You've done a good job isolating the issue. Harmonic distortion from the heat pump inverter interfering with the home inverter is a likely culprit. Finding the right filter can be tricky. Let's break down potential solutions and where to look

Understanding the Problem Deeper (Before Filter Selection):
Inverter Specs (Heat Pump & House)

Knowing the exact models of both inverters is crucial. This allows you to research their harmonic distortion specs, sensitivity, and potential compatibility issues. Post those models if you can!

Waveform Data

While amp logging is good, if possible, capturing waveform data (voltage and current) when the heat pump starts and runs would be invaluable. This would prove the presence of harmonics and help determine their specific frequencies and amplitudes. This requires more specialized equipment (oscilloscope or power quality analyzer).

Inverter Sensitivity

Some off-grid inverters are more sensitive to harmonic distortion than others. It might be worth contacting the house inverter manufacturer to ask about acceptable harmonic levels and any recommended mitigation strategies for known issues with heat pumps.

Start-up Current (Inrush)

While you logged the running amps, is there a significant inrush current upon startup of the heat pump even with the inverter soft start? This could still briefly overload the house inverter. Check with a clamp meter with inrush capability when starting the heat pump.

Finding a Harmonic Filter



Dedicated Harmonic Filters

You're right, a dedicated 240V 20A harmonic filter specifically designed for minimizing Total Harmonic Distortion (THD) can be difficult to find off-the-shelf. Here's why and what to consider:

Size and Cost

True harmonic filters are generally larger and more expensive than basic EMI/RFI filters, especially for power levels above a few amps. They contain tuned circuits to specifically target harmonic frequencies.

Application Specific

Harmonic filters are often designed for specific applications (variable frequency drives, for example). You need one compatible with a residential environment.

Alternative Filter Types and Approaches

Here are other options to consider, in order of increasing complexity and cost:1.

EMI/RFI Filter (Basic Line Filter)

While not a true harmonic filter, a good quality EMI/RFI (Electromagnetic Interference / Radio Frequency Interference) filter can attenuate some higher-frequency noise, which might be contributing to the problem. These are easier to find. Look for one with a high common-mode and differential-mode attenuation rating. Consider these search terms: "240V EMI Filter" "240V RFI Filter" "240V Line Filter" "240V Power Line Filter"

Key Specs

Look for a filter with a current rating of at least 20A (or slightly higher, like 25A for some margin) and a voltage rating of 250VAC or higher. Check the frequency range of attenuation – it should cover frequencies where heat pump inverters typically generate harmonics (hundreds of Hz to several kHz). These are usually more affordable to try initially.

Example Manufacturers

Schurter, Schaffner, Delta Electronics, Corcom (TE Connectivity)2.

Load Reactor/Line Reactor

A line reactor (also called a load reactor) is an inductor placed in series with the heat pump. It helps smooth the current waveform and reduce harmonic distortion. It's simpler and less expensive than a full harmonic filter, but also less effective.3.

Active Harmonic Filter (Advanced)

This is the most sophisticated (and expensive) solution. An active harmonic filter injects current with the opposite phase of the harmonic distortion, effectively cancelling it out. These are typically used in larger industrial applications but smaller, residential-scale units are becoming available. These usually require a qualified electrician or electrical engineer for installation and configuration.

Where to Search



Industrial Automation Suppliers

Companies that sell variable frequency drives (VFDs) and motor control equipment often carry harmonic filters and line reactors. Look for suppliers like:

Grainger AutomationDirect Allied Electronics & Automation Newark Electronics Power Quality Equipment Suppliers

These companies specialize in equipment for measuring and improving power quality. They will likely have harmonic filters and can offer expert advice.

Online Retailers

While it's harder to find specific harmonic filters on sites like Amazon or eBay, you might find some general-purpose EMI/RFI filters that could help.

Be very careful about buying electrical components from unknown sellers on these platforms! Stick to reputable brands and look for UL or other safety certifications. Specialized Electrical Supply Houses

Search for electrical supply houses that cater to industrial or commercial clients. They are more likely to have these components than a typical residential supply store.

Other Troubleshooting Steps (Beyond Filters)



Grounding and Bonding

Ensure proper grounding and bonding throughout the entire system (heat pump, house inverter, and all associated wiring). A poor ground can exacerbate harmonic problems. Confirm that all grounding connections are tight and clean.

Wiring

Check for loose connections or undersized wiring, particularly in the heat pump circuit.

Distance

If possible, increase the physical distance between the heat pump and the house inverter.

Surge Protection

Make sure you have adequate surge protection on both the AC input and DC input of the house inverter. Surges can sometimes trigger inverter errors.

Firmware Updates

Check if there are any firmware updates available for either the heat pump or the house inverter. Sometimes, firmware updates address known compatibility issues.

Heat Pump Soft Start (If Applicable)

Some heat pumps have adjustable soft-start parameters. See if you can adjust the soft start to be even slower to reduce inrush current.

Important Considerations



Electrical Code

Ensure that any modifications or additions to the electrical system comply with all applicable local and national electrical codes.

Qualified Electrician

It is highly recommended to consult with a qualified electrician experienced with off-grid systems and power quality issues before making any changes to the electrical system.

Safety First

Always disconnect power before working on any electrical equipment.

Recommendation (Starting Point)


Identify the Exact Models

Provide the exact models of the Rheem heat pump and the off-grid home inverter.2.

Contact Inverter Manufacturers

Contact both Rheem and the home inverter manufacturer to inquire about known compatibility issues and recommended solutions.3.

Start with an EMI/RFI Filter

As a first step, try installing a good-quality EMI/RFI filter (240V, 20A or higher) as close as possible to the heat pump. This is the least expensive and easiest option to try.4.

Consider a Clamp Meter with Inrush Capability

Measure the inrush current when starting the heat pump.5.

If the EMI/RFI filter doesn't work, explore Load Reactor or Active Harmonic Filter options (but consult with a professional).6. Verify Grounding and Bonding: Ensure the entire system is properly grounded and bonded.Good luck! Let me know the model numbers, and I can try to help you find more specific resources. Flag for review