Do I need expansion tank on domestic side of new HW heater?

(Hi, I also posted this on the HW heater forum, but this one is so much more active... and I need to make this decision right away... I hope it's OK to cross post like this!)

We currently have an electric hot water heater installed in our domestic HW system. We are changing to an Amtrol CH41BZ. The instructions for the Amtrol call for an expansion tank to be put into the cold water supply line. Our current electric HW heater does not have one installed and we have no problems. Do I need the expansion tank?

Thank you!
Neil

The first rule of plumbing...and a MANDATORY requirement of all pluming codes, is that equipment must be installed according to manufacturer's instruction. So YES, for all the same reasons that expansion tanks are recommended on all water heaters, you do need one. WIth the slow heating electric water heater, you may have avoided, or not noticed, significant expansion pressure. The indirect is different.

what is an Amtrol CH41BZ? I have never heard of an Amtrol water heater, they usually just make storage tanks. Whether you NEED a expansion tank has nothing to do with the water heater you install. It depends on whether you have a closed or open water system, and that is a completely different issue.

We dont see these much west of the rockies, hj, but it is an indirect heater, heated by a coil with water from a boiler pumped through it. The install instructions are quite insistent on the expansion tank, partly because there is a greater potential for overheating , if things go wrong in the circulator piping system. For example, they point out that an incorrect connection to the return can cause the water heater to be heated by the house heating loop, even if the water heater thermostat is satisfied. So the expansion tank, and of course the T/P are very important.