We have an electric RUUD Heat/AC pump. We have an intermittent rotten egg smell ONLY when the Heat is on. It never happens when the AC is on, and it isn't always present when the heat is on. It happens a couple of time per day. Any ideas on what could be causing this? It is not stronger in the bathrooms, our water is city water, and has no odor. Thanks!

GAS has an additive to it, put there by the city, so you an smell a gas leak. Gas is usually odorless, so this is a safety measure.

So, if gas is used to heat the heat the house then some of the initial gas used to start the heating process is escaping into the house and blowing out. If the smell disappates, you're fine in the short term, but you need to get the heater adjusted to eliminate the extra initial gas.

If the system is 100% electric, I'd be looking for something in the heating coils.

A lot of times some Evaporator coils smell when the heat pump goes into defrost the smell is the aluminum in the coil when the hot gas of the refigerant flows through the coil. The only way to rid this is to have the evaporator coil cleanned with evaporator coil cleanner and if the smell still exsist then the only othe option is to replace the coil

The only difference in heat or cool mode is that you aux heat is working in heat mode. It will have a bad smell the first time used for the season but not a egg smell. When your heat pump is in defrost the evap coil is in cooling mode(not heating up from hot gas). I would say you got a rodent stuck in your heatstrips and your smelling it cook.