One of the recurring joys of home ownership is plumbing. In 2021, the water heater in our home developed a small leak. Fortunately, the leak was small enough where we had some time to work out the next steps. Our long-term goal for our home is fully run on electricity for both comfort and climate reasons.

The domestic hot water system that existed in the home when we bought it used the indirect heating loop in the oil boiler and an unplugged electrical water heater as a storage tank. The boiler was setup to maintain a set point between 120F and 170F all year round. Looking at the summer usage of the boiler, we used between 0.5 and 1 gallon of oil per day to keep the boiler hot. Extrapolating this, I estimate that heating water was burning around a full tank of oil per year – around $900/yr at the $3.50/gal price in Feb 2022.

Installation

The first step in installing the HPWH was to have our electrician run a 240V 30A circuit to the location. Our electrician did this some weeks prior to the water heater’s arrival. Although the water heater’s normal consumption is around 400W, it has traditional heating elements which turn on in a few different scenarios:

• Hot water utilization is too high for the heatpump to keep up
• The ambient temperature is too low in the area the unit is installed.
• The heat pump has some mechanical problem or coolant leak which prevents the heat pump from operating.

The subsequent steps were performed by a local plumber. The existing storage tank and hardware were removed , and the indirect loop on my boiler was capped off. The new hot water heater was connected to the existing hot water line. Finally, a condensate pump and flexible tubing were connected to move the condensate created by the cold side of the heat pump to a drain.

The version of the water heater that I bought has an automatic shutoff valve and a leak detection circuit, so we installed the unit onto a drip pan. This will contain any water which may spill from a leak, and concentrate it so the leak detection circuit shuts off the cold water input.

As I recall, the manual suggests connecting the water heater to the rest of the house using a flexible line, however the plumber who installed it used rigid copper for the connections. In addition, the manual suggests using a heat trap on the cold water input. This was not done in my installation, but is on my to-do list to correct. I’d suggest reading the manual and reviewing the installation plan with the installer prior to commencing work. This isn’t to complain about our installer – they were good to work with and did clean work at a reasonable price on a fast schedule.

Costs

It always annoys me when I read great writeups of projects, and they skip the installation costs. Of course, this happened in 2021, and prices are strongly location dependent.

At the time, the Chase Sapphire Card had a Pay Yourself Back option at Home Depot. I was able to use credit card points to pay for the water heater and the other items.

The installation costs:

In 2021 Massachusetts, MassSave, the state efficiency program offered a $600 rebate if you installed a heat pump water heater. Since then, both the state and feds offer even more generous rebates to install these efficient systems.

Performance

We have owned the water heater for approximately three years at the time of writing. The elements on the tank kick on ~5 times per month on average, mostly during the wintertime when multiple sequential showers happen. The elements have been turning on more frequently since last June 2023 when I replaced our highly efficient shower head with a model which uses a much higher flow. You can see in the plot below that the summertime usage is higher after this. One item to note is that the power monitoring system was not hooked up during August / September 2022 while our electrical panel was getting upgraded.

It’s also interesting to look at a single cycle. In the plot below, you can see how the power used by the water heater increases with time as the water in the tank gets hotter.

Finally, this is a plot of a water heating cycle where the electric elements kicked on during a particularly long heating cycle during the winter. I’m not certain what was happening at this time, but I suspect several showers were taken along with dishes and laundry. There are multiple times where the HPWH power usage goes down – indicating that the temperature in the tank is lower.

Carbon Footprint

On another project, we calculated that our old boiler system used approximately a full tank of oil each year. This was done using data from a Smart Oil Gauge which reports the oil level in my tank hourly all year round. We looked back at the usage from the summertime before we replaced the water storage tank with our HPWH, and extrapolated the usage from that to the full year. Not exactly a precise measurement, given that the boiler is on in the winter time anyway but it’s a reasonable approximation.

The Energy Information Administration publishs the carbon emission equivalent for a number of fuels. Diesel or Fuel oil releases approximately 22.5 pounds of carbon dioxide per gallon. Using the estimation that heating water for domestic use burned a full 275 gallon tank of oil annually, we can calculate that hot water caused around three tons of carbon emissions from my household.

EIA also publishes the carbon intensity of the Massachusetts grid as around 952lbs per MWh. Our water heater used 700kWh (0.7MWh) in 2024, making our carbon dioxide emissions from the HPWH around 666 lbs of carbon – or around an 89% reduction in carbon dioxide! We have a PPA agreement in place to purchase 100% renewable power, which should reduce that further – though that’s mostly an accounting trick. In addition, we have solar panels on our roof which should reduce our usage further.

Conclusion

We’ve been quite happy with our Rheem Proterra HPWH so far. Others online seem to have reliability issues with the newer units, but we have not encountered any problem with ours yet. Our unit provides the hot water that we need reliably and efficiently. Hopefully we will not have to exercise the 10 year warranty!