RV Doctor – How Do I Install a Heat Strip for my Duo-Therm Air Conditioner?

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April 5, 2009

Dear Gary,

I bought a heat strip for my Duo-Therm roof air conditioning unit and I don’t know where to install it. I know it is the correct type for my unit but can’t quite figure out where it goes.

John Justice, (Lake Ariel, PA)


John, first you must remove the plastic ceiling air distribution box. Do so by first removing the return air grills covering the filters (some units may be equipped with a single grill and filter).

Once removed you should readily see two or more screws securing the plastic housing to the A/C plenum. Depending on the exact model Duo-Therm, there may be another screw behind a plastic plug on the air box itself. Very carefully pry the plug out with a small screwdriver and remove all the mounting screws.

Once you have removed all the screws, remove the ceiling distribution box to expose the metal A/C plenum. The heat strip assembly installs directly below the air discharge opening, which is that opening going up into the rooftop unit. There will be small mounting holes in the plenum corresponding to the screw locations on the heat strip. Position the heat strip directly under the discharge duct so the screw holes line up with the holes in the plenum and secure it using the supplied sheet metal screws. Plug the electrical connector on the heat strip into the existing receptacle on the AC control box. Typically the air box is notched to allow the harness to pass through it, but you may have to slit the foil or insulation or knock out a plastic plug.

Once you have made space for the power harness, you can remount the air distribution box to the plenum. Realize that add-on heat strips may take the chill out of the air, but they will not sufficiently heat the entire motorhome.

(Please feel free to comment, however, please also note that due to the volume of communications I receive from multiple channels I cannot guarantee a personal response in every instance. However, questions of an overall general interest may be considered and published in an upcoming RV Doctor column.)

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  1. Dick Spencer

    Help! My DuoTherm a/c (model 620415321) heat strip is always one when the power is on and the blower will only come on with the heat strip through the thermostat not with the a/c. I installed a new thermostat but this didn’t help.

  2. Ken in Az.

    I have a Safari Motor home that has a fuzzy ceiling surface, and I would like to know how I can clean it. Most cleaners, like carpet cleaners just leave stains.

    Thanks for any suggestions you may have.

  3. Daddio

    I have 95 Newmar Mt. Aire with with dual AC roof units. Each is controlled by a separate Dometic Duo Therm thermostats. The Rear AC stopped working, and it has been determined that the Thermostat is bad. I purchase several thermostats including Dometic Duo Therm. None of these have a ribbon (like a hard drive ribbon) and could you advise me how to pluga ten wire ribbon connector to a five or seven wire replacement thermostat. The dometic replacement includes the AC control box but I don’t think it will plug-in to unit. Also, AC units are ducted, so I can’t access it from the inside.

    Thank you,