Thanks for the question. I will say that I have used them in single family, multi-family, and ADU projects. We used to use on demand gas water heaters, typically mounted on the exterior of buildings, low cost, space efficient, relatively quiet, quick.... During the last few years, as Berkeley was in its no gas phase, we switched to heat pump water heaters. None of my clients could afford the two part system with the condenser outside and the tank inside, so we were using the tanked heat pump with the condenser on top of the tank. Those take up a great deal of space, are noisy, and make cold air in the process of making hot water. Even those that we have created mechanical rooms with outside access have annoyed some of my clients to the point they run them in all electric resistance mode, or want them swapped, which we have done. If you don't want the ducts and the building penetrations, you need to have a place to dump cold air into the space, and if that happens to be a space accessible from the interior, you are dumping cold air into the interior of sometimes very small ADUs. We are doing one now that is so small we are using an on demand resistance water heater. It seems to me that a gas on demand would be better for the environment and for operational costs, but that was not an option. Is it now? Now that the City has been rebuffed by the District Court on appeal?

I will admit, part of this is new technology adoption and no experience in the marketplace for how to handle things. However, heat pump water heaters are not really ready for primetime in my opinion, and the alternative nat gas on demands solved so many issues. At $1200 a square foot to build super small ADUs, 10 square feet of space to accommodate a heat pump water heater gets very expensive and net net to the client it is only downside for the same result - hot water.

Dan, for the very small ADUs we sometimes recommend electrifying the main home water heater, and upsizing by one tank size (eg if 50 gallon HPWH would do for just main home, go up to a 65 gallon product) to also serve the ADU. This can work if the run is short enough and an on-demand recirc loop is added. Wondering if this scenario would be an option that could have worked in any of those problematic situations? That aside, would like to better understand which brands you've used and had the best and worst experiences with... if you go to the "expanded list view" of the HPWH product page here (https://buildinggeni.us/products/heat-pump-water-heaters-hpwh), info on space and clearances is included. We still need to add the extra small products that are only 20" diameter, although those also seem really tough to track down from suppliers...

We have done what Katy suggests on 2 recent ADUs. On A third ADU we just kept the existing gas water heater and connected it to the ADU. All three have a demand operated recirculation pump. It never made any sense to put a heat pump hot water heater in an ADU...biggest ADU I've done is about 500 SF. I think I read that electric on demand are now allowed in small buildings. Katy...did I hear that from you or your guide? I forget how small the building has to be to allow that, but it might make environmental sense in many cases.

Sarah, in theory, electric tankless WH are allowed for homes ≤500 sq. ft. but as explained in the e-guide (see pg 104) this may not fly from the standpoint of the entire home meeting Title 24 energy code, since all of the compliance now needs to come from the envelope but even with great windows, slab insulation and other above-code specs, you may exceed both the heating and cooling budget and ultimately not be Title 24 compliant... (mainly since ADUs have more window area than allowed prescriptively due to how the code works.)