Combustion air for cast iron boiler

The damper you show is a barometric damper. That will not work.

Manuals are awesome.

Read carefully. Install more carefully.

The openings to the outside for combustion air do not need to have dampers. The openings can be permanent fixed openings with the minimum free area of 1 Sq" per 4000 BTU of input. so a grill that is 4x10 with 80% free area (not blocked by the louvers) will be more than enough. you need to put one at the top of the room and one at the bottom of the room that connect directly to the outside as illustrated in the diagram.

Here are 4 different diagrams that show different sources of combustion air. two are from outside and two are from the attic. the Attic method requires that you drop one of the openings with a duct to within 12" from the floor. since that is a ducted combustion air inlet from the attic, the 1sq" per 2000 BTU number is recommended. The attic must also have sufficient ventilation to provide the combustion air. Some attics have exhaust fans that control the attic temperature in the summer. Since that burner will be making DHW in the summer, you can not use the attic if you have an exhaust fan moving air thru the attic.

If you are afraid of that cold air causing the pipes in the boiler room to freeze, and you want to put motorized dampers on the openings to reduce the probability of freeze-up, then those dampers need to have a switch that will prevent the burners from operating if the campers fail to open. If you choose not to use dampers, that is ok too.

One other observation…

It appears that you are using well water with some of the well water equipment tucked in behind the boiler and the DHW tank. After you build the boiler room wall around that area, will you be able to access the well water apparatus when it requires service? If you are going to use the two 4x10 openings directly to the outdoors, perhaps moving far enough away from the water & heat piping so freezing is not a problem would be a good idea, even if that means that your boiler room needs to be 10' x 15' with 8' ceilings.

I would check the volume of the building first, it probably won't qualify as confined space. Most cases, these don't need outside vents.

People tend to overestimate how tight a spray foam building is. Usually is better than regular construction but can still leak a fair bit if a couple of details are not handled well. Even if super tight, a power vent unit doesn't exhaust all that many CFM of air and it will take a heck of a tight building to get it any measurable depressurization in a large workshop. Power vent units are also much less sensitive to building pressure.

If the boiler does end up enclosed in a small room, than it needs a high / low vent to the open workshop space provided that workshop space is big enough.

@ajping3830 asked: "How can I figure if there is enough combustion air in the shop space? There will not be any additional rooms, just wide open."

A confined space is considered any space that does not have at least 50 cubic feed for every 1000 BTU of burner input. The Shop is much larger than that, so you have sufficient amount of combustion air if you use two 12 x 12 louver openings from the boiler room to the shop.

The total shop + boiler room will have a total of 14,400 cubic feet volume to draw combustion air from. that means that you can have up to 288,000 BTU input and still not have a confined space. but as tight as you are making that space, I would not go much over 220,000 BTU. Since you boiler has a 100,000 input and it doesn't look like you will be adding a separate water heater any time soon, I think you will be fine.

Now you don't need to worry about cold air from the outside louvers freezing any pipes. Some welding shops do however add exhaust fans to clear fumes from the work space. especially if you have the ability to bring automobiles or other combustion engine equipment inside to work on them. That can be a real problem. I was working on a problem boiler that was connected to a shop that was 5 times larger than your shop. The exhaust fan would wreak havoc on the flame and the venting system whenever they operated the exhaust fan with all the garage and shop doors closed. The exhaust fan would suck air the wrong direction from the boiler's exhaust vent pipe. That would blow out the electronic ignition pilot and cause a flame failure signal to stop the burners from operating.

Does the boiler need to be elevated 18” if it pull air from a garage or shop?

my understanding was that if it had overhead door that a vehicle could be stored inside, a flame needed to be elevated 18”

We did this with water heaters in garages or with attached rooms until the flame protected style water heaters became available

No harm in checking with the current codes in that area. Stay safe.