I’ve now had a year with the Geyser heat-pump water heater (HPWH). With the exception of the puddle on the floor in July 2011, it has performed consistently.
Its performance has not been thrilling, though. In the summer, it was making hot water at about 0.13 – 0.15 kWh/gallon, with incoming water in the mid-60°Fs and basement air temperature around 70°F. In the winter, with basement temperatures in the low to mid 50°Fs, and incoming water at 50°F or a bit below, this consumption ratio increased to 0.25 kWh/gallon.
I switched to using only the upper electric element in mid-January 2012, and the consumption ratio was 0.31 kWh/gallon, so the HPWH was saving about 20% — actually more, since the HPWH was heating the entire tank, and the electric element only was heating the upper 30% of the tank. This was verifiable by the way with the infrared camera — a sharp temperature gradient below the element location.
Better performance from a Stiebel Eltron unit
If I didn’t give myself the flexibility with the 85-gallon tank to do the HPWH or add solar thermal hot water I would have installed a 50-gallon Marathon instead. We have very good data from the Eliakim’s Way homes that show about 0.21 – 0.23 kWh/gallon. So over a year I’m not sure my HPWH and the larger tank saved me anything over a smaller electric water heater.
One very significant factor is our very low hot water usage of about 13 gallons/day. This means that the HPWH spends a significant portion of its operating time working against the standby losses, which means that it’s cycling in the 110°F – 120°F water temperature range, where it is least efficient. And of course that energy is not being used to heat hot water to replace hot water we’ve used.
I have data on another HPWH, the Accelera made by Stiebel Eltron.
It was installed late this past winter in a deep energy retrofit that South Mountain Company did on a small house in Chilmark. The basement had about R-20 walls and an R-3 floor. There is a ducted minisplit heat pump air handler and insulated ducts in the basement as well as the Stiebel Eltron.
The Stiebel Eltron has an 80-gallon tank which has the refrigerant heating coil wrapped around the outside of the tank beneath the insulation. It has a 1.7-kW backup electric element with a separate thermostat. This unit was set to make 130°F water. We installed a water meter on the cold water inlet and measured the electrical usage with the Powerhouse Dynamics eMonitor. Over the first six months, the household averaged 45 gallons/day of domestic hot water usage.
As in other Martha’s Vineyard homes, the incoming water temperature varies, starting at 50°F in early March and rising into the low 60°Fs in August. Basement temperature began in the upper 50°Fs and rose to the upper 60°Fs.
The HPWH made 7,980 gallons of hot water and used 477 kWh of electricity, a consumption ratio of 0.060 kWh/gallon — over three times more efficient than the 50-gallon Marathon tanks at Eliakim’s Way, which used 0.20 kWh/gallon over the same months in six houses that averaged 43 gallons per day.
This performance is in a whole other ballpark than that of the Geyser. Also, the unit seems to have low standby losses. On days with no usage it was using about 1/2 kWh. My biggest question is, how long will this expensive device (the list price is about $2,600) last?
Where does a heat-pump water heater make sense?
It’s worth noting that a HPWH takes heat from the house, at least during the heating season, so how you heat the house matters. Here are some cases to consider:
1 – The HPWH is in a basement with a gas furnace and leaky uninsulated ducts that keep the basement at 70°F. The HPWH is operating efficiently because it is taking heat from nice warm air, and that heat is only indirectly getting to the living space. Probably a good application.
2 – The HPWH is in the thermal envelope of a direct gain passive solar house with a wood stove backup. Again, the heat pump is operating in a favorable temperature regime, and the source of the heat is either the sun or firewood. And often during the winter the space may be overheated and the cooling is not objectionable.
3 – The HPWH is in the thermal envelope of an electrically heated house. Each unit of energy removed from the air is replaced by electric resistance heat. Not a good choice.
4 – The HPWH is in the thermal envelope of a house heated with minisplit heat pumps that operate at a COP of 2.5. The HPWH COP of 2 is effectively reduced to 1.4 because of the energy required by the heat pump to offset the cooling effect of the HPWH. If the house is in heating mode for six months of the year, and the rest of the time the cooling effect of the heat pump is negligible or welcome, then this changes to 1.7.
And finally, the more the climate shifts towards being cooling-dominated, the better the HPWH looks. A HPWH in your house in Florida supplies free cooling and dehumidification as it heats water.
A HPWH is also a dehumidifier
The other thing we’ve learned with the Stiebel Eltron HPWH is the effect it has on the basement humidity. We know a HPWH will both cool and remove moisture from the air, but we didn’t know if it would make that air higher or lower relative humidity. It could possibly cool the air and not remove enough moisture to keep the relative humidity from rising as the air was cooled.
Image 2 (below) shows data recorded during a 3 1/2 hour run of the HPWH, and the conditions of the air at the start and the end.
What we see is that the basement both cools and drops in relative humidity. As my friend and South Mountain colleague John Guadagno says, good stuff, good stuff! The reason it’s good is that the moisture content of materials is based on the relative humidity of the surrounding air, and lower moisture content means lower opportunity for mold. I agree with JG!
Water heating strategies
To sum up how I’m thinking on how to make domestic hot water, given my preference to think in terms of electrically powered buildings to mate with renewable power generation:
– Low DHW users, say up to 20 gallons/day, should use electric resistance in either a superinsulated tank or maybe distributed instantaneous electric heaters. (Caveat emptor: lots and lots of amps!)
– Medium DHW users, say 20 – 50 gallons/day, should consider a heat-pump water heater. Pick the highest efficiency and one with a large tank, which keeps the electric backup off.
– Large users of DHW should consider a solar thermal system. Look at the Wagner system, which is a clever packaged drainback system, as one possibility.
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.
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