In my last blog, I discussed the basics of our domestic hot water (DHW) load, and looked briefly at adding a solar hot water system to satisfy most of that load. What I decided to do first was to try a heat-pump water heater (HPWH), partly because it was a much simpler and less costly installation, and partly because I was just curious to see how well one would work.
Since I already had the 85-gallon Marathon electric water heater, I didn’t need additional storage, so I looked at the add-on products available and chose the Geyser, which is made in Maine by Nyle Systems. It costs in the range of $1,000, plus installation.
An add-on heat-pump water heater
The Geyser is a small cube (about 1 1/2 feet on a side) which can sit on the floor or on a platform adjacent to the water heater tank. The way it (and other HPWHs) operates is that it removes heat from the air in the space in which it is located and transfers that heat to the water.
The Geyser has a compressor to operate the refrigeration cycle to take the heat out of the air (like an air conditioner or dehumidifier), a fan to move air across its refrigerant coil, and a pump to circulate water to and from the adjacent storage tank. It connects to the tank at or near the bottom, so it heats the coldest water, maximizing the efficiency of the heat pump. Image 2 (below) shows a the interior of the Geyser, including its key components.
As a HPWH extracts heat from the air, it also cools it, which means that it acts as a dehumidifier as well, with one key difference. Conventional dehumidifiers cool the air, condense moisture out, and then reheat the dehumidified air, so they remove moisture and add heat to the air. Since warming the air lowers its relative humidity, the twin effects of moisture removal and heat addition are exactly what we want — lower relative humidity means lower moisture absorption by materials in the space being dehumidified, and therefore less mold.
A HPWH removes moisture from the air, but cools it rather than heats it, because the heat goes into the DHW, so it isn’t as effective a dehumidifier. I put a Hobo four-channel data logger in place to look at the effects of the HPWH on the basement environment.
The normal Geyser installation uses a clever tube-in-tube fitting that is inserted in the drain fitting on a conventional electric water heater (or other water heater tank). This allows the Geyser to both extract water to be heated, and return heated water through the same port on the water heater.
Removing the Marathon’s lower heating element
In the case of the Marathon, the drain port location precludes this. I chose to do something a bit unconventional — I removed the lower electric heating element (residential electric water heaters have an upper element and a lower one, and only one operates at a time) and used an adapter to connect the heated water returning from the Geyser to this location. The water to be heated comes from the drain connection.
This approach loses the functionality of having the lower element as back-up to the HPWH — only the top element is usable. The Geyser uses the existing line voltage thermostat on the water heater to tell it when to operate. In a typical electric water heater, the controls first ensure that the top thermostat, that controls the upper element, is satisfied, then the lower thermostat and the lower element, which properly prioritizes the water at the top of the tank, which is drawn off first.
In my set-up, once the upper thermostat is satisfied, the power is switched to the lower thermostat, which can call for the Geyser to operate if needed. I’ve set the upper thermostat to about 90°F, and the lower thermostat to 120°F, which forces the heat pump to do all of the water heating.
I installed the Geyser on July 5, 2011, and we haven’t used any power for electric resistance heating since then; all water heating has been done by the Geyser. Note that HPWHs are slow heaters relative to typical water heaters. The Geyser’s output is under 2 kW, whereas an electric water heater is typically 4.5 kW, and gas or oil heaters are much more. This is why the tank size is large for a HPWH. We’ve had a house full of guests a number of times this summer and fall, and no problem with enough DHW.
Monitoring performance with water meters and a Kill-a-Watt meter
Since this is a new technology for me, I wanted to understand how well it was working. I installed a DLJ75 water meter on the cold water feed line to the water heater, before it splits to the thermostatic mixing valve, so I would be able to measure how much DHW we were using. This is a simple, time-tested mechanical water meter. We’re installing a couple of others right now that have a pulse output that can be counted and totaled, so they won’t need to be read by a human daily to get a sense of the usage patterns.
The Geyser is a 120-volt machine that simply plugs into an outlet, so I’m tracking its electrical usage with a Kill-A-Watt meter. I also have been using the two exterior channels of my Hobo logger to measure cold water inlet and DHW outlet temperatures. With flow and temperature differential I can calculate the energy demand of the DHW system, and with the kWh I can calculate the system efficiency at satisfying that energy.
Handling the condensate
Like dehumidifiers, HPWHs remove moisture from the air and therefore generate liquid water as that water vapor condenses on the coil. The Geyser has a drain pan to catch this condensate and a tap that can be connected to a plumbing trap or a condensate pump. Mine is simply draining to a 5-gallon pail.
Interestingly enough, in the middle of the summer, when the basement temperature and relative humidity was highest, there was a puddle on the concrete below the unit, which went away as we left the peak cooling season behind. I tried to determine the issue and the folks at Nyle were very helpful, but in the end I think that somehow there is a leak in the drain pan, probably at the corners, that only is operative when the rate of condensate is high. Alternatively, there may be condensation on a component that is not above the drain pan. I may need to wait to next summer to resolve this one.
I have been pleased that the machine is fairly quiet, and so far, completely reliable.
Add-on or integrated unit?
If I had decided at the beginning that I wanted to install a HPWH, I probably would have selected an integrated unit, that has the heat pump built-in on top of a storage tank. These are made by Stiebel Eltron, GE, A.O. Smith, and Hubbell, amongst others.
There is an excellent report on the field performance of HPWHs authored by Steven Winter Associates.
In the next post, I will comment on other aspects of the DHW installation; then in a subsequent post I’ll share some data on the performance of the HPWH and its effect on the temperature and moisture level of the basement air.
[Editor’s note: This series will continue; stay tuned for “Getting into Hot Water — Part 3.”]
Marc Rosenbaum is director of engineering at South Mountain Company on the island of Martha’s Vineyard in Massachusetts. He writes a blog called Thriving on Low Carbon.