[EC900]

You’re seeing the heat flapper outer rotating open/close but can you see inside the flapper box if the inner flapper is closing all the way? Pull hoses off and get a look inside the flappers, inner flap might be rusted or impeded from closing all the way. Don’t underestimate the tenacity of mice exploring an open flapper to nest.

My old VW flapper boxes were rusted beyond fix, every spring I’d pull off the hose and reattach in winter for heat.