Tommy here is the H1 manual which has a IPL down on the last couple of pages:
https://www.hearth.com/econtent/images/uploads/H-1-2manual.pdf
The diagram of the HI is not that great and does not really show the Primary air vents that are at the bottom on both sides...but they are there and ash can pack those vent slots so good you can't even tell there are supposed to be slots. If you follow the Primary air path from where it comes in at the rear flapper, you can see it goes in, straight down, then forward(lower left of firebox) then through the crossover tube, then straight back(lower right of firebox).
You don't need to do a complete dis-assembly. I would remove the baffle first, through the front doors look up and on each side is one bolt sticking through the baffle with double nuts on it. Spray them down a few times with a penetrator like Blaster or Kroil & let them soak.
Get out your shop vac & a roll of duct tape & a small (1/2" or 3/4") diameter rubber hose 1' long or so. Tape the hose to the nozzle of the vacuum and start cleaning the Primary air slots on both sides of the firebox floor. If they are really packed and you can't get them clean there are visible bolts to remove the two side air vents. The crossover tube in front(floor of the firebox) that connects the two does not have air slots so it usually does not need to be removed.
Spray a penetrator on any bolt/nut that you are going to try and remove. Years of burning really makes them stick and forcing them will snap the bolts. Remove the nuts on the baffle and the baffle drops forward. It's two piece, so you can remove it through the door. Vacuum & clean the baffle and it's mounting area. I didn't bother to take off the rear and right side cast iron heat shields, as I figure more ash behind those is a good thing.
Make sure the Primary flapper & it's air intake bi-metalllic coil is set and functioning correctly. If it's not then you are either not getting enough air or too much. Check your damper and make sure it is functioning correctly. Inspect the secondary air tube for burn through or warping.
Check your flue & sweep it while your at it. Reassemble everything & you should be able to cruise between 420 and 500 degrees flue temp. This whole clean out should only take an hour or two. If you do break a bolt, just go to the hardware store & replace with an equal sized stainless steel one.
Good luck, take some pictures and post them up on a new thread....we like pictures