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Getting your Trinity Audio player ready...This is going to be a pretty short blog post because it turns out it's not all that interesting living with a heat pump water heater. It works, it's quiet, you pretty much forget about it. When I ask my parents about theirs, it's clear they haven't given it any thought at all since it was installed. It just works. But I will cover the sound, the electricity use, and the cost of my installation in some detail for those who are interested.
In May I installed an 80-gallon, 15-amp Ruud heat pump water heater in a closet in my garage, replacing a gas tank water heater that was in there. I got the 15-amp model to save room in the electric panel (vs the more traditional 30-amp model), and I got the large 80-gallon model 'just in case'. (That was definitely overkill. A 65 gallon model would have been more than enough for our house.) When first installed it was louder than spec'd, but I reached out to Ruud and the contractor put in some sound-proofing as I described in an earlier blog post.
Since then, I've noticed the sound maybe twice, each time when I was in the garage and heard a low hum. I have never noticed it from inside the house. What is the sound like? If I stand in the closet right next to the heat pump water heater, the sound is a low-frequency hum of about 60 dB. Below you can see the sound meter reading, and you can listen to it here. Six feet away, holding the closet door open, the sound is around 54 dB.
If the door to the water heater closet is closed, then the sound in the garage is a quiet low hum, anywhere from 42 to 48 dB depending on where the sound is measured. You can see a meter reading below from about 8 feet from the door, and listen to the sound here and here.
From inside the house, in the family room adjacent to the back wall of the closet, you can just barely hear a hum if you listen very hard. I have never noticed it, and neither has my keen-eared daughter. The sound meter might read 38 instead of 35 when there is no other ambient noise. You can listen to it here, though I couldn't hear anything when I made that video. But when I listened very carefully as I used the water heater app to turn the heater off, I could hear a slight decrease in sound. (1)
Operationally the water heater doesn't use much electricity. If you look at the specs for the appliance, it generates BTU/hour of power when the compressor (heat pump) is running. That is equivalent to watts (1 watt = 3.41 BTU/hour).
Since the heat pump generates about 3.7 times more power than it takes in, that means it uses about /3.7 = 333 watts. That is about the same as a coffee maker or plasma TV, according to this chart from Silicon Valley Power.
If a house has used a lot of water and the tank is substantially emptied, the resistance element will kick in to quickly fill up the tank. (2) That element uses about seven times as much electricity as the compressor, or watts, and would use double that if you have a 30-amp model! That is why you don't want to have a very small tank. With an enormous 80-gallon tank and relatively low household hot water usage, our resistance element has never turned on in the 3+ months the water heater has been running. (3)
In practice, what does this look like? Below is a chart showing a typical day's power draw (in kW). The pump ran three times for about an hour each at around 400 watts. Most days it runs at around 6am and 9pm because we have a recirc line ' hot water is sent through a recirculating line to minimize wait time for hot water in mornings and at night. It is on a timer and runs from 6-7:30am and from 9-10pm. You can see the heat pump's compressor operating during that interval. During a busy day with several showers the heat pump water heater might run for 6 hours instead of 3-4.
Below is a chart showing the water heater's daily energy usage (in kWh) over the last 60 days. (Can you see when my daughter and her long showers came home?)
If you look at the last 12 days, energy use averaged about 1.7 kWh/day. Over the last 60 days, it averaged about 1.4 kWh/day. That is lower than I expected based on my gas water heater use. Last year's utility bills suggested the gas water heater was using between one-third and one-half of a therm per day. That translates to between 10 and 15 kWh/day. But because the gas heater was only 0.63 efficient, and the hybrid heater is 3.7 efficient, I expected the hybrid heater to use between 1.7 and 2.6 kWh/day (10 * 0.63 / 3.7 and 15 * 0.63 / 3.7). But it's using less. The weather has been warm, meaning the heater is running even more efficiently than average. I expect that during the winter, daily energy use will get closer to 2.5 kWh, especially when my daughter is home!
The cost to run the water heater for a month, at Palo Alto's Tier 2 rate of $0.20/kWh, is somewhere between $9/month (if using 1.5 kWh/day) and $15/month (if using 2.5 kWh/day). Our gas water heater, at Palo Alto's Tier 2 rate of around $2.20, cost between $22/month (if using one-third therm per day) and $33/month (if using one-half therm per day). At the Tier 1 rate of around $1.30, the cost would be between $13 and $20 per month. So the new heater is cheaper to operate, but it's pretty negligible since we don't use all that much hot water.
The only downside of this whole project was the installation cost. It cost about $ net to install. Materials were about $, labor was about $, the permit was around $500, extra soundproofing was around $500, and Palo Alto's rebate was $. (4) That cost is okay for some households but absolutely not okay for others. Higher incentives are available in some areas, but costs must go down as more people install these. Sunwork installations are more moderately priced. The new federal incentives will help push the market along as will more aggressive state and local building codes.
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Rheem/Ruud has two new 'plug-in' models that should cost less to install because they require less electrical work. The HydroBoost seems to be the best fit for our area. It needs only a standard plug, and it uses higher water temperatures and a mixing valve to increase the effective capacity of the tank so that it is less likely to run out of water. The tank has no resistance element, so it refills at a rate of only about 12 gallons per hour. The other model they offer has an extra-large heat pump that fills the tank at a rapid 28 gallons per hour. However, it requires a dedicated circuit and I worry that the powerful compressor will be loud, though I haven't heard any reports. If you are interested in that model, I would ask about the sound.
Anyway, my heat pump water heater has been a non-event since it was installed. We don't notice it, it works great, and the utility bills are slightly lower. I think in five years we are going to think it strange that anyone would bother to read a blog post about these appliances. They will just be standard and no one will care much about them. Palo Alto has already taken some steps in that direction by requiring remodels and additions to use them. It makes sense to avoid installing new long-lasting gas appliances when practical, as it is in these cases. With gas water heaters contributing about one-third of our household gas use in this area, switching to the heat pump water heaters can make a big dent in building emissions.
I'd love to hear your experience if you have installed one, or any questions you may have if you are considering one.
Notes and References
1. The wall between the closet and the family room is insulated and also has QuietRock on the garage side, glued with some sound-proofing glue. I'm not sure how much of a difference that made.
2. I am operating the water heater in the recommended 'Energy Saver' mode. The 'High Demand' mode will cause the resistance element to kick in more aggressively, for example if you have house guests and need to accommodate an unusual number of showers or other hot-water loads.
3. I know this because I have an Eyedro circuit monitor on the water heater circuit, so I can see how much electricity it is using and when. I recommend installing one of these if you get a heat pump appliance, since it can help you to understand your usage and ultimately your electricity bill.
4. These costs are approximate because the electrical work was combined with other things, as was the permit. The cost is also on the high side because the contractor did a bunch of drywall work in the garage in addition to installing the heat pump water heater.
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This is part of an expert best practices web guide to Heat Pump Water Heaters published by the ENERGY STAR Residential New Construction program. Read Part I of the guide.
Heat Pump Water Heaters are designed to be used in spaces with ambient temperatures ranging from about 40 to 120 degrees Fahrenheit. This can include garages, basements, laundry rooms, and closets with louvered doors. The warmer the air, the more efficient the operation. HPWHs should always be protected from the elements.
The sound of a Heat Pump Water Heater is similar to that of a dishwasher'between 45 dBA and 55 dBA'which may be noticeable if located near a bedroom. Since Heat Pump Water Heaters will also have a localized cooling effect when the heat pump is running, do not place them in frequently occupied areas with temperature-sensitive activities.
The key to successful Heat Pump Water Heater operation is to provide enough air for the water heater. Nothing needs to be done if the space around the Heat Pump Water Heater meets manufacturer requirements (450 to 700 cubic feet). If installed in a small closet, a full louvered door can be used to circulate air into a larger space. If grilles are used, they should provide a total of 240 square inches of 'free air,' which is the space between the fins. Placing a grille at both the top and bottom of a door or closet is the best application. Alternatively, in small closets, ducting can be used for the intake and/or exhaust air from another nearby location.
In cold climate regions where water pipes can freeze, integrated Heat Pump Water Heaters should not be installed outside. If located in a warm climate, a Heat Pump Water Heater may be installed in a carport, garage or an attached outdoor closet provided with properly sized vents or louvers.
Heat Pump Water Heaters can be used in conjunction with solar thermal. Refer to the manufacturer's recommendations for installation methods.
If the power goes out, a Heat Pump Water Heater will continue to supply the remaining hot water in the storage tank. However, it will no longer heat hot water because neither the heat pump nor the resistance heating element will work without electrical power. When power is restored after the outage, the water heater will automatically revert back to the most recent user settings. In cases where a home is also equipped with batteries and disconnect from the grid, the Heat Pump Water Heater can often continue to run in heat pump-only mode as it draws less than 500W of electricity.
Credit: Efficiency Maine
Heat pump water heaters have a fan and compressor, both of which make some noise. When installed in less frequently occupied areas'like garages and basements'most people are unbothered, or even unaware, of the sound. However, noise perception varies between individuals. To reduce the impact of noise, add sound insulation to the space where the water heater is installed or select a quieter model. The ENERGY STAR NextGen program requires that heat pump water heaters in occupiable spaces have sound at levels less than 55 dBA, which is the volume of a background conversation. Even quieter models (rated at 45 dBA and less) are also available.
Studies have shown that Heat Pump Water Heaters often do not significantly increase heating system runtime when installed in conditioned space because their cooling impacts are small and localized. Interactions are minimized when the Heat Pump Water Heater is installed farther away from the thermostat and air handler, since the cooling from the Heat Pump Water Heater is not mixed with the rest of the home. When a Heat Pump Water Heater is actively heating water in heat pump mode, it generally provides 2,500 to 5,000 Btus/hour of cooling to the surrounding air. With an efficient space-heating system, energy savings from a properly installed Heat Pump Water Heater will exceed any marginal space-heating energy increases, and in the summer, the home will gain some 'free' localized cooling and dehumidification as heat is moved from inside the home and put into the Heat Pump Water Heater. Even when there is some interaction, the Heat Pump Water Heater overall provides energy savings compared to less efficient alternatives like gas and electric tank and tankless units. The small impact on heating and cooling is especially beneficial in warm climates.
Heat Pump Water Heaters have been available for more than 40 years. Current Heat Pump Water Heaters are designed like refrigerators to operate as a fully sealed system. Heat Pump Water Heaters have similar life spans to electric storage water heaters, and many come with 10-year warranties.
Heat Pump Water Heaters have hot water delivery capability comparable to (or greater than) electric-resistance tanks. Follow the tank-sizing guidelines on page 8 and review each model's First Hour Rating. Thermostatic mixing valves can also be installed to allow a higher set point on the water heater and increase the total capacity of the overall domestic hot water system.
If space, budget, and location allow, a larger tank is typically a better choice. This allows more water to be heated during off-peak times for future hot water use at a later time. A larger tank size will also rely more on the heat pump rather than using the less efficient resistance heating element. The ENERGY STAR NextGen program requires a minimum tank size based on the number of bedrooms.
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