A heat pump can not only be used to heat a house, but also to heat domestic water. In fact, heat-pump water heaters are among the most efficient type of water heaters on the market today.
One of the interesting things about a heat-pump water heater is its versatility. In a hot climate, it is best practice to place the heat pump indoors. This way, it will pull heat from inside the house and use it to heat the water. In a cold climate, however, it is best to put the heat pump either in a garage or use ducts so it draws air from the outside. This way it doesn’t steal any heat from inside the house.
Because I live in a colder climate, I decided to place my heat-pump water heater indoors and duct it to the outside. This allows me to have the best of both worlds. The heat is drawn from the outside, so it doesn’t steal precious heat from inside the house, yet the tank of hot water is indoors; if any heat escapes from the tank it will only make the house warmer.
I bought a hybrid heat-pump water heater. Much like a hybrid vehicle, which has the ability to either power itself with gasoline or a charged battery, the hybrid water heater can heat water with either the heat pump, or a traditional electric heating element. Ideally, one would select the proper setting to run the hybrid water heater in only-heat-pump mode, and utilize the traditional heating element only in the extreme event that outside air is too cold for the heat pump to function.
The Rheem heat pump is able to function properly even at 37°F. While there are definitely days here where the temperature drops below 37°F, it is quite rare that the high temperature for the day is below that. Since the water heater has a 50-gallon tank, it only needs to function for a few hours a day to heat the water in the tank. As long as the temperature is above 37°F for a few hours each day it should operate just fine on the heat pump setting. That having said, it is always nice to have a backup plan.
Installing the water heater
Installation of a heat-pump water heater is basic plumbing, electrical, and HVAC all in one. The first step is to run the water lines. Way back when I ran the PEX for the water supply lines in the house, I had run a line from the main supply, formed it into a loop in the utility room, and then into a manifold. From this manifold, I ran PEX to every fixture needing hot water in the house.
The loop was necessary for this very step. I cut the loop, and attached one side of it to a flexible metal line that ran to the hot water outlet on the water heater. Building codes in my area do not allow PEX to be connected directly to the water heater so the 18-inch-long flexible metal is necessary. I ran the other side to an expansion tank, then to a shutoff valve, and then into the flexible metal for the cold water inlet of the water heater.
The expansion tank is important to allow room for the water to expand and contract as it heats and cools. As we learned when discussing the heat pump, the volume and temperature of a liquid are directly related, so when the temperature of a liquid rises, so does its volume. Without an expansion tank, the water in the supply lines will create excess pressure as it is heated and cools and can cause the PEX to deteriorate faster.
I installed the waste lines next. Earlier in the build, when I was installing the DWV plumbing for the house, I had installed an indirect waste receptor in the utility room. An indirect waste receptor is simply an open pipe with a trap to ensure that sewer gases don’t run out of it. As you can see below, the code allows waste lines into the open pipe as long as they have an air gap. This ensures that if for whatever reason the pipe gets full of water, no water can be siphoned out of the pipe by one of the waste lines.
I directed a total of three waste lines into the receptor. The first was from the drain pan, which I had placed the water heater in. The second was from the condensate line. All heat pumps create a fair amount of condensation due to the temperature changes and the heat-pump water heater is designed to catch that condensate and direct it towards this pipe.
The last waste line was for the temperature/pressure relief valve. This valve is found on all modern water heaters. If for whatever reason the pressure or temperature in the water tank gets too high, the valve will open and release the pressure by directing water into the pipe. Without this valve, pressure could possibly get so high in the tank that it would rupture, and scalding hot water would be released.
The next part of the installation was to install the electrical wiring. I had already run a 30-amp circuit to the utility room way back when I did all of the wiring for the house, so it was simply a matter of connecting some conduit to the box, running wire through it, and connecting the wires.
The last step was to install the ductwork. I had already installed the vents for both the intake and exhaust for the water heater, using the same vents that I had used for the ERV. All that was left now was to simply connect one end of some insulated flexible duct to the vent, and the other end to the water heater.
This is one in a series of blogs detailing the construction of a net-zero-energy house in Point Roberts, Washington, by owner-builder Matt Bath. You’ll find Matt’s full blog, Saving Sustainably, here. If you want to follow project costs, you can keep an eye on a budget worksheet here. All photos courtesy of the author.
Previous posts by Matt Bath: