly to the blower lead, it fires up no problem.

My theories: Something in the circuit board is drawing too much power, causing a voltage drop and triggering the breaker? Maybe the capacitor? Or maybe the board doesn't like the power supplied by the converter? It checks out as 12v with a multimeter. Drops to 10.7 when trying to fire up?!

I didn't have a 12v battery on site to test with, we use converters for everything.

Converter: https://www.amazon.ca/SunbowStar-Switch-Supply-Driver-Transformer/dp/B018DZQZ1Q/ref=sr_1_2?ie=UTF8&qid=1508769024&sr=8-2&keywords=12v%2B25a&th=1

Should be more than enough.

I'm stumped!

You will probably need a power supply that's quite a bit larger. The motor will draw several times the running amps to start.

Ok, that's great to know. Any idea how much larger?

Not off the top of my head. Personally, I would add an AGM battery and a smart 3 stage converter to the home and be done with it for 5 years or so.

If the problem is insufficient starting current, adding a small battery (such as a motorcycle battery) will be a lot less expensive than buying a bigger power supply.

Joel

Have the same issue. Appears to be a circuit short on the control board.

I have gotten it working by removing the Ground wire from the 4-place ground bus on the side of the housing (just above the circucit borad) the blower is still grounded via the case so it runs.  But I've not yet had time to dig farther.. .Too many othe repairs at this time. Perhaps next week.

Just got a bypass for the Lube Pump controller installed.  Had to replace the pump last summer, New pump occasionally "Blips" a short.. This blows the fuse.. Tried a circuit breaker figured the short delay might save the day. Blew the controller.. Fixed that (Simply trace fail, soldered a jumper over the trace.. Yes I know how to do that) so the controller now works but I'm concerned about drawing that kind of current via the controller.. So I installed a relay

Controller powers Relay. Relay powers pump and sensor switch.  Should work.

I may, however, have disabled one of the sensor functions.. IF SO a light bulb will fix it.. Just have to splice one in.

Are you seeing a flashing error code on the unit? that may help you diagnose, as for motorcycle batteries, I love the Big Crank from batterymart.com, they're sealed and that's what I use in my Dyna

Yes the error code is for airflow/limit switch but I went through that all with Atwood over the phone. Sail and limit switches are both good. The vents are wide open and there's way more return air space than the specs ask for.

The thing I'm wondering about, regarding the starting current, is that there's a 15A DC fuse (from a car) in line between the power supply and furnace as well as the furnace's breaker switch, also 15A. So if it's pulling from a 20A converter, how can it be pulling more than it can supply, without blowing either of the 2 fuses?

Great Horned Owl said:

If the problem is insufficient starting current, adding a small battery (such as a motorcycle battery) will be a lot less expensive than buying a bigger power supply.

Joel

Click to expand...

Joel, I think a 40A power supply (2x the size) is about $25 more. The issue with a battery is that there is no DC system on this unit, it's all AC. In order to use the nice little Atwood furnace we needed to install a converter. So if we put a battery in, it would discharge (the furnace pulls 11A) and not charge (Need more than 12V to charge a battery)

John From Detroit said:

Have the same issue. Appears to be a circuit short on the control board.

I have gotten it working by removing the Ground wire from the 4-place ground bus on the side of the housing (just above the circucit borad) the blower is still grounded via the case so it runs.  But I've not yet had time to dig farther.. .Too many othe repairs at this time. Perhaps next week.

Just got a bypass for the Lube Pump controller installed.  Had to replace the pump last summer, New pump occasionally "Blips" a short.. This blows the fuse.. Tried a circuit breaker figured the short delay might save the day. Blew the controller.. Fixed that (Simply trace fail, soldered a jumper over the trace.. Yes I know how to do that) so the controller now works but I'm concerned about drawing that kind of current via the controller.. So I installed a relay

Controller powers Relay. Relay powers pump and sensor switch.  Should work.

I may, however, have disabled one of the sensor functions.. IF SO a light bulb will fix it.. Just have to splice one in.

Click to expand...

Hey John, thanks for the info. What sort of vehicle is your furnace in? Standard RV? You got me thinking on the path that maybe there's a grounding issue, because there's no ground to frame, the ground wire just runs to the converter common terminal. I could try your trick but the unit is brand new so I'd nervous to void the warranty

rfeeny said:

Joel, I think a 40A power supply (2x the size) is about $25 more. The issue with a battery is that there is no DC system on this unit, it's all AC. In order to use the nice little Atwood furnace we needed to install a converter. So if we put a battery in, it would discharge (the furnace pulls 11A) and not charge (Need more than 12V to charge a battery)

Click to expand...

The converter that you installed is intended to charge batteries  I will keep ayours charged with no problem.

Joel

My guess is the fan motor's starting surge is momentarily pulling down the converter's voltage, and the voltage dropout is glitching the controller board.

I'd try adding a car stereo capacitor across the power leads at the furnace.  These are high value capacitors (1-10 farads, not microfarads) and their purpose is to store enough energy to keep the voltage up during those bone rattling bass booms.  It should do the same for your furnace during the fan motor's starting surge.

As far as the 15 amp fuse not blowing, it takes more than a 25% momentary overload to blow a fuse.  The fan motor's starting surge could be exceeding the power supply's current limit but it's over soon enough that it doesn't blow the fuse.

Are you sure all wires to/from the convertrr are large enough for the initial current draw?

Ernie

10.7 volts is probably NOT enough to start and run the furnace, that's like trying to run the furnace with a dead battery. That's too much of a voltage drop. adding a small battery will probably work, but if this is a new installation and you're having trouble now, it'll always give you trouble. I'd use a AGM scooter battery, they can be bought pretty cheap on Ebay, then get a battery tender to take care of the AGM battery. It'll cost you more up front, but it will give less trouble in the long run. Another thing, the way you have it set up you have no heat if you lose 120 volt power. By using a 12 volt battery setup you would still have heat for a short time without 120 volt power.

[quote author=Ernie n Tara like nk=topic=110390.msg995026#msg995026 date=1511363576]
Are you sure all wires to/from the convertrr are large enough for the initial current draw?
Ernie
[/quote]

Yes, the manufacturer specs 22ga minimum and we're using 18ga

The wire gauge will be determined by the amperage needed AND THE LENGTH OF RUN.>>>Dan

18 gauge is too small for the 12V line almost surely. I used 6 gauge on a 600W inverter and had to double it!

Ernie

Ernie n Tara said:

18 gauge is too small for the 12V line almost surely. I used 6 gauge on a 600W inverter and had to double it!

Ernie

Click to expand...

Are you sure? I double checked and they aren't wires we've added, the 18ga we added were for the thermostat (extended the stock wires 6 inches, the manufacturer said use 22ga) and the power supply lines for the thermostat are the original ones that came attached to the furnace.

Utclmjmpr said:

The wire gauge will be determined by the amperage needed AND THE LENGTH OF RUN.>>>Dan

Click to expand...

Hey Dan, I used the stock leads that were attached to the furnace and they run about 12 inches. I mounted the power supply right above the furnace.

Right now I'm waiting to go back to site, I'm now wondering if the fact that the entire unit is powered by a 15a extension cord could be the problem, the low current has been causing problems with the LED lights. The cord is a temporary thing that will be replaced with a 50A 120/240V proper RV plug when we're done construction. It's currently only running the furnace and about 10 LED pot lights.