gas water heater passive vent damper

I read about a passive vent damper on for a gas water heater that is essentially a very light plate that rises to open the flue when the heater fires up and settles down to close the flue when the heater stops. Its non-electric, completely mechanical. Its supposed to cut standby losses quite a bit. I've been unable to find one for sale. Anyone every see this? Know where it could be purchased? ref: Thanks Eric
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