Do You Need That Black Box on Your Water Heater?
(plumbing101.i234.me) — If you’re replacing an old electric water heater in a mobile home and notice a small black box wired in with it, you’re not alone. Many older systems include extra electrical devices that can look mysterious if you’ve never seen one before.
Question:
“I found a small black box wired into my old electric water heater in my mobile home. Do I need to keep it for my new water heater? What does it do?”
Answer:
It depends on what the box is — but most likely, it’s a relay or control device for power management or safety.
Here are the most common possibilities:
- Energy management relay: Some older mobile homes had energy-saving relays that let the utility company control when the heater ran. If that’s the case, you can remove it and wire directly to the new heater following modern electrical codes.
- High-limit safety switch: It might be a thermal cutoff that prevents overheating. If it’s separate from the water heater’s own thermostat, your new heater already has this built in, so it’s no longer needed.
- Load control timer: Occasionally, it’s a timer or smart relay that runs the heater only during off-peak hours. If you aren’t on a time-of-use plan, you can safely omit it.
What to do next:
- Turn off the power at the breaker before touching any wiring.
- Take a clear photo of the wiring and box for identification.
- If unsure, have a licensed electrician verify whether it’s needed for code compliance in your area.
- Modern heaters usually connect directly to a 30-amp double-pole breaker with a dedicated line.
You can find more information about safe electric water heater wiring in Family Handyman’s electric water heater installation guide and the Home Depot wiring guide.
Bottom line: If your new water heater passed inspection specs and has built-in thermostats and limit switches, you likely don’t need the old black box. Always confirm with an electrician before removing it completely.
I’m Chris Mayer, writing for Plumbing 101.