The Rheem ProTerra hybrid heat pump water heater
My gas-fired tank water heater (or boiler) developed a leak in 2021 and had to be replaced. After trying out an oversized tankless gas water heater for a couple of months, I decided to install a 50 gallon Rheem ProTerra hybrid heat pump water heater. The top of the heater houses a heat pump that blows air over refrigerant to transfer heat from the environment into water. The "hybrid" in the name means it can fall back to using electrical resistive heating elements to keep up with demand if there's not enough heat to scavenge from the air. Electric resistive heating is much less energy efficient than a heat pump.
A heat pump's main advantages are:
It has drastically reduced energy usage compared to conventional electric water heaters using resistive heating elements. It's even better when combined with a solar system or running it with a rate-sensitive schedule.
It doesn't burn natural gas and need a flue or ducting system to vent carbon monoxide exhaust.
It's relatively quiet, compared to a tankless gas-fired water heater. But it still has the equivalent of an air conditioner in it, and the Rheem doesn't seem that worried about sound isolation (like freezers, also heat pumps, are), so it's not silent.
The heat pump cools the surrounding area, like an AC unit. The air outlet can be vented outside though.
There are fine-grained controls for how to heat the water, including a heat pump-only mode for maximum efficiency.
Some jurisdictions have rebates for buying an energy-efficient water heater.
But, I also understand now why these water heaters aren't more common:
The less expensive ones require a dedicated single-phase 240V 30A circuit, which could stress your electrical service capacity. The electrician who installed the circuit had to add a load controller box to shut off power to other circuits in case it draws too much current, to avoid overloading the main service line and pass inspection. Practically, in heat pump-only mode, it only draws a few amps when running, but the backup electrical heating elements are rated to 4500 Watts.
In 2022, Rheem started selling a "plug-in" model that ditches the backup resistive heaters and works on a standard US 120V 15A circuit. I would have opted for this model if it had been available in 2021, since I've only used it in the heat pump-only mode so far.
The plumbing connections are horizontal, on the front sides of the unit, taking up valuable space and making the plumbing lines a bit awkward. For water heaters, plumbers typically put the pipe outlets high on the wall so they can enter the unit at the top. Since the heat pump is taking up space on the top, and running lines through it could be counter-productive, they put them on the side. The cold water line enters at the bottom of the unit, near the floor. A.O Schwartz, sold by Lowes on the US west coast, has connections on the top of the unit.
The plumbers in my area didn't offer a heat pump hybrid water heater as an option, even when it was clear that a gas-fired heater would be difficult to vent. When I called an electrician to run the circuit, they assumed this was a typical electric water heater, and tried to quote a complete service upgrade. Plumbers either quote extremely high because they expect to receive the rebates or just refer the work out to another plumber. For instance, Home Depot's contractor (when you select installation at checkout) quoted more than $7,000 to install a $2,000 water heater with existing electrical and very limited plumbing needed.
In heat pump-only mode and a 50 gallon tank, its recovery rate might be too slow for a family of 4 or more. The first hour rating of these water heaters are often below the tank capacity, so to meet code requirements, you might need to size up the tank from a gas water heater. I didn't realize this and inadvertently used a 50 gallon replacement, when I should have used a 65 gallon unit. I couldn't find any gas water heaters of this size, but it's a natural size step for heat pump water heaters due to its code-compliant first hour rating for small homes.
The heat pump doesn't work as efficiently when the air temperature is below about 40ºF. It doesn't usually get that cold in enclosed spaces where I live.
A heat pump water heater is more than twice as expensive as the same-size gas water heater and lasts the same amount of time due to tank corrosion.
Store warranties
If you buy a water heater from a home improvement store like Home Depot, be prepared to have a tough time if you need to exercise the warranty. The Rheem water heater I original bought came with a 7-year warranty and the unit developed a leak after about 3 years. I called the number of the tank, which took me to Rheem's support line. I sent them photos over email and then called them back to talk about the damage. They determined that the unit would need to be replaced, but then said that I had to work with Home Depot.
This is where things fell apart. First of all, the warranty provides replacement at cost, pro-rated to the remaining warranty. So you basically get a discount on a new water heater (but also an entirely new warranty). You also need to exchange the defective water heater for a new one to be able to apply the warranty.
When the water heater first failed, I mistakenly hired a plumber to just replace the water heater and take it back to Home Depot to be exchanged. That was a bad move; I should have made sure Home Depot had any heat pump water heaters in store. The plumber had to take the leaking water heater back and reinstall it, breaking the heat pump function in the process. Luckily, the leak was really slow and intermittent, so we could wait a bit longer.
Home Depot doesn't stock these water heaters in store and you can't get one shipped there without actually buying one. So I had to buy a new water heater, wait for it to be shipped to my house, and have a plumber replace the water heater and take the defective one to the closest Home Depot. Then, Home Depot needed me to come in and accept a store credit for the pro-rated warranty replacement cost. Home Depot's online store and retail locations use different inventory management and ordering systems, which makes dealing with this process more interesting.
Anode rod
A common complaint about this water heater is a sulfur smell from its water as its stock magnesium anode rod corrodes. Anode rods are crucial to extending the life of a tank water heater, but they need to be replaced about every two years because they're sacrificial and designed to deteriorate before the lining of the tank. One fix is to chlorinate the incoming water, but this still leaves me with the biannual rod replacement.
To avoid the potential for smelly water and hassle of replacing an anode rod down the road, I bought a Corro-Protec powered anode rod. The rod itself is a lot smaller than a conventional anode rod and needs electrical connections to the rod and the tank, to act as a ground. This was by far the most complicated part of the install and it's unclear if it was worth it.
The anode rod is located inside the heat pump chamber on the water heater, so the top cover plate needs to be removed to access it. The stock anode rod has a 1 1/16" bolt that's surrounded by a plastic cover, probably to protect the insulation around the tank. This plastic cover needs to be removed to make it possible to fit a socket around the bolt and let it turn freely. Some people use a Dremel tool to cut away the plastic, but this can be messy. The best way to do this is to use a small plastic spudger to lift up the edges of the plastic and then grab it with linesman pliers. I was able to twist the plastic out of the hole after 15 minutes of yanking and spinning.
The cavity into the unit is about a foot deep so the socket needs an extension bar to reach the bolt. Even with a 15" ratcheting breaker bar, I couldn't budge the bolt. I rented a 18V cordless impact driver and was able to dislodge it fairly easily. I put some teflon tape on the new anode rod and tightened it down with the ratchet.
The cavity where the anode rod is located is air-tight for efficiency, so I had to drill a ½" hole in the top cover plate to run the wire. The sides of the hole will be sharp, so it's a good idea to put a plastic bushing or masking tape to prevent fraying the cord. For a ground connection, I used the green screw that's used to ground the unit with the incoming circuit.
PEX plumbing
Copper, CPVC, and cross-linked polyethylene (PEX) piping are the current state-of-the-art for making plumbing connections for potable water. Properly soldering copper pipe connections can take months of practice, leave a mess, and fixes the water heater in place. CPVC is another option that's easier to install, but is also messy and relatively rigid. Because drywall has to be installed behind this unit after it's been inspected, I opted to use flexible PEX pipes. The PEX-B composition is said to leach less chemicals into the water, but no PEX pipe should be used where it could be exposed to the sun or UV light sources. PEX is also much less expensive than copper piping.
There are three typical ways to make connections between PEX lines:
Use a crimping tool to compress a collar over a PEX line wrapped around a barb from a fitting. The barb must be smaller than the inner diameter of the PEX, adding a narrow point in the plumbing and hurting circulation.
Use an expansion tool to open the PEX tubing over a fitting. Richard Trethewey from This Old House recommends this method for How to Replace Polybutylene Piping with PEX when the connections will be in the wall.
Use Sharkbite's push-to-connect fittings. These use a durable o-ring and hooks to make a water-tight seal around the outside of the pipe and prevent the PEX from pulling away from the fitting. It's unclear how many times they can be reused because of a special lubricant on the o-ring that's partially consumed on installation.
I opted for Sharkbite because of how easy it was install and all of the connections are outside of the wall, so they can be monitored.11 Sharkbite isn't recommended when the fittings will be hidden inside a wall. The fittings alone were about $10 each, and we needed about half-a-dozen elbows to accommodate the hot and cold water connections.
Other installation details
You probably need a disconnect switch on the hardwired electrical line coming from the wall for a 240V water heater.
It's unclear if a drain pan is necessary with the unit installed on a concrete slab, but a lightweight plastic pan helps scoot the unit around during installation. Make sure to attach the hose fitting to the pan so, in the event of a leak, a hose can be connected to drain the unit outside the garage.
If you live in an area prone to earthquakes, make sure to strap the tank to the wall with metal bands.
Add an expansion tank to the cold water line to avoid excess pressure in your home water lines. Also make sure the pressure relief valve on your main water line is within 60 to 80 PSI.