Fridge frame heater switch

[tinkeringtechie]

It has already been mentioned in another thread that the Dometic RM3762 (and possibly other models) has a 12V frame heater that uses a considerable amount of current. This can be quite a burden on the battery while boondocking and the frame heater itself is more of a luxury than a necessity. I did a bit of investigation in the manual and it appears to be easily accessible. According to the wiring diagram it's connected using a light blue wire and starts and ends in the rear access panel with nothing but a resistive heater wire between the two ends (no complex circuitry). I poked around in the access panel today and it looks like a pretty easy modification:

[attachment id="71" thumbnail="1"][attachment id="72" thumbnail="1"]

You can easily see the light blue wire. The negative end is just grounded directly to the chassis. I haven't made any modifications yet, but my plan is to add a relay to the grounded end that connects it when 120V is present, but leaves it disconnected when only on 12V. I'll post more details once it's complete.

[fasteddieb]

Nice!

You are certainly living up to your screen name!

[tinkeringtechie]

Thanks, I'm just getting warmed up :-)

[fasteddieb]

In all honesty, I think the most elegant solution here may just be the simplest...

...a weatherproof SPST switch mounted right on the refrigerator access panel labeled "SHORE POWER" and "BOONDOCKING".

The whole relay thing seems a bit much.

Then again, I always enjoyed Rube Goldberg contraptions, so let us know how it goes!

[charliem]

[font size="3"][font face="arial"]I agree the SPST switch will suffice, but you forget Tinkeringtechie is a real tech junkie. No solution is complete without some If/Then logic  . Charge on, Travis.
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[tinkeringtechie]

The SPST approach would work just as well, but I have enough things to remember as it is. I'm a big fan of automation whenever possible. In this case the relay doesn't add too much complexity. A relay with a 120V coil plugged into the fridge outlet would complete the build.

[charliem]

[font size="3"][font face="arial"]I finally made the refrigerator frame heater mod to my 21RBS. I mounted a toggle switch in the external fridge access area. That should save 12 AH per day when dry camping.

One caution: the blue wire connected to the ground stud is not ordinary wire. It is spiral resistance wire consisting of very tiny wire wrapped around a non conductive core. As such it cannot be soldered. If you must cut it, as I did before I knew, use a crimp terminal on the end. The best approach would be to use the existing ring terminal with a screw terminal switch or other screw terminal device. Be careful when attempting to strip the insulation. The wire is easily damaged. 
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[tinkeringtechie]

[quote source="/post/2012/thread" timestamp="1405468836" author="@charliem"][font size="3"][font face="arial"]One caution: the blue wire connected to the ground stud is not ordinary wire. It is spiral resistance wire consisting of very tiny wire wrapped around a non conductive core. As such it cannot be soldered. If you must cut it, as I did before I knew, use a crimp terminal on the end. The best approach would be to use the existing ring terminal with a screw terminal switch or other screw terminal device. Be careful when attempting to strip the insulation. The wire is easily damaged. 
[/font][/font][/quote]Interesting... so the entire wire is the heater? Even that rear compartment is receiving some portion of the heat?

[charliem]

[quote source="/post/2015/thread" timestamp="1405486930" author="@tinkeringtechie"]Interesting... so the entire wire is the heater? Even that rear compartment is receiving some portion of the heat?[/quote][font face="arial" size="3"]Apparently so. Aside from my basic philosophical [/font][font face="arial" size="3"]disagreement with heaters in fridges, now I find I'm heating the great outdoors when it's already 100F. Hmmmmm.
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[tinkeringtechie]

That's even more disappointing. You wouldn't think it would be that difficult to use low impedance wire until it gets to the frame itself. I wonder what percentage of the 500mA actually makes it to the frame. Now I'm really glad that I've disconnected it. I still haven't noticed any difference.You'll have to let me know if you see any changes with it off. Maybe if you're full-timing in a swamp it might condense enough to freeze shut (silicone grease on the seal?)

[charliem]

[quote source="/post/2021/thread" timestamp="1405516908" author="@tinkeringtechie"]That's even more disappointing. You wouldn't think it would be that difficult to use low impedance wire until it gets to the frame itself. I wonder what percentage of the 500mA actually makes it to the frame. Now I'm really glad that I've disconnected it. I still haven't noticed any difference.You'll have to let me know if you see any changes with it off. Maybe if you're full-timing in a swamp it might condense enough to freeze shut (silicone grease on the seal?)[/quote][font face="arial" size="3"]According to the Dometic brochure the two door models had a switch inside, called climate control, with instructions to [/font][font size="3"]turn it on for high humidity environments. Probably generated too many calls to customer service asking "what's this switch for?" So, to save a few cents and some call center labor, they changed it to "automatic front frame heater" and eliminated the switch. I think the only problem would be cold, damp environments where the dew point is close to air temp and close to freezing. Warmer high humidity environments, where the ambient air temp is high, would only result in condensation. If the trailer inside temp is comfortable freezing should not be a problem. More to follow after I gather some experience.
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[charliem]

[font size="3"]I can now report on the effect of the frame heater in the Dometic 7 cu. ft. fridge. As previously reported I installed a switch to disable the heater to save battery power when dry camping. On a recent trip to central Florida during a hot, humid, rainy week I experimented with and without the heater. Without the heater connected I observed water condensing on the front frame piece between the top of the refrigerator door and the bottom of the freezer door. It was enough to cause water to collect and stand on the top edge of the refrigerator door. No frost or ice was observed. When I turned the heater on the condensation appeared to cease. I must stress that this was under extreme humidity conditions: 95%-100% RH with temperatures at 90F plus and periods of rain outside (standard Florida summer); A/C running inside.

My conclusion is the switch addition is definitely worthwhile. Apparently Dometic thought so too, until they recently deleted it. If it's hot/humid enough for A/C you will probably have shore power available and can run the frame heater. If it's cool/dry enough to dry camp without A/C you can save 12 amp-hours per day of battery power by turning off the heater. Even if the condensation does form it's more of a curiosity than a problem.
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[drake]

Great thread. I'm throwing in a relay to the ac so it will run when I'm on shore power anyway. What a waste of juice when boondocking, anything else like this?

[tinkeringtechie]

[quote timestamp="1468553055" source="/post/23037/thread" author="@drake"]Great thread. I'm throwing in a relay to the ac so it will run when I'm on shore power anyway. What a waste of juice when boondocking, anything else like this? [/quote]Let us know how it goes. I never got around to adding the relay because I didn't notice any difference when I just disconnected the heater entirely. We used the fridge non-stop for an entire year and never once had issues with condensation or freezing on the frame.

[daplumbr]

I'd like to install the heater switch, but can't find the blue wire. The connections on mine don't look like TT's photos. Last night the frame my fridge was very warm and it violates all my practical nature to have the door frame of a REFRIGERATOR heated. I guess I'll have to track down the heater wire with my meter.