Hi all - newbie here. I think I got conned by the plumber yesterday. I have two 50 gal hot water heaters set up in parallel with a single 4.75 gal expansion tank in the common supply line, downstream from the shut-off valve. The tank failed at the nipple weld (horizontally mounted with no extra support - ugh). The plumber said that I needed two expansion tanks (one for each heater) - making it sound as if the one 4.75 gal tank that I had was inadequate with repsect to expansion capacity. I made the mistake of trusting him, as I didn't know the required expansion tank capacity per heater off the top of my head. Later, I see that he merely installed two 2.1 gal tanks on the common supply line, clearly at a higher cost to me compared to installing one larger new tank (paying for two tanks vs one plus the extra labor). Here's where the engineering comes in: the system doesn't "know" how many tanks there are - it only "sees" expansion capacity. That being said, you can get the job done with 1,2 or 4 tanks for that matter, as long as the aggregate expansion capacity is adequate. Clearly one tank is the cheapest way to go, but the plumber said "one tank per heater is what we've always been taught" - translation - "we've always been taught to rip people off." I used to be a marine engineer, so they can't try to BS me and justify why they ripped me off after the fact. The only semi-plausible explanation, one that I finally got from the plumber's estimator after a round of BS answers from the plumber (along the lines of the tanks "balancing each other out" - what? Utter gibberish!), is I now have a back-up if one tank fails - but, alas, a single 2.1 gal expansion tank is inadequate by roughly 50% in terms of required expansion tank capacity for two 50 gal hot water heaters....at some point, both heaters will run at the same time and I'll likely get to see if the relief valves are working properly - much the same way if I only had one 4.75 gal tank that failed. Again, I got conned - silly me for trusting someone..... Thoughts anyone? Differing views?