Installing a Valor stove flush mount inside a wood fireplace surround. Venting will be 8' vertical transitioning to 8' horizontal. The unit came with a framing plate, but also bought a closure plate for some reason. What is the difference between the two? Do I use one or the other, or both. Have instructions for the framing plate, but none for the closure plate. Using 5/8" cement board. Does the 530IN heater mount to the cement board or to the framing plate? Will be finishing with marble on top of the cement board.