You will have to seal the entrance, air inlet and exhaust vents. They must be sealed to keep fumes out. You will need an alternate air supply such as Scott air packs.
Things to consider about underground shelters and fires:
- Air packs are usually good for about 30 minutes.
- Each person breathes at a different rate.
- There is no way to know how long the fire will last. Wooded areas burn much longer than grassy areas.
- How will you know when it has passed?
- There is no way to know if the toxic fumes from the fire have dissipated before you open the vents. Air packs will need to be used until you are certain toxic fumes have dissipated.
- Vents could become clogged with ash and debris, making them difficult or impossible to open or use.
- Escape hatches could be blocked by downed trees.
- Stored goods could become contaminated by toxic fumes when the vents are opened or the seals fail.
- Once you are below the fire, you are stuck there for an unknown amount of time. No one will be able to come to your aid.
- Many shelters are sealed with rubber and nylon seals and o-rings. The heat from the fire could cause them to fail. If this happens there is great risk of the air being sucked out and toxic gases would fill the void. Persons inside would be exposed to these gases. Protective clothing will be needed as well as the air packs.
Another question was about using scuba gear and tanks instead of the air packs. That is much more risky than when sheltering underground with the air packs. Technically it “could” be done, but parts of your body will be exposed to the toxic gases that may be entering your shelter.
Considering this information, the risks seem too high. Wouldn’t it be better to seal the vents to the shelter before evacuating? Return after the fire is extinguished to ventilate and clean your shelter. You might need to live in it until your house is repaired.