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Noisy Range Hood Fan

Vent Hood Motor Modification

By Les Adams

Caution!

This installation procedure is NOT all encompassing and it is assumed that the installer will have at least the basic electrical/mechanical skills to accomplish this procedure. If you do not have these skills, seek out a friend or technician that does. While the modification is not difficult, you could damage components inside the RV should the procedure be done incorrectly.

Forward

The kitchen vent fan found in most RV's is a simple 12 volt motor that rotates a fan blade and exhausts cooking odors and steam out of the vent hood exhaust vent. In most cases, the fan motor runs quite fast and makes an excessive amount of noise. The motor can be slowed down and fan noise reduced to an acceptable level by installing a wire wound resistor in series with the fan hot lead (black). The resistor chosen for my modification were 2- 15 ohm, 20 watt resistors. This yields a decent reduction in motor speed while still maintaining a reasonable amount of exhaust airflow. If you desire more or less air flow simply adjust the resistor values. 2 - 15 ohm resistors placed in parallel equal 7 ½ ohms. Any two equal value resistors placed in parallel will halve in value.

The resistors have to have a large wattage rating (20 watt minimum) as the fan motor draws a considerable amount of current. This type resistor can be found at most hobby type electronic supply stores such as Radio Shack and others. It is usually rectangular in shape and appears to be of a sand coated substance (ceramic). When installing these resistors make sure they are not touching or laying on other objects or existing wires as the resistors will become hot after extended use.

When you slow down the fan speed you create another problem in the form of insufficient air movement to open the flapper door of the vent. By adding a simply light spring to the flapper door, it is possible to hold the door open so that the door operation is no longer dependent on airflow. To facilitate closing the door with the spring installed, a chain pull is added to the door and the door is pulled down and locked in place by means of a combination hole and slot that the chain body locks into.

Step by Step

  • 1. Start by removing the vent hood from the trailer (Motorhome)
  • 2. Disconnect the 12 volt power making sure to tape exposed power leads to avoid shorting them
  • 3. Remove sheet metal cover that protects the fan/light switch
  • 4. Locate the lead that goes ONLY to the fan motor
  • 5. Cut this lead about in the middle of the wire and install the 2 - 15 ohm, 20 watt resistors
  • 6. Solder these resistors in place and tape or install heat shrink tubing over the exposed resistor leads
  • 7. Drill a small hole in the flapper door and vent hood and install the light expansion spring and chain pull
  • 8. Drill a hole and slot in the lower part of the vent hood and run the chain pull through the hole. You should be able to pull down the flapper door and slide the chain pull into the slot to hold the door closed.
  • 9. Reinstall the switch protective cover, reconnect 12 volt power leads and reinstall vent

  

 

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