I used my handyman once to replace without a permit but then I got to thinking about if for some reason the tank malfunctions and causes injury, I'd be liable and insurance wouldn't pay so for the small cost compared to leaving yourself exposed, its worth having a plumber do it and pull a permit.

Because no device can be 100% reliable, you have to balance the benefit of the device in the market (people get healed) with the risk of device malfunction (worst case, people die).  

If the clog is due to malfunction I fix it.

But I can tell you that is perfectly safe 99% of the time unless there is a facility malfunction resulting in a catastrophic failure (blowing up).

They have the tools to run diagnostics and tell you exactly what is malfunctioning.

I attempted to do so and it keeps malfunctioning.  

It is possible there is a malfunction that short cycles the system, or, the tenant is just overly sensitive/demanding.

My dad owns an Automotive shop, so I asked him when I turned 16 what vehicles he saw the least of when it came to major malfunctions and expensive repairs.

We actually had our dryer malfunction a few months back and had it serviced/repaired, so I imagine it is being used a lot.

Boiler System:How it works: heats water to produce steam for heating and hot waterWhat it does: provides heat and hot water for a household or buildingWhat can go wrong: malfunctioning components, leaks, pressure issuesAverage repair cost 2023: $500 - $3,000Furnace:How it works: burns fuel to heat air that's distributed through ductsWhat it does: provides heat for a household or buildingWhat can go wrong: malfunctioning components, dirty filters, pilot light issuesAverage repair cost 2023: $300 - $2,000Water Heater:How it works: heats water for use in sinks, showers, and appliancesWhat it does: provides hot water for a household or buildingWhat can go wrong: malfunctioning components, leaks, sediment buildupAverage repair cost 2023: $200 - $1,500Gas Meter:How it works: measures the amount of gas being used by a household or buildingWhat it does: helps accurately bill for gas usage and monitor usageWhat can go wrong: leaks, malfunctioning components, inaccurate readingsAverage repair cost 2023: $100 - $500Electric Meters:How it works: measures the amount of electricity being used by a household or buildingWhat it does: helps accurately bill for electricity usage and monitor usageWhat can go wrong: malfunctioning meters, faulty connections, inaccurate readingsAverage repair cost 2023: $100 - $500Plumbing:How it works: distributes clean water and removes wastewater and sewageWhat it does: provides access to clean water and maintains sanitationWhat can go wrong: leaks, clogs, low water pressure, faulty fixturesAverage repair cost 2023: $200 - $1,000Roof:How it works: provides protection from weather and elementsWhat it does: keeps a household or building dry and structurally soundWhat can go wrong: leaks, damage from weather or debris, aging materialsAverage repair cost 2023: $300 - $3,000