Need Help!! Building custom wood burning fireplace

[Cpal331]

I am currently building a new block veneer in front of an existing firebox built back in 1978 to code. As u can see in the picture below I had a custom pocket door unit made which has to be enclosed by block in order to achieve the desired look.
My concern is: the unit is 4 inches wide with a 3 inch gap that I have been filling with type S mortar to eliminate air pockets. I am at the point where I need to put a lintel in and I am not sure which type of lintel will work. If continue building block to the ceiling once the lintel is in, I will have a huge air pocket above the unit behind the block wall about 7ft long, 7inches wide and 5 ft. Tall. Because this cavity will be sealed in: will I have any problems with the warm air from the fire heating the air in the cavity and building pressure? Do I need a pressure relief hole somewhere?
Could this cause combustion? Could use some advice friends. Thank you!

 

[bholler]

My concern is: the unit is 4 inches wide with a 3 inch gap that I have been filling with type S mortar to eliminate air pockets.

What do you mean where is the gap? As long as it is not adjoining the firebox you dont need to worry about it.

I am at the point where I need to put a lintel in and I am not sure which type of lintel will work.

What do you mean what types are you considering?

If continue building block to the ceiling once the lintel is in, I will have a huge air pocket above the unit behind the block wall about 7ft long, 7inches wide and 5 ft. Tall. Because this cavity will be sealed in: will I have any problems with the warm air from the fire heating the air in the cavity and building pressure? Do I need a pressure relief hole somewhere?

It will not be sealed that tight no need to worry.

Could this cause combustion?

is there anything there that can combust? if not you will not have combustion.

I am more than a little concerned about the firestop foam I can see is any of that within the required clearance to combustibles? If it is it needs to be removed it is very flammable

By the way here is a link to the applicable codes. the section about fireplaces is further down but you should also make sure your chimney is code compliant as well. Many are not.

http://www.rumford.com/code/clearances.html

 

[Cpal331]

Bholler: the gap in the side view pic. The block will continue to rise to the ceiling and there will be air space above the unit

 

[Cpal331]

I was going to fill in the cavity where the fire block foam is with type S mortar and make it flush with the cement board. I'm concerned the heat will radiat through the door unit into the open cavity above. I just don't know if that's good or bad.

 

[Cpal331]

As far as lintel I wasn't sure if I should get angle or an I beam

 

[bholler]

I was going to fill in the cavity where the fire block foam is with type S mortar and make it flush with the cement board.

Yes but the fireblock foam need to be removed it is very flammable.

As far as lintel I wasn't sure if I should get angle or an I beam

Check the ratings of them and you will be able to figure it out. you could also use a concrete lintel.