The heat traps are plastic devices that are in the ends of the in/outs of the heater as -purchased. I suspect them to be the items responsible for this noise. The heater is electric and is brand new, but not using electric as it's run off a heat exchanger from the outside boiler. It will clack even when a location in another part of the house is used and also clacks when the well pump cycles. I'm about ready to take that heater apart and rip those heat traps out!!
Ah...HA! Although this goes against everything I am, I broke down and looked at the book that came with this heater. In the "Before You Call For Service" section:
Rattling noise during periods of water usage ~ Internal heat traps in operation ~ This is normal for heat trap fittings when in operation and does not indicate a need for service.
So there it is. Hidden in the book of directions. No wonder I never saw it!
I asked lazypup what heat traps were. He said in the dielectric unions are plastic cross pieces cast into the plastic in the form of an X much like what's in your bathtub drain basket. how they function or what they do I have no idea.
The tank is heated using a heat exchanger, so there are no elements and no flame. (It is an electric heater, though.) As the directions/faq's for the heater state, the heat traps do make noise when operating correctly. But it's still irritating!
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