Ohm’s Law Kicks In

Yes, you can wire a 240-volt rated water heater with 120-volts, exchanging the second hot (red) wire for a white (neutral) wire to the neutral bus. Strange, but true. Because of Ohm’s Law and the fact that the resistance of the circuit stays the same, half the voltage means the watts of heating energy generated is reduced to one quarter of what it was at 240 volts. 

     We came upon that situation today at a homeowner-wired-and-plumbed water heater replacement for a 1970s mobile home, shown above. One look at the plumbing configuration and you know it was not done by a pro. The rating on the data plate is 4500 watts at 240 volts, so it produces 1125 watts with the 120-volt wiring and takes four times as long to heat the water. But it works. Older mobiles often had small, 120-volt water heater heaters and we suspect the existing wiring was reused for the replacement.

    The thermostat is mechanical, not electric, so it is unaffected by the changeover. We doubt that the manufacturer would approve, however, or honor the warranty for a unit wired like this.

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Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code?

• Can you use a light switch for a water heater disconnect? 

• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

• Can you wire a 240-volt water heater with 120 volts? 

• Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?  

• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater? 

• What is required clearance for access and working space in front of an electric water heater? 

• What is the purpose of a thermostatic mixing valve above a water heater? 

• Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

• What is the difference between a manufactured/mobile home water heater and a regular water heater? 

• Does a tankless water heater require a pressure relief valve?

• When is a water heater drain pan required?

• Why is there water in my water heater drain pan?

• What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

• What is the difference between a single element and dual element electric water heater? 

• What is an FVIR water heater?

• What is a heat pump water heater?

• What is a dielectric union?

• What's that powdery crust on the pipe connections at the water heater? 

• What are the most common installation mistakes with water heater replacement?

• Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises? 

• What can I do to make my water heater last longer?

• How can I determine the age of a water heater if the serial number is missing or decoding it is impossible? 

• How does a hydronic heating system work?

• What is the difference between a regular water heater and a direct vent water heater? 

• What is the difference between a regular water heater and a power vent water heater?

• What is backdrafting at a gas water heater? 

• How do I determine if a water heater is gas or electric?

• What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?  

• What's the valve with the flip-up handle on the water heater for?

• Why is an older water heater an insurance problem?

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.