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Miller Manufacturing, Glencoe, MN 55336 USA  •  www.miller-mfg.com

Read Instructions Thoroughly Before Use

CAUTION

1. Have a qualified electrician install a properly grounded 

receptacle outlet, acceptable for outdoor use and protected 
from snow and rain, immediately adjacent to the location 
where the heater will be used.

2. Route the supply cord and locate the heater so as to be 

protected from damage (by livestock if using 7521 and 7621)

3. Do not use extension cords.
4. Inspect cord before using.
5. Unplug heater at receptacle outlet when not in use or before 

removing from tank or pond.

6. Store heater indoors after winter season.
7. Models P7521 and P7621 are intended to be used only in ponds 

for keeping water from freezing during the winter months. 
These units have a 15 foot supply cord which allows the unit to 
reach the middle of the pond for efficiency reasons. Position the 
supply cord around plants and other objects in the pond. Any 
excess cord must be stored and protected from damage.

8. Heating element MUST be completely submerged while in use 

to prevent risk of fire hazard.

9. Do not tether unit.

CAUTION:

 to ensure continued protection against electric shock 

hazard, connect to properly grounded outlets only. 

Connect 

only to a circuit that is protected by a Ground Fault Circuit 
Interrupter (GFCI).

 This is a sensitive device that cuts off the 

electricity if there is any leakage of current. This device may be 
obtained from any electrician or hardware store.

INSTALLATION

1. 

If the unit is to be used in a non-metallic tank or pond, a 

Model 88R guard must be attached

 in order to prevent the 

heating element from coming in contact with the walls of the 
tank. The 88R guard must be purchased separately.

2. The unit should be placed in a tank or pond with water and 

then 

plugged into a Ground Fault Circuit Interrupter.

3. Some animals--particularly horses--may try to remove the 

7521 and 7621 de-icers from the tank. To counteract this, you 
may wish to run the cord through PVC or galvanized tubing 
extending over the tank. Be sure to allow slack in the cord to 
compensate for varying water level.

4. The unit is thermostatically controlled so that it will not turn 

on until the temperature drops to around 40°F. If it is removed 
from the tank, it will turn on and then shuts itself off in about 
60 seconds and will then cycle on and off.

MAINTENANCE

Do not allow lime or other impurities in the water to build up on 
the heating element. The frequency of cleaning depends directly 
upon the composition of your water supply. To remove buildup, 
soak the heating element in vinegar or a lime removing cleaner 
obtained at any farm or hardware store. Then use a rough brush 
to scrub the element. Do not use steel wool or something that will 
scratch the element. Inspect the element for any cracks before 
reinstalling.

IMPORTANT 

The use of the Model 88R guard is mandatory to prevent the 
heating element from coming into contact with combustible 
materials such as the side or bottom of a non-metallic tank.  
Read and follow all instructions. 

ATTACHING THE  

GUARD TO  

THE DE-ICER

Turn the unit upside down and  
place the guard as shown.  
Rotate the short spring to thread  
it onto the guard and the straight  
section of the heating element.  
The spring should capture  
the cross-wire on  
the guard and be  
centered around  
the copper strap  
as shown.

WARRANTY

This de-icer is warranted for a period of 1 year from the date of 
purchase. If you believe your de-icer is defective and still within 
the warranty period, return it to the factory or place of purchase for 
inspection and possible replacement. The warranty is voided if (1) 
the ground terminal on the plug has been removed, (2) excessive 
deposits have been allowed to accumulate on the heating element, 
or (3) there is evidence of general abuse such as animals chewing on 
the cord. This warranty does not cover incidental or consequential 
damage resulting from either a defect in parts, materials, or 
operation failure. Some states do not allow the exclusion or 
limitation of the above damages so the above limitation may not 
apply to you. No agent , employee, or representative of Miller 
Manufacturing has any authority  to bind Miller Manufacturing to 
any affirmation, representation or warranty directed towards any 
products bearing the Miller Manufacturing name, except as stated 
herein. This warranty gives you specific legal rights--you may also 
have other rights which vary from state to state.

LOCKNDRY® DETACHABLE 

POWER SUPPLY CORD 

(AVAILABLE SEPARATELY)

The LOCKNDRY® detachable power supply cord provides a secure, 
water-resistant connection that is UL approved for use with stock 
tank de-icers whereas conventional extension cords are not. The 
LOCKNDRY® cord set features:
- Plugs into any normal 3-prong outlet
- Mates with LOCKNDRY® plugs to form a water-resistant seal
- Keeps connection from separating during use
- Second, locking nut for secure connections
- Outdoor approved cord set
- UL listed

CAUTION 

To provide continued protection against shock: 

a) Connect the heater appliance and LOCKNDRY® per these 

instructions and those provided with the LOCKNDRY®  
cord set.

b) Connect only to a properly grounded GFCI protected  

receptacle outlet.

c) Use no more than one LOCKNDRY® cord set – for an installation. 

Do not use an extension cord if the receptacle is located such 
that the heater and/or LOCKNDRY® cord set cannot be plugged 
in. Have a qualified electrician install a properly grounded GFCI 
protected receptacle outlet, acceptable for outdoor use and 
protected from snow and rain where the heater will be used.

d) Use only the LOCKNDRY® cord set referenced in the instructions.
e) Locate the heater and route its supply cord and LOCKNDRY® 

cord set so as to be protected from damage.

f) Keep the LOCKNDRY® and heater supply cord set connection a 

minimum of 6 inches above ground to avoid wetting by pooling 
water or accumulated snow.

g) Inspect heater supply cord and LOCKNDRY® cord set before 

using. Do not use if the supply cord and/or LOCKNDRY® cord set 
is damaged.

h) Do not unplug heater from the LOCKNDRY® cord set. Unplug the 

LOCKNDRY® cord set only at receptacle outlet when not in use or 
before moving location of heater.

i) Store the LOCKNDRY® cord set and heating appliance indoors 

after the winter season.

INSTRUCTIONS FOR  

THE LOCKNDRY®  

CORDSET

(1) Insert the plug into the receptacle 
just as you would a normal extension 
cord making sure the plug is fully 
inserted.

(2) Slide the large nut forward to 
engage the threads on the plug.

(3) Rotate the nut to tighten the 
plug against the receptacle. 

(4) Slide the second, thinner, red 
nut forward to engage the threads.

(5) Rotate the red nut snuggly 
against the larger nut.

FLOATING DE-ICERS

 Item # 7521 & # P7521 - 1500 watts, 120 VAC, 60 Hz 

Item # 7621 & # P7621 - 1000 watts, 120 VAC, 60 Hz

Item # 521G - 1500 watts, 120 VAC, 60 Hz

 304114 

SPRING

CROSS-WIRE

GUARD

COPPER 

STRAP

HEATING

ELEMENT

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