Mod #40: Holding Tank Heaters

Mod #40: Holding Tank Heaters

Submitted on: 07/17/08

     Category: featured, heating
Mod Rating: 12345

(203 ratings)

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Mod Description:

There is so much to see and do during the winter season and enjoying these activities from the comfort of your RV can be a refreshing experience. But it doesn’t have to be winter in order for the temperature to fall below freezing. If you camp at high altitudes, even during the summer months, it is possible for the temperature to approach the holding tank freezing mark. By doing this mod, you can be assured that your holding tanks won’t freeze in all but the most extreme cold weather.

Mod Difficulty:

RV plumbing systems are pretty much the same in almost every RV. They consist of a pump, some plastic PEX (cross-linked polyethylene - you can use that one at your next party!) tubing, faucets, a water heater tank, and fresh, gray, and black water tanks. Some RVs have the holding tanks inside the RV while others are mounted below the floor, exposed to the elements. Although some manufacturers claim that their RVs have “enclosed and heated” holding tanks, that may be only partly true.

Enclosed typically means a thin piece of plastic sheeting called Coroplast that is screwed to the bottom of the frame rails running the length of the RV, covering the holding tanks. There may or may not be insulation in between the the sheeting and the flooring. Heated usually means that there is a small duct from the furnace placed through the floor in to the enclosed space for heating the holding tanks. In combination, the sheeting, insulation, and heater duct can help in temperatures down to about 25° F, that is as long as the furnace is running. When it’s that cold, most RVers use supplementary electric or catalytic heaters to save propane, which make the furnace run less, contributing to less heat supplied to the holding tanks.

OK, now that you know everything there is to know about the basic way holding tanks can be heated, let’s get on with the mod. To handle temperatures below freezing without having to resort to running the furnace full time, you can affix thermostatically-controlled heating pads specifically made for plastic holding tanks. These pads have an adhesive backing that is used to stick the pad to the holding tank. Simply peel and stick, after cleaning the bottom of the tanks of course.

You have three options for powering the heating pads: 120 VAC, 12 VDC, or a combination of both. If you have a huge battery bank then go with the 12-volt option. The 12-volt option can draw a lot of juice so plan your tank heating time accordingly so you don’t wake up to a tanksicle. If you know you will always have a 120 VAC electrical source, then go with the 120-volt option. These don’t draw nearly the current as their DC counterparts and are a good solution if you are a full-hookup kind of camper. To cover all your bases, choose the combination model.

You also have options for the size tank you need to heat. The larger the tank, the larger the heating pad, and the larger the current draw. And while some may think it’s only necessary to heat the fresh water tank(s), it’s imperative that you consider the black and the gray tanks as well. Damage from freezing can just as easily occur with the gray and black tanks if left unheated in prolonged freezing temperatures, let alone not being able to drain your tanks if they are solid ice (or whatever is in there).

Wiring is straightforward. For the AC model, simply plug in the heating pad cord in to an AC outlet. The DC and AC/DC combination pads require that you dedicate a circuit in the RVs 12-volt panel. There are usually enough spare circuits available in the RV power center to accommodate multiple heating pads. Simply run a minimum of 14 gauge wire from the pad to the panel, giving careful consideration to the polarity of the wiring: red to red (+ to +) and white to white (- to -). These are the most common wire colors for most heating pads. Don’t forget to place a switch between the positive wire from the pad and the power center. You need a way to turn the pads off! Any single-pole-single-throw (SPST) switch rated for at least 15 amps DC will do.

TipExamine your RVs plumbing to determine what measures may be needed to prevent damage from freezing temperatures. Some RVs have plumbing exposed to the outside elements. In this case, you should wrap the exposed plumbing with heat tape and foam pipe insulation.

TipIf possible, empty the holding tanks if they will be subject to freezing and pour a couple of quarts of non-toxic, biodegradable antifreeze into each holding tank (except the fresh!). This will protect the dump valves. Pour in more antifreeze as waste water fills the tanks.

ModMyRV recommends these parts for this mod:

General Reference
How to Cold Weather Camp in Your RV
UltraHeat RV Holding Tank Heaters

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18 Comments For This Mod

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  1. jmt

    Has anyone done this mod and did it work like you expected?

  2. Bob Vaughn

    I wonder if this mod would increase the smell coming from the holding tanks…..I always noticed that the outhouse stunk more in the summer than it did in the winter. Heating the black water UCK….

  3. C Condit

    You carefully clean out all the tanks when expecting freezing weather, add RV antifreeze to the black and gray tanks, and turn on the heating pads. There will be no smell. I clean my tanks regularly. It is not hard.

  4. Trailblazer

    I have just completed this mod using the combination 12v and 120v units. The pads were added to both the black and gray tanks, but not the drain pipes. Our fresh water tank is above the floor in the rig so I did not have to worry about adding a tank heater to it.I plan to use heat trace tape on the drain pipes and then wrap the pipes with insulation. We use our rig during the winter months and want to be able to use our water system.

    I will be happy to report back the results after a season of using the tank heaters.

  5. Mike Cook

    How does the heating pad adhere to the tanks? Adhesive? Since my tanks are on the bottom, and the pads are on the bottom of the tanks, they’d have to be VERY securely adhered - there would be strong wind on them from the RV driving speed.

  6. Trailblazer

    Mike, the pads are held on by a very strong adhesive. You basically clean off the exterior of the tanks with isolproply alcohol. Then peel the paperbacking off of the tank heater and press it into place. Just make sure where place them where you want them as once the adhesive touches the tank you can’t remove the heat pad to re-position it. Like you I have exposed tanks under my trailer but I have no concern with them coming off by the wind driving down the highway.

  7. joel northen

    rather than deal with a heating pad, i put a small electric heater on low in the space with the water tanks. I use heating tape and pipe insulation on the water going into the rv. i do not use the white water tank b/c of the risk of it freezing due to a power outage/propane outage. I also pour 1/4 bottle of rv antifreeze down the shower (grey tank) and 1/4 down the toilet (black tank). I live in Breckenridge Co, it has not been above freezing since before november. I also dont let the tanks get above 3/4 full, incase my system fails for any reason and my tanks freeze. I also take great care when emptying the tanks to make sure the sanitation line is completely empty to prevent ice dams from building up and causing a back up.

  8. Pianotuna


    If you have enclosed tanks a simple mechanical thermostat controlling a small fan based heater may be a much less expensive upgrade. I installed an AC outlet in the compartment which is powered by the inverter.

  9. ModMyRV

    I’ve heard of this but haven’t seen one at work in the wild. Do you know what ambient temperature the fan-based heater is effective down to? Of course, this would depend on the tank size and amount of water in the tank.

    Also, how long can you run the heater on the inverter? I would imagine that an AC heater large enough to prevent freezing of the tank would have a large current draw.

  10. Wolfwalker

    I wonder if I need to do this mod…we live in our Keystone Montana and I take great care of keeping the tanks clean and emptied..I did however heat trace all the exposed drain lines beneath the sealed underbelly and hard lined the drain to the campsite inlet. I have three tanks, black, grey and galley. So far I have yet to have any problems. However, I am a firm believer in prevention…


    Completed this mod today on my class C with a set of heating pads that I bought off of ebay. I currently hot wired the system into the existing wiring harness for the factory installed pads complete with a switch. Seems to be working fine. I am currently waiting on the OE wiring harness with the heating pad relay to finish off the job.

  12. Under floor heating

    interested in a flat built in the seventies with underfloor heating, wonder how long it will last.

  13. dartmouth01

    I’m in the process of doing this mod and I want to try to wire in an LED that will light only when the heating pad is triggered to turn on by the thermistor. This would go along with the illuminated switch I have so I would both know when the switch wsa ON, as well when the heating pads were actually getting turned on. Anyone have any ideas? I’m thinking of putting a relay in between the + amd - wires of the heating pad. Then the circuit is “closed” by the thermistor, the relay would activate and then send power to the LED. Can anyone tell me if this would work?

  14. Wolfwalker

    Just thought I would let you all know, we are dealers for ultraheat…contact me for pricing..special for memebers

  15. Timbo

    So I am wondering why it requires a dedicated circuit and why i cant just splice into the 12v light fixture thats located in each of my cargo bays with my tanks.


  16. cliff

    Current carrying capacity is the issue. You Cargo bay lights are probably not a large enough gauge wire. However, research it all on Google. See what the current draw is specified for your tank heaters, add up all the draw, then, determine your cargo light wire gauge (probably 16 gauge), determine the current carrying capacity of that wire gauge, this lets you know if it will carry the load. Also, be aware that if the heaters are ON and you turn the bay light ON, the wire/circuit has to be sized to carry both loads simultaneously(or , be sure to not run both simultaneously).

  17. David Nokes

    We’re the Comox Valley on Vancouver Island, and are planning to do the modification. I like the idea of a illuminated switch, in-line LED and thermisatat as part of the design. Thanks for the helpful tips!
    David Nokes

  18. Beth

    my water tank is under bed so not exposed, but am I to understand that if I put one quarter RV winterizing fluid in my grey and black tank they will be OK down to about 25 degrees?? what about the lines??

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