rotten egg smell. to me here in united states = liquid propane gas leakage. they add a chemical to the gas for heaters. and if you get a gas leak, you could possibly smell rotten egg smell.
if not a gas leak, i would suspect, sewage leak. ya have normal drain in bottom of sink, and if bathroom sink normally an overflow in the sink. i have seen some rare sinks were overflow and bottom drain on sink required you to TEE the 2 areas together before going into the P trap. and then having enough water in the P trap to fill the P trap to keep gasses from exiting out of the bottom drain or overflow...
i have seen it were just running enough water has caused the "vents" on the sewage line, cause a back draft per say. which allowed gases from the sewage line to push back through the P trap and back into the home.
i do live on a well but, i have also delt with limited city water. city water is not always good clear / clean water. if ya ever had a "boiling order" per say. normally after the fix or dealt with a new line in the area. to just plain filtering of water at the given water plant. you can get some "fines" in the water small size particles. and the water heater just loves catching the stuff. ya use some hot water. the normal small size particals that would not sink in flowing water now has hours to seat in the water heater and sink down to the bottom before next use. over time, this crud begins building up in the bottom of the hot water tank.
a simple flushing of the hot water tank with some sort of chemical say bleach or like. can disinfect and help remove the crud in the bottom of the tank but not always. if you ever had an aquarium for pet fish or a small backyard pond for say goldfish / koi. and have dealt with algae, and taken stuff apart and seen algae growing inside the pipes and every were else... and have seen what it takes to clean the stuff off. you get a better perspective of just a simple "draining" of the hot water tank and flushing it out with some chemicals, does not really clean it.
this goes with old pipes in the house as well. more so with wells with heavy metals in them. on this old farm house went to replace water pipes that were 1/2" diameter inside diameter, but were barely a tooth pick hole left from all the crud and like built up over the years.
so my question is how bad is your water heater, and how bad are your pipes with crud build up?
if you replaced a line, then how much of that crud, did you end up beating loose from inside the pipes, and now some place stuck in the main flow of water, giving off some ugly smell?
just replacing the anodizing rod doesn't really do much good, if it is being overwhelmed by all the crud that has built up over the years.
for myself, 2 or 3 water heaters here on the farm well, about i guess every 10 or so years. replacing the rod never made much good, it ended having to replace the water heater themselves. of course the water heaters in most part were rusted bucket by then on the outside, so in that respect it was easier call to make, though pocket book, it was a hard call.
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contact your local water company, and ask for a "test kit" some times it is free some times it is not. there is normally a time limit, between time you fill test kit with water, and by the time testing lab receives it and deals with it. if to much time happens, then it may cost you more to redo the test.
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since i am on a well, i already have a "priming" area inside house. to help prime the pump. this is also one of the places, i can dump some bleach into the system, to disinfect things, if i just redid stuff in the house.
1 T, place in main line coming into home, and to everything else.
2 valves. (one to shut off main water line), other one coming off of tee for priming.
1 reducer and/or bushings. go from say a 1/2" valve up to say a 2" to 4" pipe size. basically you are making a "funnel" out of reducers/bushings (going from small size up to larger size), on the very top put a female / male threaded connector. with a screw on end cap. the screw on cap basically to keep crud, dust, etc.. from falling in funnel when not used. (which may be years for some folks between use).
you now have a easy valve / funnel, to dump some cleaning chemicals into your system.
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as far as why that sink? for myself, the way my water lines are ran in house. if i open the valve to fill down stairs pond wide open (very end of everything else). the well pump will begin to cavitate, (pulls air out of the water), and while nearly all bubbles due collapse back into the water, i do get a smallish air pocket in the bathroom sink. granted i have an outside garden hose connection, and kitchen sink, and a old dish water connection before bathroom sink. but those connections. do not come directly off the top of the main water lines. instead i teed into the water lines and had run some feet across the basement ceiling and then up to them. unlike the bathroom sink, were i teed and ran a pipe straight up to the bathroom sink. guess were the bubbles went? bathroom sink. air bubbles = top of pipe. and that is were they ended up at. all the other lines never allowed the air bubbles/gas to escape up and out of the lines, due to water always ran down and out, and then over and up to them. vs straight out off the main line.
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with above said, i am going to say crud that is bottom of the hot water heater, and possibly crud that has built up over years in the regular drinking water pipes, that is the cause.
