Most of the time, that rust on the hot side is from the water heater.
I noticed today that the hot water from my faucets (kitchen, both baths) has a rusty color to it once I let it run hot for about 3-5 seconds. When I turn the faucets to cold, the water runs clear. Is this a hot water heater problem or something else? My heater is gas, is about 11 years old now, and I was already looking to replace it. I hadn't seen any signs of failure until today...unless this is caused by something else. Any ideas?
Your water heater is rusting on the inside. You can either change it now, or when it starts to leak.
Thanks for confirming what I thought. Looks like I'll be changing out a water heater this week!