Harman mag with hot water coil problem

I'm getting an odor coming from stove when all the hot water is used from 40 gal tank.
When the cold water starts going through you can smell it (almost like a sulfer smell)
CO detector is not registering anything so its not a venting problem like I first thought it was.
If anyone can help me with this please reply

Due to very low temp. of water coil now with cold tap water not allowing much temp. rise, I'd say you are condensing the hot coal gases. The longer it takes to get above a condensing temp. the longer the smell exists. You are condensing many, if not all, components in the gases...sulpher is just the more noticeable.

we started to stagger hot water use yesterday, it worked, no odor last night
thanks for your reply