Hot heater 110 volts 20gal water smell bad not used ofton

Please clarify your question

I'll take a leap here and guess they are talking about an under the counter 20 gallon HW heater operating on 120VAC. I will also guess you are on a well. If this is the case, it is from decomposing sulfur bearing minerals in the soil that leach into the well water, generating Hydrogen Sulfide in trace amounts. Another possibility is the Sulfide reacts with the anode rod and causes the smell. The easiest solution is to run water through the tank, heat off, to flush it well. Then try to heat the water. You may need to replace the anode rod, particularly if you have hard water. This happens to my HW heater in our GA mountain home when we don't go there much in the winter. Flush it for a while and it goes away.