That's a good question and a common scenario. Here's a breakdown of what's happening and how to interpret the results

Understanding the Test
Insulation Resistance Testing

This test measures how well the insulation between the energized heating element and the grounded metal tank is preventing current leakage. A high resistance reading indicates good insulation, while a low reading suggests insulation breakdown.

Why It Matters

Leakage current can be dangerous, leading to shocks and potentially tripping a GFCI breaker. It also wastes energy.

Factors Affecting Readings Water

Water is conductive. Even pure water is more conductive than ideal insulation. Water around the element significantly reduces the measured resistance.

Temperature

Higher temperatures generally decrease insulation resistance.

Age

Insulation degrades over time due to heat cycling, mineral deposits, and general wear.

Contamination

Minerals from hard water, scale buildup, and other contaminants further reduce insulation resistance.

Element Design

Different element designs and manufacturing tolerances will have slightly different initial insulation resistance values.

Test Voltage

Using a higher test voltage (like 500V) can sometimes reveal weaknesses in the insulation that might not be apparent at lower voltages.

What to Expect (and Why It's Tricky)

It's difficult to give a definitive "perfect" reading for a new element because of all the factors mentioned above. However, here's some general guidance: New, Dry Element (Ideal)

A brand new element tested before installation should ideally read hundreds of megohms or even into the gigohm range (using a 500V tester).

New, Submerged Element (Still in Package)

Even submerged in the package, the reading will decrease a bit, but you'd still expect above 100 Megohms.

Used Element (What You're Dealing With)

A used element in a water heater tank filled with water is a different beast. Readings will be significantly lower.

Your 20 Megohms Reading: Interpretation and Action 20 Megohms is low but not necessarily immediately catastrophic. It's in a gray area. Safety Considerations



GFCI Protection

Critical. If the circuit feeding the water heater doesn't have GFCI protection, install a GFCI breaker immediately. This will help protect against shock hazards.

Bonding

Ensure the water heater tank is properly bonded (grounded) to the electrical system.

Troubleshooting/Next Steps


Disconnect Power

Always turn off the power to the water heater at the breaker before doing any work.2.

Drain and Inspect

Drain the water heater. Remove the element and carefully inspect it. Look for:

Visible Cracks

Any cracks in the ceramic or other insulating material.

Bulges/Deformation

Signs of overheating.

Scale Buildup

Excessive mineral deposits. This can be a major cause of leakage.3.

Clean (Carefully)

If the element looks otherwise sound, carefully clean off scale and mineral deposits. Use a soft brush and a descaling solution designed for water heaters (follow the solution's instructions). Be gentle!4.

Re-Test (Dry)

After cleaning and drying the element completely, re-test it with the megohmmeter outside the tank. A dry, clean element should give a significantly higher reading than when it was submerged and dirty. If it still reads low even when dry, the insulation is likely damaged internally.5.

Consider Replacing

Even if the cleaned element reads slightly better, 20 Megohms is still a yellow flag. Given the age and the fact that it's been submerged, strongly consider replacing the element for peace of mind and safety. Water heater elements are relatively inexpensive and easy to replace.6.

Inspect Tank

After removing the element, inspect the inside of the tank for excessive rust or sediment buildup. This can give you clues about the overall condition of the water heater.

When to Replace Immediately

If you see any of the following, replace the element immediately: Visible cracks or bulges in the element. The element is tripping a GFCI breaker. You feel even a slight tingle when touching the water heater tank (this is a very dangerous sign).

Documentation

Keep a record of your megohmmeter readings, the element's condition, and any actions you take. This will help you track the health of the water heater over time.

In summary: 20 Megohms is a low reading that should be investigated further. It's not a guaranteed "replace now" situation, but it suggests insulation degradation. Implement safety measures (GFCI protection), thoroughly inspect the element, and consider replacement, especially if the element is old or shows signs of damage. Flag for review