Wood furnace fire going out

[Quietwelder87]

Hi all, I'm new to wood indoor wood burning. We bought a house with a falcon f-2500 wood electric furnace, company no longer makes stoves/furnaces. House is built in 2014 and would be pretty airtight which may be another issue. Plumber said should be a 4" fresh air intake near furnace, but there isn't. (might help, don't think it's the problem)

Last night overnight temp was - 10C (14f)

Wood is oak, moisture 15-16%.

We are having a problem with the fire going out when the time between Heating intervals is too long(I believe). When wood thermostat calls for heat it opens the fresh air damper at back of furnace.

Would there be another small vent which let's enough air to get in to keep fire from smothering?

Chimney was cleaned out, fire bricks had some cracks so I didn't take out. Do I have to take those out to clean? If so I'll get new bricks and can do that.

Sorry for long post, wanted to get as much info as I could.

Thanks