You may know that heat pump (HP) technology has come to domestic hot water heaters (DHWs). Like all heat pumps, they use a refrigeration cycle to leverage latent ambient heat. This can result in half or even a quarter of the electric consumption used to create the same amount of hot water compared to traditional electric DHWs that use resistance coils. Unlike space heating heat pumps, whose performance is based on available outdoor heat, heat pump domestic hot water heaters (HP-DHWs) are designed to utilize the heat available inside your building.
Is a HP-DHW right for you?
It’s tempting; a HP-DHW will lower your annual hot water cost, which can be 15 to 18% of total home energy. That’s the second biggest portion after space heating! HP-DHWs can be pricey, so some states offer rebates. (A list of states offering HP-DHW rebates and a DHW usage cost comparison tool is here, sustainableheating.org/heat_pump_domestic_hot_water_heater.)
As a rule of thumb, HP-DHWs require 700ft2 of airflow around the appliance in order for the heat pump to function effectively. Various manufacturers may have different specific space requirements. While DHWs are typically placed in basements where there is plenty of ambient air, they may also be placed in locations as small as a utility closet, so some manufacturers offer add-on kits to vent the HP-DHW’s inlet and outlet ambient air to a larger room. Further on, I will explain how to hack those vent kits for air conditioning, but first, let’s set some general expectations. Different homes have different basement layouts, basement uses, and hot water consumption patterns.
These things matter. Here is why:
While the HP-DHW is using the available heat in your basement air, it is blowing out air that is 25 to 30°F below the basement’s ambient temperature. Think about your basement activities and whether they’re seasonal. What are the factors that influence temperature in your basement? Do you have any heat emitters down there or is winter heat a function of heat loss from the boiler, ductwork, and hydronic heating runs? How much cool air the HP-DHW generates will depend on your hot water use, the appliance’s recovery time, and the user-selected operating mode.
HP-DHW operating modes
Specific HP-DHW operating modes differ between manufacturers and models, but they are usually some version of the following.
Heat pump mode utilizes the refrigeration cycle exclusively. The electric resistance elements will not come on at all. This is the most energy efficient mode and also requires the longest hot water recovery time.
Hybrid mode prioritizes the heat pump refrigeration cycle and uses the electric resistance elements as little as possible.
This mode uses heat pump and electric resistance elements simultaneously. This results in the quickest recovery time while still providing some energy savings.
In this mode, the electric resistance elements do all the work. This results in the highest energy consumption. Electric mode is useful during heat pump maintenance and can also be used if the cooling effect or noise of the heat pump is unwanted as long as utility cost is not an issue.
Instead of turning off your hot water and returning home to a cold shower, the vacation setting allows you to set the HP-DHW to get up to temperature the day before you get home.
- Traditional stand-alone hot water makers are silent. HP-DHWs sound similar to a refrigerator.
- HP-DHWs require a drain for condensate. If they cannot be gravity-drained to a floor drain, a condensate pump will be required.
- The amount of dehumidification will be based on the appliance runtime and other factors. The dehumidification performance cannot be specifically set.
Now, about that air conditioning hack
In order to get around the 700ft2 airflow requirement for DHWs located in a utility closet, some HP-DHW manufacturers offer an add-on kit which allows you to attach ductwork to the inlet and outlet ambient air vents. Instead of operating with basement air, you draw clean air from conditioned spaces and blow cool, clean, dehumidified air back into living spaces. The fan inside a HP-DHW is not intended to push air through duct runs, so this hack requires the addition of an in-line booster fan and an electric outlet to power it.
In my personal experience, with two people living in a 2,000ft2 building, this provides some modest air conditioning and significant dehumidifying. On hot, humid summer days, this keeps the downstairs comfortable, but we still use air-conditioning in the upstairs bedrooms at night.
Winter operation (no air-conditioning)
In the winter time, we either unhook the ductwork so that the HP-DHW uses basement air, or we just turn it off and switch over to the old indirect hot water maker. We could also use the HP-DHW in electric mode, although this would increase our electric bill from about $15 per month to almost $50.
I have also seen people hack their way into bringing more hot air around the HP-DHW by placing the appliance near a commercial-grade refrigerator with a compressor that pumps out a lot more heat than consumer units. Another strategy is to locate the HP-DHW near the ambient heat from the boiler or furnace, or figure out some other creative, site-specific heat reclamation strategy.
Thinking through your HP-DHW project
There is no one-size-fits-all answer. Adding any hot water maker requires a plumbing and heating professional. That said, there’s plenty you can do to plan the project. Think through placement and position of the tank to minimize duct bends, hot water runs, and condensate drainage while still allowing access to HP-DHW controls. If you’re handy, you may be able to self-install the flexible duct runs, in-line fan, floor penetrations, and registers.
We are at a moment in time when new HP-DHW technologies enable us to reduce our energy consumption. Understanding our options and applying them creatively allows us to save money and reduce pollution.
Jeff Rubin is Executive Director at Sustainable Heating Outreach & Education, a nonprofit 501(c)(3) sustainableheating.org.