You've clearly done your homework on this! You've identified the key sections of the NEC and correctly calculated the potential load. Here's a breakdown of my thoughts and feedback, along with a few potential avenues to explore

Your Analysis is Accurate:
You've correctly identified NEC 422.13 for continuous loads and 210.23(A)(2) for the 50% rule. Your calculations for amperage draw are accurate based on the voltage and wattage you provided. You're right to be cautious about the 125% continuous load factor.

The Core Problem

The crux of the issue is the combination of:1.

HPWH considered a continuous load

Meaning you need to multiply its amperage draw by 1.25.
2.

Multi-outlet branch circuit

Invoking the 50% rule for cord-and-plug connected, fastened-in-place appliances on circuits that also supply other outlets.

Likely Code Violation

Based on your figures and interpretation of the NEC, you are likely correct that installing the 1100W HPWH on this shared circuit would be a code violation. Applying the 125% factor brings the calculated load over the 50% limit of the 20A circuit.

Why it's Tricky

The code is designed to prevent overloading the circuit and causing a fire hazard. Even though in practice the washer and HPWH might rarely, if ever, run at full load simultaneously, the NEC errs on the side of safety by assuming a worst-case scenario.

Possible Solutions and Considerations (in order of preference, ideally)


Dedicated Circuit (Best Solution)

This is the cleanest, safest, and most code-compliant option. Run a new, dedicated 20A circuit with 12/2 wire from the panel to a dedicated outlet for the 1100W HPWH. This eliminates the shared-circuit issue and allows you to confidently use the more powerful HPWH. This is what you should be quoting. Upside: Fully compliant, safest, best performance from the HPWH. Downside: Most expensive, requires running new wiring.2.

Replace Receptacles with 20A

While the wire and breaker are rated for 20A, the 15A receptacles are a limiting factor. Replacing the existing receptacles with 20A receptacles might be an option, but it doesn't address the 50% rule issue. And, honestly, you'd want to be absolutely sure that all wiring, boxes, and connections are suitable for 20A before doing this. This is generally not recommended if there's the possibility of future misunderstanding and use of higher-rated devices. This does

NOT make the installation code compliant.3. Informal Load Calculation/Appealing to the AHJ (Authority Having Jurisdiction)

This is risky and depends entirely on your local inspector. You could argue that the washer and HPWH will never run at full load concurrently, and present data or a well-reasoned argument to support this. You might even use a clamp meter to monitor the actual current draw of the washing machine during a typical wash cycle. If you can demonstrate that the total load will always be within safe limits, the inspector might grant an exception. However, don't count on it, and get it in writing beforehand. Upside: Could save the client money. Downside: Highly dependent on the inspector's interpretation, could be rejected, creates potential liability if things go wrong later.4.

Use the Less Powerful HPWH (Most Reliable Solution)

This is the most straightforward solution to guarantee code compliance and avoid any potential issues with the existing circuit. This is also going to be the least labor-intensive solution. Upside: Avoids code issues, less work. Downside: Client doesn't get the desired performance.

Important Considerations



Local Codes and Amendments

Always check for any local amendments or interpretations of the NEC that might affect this situation.

Homeowner's Future Behavior

Consider the possibility that the homeowner, or a future owner, might plug in other appliances into the same circuit, further increasing the load.

Documentation

Document everything. If you choose a solution that deviates from strict code compliance, get it in writing from the AHJ (Authority Having Jurisdiction).

Recommendation

Based on the information provided,

the safest and most responsible course of action is to recommend the dedicated circuit. It's the only way to guarantee code compliance, ensure the HPWH performs optimally, and avoid potential safety hazards down the road. While it's the most expensive option, it's the best long-term investment for the client's safety and peace of mind.If the dedicated circuit is absolutely not feasible due to cost or other constraints, using the less powerful HPWH is the next best option. It avoids any potential code violations and provides a safe and reliable solution. The "appealing to the AHJ" route is very risky and should only be considered as a last resort. Flag for review