Adding a heat pump to your water heater can significantly improve its efficiency, potentially saving you money on your energy bills. This solution uses the ambient air temperature to heat water, reducing the reliance on traditional electric heating elements. In this article, This Old House plumbing and heating expert Richard Trethewey demonstrates the process of retrofitting a heat pump to an existing water heater.
Understanding Heat Pump Water Heaters
Heat pump water heaters are an effective alternative to traditional electric water heaters. They work by extracting heat from the surrounding air and transferring it to the tank, using less electricity than conventional heating elements.
How Heat Pump Water Heaters Work
Heat pump water heaters operate on a simple principle: they move heat rather than generate it. This process is similar to how a refrigerator works but in reverse. The heat pump draws in warm air, compresses it to increase its temperature, and then transfers that heat to the water in the tank.
Benefits of Adding a Heat Pump to Your Water Heater
Adding a heat pump to your existing water heater offers several advantages:
- Increased energy efficiency
- Lower electricity bills
- Dehumidification of the surrounding area
- Potential for longer water heater lifespan
Pro Tip: Bruce, a This Old House plumbing and heating expert, explains the efficiency gains in simple terms: With a standard electric resistance water heater, “a unit of electricity goes in, you get a unit of hot water out. A dollar in, dollar out.” A heat pump water heater, by contrast, harvests energy from warm ambient air, dramatically cutting electrical consumption — making the upgrade one of the most impactful efficiency improvements a homeowner can make.
Types of Heat Pump Water Heater Systems
There are two main types of heat pump water heater systems:
- Integrated units: These combine the heat pump and storage tank into a single appliance.
- Add-on units: These retrofit systems can be installed on existing water heaters, which is the focus of this article.
Pro Tip: Bruce, a This Old House plumbing and heating expert, notes that integrated heat pump water heaters are also known as hybrid water heaters: “This is a conventional hybrid or heat pump water heater. They’re called air source because they take warm air from the room. They harvest the energy through the compressor or the heat pump unit on top.”
Preparing for Heat Pump Installation
Before beginning the installation process, you’ll need to assess your current setup and gather the necessary tools and materials.
Assessing Your Current Water Heater
Examine your existing water heater to ensure it’s compatible with a heat pump retrofit. Most electric tank-type water heaters can be adapted, but it’s a good idea to check the manufacturer’s specifications and local building codes.
Tools and Materials Needed
Gather the following tools and materials:
- Condensate pump
- Drill with 1/2-inch bit
- Electrical cable
- Foam-rubber pipe insulation
- Heat pump unit
- Pipe wrench
- Plumber’s tape
- PEX tubing and fittings
- Single-entry valve
- Wire nuts
Safety Precautions
When working with water heaters and electrical components, safety should be your top priority. Always:
- Turn off the power supply to the water heater at the circuit breaker
- Shut off the cold water supply
- Wear appropriate personal protective equipment
Don’t Overlook the T&P Valve: As explained during a This Old House water heater installation, the temperature and pressure relief valve is “the most important device” on a water heater. It’s designed to relieve at 210 degrees Fahrenheit — just below steam — or at 150 pounds of pressure. “In either case, if you got to those levels and you didn’t have a valve like this, it could almost become a bomb,” the installer noted. Make sure this valve is properly installed and functioning before putting the system back into service.
Step-by-Step Heat Pump Installation Process
Follow these steps to add a heat pump to your water heater:
- Shut off the cold-water line and electrical circuit to the water heater.
- Pump all the water out of the water heater’s tank.
- Open a hot-water faucet in the kitchen to release pressure and break the vacuum in the tank.
- Use a pipe wrench to remove the drain valve from the water heater.
- Replace the old drain valve with a single-entry valve. Wrap plumber’s tape around the female threads and tighten the new valve into the drain-valve hole.
- Attach two lengths of PEX flexible tubing to connect the heat pump to the single-entry valve on the water heater.
- Slip foam-rubber pipe insulation over each length of PEX tubing.
- Drill a ½-inch-diameter hole through the thermostat cover on the water heater, then attach a rubber strain-release fitting.
- Feed an electrical cable from the heat pump through the strain-release fitting and attach it to the thermostat on the water heater.
- Screw the thermostat cover back onto the water heater.
- Install a condensate pump to collect and discharge condensation from the heat pump to the washing machine drainpipe.
- Turn on the cold-water line and the electrical circuit.
- Fill the water heater’s tank with water.
- Turn on the heat pump.
Making Electrical Connections to Your Heat Pump
When setting up the electrical connection to your heat pump, be sure to consider the following:
Wiring the Heat Pump to the Water Heater
The heat pump needs to be wired to the water heater’s thermostat to override the traditional heating elements. This allows the heat pump to take over the primary heating duties, with the elements serving as backup.
Safety Considerations for Electrical Work
When working with electrical components:
- Ensure all power is disconnected before beginning work
- Use appropriate wire gauges and connectors
- Follow local electrical codes and regulations
- Consider hiring a licensed electrician if you’re unsure about any aspect of the wiring process
How the Wiring Works: On a This Old House project, the team drilled a hole in the front cover of the water heater and installed a strain relief — the ideal entry point for running wire between the heat pump and the water heater. The wire connects directly to the thermostat, bypassing the heating element. One connection is made with a wire nut, and the other is secured by tightening a screw on a metal fork terminal. This setup ensures that when the thermostat calls for heat, it activates the heat pump rather than the resistance element.
Don’t Forget the Condensate Pump: Once your electrical connections are made and the breaker is back on, there’s one more critical component to address. As shown during a This Old House heat pump water heater installation, a condensate pump must be installed below the unit. Any time the heat pump runs, it scavenges heat from the surrounding air, and that process also pulls moisture and humidity out of the air — just like an air conditioner. That condensation needs somewhere to go, so the condensate pump collects and removes it.
Managing Heat Pump Condensate
Heat pump water heaters produce condensate as they extract heat from the air. You’ll need to manage this water to prevent damage and maintain efficiency.
Installing a Condensate Pump
A condensate pump collects the water produced by the heat pump and safely discharges it to a drain. Richard Trethewey demonstrates how to connect the condensate pump to the washing machine drain, providing an efficient solution for water disposal.
Proper Drainage Solutions
When selecting a location for your condensate drain:
- Choose a nearby drain or utility sink
- Ensure the drain line has a continuous downward slope
- Avoid draining into areas that could freeze in cold weather
Optimizing Heat Pump Performance and Efficiency
To get the most out of your heat pump water heater, consider these tips for optimal performance and efficiency.
Ideal Placement for Heat Pump Units
Place the heat pump unit in an area with ample air circulation and consistent temperatures. Basements or utility rooms often work well, as they provide a stable environment and can benefit from the dehumidifying effect of the heat pump.
Maintenance Tips
Regular maintenance can help ensure your heat pump water heater continues to operate efficiently:
- Clean or replace air filters as recommended by the manufacturer
- Check and clean the condensate drain line periodically
- Inspect electrical connections annually
- Keep the area around the unit clear of dust and debris
Energy-Saving Strategies
Maximize your energy savings by:
- Insulating hot water pipes
- Scheduling regular maintenance checks
- Setting the water temperature to 120°F or lower
- Using low-flow fixtures and appliances
Our Conclusion
Adding a heat pump to your water heater is an effective way to boost efficiency and reduce energy costs. While the installation process requires some technical knowledge, the long-term benefits can be significant. By following proper installation procedures and maintenance practices, homeowners can enjoy reliable, cost-effective hot water for years to come.
