Have you ever been somewhere, turned on the tap and smelled that faint (or stronger) odor from the water faucet? Maybe at your own house? That's bacterial crud growing in your pipes.
Years ago, I did the realtor gig for a while. I always knew which houses had the water heater turned way down and that they didn't use the hot water tap in that sink often enough. Yummy, mold stench...and someone is rinsing their mouth with that water??
We had the same problem here, but we plumbed our house with a Pex Manifold system, so we were able to disconnect each pipe, run a bit of bleach water into it and turn the faucet on to a dribble for a while to disinfect the pipes. After that, the water heater went back up a notch and I started using the hot water tap more often.
If you don't have a way to clean those water pipes, you might want to consider different ways to save power. Perhaps a timer on the water heater, insulating blanket if it's electric, pipe insulation over exposed pipes, or 'on demand' type water heaters in the bathroom and kitchen. Beats mold in the pipes.