You mentioned a few issues/concerns with your boiler that perhaps I can help with.
1. Overheating your boiler and triggering the popoff is of course a think to avoid, but can be handled quite easily.
First, do you have an automatic fill valve from your fresh water supply to your boiler, and pipe the drain from the popoff (TP Valve to be technical) to a container or indirect drain?
I prefer not to plumb directly to a drain, as it would be easy to miss a malfunction that causes frequent draining.
The fresh water cools the system and prevents air from entering the system. *Some* oxygen is introduced with the new water, so you don't want this to happen frequently.
A second method of dealing with over firing/over temp is to have a loop that can pull a lot of heat from the system. A separate, cool basement, large slab, or some even use an outside, inground loop. The idea is when the water temp gets too high, just before popping off, this loop opens and draws heat out of the system.
It's often better to draw heat out of the system than to introduce cooling procedures (ash on fire, fresh water inlet, etc). Slow and controlled draw down vs fast and extreme measures.
2. Adjusting your baro. It MUST be mounted level. There should be an adjustment knob that moves the weight, but this should ideally be done by using a manometer like the Dwyer that many people here favor. Do a quick search for more info and detailed procedures.
BE CAREFUL with adjusting a baro. Do it wrong and you won't have enough draft, potentially introducing combustion gases into your living space
3. Lower draft door. Are you opening the draft control door, or the ash pan door? I recently found that my previous efforts at firing my handfed bit furnace by opening the ash pan door because I *thought* I didn't have enough draft and combustion air was WRONG.
I actually had TOO MUCH draft and introducing too much underfire air was causing the fire to burn quickly and heat in spurts, but not run steadily- leading to a 'cool fire' that burns but sends much of its heat up the chimney!
Adding a baro this year and using less underfire air (and more PATIENCE) once the fire was going strong led to more consistent heat output and longer burn times. Lower heat output, but much more steady.
Takes longer to heat a cool house, but easier to keep the house warm constantly.
I am struggling with a load of coal that has far too many fines in it for the next 2 or 3 weeks, but then back to larger lump/stove coal is likely. The exception would be if I can find consistently sized nut, then *maybe* will work to get it going.
Berlin suggested Valier Stove sized, near Home, PA. It worked really well, but a little far and harder to shovel than nut. Also very smokey, but it has given me the best burns so far. I may be going back to that, since this single 'try out' load from elsewhere has too many fines and is giving some trouble.
In the end it may be better to weld a curved point on my shovel to make it easier to handle and use the bigger lump.
If you do a quick search on my limited number of posts, you can read more about the steps (some good, some misteps
) I took over the past couple of years to get to this point. Maybe something here will trigger some ideas for you.
Hope this helps.