Heat Pump Water Heating: New Washington State Standard

Washington State the first to require all-electric water heating

By Tyler Van Dooren, PE – Mechanical Engineer | Seattle Office

In an effort to meet future climate goals, the Washington State Building Code Council voted to update the state energy code to require all-electric water heating in new multifamily and commercial buildings. Washington is the first state to do so. The requirement will go into effect in 2023 and is already in effect in the City of Seattle.

Multifamily and Commercial properties will need central heat pumps

Practically, this means most buildings will need to heat water primarily with heat pumps. This is a familiar concept for single-family homes or townhomes, where small heat pump water heaters are widely available and used frequently in new construction. However, the scale of heat pumps needed for large multifamily and commercial products requires a significantly different system than the small-scale heat pump water heaters found in standalone homes.

Design considerations for central hot water plants

There are additional factors to consider when designing a central hot water plant with heat pumps. The first is heat pump performance varies with the ambient temperature conditions of its location. The colder the air temperature, the lower the heat pump capacity. For buildings with an enclosed parking garage, it is often recommended to place the heat pumps there where temperatures typically don’t fall below 40-45°F. However, if the heat pumps are outdoors, they will either need to be sized for much colder design temperatures (creating a larger plant) or supplemented with electric heat. For example, Washington State Code requires heat pumps to provide 100% or 50% of a building’s water heating demand at an ambient temperature of 40°F or 24°F, respectively. 

A second important consideration when designing a heat pump plant is sizing storage tanks properly. Often, the overall capacity of a heat pump plant is limited (due to space concerns, cost, or equipment limitations), and may not be able to provide enough heating to keep up with the highest demand times of the day. The heat pump itself is decoupled from the main hot water circulating loop via the storage tank and can steadily feed heat into the storage tank throughout the day. Using a large storage tank can provide enough buffer to accommodate peak demands. This can lower project costs (tanks are less expensive than heat pumps) and protect the owner from peak demand electricity costs.

In addition to the previously discussed factors, other topics to consider when designing a heat pump plant include piping configuration, single vs. multi-pass heat pumps, and storage tank orientation.

Planning for the future

Multiple jurisdictions, including California, New York City, and several other cities, are now considering or have implemented this same all-electric water heating requirement. As this becomes more common, owners, property managers, and plumbing design engineers will need to be familiar with these systems and concepts.