Step 25b – Install the Heat Pump Water Heater
The beautiful efficiency of a heat pump can not only be used to heat the house, but also to heat the water. A heat pump water heater is the most efficient type of water heater on the market today. One of the interesting things about a heat pump water heater is it’s versatility. In a hot climate, it is the 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. Being 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, so if any heat escapes from the tank it will only make the house warmer.
The specific type of heat pump water heater I bought is called a “hybrid” 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 case that the air outside is too cold for the heat pump to function. The Rheem heat pump is able to function properly even at 37 degrees F. While there are definitely days here where the temperature drops below 37, it is quite rare that the high temperature for the day is below 37 degrees. 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, so as long as the temperature is above 37 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.
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 do not allow PEX to be connected directly to the water heater so the 18 inch long flexible metal is necessary. The other side I ran into 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 it’s 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 sewer gases don’t run out of it. As you can see below, the code allows you to run 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 indirect waste 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 condensation 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 and ensures that 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 at high speeds.
The next part of the installation was to install the electrical wiring. I had already ran a 30 amp circuit to the utility room way back when I had run all 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 vents. 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.