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Tank Heater Sizing

How to Select UltraHeat RV Tank Heaters
for Your RV's Holding Tanks

All RV holding tanks mounted below the floor of a motorized or towable RV or Leisure Vehicle will potentially freeze solid in cold weather. UltraHeat RV Holding Tank Heaters sometimes called RV Tank Blankets or Tank Pads are designed to protect your Unit’s black, gray and fresh potable water holding tanks from freezing.

While each RV or Leisure Vehicle is different, our overall product design benchmark and testing of heat blankets for RV holding tanks is -11°F (-23.9°C). This is on an RV holding tank (with the properly sized heat panel), mounted below the floor and totally exposed to the elements.

Lower more extreme temperatures (like down to -40°F (-40°C)) can be achieved by additional modifications to the RV like enclosing and insulating, once the UltraHeat system is installed and tested.


This allows the owner to use their major conveniences like the shower, sinks and toilet. Correctly choosing holding tank heating pads from UltraHeat is simple.


1.    Select tank heater sizing by the WAY YOU USE your RV or Leisure Vehicle.

If you are traveling with your unit, moving from place to place, not staying in one place for any extended period of time the recommended UltraHeat RV tank heater is either:


  • Only an 13.5 VDC powered tank heater(s)

  • or use the COMBINATION AC/DC voltage, AM-M3600 heater (This heater has two (2) separate heating elements that operate INDEPENDENT of each other)


**check DC power source availability and converter size before selecting DC only heaters**


If choosing DC only models, they would work thru your converter while traveling and also once plugged in to 120 VAC.  If you choose a dual voltage heater, you can use the DC side while traveling, and switch over to the 120 Volt side once parked and accessing power from either a portable generator or you are plugged into shore power (campground plug).

 If you’re not concerned about freezing during travel, like most construction trailers, some park models or permanently stationary recreational units, consider 120 VAC only models.  


13.5 Volt DC RV Tank Heaters

These Heaters are wired directly into the 13.5 Volt DC power systems (the 12 VDC converter) of an RV or Leisure Vehicle. Installed on a fused 13.5 VDC power circuit and operated with a 12 Volt toggle power switch, the 13.5 VDC tank heaters are easy to operate. Once installed, turn the power switch "ON" when the outside temperature approaches freezing and only when fluids are present. Turn the power "OFF" when the outside temperature stays above freezing or when tanks have been emptied.


120 VAC (Household Current) Retail Aftermarket  RV Tank Heaters

These model heat panels are equipped with a 15-foot (4.6m) power cord and a standard two-prong molded electrical plug. An appropriate extension cord and access to 110-120 Volt AC standard shore power or generator power is all that is required to power and operate. Unplug the power cord when the outside temperature stays above freezing, or when the tank is void of fluid contents. 

120 VAC models can also be ‘hard wired’ directly into the 120 VAC power panel from a circuit protected by a circuit breaker.  Wired thru a 120 Volt rated toggle power switch, and controlled “on/off” meeting the same criteria as the DC heaters.  


Note: 240 VAC models are also available; please contact us if you have interest in them.



2.    Determine the size(s) of holding tank(s) that need protection.





Holding tank capacities are normally listed in your RV's owners manual, or in the specifications for your RV.

The capacity of a holding tank can also be calculated.



  1. Measure the length, width and average height of a holding tank in inches.

  2. Multiply the length, width and average height, in inches to find the volume in cubic inches.

  3. Now divide the volume in cubic inches by 231 cubic inches per gallon to find the capacity of your holding tank in gallons.



  1. Measure the length, width and average height of a holding tank in centimeters.

  2. Multiply the length, width and average height, in centimeters to find the volume in cubic centimeters.

  3. Now divide the volume in cubic centimeters by 1,000 to find the capacity of your holding tank in liters.




Once you determine the size, use this breakdown on the different Aftermarket heaters available in North America to determine your tank heater sizing:


  • Up to 25 Gallon (95 Liter) Capacity:

  • Up to 40 Gallon (110 to 151 Liter) Capacity:

    • Use AM-M3600     120VAC/12VDC Combination dual voltage/element

  • Up to 55 Gallon (208 Liter)

  • Up to 75 Gallon (283 Liter) Capacity

    • Use AM-M2400 13.5 VAC heater

    • Use AM-M4800 120 VAC heater

    • AM-M3600 Dual Voltage Combination heater – (2 heaters required)


Larger than 75gallon (283 Liter) Capacity, refer to Tank Heater chart below in this section. (multiple heaters must be  wired in parallel or independently controlled based on Voltage and Amp draw of circuit )



3.    Once you have determined each tank size and tank heater sizing, don't forget to consider the drain pipes and elbows between the tank and termination valve. Any pipe lengths greater than 6 inches (15.24 cm) between the Holding tank discharge and valve must be completely heated to avoid freezing. On the RV's fresh water system, inspect the 1/2 inch feed line between the tank and where it goes through the cabin floor, take note if it’s a vinyl reinforced line or ridged “Pex” style tubing.  Our  UltraHeat PH513, 1/2 inch pipe heat panel can only be used on ridged type plumbing. On supply lines that are vinyl or long length runs, consider the UltraHeat 13.5 VDC Smart Heating Cable.   Also map out your electrical feed and connections to define what  switch kits or other accessories that you will need  to control and complete your system.

Tank Heater Chart.jpg

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