We are increasingly considering all-electric designs for homes. This primarily stems from two sources:
Several counties and cities adopt reach codes (local requirements to exceed the state code). Many of these reach codes mandate all-electric designs.
A map of these reach codes can be found here.
Electrification of buildings in order to minimize carbon emissions and the impact of buildings on carbon footprints.
This post will focus on all-electric home design, with an emphasis on the impact of electric water heaters on Title 24 compliance.
Note: This post is specifically focused on the 2019 Energy Code and does not consider the 2022 Energy Code (which goes into effect on January 1, 2023). Subsequent posts will cover hot water heating for this energy code.
Executive Summary
We ran nearly 3 dozen Title 24 analyses on an example new construction ADU across all California climate zones, considering both tankless electric water heaters and electric heat pump water heaters. We found that electric heat pump water heaters are more effective at passing Title 24 code than electric tankless water heaters in all cases. Furthermore, when we attempted to get a tankless electric water heater to pass the code, we found that significant, costly improvements to the construction of a home still did not allow the tankless electric water heater to pass.
When considering all-electric residential buildings under the 2019 energy code, we recommend utilizing electric heat pump water heaters in all cases. This is further supported by the 2022 energy code (effective Jan 1, 2023), which considers electric heat pump water heaters as the standard for hot water heating.
Example Home
We will utilize an example home to demonstrate the effect of electric water heaters on Title 24 compliance.
Let's consider a new 749 sqft detached ADU with the following insulation values:
Exterior Walls: R-15 Batt Insulation. Wooden 2x4 Stud Framing @ 16" O.C.
Ceiling: R-30 Batt in an Attic Roof. Wooden 2x4 Framing @ 16" O.C.
The foundation is slab-on-grade and the front wall faces SW and the overall glazing percentage is 18.3%. The owner plans to add new panels onto the main home to meet the solar requirement for the ADU (read more about this here) and use a variable capacity heat pump (ductless mini-split) in the home for space conditioning.
Water Heater Types
The owner is primarily concerned about the type of water heater that they will use. Due to a local reach code, they must use an all-electric design and have been discussing two potential options:
Tankless Electric Water Heater - An instantaneous water heater that provides hot water only as it is needed. Here is an example.
Electric Heat Pump Water Heater - A large water heater that heats water by moving it from one place (the surrounding air) to the water held in the unit's storage tank. Here is an example.
Here is an excellent write-up on heat pump water heaters that dives a bit more into how they work.
Even within the same "type" of water heater, not all water heaters are made alike, and this is especially true of heat pump water heaters. When considering heat pump water heaters, there are three main things that affect Title 24 compliance:
The efficiency (referred to as the Coefficient of Performance)
A higher value reflects a water heater that is more efficient.
The heat pump type (NEAA rated or not)
Note that using an NEAA-rated water heater provides a large credit for Title 24 compliance. However, this also means that the specific brand/model (or equivalent) must be installed and will likely be verified by a building inspector.
The location of the water heater
For our purposes, we will consider the most conservative case, that the water heater is located outside. Note that it is more common to install a heat pump water heater in a garage or mechanical closet.
Analysis Set-Up
We will run thirty-two individual Title 24 analyses on this home, one for each climate zone across all
sixteen California Climate Zones, and change nothing else except for the water heaters. For the purposes of this analysis, we will consider the two water heaters linked above, namely:
For the tankless electric water heater, we will use the Bosch WH17 as an example.
For the electric heat pump water heater, we will use the Rheem XE65T10H45U0 as an example. Note that this water heater is NEAA rated.
As previously noted, we will assume the water heaters are installed outside the home.
Results & Discussion
Comparison between Water Heater Types
The figure below shows the resulting Efficiency Design Rating (EDRe) of each model. Recall that EDRe is the "efficiency" EDR metric which includes energy used by the envelope, IAQ, HVAC, DHW, and unregulated loads but does not include photovoltaics or batteries.
When reviewing this figure, we can note three important things:
In no case does an electric tankless water heater pass - all cases show failure.
Heat pump water heaters are more effective at passing Title 24 code than electric tankless water heaters in all cases.
Heat pump water heaters are not a panacea! While they are more efficient than electric tankless water heaters in all cases, they prove to be even more effective in specific climate zones.
The third point is easier to see in the next figure, which shows the difference between the EDRe of the heat pump water heater and the electric tankless water heater. This is calculated by taking the EDRe of the heat pump water heater and subtracting the EDRe of the tankless water heater in the same climate zone.
What would it take to get an electric tankless water heater to pass Title 24?
We are regularly asked - what would it take to get an electric tankless water heater to pass Title 24?
Let's test out an example of the same example home in climate zone 3, where the heat pump water heater already allows it the project to pass Title 24 code. In this case, we will consider increasing the insulation values as follows:
Exterior Walls:
Originally R-15 Batt Insulation. Wooden 2x4 Stud Framing @ 16" O.C.
Now, R-38 Spray Foam Insulation with R-10 rigid foam insulation. Adjust framing to be wooden 2x6 Stud Framing @ 16" O.C.
Ceiling:
Originally: R-30 Batt in an Attic Roof. Wooden 2x4 Framing @ 16" O.C.
Now, R-38 Batt in an Attic Roof with R-10 Insulation Under the Roof Deck. No change to framing.
Additionally, let us call for HERS verified Quality Insulation Installation (QII) for all insulation installed. Further, we will add in a 13.5 kWh battery storage system with a time-of-use control strategy
With all of these measures, we increase our EDRe compliance margin from -15.5 to -11 (an improvement in EDRe compliance of 4.5) meaning we still don't pass! In order to get this to pass, we would have to consider additional costly measures that will continue to pass on the additional cost to the project owner.
In contrast, for the same project, the improvement in EDRe compliance when changing from an electric tankless water heater to an electric heat pump water heater is 16.1!
Summary
When considering all-electric residential buildings under the 2019 energy code, we recommend utilizing electric heat pump water heaters in all cases.
C&S Energy Pro is committed to getting a fast turnaround on your Title 24 documents which will help ensure construction projects are completed on time and that consumers will be able to move into their homes quicker. Avoid the hassle of long wait times generally associated with Title 24 documentation and call us today!
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