What size is your firebox/How many lbs in a full load?

I'm planning on using my wood/coal boiler as a coal boiler for the first time this coming winter. It is a Hytec boiler made in Ma. in 1981 and the model name is Hydra Fire. I bought it new 27 yrs ago and only tried coal 1 time (had no clue what I was doing). I burned alot of wood in it over the years but gradually got away from using it and instead used a Tempwood wood burning stove in my living room-which isn't that pretty but burns unbelievably good! I described the wood boiler in another post looking for info but I got no response. The draft is controlled high and low aquastats made by honeywell that monitor the water temp. The unit is plumbed in parralell with my burnham oil burner. Their is a Grundfos circulator pump connected to an aquastat that pumps water through the boiler and these pipes connect to the main pipes that enter and exit the oil burner. Essentially, the coal boiler keeps the oil burner hot. For baseboard circulation the same circulator that moves the water from the oil burner also moves the water through the radiators even if the coal boiler is doing the work. There is one heating zone that opens to dump excess hot water if temps get too high-controlled by a valve and aquastat. The system always worked flawlessly after I tweaked the settings to work well with the oil burner. I did get away from using it though because I found my Tempwood in the living room to be so efficient. I burned less wood but of course I got no hot water.
Here are my main concerns. The burning rate is controlled by an aquastat that turns on or off a Honeywell Draft damper operator motor (a full on or full off motor with a chain attached to a flapper that controls whether air is allowed into the damper or not.) So basically it is either wide open or closed shut. I could adjust the limits of open and closed by playing with chain length and location. I just wonder if coal will stay lit if the damper door is closed for an extended time like when the weather is not too cold or if I were trying to just heat water in the summer??? With wood it was never a problem.
My other concern is the length of time that a load of coal will burn in this unit. The fire box dimensions are roughly 13-14" X 25" x 8". The 2 long sides of the firebox are angled so that at the bottom the width is 9" and at the top of the brick it is 16". The nameplate on it says it is designed for wood and anthracite. There is a long arm on the front to shake the grate. When you open the ash door there are 2 slots for running a long, flat bar just over the top of the grates for ?? It also came with a long rod with a bend on the end for cleaning between the grates I guess. The boiler has insulation under steel panels on the 2 sides and the back. I don't know how long this size firebox will be able to burn in cold weather. My house is well insulated-I built it myself 27 yrs ago. I already made the commitment to coal and just stacked 120 bags of Balshack nut coal the other day. Grates have about 1/2' spacing and I am guessing on what size coal to use since I lost all paperwork on this stove. There is NO INFO on this boiler on the internet at all!.
In summary, How long might this burn with that firebox size and do you thing the damper on/off control will work sufficiently well or might I have to always leave it open a little???
Sorry for the lengthy post but I'm new here and excited about going this route and I've been reading this forum non stop for days. I know I'll at the very least have to learn the art of coal burning and forget all my wood burning ideas-except for my ugly,trusty,tempwood-even the ashes disappear in that thing.

I think you will have problems losing the fire with the draft control completely closed, you will probably need to modify it to allow a small amount of air to pass up through the coal bed. You need to make sure the door gaskets are in good shape and any over-fire draft controls are closed for burning coal (the air must be drawn up through the coal bed, not over it).

Nut size is a good place to start, as I said above you will need to experiment with the draft control to find out how much air you need to sustain the fire and not over-temp the boiler when it's not calling for heat.

Your burn time will ultimately depend on the load placed on the boiler, I would guess that 12 hour reload intervals should be possible with an 8-10" deep bed.

What do you have for a chimney? Do you have a barometric damper installed?

Thanks for responding. The furnace exits through 2-8" elbows and a little over a foot of horizontal run into a masonry chimney in the center of the house- no damper. I estimate that there is 32 feet of chimney above the entry point and the chimney ends 5 ft. above the roof peak. Its a 3 flue chimney. Woodstove on first floor, oil burner and wood/coal boiler in their own flues in the cellar. I forget the flue size but none of them are small. There are 2 holes 3/16" diameter ea. in the front door with a steel plate directly behind the entire door. They could be blocked very easily. I could adjust the damper so that it won't close all the way-thats just the way I ran it with wood for years. I guess wood can bounce back more quickly? I am actually heading out to a local place today to buy all new gaskets. I had a small crack near the corner of the door opening and I was using some stove cement to seal it up this morning. It comes in a caulking tube. When I was caulking it it suddenly splattered out of the tube and went right back in my eye! Hurts like hell!!!! Never had a tube of anything in a caulking gun splatter like that before. That is mean stuff-eye won't stop watering after rinsing like crazy. I'm a mess I'm glad to here that you think the box should be able to burn for 12 hours or so. I was hoping for about that. The area looks like it would hold a little more than a 40 lb bag. I guess that's reasonable. I'm open to any advice.

Here's a pic. I'm cleaning and spiffing it up. Just started.

As I said, your burn time will depend on how hard the boiler is run. What type of wood did you burn in the past and how long could you go between reloads?

If the holes in the loading door are only 3/16" in diameter, try it the way it is. They won't allow much air to pass through and what does will help burn the gas off after a reload. One cubic foot of coal weighs approximately 50 lbs, based on that you can calculate how much your firebox will hold.

You are right about the wood, it will react much faster to a change in damper position. If you aren't planning to burn wood this season, I would install a barometric damper.

Hellobob86, you should be able to get 12 hours or so of burn time, but as stated above, if it is 0* outside, or if it is 28* outside will make a huge difference in how long a load of coal lasts..

You really must install a barometric damper, especially with that tall chimney.. read the several topics on baro dampers.. You could have a strong wind come up overnight and cause your fire to burn out very quickly because of the increased draft. That tall of a chimney is capable of over .15" wc.. and you want your baro set to regulate the draft at about .05-.07"wc.

About the flapper on the combustion blower.. if it really does seal off all the air, then you may want to put a timer on the combustion fan.. set the timer to run for about a minute each hour. This is what most boiler systems have to keep the fire alive during summer use for DHW, and warmer winter weather..this works better than just proping the flapper open,, you can adjust the timer to keep the fire alive without risking feeding too much air to the fire..

My boiler has not only the timer, but an additional aquastat that will not allow the timer to run the combustion fan if the water is already over 200*. This protects against overtemps.

Hope this helps.. Greg L.

The damper we are talking about are non-electric right? Are they very expensive? This boiler does not have any combustion blower. Did they have those back in 1981? I suppose I could try it w/o blocking the 2 small holes in the door. They can be blocked simple enough.
I was going to get new gasket material today but I ended up having to go to the emergency room because my eye swelled up like it was going to explode! I will never use a tube of that stove cement without safety glasses again. The hospital cleaned it out again and then sent me to an eye Doc. He made alot of noises like Hmmm, and OOOHH and rattled off some names to his assistant. Gave me 2 prescriptions and I have to go back to see him tomorrow. At least his office is near the stove place

Sorry to hear about your eye, that must be some good stuff! Here is a link to a barometric damper, they are very simple and inexpensive. http://www.fieldcontrols.com/draftcontrol.php

Just ordered an RC model for 8" pipe from Simply plumbing.com
The stuff in the stove cement was sodium silicate. Still burns like a son of a gun. I'll see if I can get some things done tomorrow without my weekly visit to the emergency room. Its been a bad few weeks.

I relined the firebox with new firebrick. I now have some exact dimensions and it works out to 1.625 cu.ft within the firebrick area. That should hold 81 lbs of anthracite. Assuming I am replacing 1/2 at a time (while adding it slowly to allow it to burn off gasses and get going) I'll be adding about a 40 lb bag each refill. I assume that should go near 12 hours under the right condition? Of course that is pretty simplified and I'm sure I may be adding less sometimes and more other times.
I used to burn oak and maple and it would go about 10 hours on a really cold day but of course the boiler was filled up as far as I could go with logs. It is heating a 2500 sq ft colonial with open foyer and 5 separate heating zones plus DHW. I am sure I will still be throwing logs in the Tempwood just to keep it toasty for the cat and 3 cocker spaniels-uh and the old bag.
Hopefully the new damper will make it more efficient too. Its all cleaned and painted up. When I get the damper in I'll upload a new pic of the beast-Oh, I wasn't talking about you Honey