converting old coal boiler to wood pellets

I have a large 100 year old house which was originally heated with coal burning fireplace inserts, a coal stove in the kitchen and a large gravity flow open system (unpressurized) cast iron coal burning boiler in the basement with large cast iron radiators.

At some point in the past, the boiler was converted to oil burning by installing an oil burner. The mid-level doors were removed (loading doors?) and a plate with the oil burner was bolted on over the opening. The upper and lower level doors were sealed shut with what looks like high temperature silicone. The oil burner, based on flow rate, is running at a fuel consumption rate of about 150,000 BTU/h. The entire system (boiler, pipes and radiators) holds about 250 gallons of water (I measured when I flushed it out and refilled it).

The existing system takes about 3-4 hours to get all the radiators nice and hot to the touch if I let things cool down to about 50 degrees F (like when I go away for a weekend and turn down the thermostat). I find that setting the aquastat to 155 degrees F max works well if it is above freezing outside and 170 degrees F if it is below freezing outside. I have a good "smart" thermostat that takes outside temp into account and "learns" how long it takes the system to heat things up so I can set it to say what temp I want the house to be at what time and it figures out when to ask the boiler for heat to hit the temp I want at the time I want and also when to stop asking for heat so the house temp doesn't overshoot the target temp (the radiators don't "stop" heating until the water cools down to room temp). The boiler, radiators and 250 gallons of water have pretty huge thermal inertia.

Some electric baseboard heaters and forced air electric heaters were also installed in the colder parts of the house at some point in the past.

I have installed some pellet inserts into the larger fireplaces and a pellet stove in the kitchen for space heating in the most used and coldest parts of the house. I have also done a lot of insulating and air leak plugging (doors/windows etc)

At this point, I am using about 800 gallons of oil per year in the boiler. This is down from about 1600 gallons when I started insulating and putting in the pellet stoves (Oil is currently about $4.60 a gallon here) . My average electric consumption in the winter has also gone down from about 3500 kwh per month to about 2800 kwh/month ($0.125 per kwh). My summer consumption is about 2000 kwh/month. I am burning about 5 tons of pellets per year ($200/ton delivered).

So, right now I am using per heating season:

5600 kwh (over the summer baseline) = $700
800 gal of oil = $3680
5 tons pellets = $1000

Doing a rough cost per input btu calculation on oil vs pellets vs electric, figuring in the burner efficiency (but not the boiler efficiency which is probably low but I have no idea what it is) I get the following:

$1 of pellets = $2.72 of oil = $2.86 of electricity.

So, replacing the oil burner with a pellet burner like the Pellergy 3550 or the Woodmaster Renovator (maybe too small?) would mean getting rid of the $3680 spent on oil and adding only $1353 in pellets, so saving $2327 per year.

The price of pellets has been pretty stable here for the last few years and the price of heating oil has only gone up. When oil was $95 a barrel, heating oil was about $5.05 a gallon. Now, the price of oil is around $44 a barrel and heating oil is still $4.60 per gallon here. I am scared to see what would happen to the price of heating oil if oil went back up to $95 a barrel!! The electric company wants to increase the rate to $0.16 per kwh over the next 4 years.

I know that I can probably double the efficiency of my ancient boiler by replacing it with something new and efficient, but the cost to do so would be at least $10,000 more than just changing the burner because of all the other things I would then have to change (adding storage, heat exchanger, pumps, valves, piping, renovations to get the new boiler into the basement, etc) The most it could save me is under $700 a year in pellets, so not a great deal.

I know I will have to change the flue pipe to double wall and put a SS liner into the chimney.

Has anyone converted an old (>100 years old) coal boiler to pellets using one of these burners? Are there any problems with doing it? Are there any problems in my logic and/or math?

Sorry for such a long post!! Thanks for your time reading it and huge thanks for any helpful comments or advice!!

maple1 said:

Don't think I can help you much but quite interested in feedback you hopefully get. I have wondered why we don't hear more about the pellet head conversions - they seem to me to be a pretty good solution for someone in your spot.

(I might also check out water treatment stuff - an open system would be quite prone to corrosion, I think. And the flush & refill process I think would be introducing even more oxygen into the system - but I have no experience with open systems.)

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I did add boiler water treatment when I flushed and refilled the system. I don't think it had been flushed in a long long time and I got about 20 gallons of "mud" like sludge out of the bottom of the boiler. I noticed that the system seemed to work better and faster after I flushed it out. I used a reverse osmosis disposable cartridge thing when I refilled and then added treatment chemicals based on my water volume (I got both from a boiler treatment supply company). I do need to pull off a sample and send it out for analysis to see what other specific treatment needs to be done.

leon said:

Converting coal boilers to pellets is a very very expensive proposition
do to the required welding and sealing the firebox airtight.

The pellet stokers are a totally different design with a sloped upward
stoker grate under the "steam chest"

The new pellet boilers are also very expensive for the "total" install cost

You have to look at this on a dollar for dollar basis.

I do not represent Keystoker but the next smallest Keystoker being the KAA-4-1
WHICH is the boiler I am installing in my very old school house monstrocity
will cost you about half of the cost of a modern pellet boiler.

The Cost for a keystoker KAA-4-1 is $4,723.00 US with New York State Sales Tax of 8%.

The KAA-4-1 has a 24 gallon water volume when used for hydronic heating.
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NOW please expand on your current system were the radiators converted to hot water heat
with a two pipe system or were they installed as steam radiators and kept as single pipe steam radiators????
I dont see that mentioned in your post???

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The firebox was sealed up when the oil conversion was done. I have noticed that when the oil burner turns off, there is basically no air going up the chimney anymore and the flue pipe cools down pretty quickly so (amazingly) I am not losing too much heat that way. There is no damper on the flue, so the firebox must be pretty airtight. The flue pipe is a 9" ID pipe going up a 56' chimney so the draw must be like opening a door on the international space station. To attach a pellet burner (Like the pellergy 3550) I think all I need to do is get some sort of new swing door made for the boiler with the burner bolted on?

When it was a coal boiler, the "stoker" was basically a bucket and shovel manual system. ie: You use the shovel to fill the bucket and then you dump the bucket into the burn chamber of the boiler.

The system was always hot water, never steam. The boiler has 2 pipes coming out of the top, one is a 2-1/2" pipe that branches off and feeds all the 1st floor radiators in parallel and the 2nd is a 2" pipe that branches into several 1-1/2" risers that go up to the 2nd floor. Then there are 3 pipes connected to the "bottom" of the radiator, all 1-1/2". One of them has a T in it and has the filler connection as well as having the return lines from the 2nd floor coming into it and the other has all the return lines from the 1st floor coming into it the last one narrows down to a 1" line going up to the vented overflow tank in the attic.

Last summer I added 2 Ts, 3 valves and a large section of 3" pipe with a T at the top and bottom with plugs in parallel to the 1-1/2" return line that doesn't have the filler or the overflow on it. I can close off the valves and open the plugs to drain the 3" ID pipe section and then put the bottom plug back in and that section will hold 1 gallon of water treatment chemical. I can then put the top plug back in and open 2 of the valves and the water will flow through my 3" bypass to add the chemicals to the system. In normal running mode, the 2 bypass valves are closed and the valve on the main line is open. Basically this gives me a way to add up to 1 gallon of chemicals, one chemical at a time, to the system. I drained and flushed the system to add this piping. I can also use this bypass to add a secondary boiler in the future without draining the entire system, if I need to.

maple1 said:

It is possible to succeed without buying & installing a new coal boiler, I think.

Do you even know if coal is readily available where he lives? It's not here.

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I did a quick check and despite the fact that I am within about a 2 hour drive of several coal mines, it is all loaded right onto ships and exported to the US and Asia. You can only buy it by the barge full here, there are no companies doing residential delivery. There is a company that sells pallets of burlap backs of hard anthracite coal that can be tailgate delivered (for a fee) but not rice coal.

Also, the local municipalities have banned new installations of "coal burning appliances for residential use". There are a lot of eco types trying to get all the coal mines shut down too.

The funny thing is, there are also 4 big pellet mills within a 2 hour drive of me but none of them sell in small quantities, it's all bulk exported in big ships. The pellets I use come in by train then container ship from a pellet mill about 300 miles away. I'm hoping someone will eventually start doing bulk residential delivery from the mills.

Previously, about 25 years ago or so before the oil conversion was done, the coal was brought in in big burlap bags that were stored in a big empty room in the middle of the basement that has no outside wall. There is a small elevator running up the middle of the house (like a dumbwaiter) from that room that was used to bring buckets of coal up to the 1st, 2nd and 3rd floor fireplaces and to the kitchen coal stove.

The closest a delivery truck could get to the house is about 100 feet, the oil deliveries have to use the truck with the "extra long hose".

My pellets get delivered on pallets and the guy has a small forklift on his truck that he uses to bring them to the house and stack them in the storage shed.

I am really hoping that I can remove the oil burning "gun" type burner from the boiler and replace it with a pellet burning "gun" type burner. I have a good location for a large pellet hopper to feed the burner.

Basically I want to remove the oil burning gun and the oil tank and replace them with a pellet hopper, auger and pellet burning gun.

I'm not worried about backup heat because I can use the electric baseboard and space heating and the pellet inserts and stove in the event of the boiler being down, it's just more $$ to use the electric.

I don't have an extra unused chimney near where the boiler is, just the one that the oil is currently using. I know I'd have to put a SS liner into it to burn pellets but that isn't too expensive or complicated.

S.Whiplash said:

The following thread covers the discussion pretty thoroughly.

https://www.hearth.com/talk/threads/pellet-burner-conversion.58246/

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Thanks! That thread gave me pretty much all the info I needed!!

I took the oil burner off my boiler and opened up all the doors, the high heat silicone that sealed them needed redoing anyway so I scraped it off carefully and got everything opened up. It is a 3 pass cast iron boiler, with a very large burn chamber and a very large ash compartment under the burn chamber that used to have a grate that opened and closed to dump the coal cinders (I still have the grate but it's not in the boiler right now). There are 3 sets of doors on the front, the bottom ones are for cleaning out the ash chamber, the middle ones have been removed (I still have them) and they were for putting coal in and they have adjustable air inlets on them. In their place, a big single door has been put on with the oil burner bolted onto it. I can remove that door really easily by undoing 2 bolts that were put where the hinges for the old doors went. The top pair of doors opens up to the heat exchanger and cleaning it from those doors is really simple and easy. I am thinking of making a little cleaning device by attaching a compressed air nozzle and a vacuum attachment to one of those snow brushes for cleaning off cars. I could hook that up to my ash vacuum and air compressor and use the brush and shots of the compressed air to clean it out, with all the mess being sucked up by the ash vacuum. It was DIRTY in there from the oil!! It was spotless last October before the heating season, my service guy cleans it out in Oct each year.

There is a turbulator in the heat exchanger, pretty primitive but it's there. Also the huge flue opening at the back of the heat exchanger has a couple of firebricks blocking the bottom half of it. I guess the draft was too much? It is a 9" ID flue... There is another big cast iron turbulator plate thing that isn't currently in there, which I assume was removed when it was converted to oil. I still have it and can put it back easily if needed.

Some modifications were made when it was converted to oil. The door with the oil burner, the sealing up of the doors with high temp silicone, the addition of rope gaskets to seal it up, and the lower ash box was filled with sand and some big firebricks were siliconed in place to cover the ash door opening. The coal grate was removed from the bottom of the burn chamber and a big cast iron plate was slid inside on top of the sand. It's not welded in or anything, easy to remove if I want.

The boiler is BIG and holds about 80 gallons of water (total system volume is 250 gallons) so I am not too worried about short cycling. Just heating up the water in the system from room temp would take over 250,000 BTUs with zero losses and 100% efficiency so even a 160,000 BTU/h burner would take at least 2 hours to heat the water up to full temp from cold and at least an hour to heat it up to full temp from standby temp (140 F). The max heat output of the distribution system (assuming a room temp of 50 F and a water temp of 170F) is 250,000 BTU/h based on the BTU capacities of the radiators, and doing all the fancy calculus, the minimum time for the system to drop the water from 170F to 140F (assuming a constant air temp of 50F) would be just under 2 hours. Of course, the air temp wouldn't be constant at 50F and would rise (the heat is going somewhere!) so the real time would likely be more like 4 hours. I have a fancy thermostat so things like someone leaving a door open for 5 minutes wouldn't trigger the thermostat to call for heat and cause a short cycle (I fixed that problem VERY early on because the oil burner was short cycling like crazy)

I contacted the Pellergy people, they won't sell me a conversion burner because there is no dealer in my area. They offered to sell me a full new boiler which would end up costing me at least $30,000 for the boiler and everything I would need and all the work I would have to do to get it installed... no thanks...

That basically left the Woodmaster Renovater, the Pellx, the Ulma and the Swebo.

I couldn't figure out if the woodmaster renovater and the ulma are the same thing? Looks like it and Ulma lists woodmaster as their US distributor so I think they are?

For the Swebo, from what I could tell, it doesn't have the air compressor hook-up to do auto cleanout. Can anyone confirm if it does or doesn't? The PB/50 looks like it would be perfect otherwise, the higher BTU/h range would be nice to have.

For the Pellx, the 35kw model looks like it has and does everything I need. The BTU/h are a little bit lower than I would like but can work.

The Woodmaster Renovator 30kw also looks promising, a bit low on the BTU/h but from what I have been able to figure out I can compensate for that with my aquastat and thermostat settings. The Ulma 3000 TCA seems to be basically the identical burner? Ulma makes some bigger burners, the Eco 60kw in particular, but I don't know if anyone in North America is carrying them and also I wonder about if they are available for 60 hz power?

Anyone have any thoughts or comments? In particular, can anyone confirm if the Ulma and the Renovator are the exact same thing? Or if the Swebo does or doesn't need more frequent cleaning due to the lack of an air compressor fed auto cleanout function?


I also found something REALLY cool, but that's an issue for an entirely different thread! I'll post the link here and start another thread about it in the right forum category because it's a general pellet thing not a boiler specific thing. http://www.pellvac.com/e/system1-e.php

bholler said:

I don't know but it doesn't seem like the best idea to me to dump allot of money into a 100 year old boiler. Yes they are tanks and last a long time but they do eventually give out. I would look into new boilers seriously as well. i am sure you could convert it but the newer ones will be more efficient i am sure. I could be totally wrong but that is my 2 cents

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Yes you are correct, but my idea was to get something that could later be attached to a new boiler, so almost nothing is being spent on the oil boiler that doesn't just move over to the new one when the old one gives out. For example, the pellet hopper, auger, burner etc, would all stay the same. The mounting plate would have to be made but that isn't very expensive.

The radiators and distribution system needs a really high mass and high volume boiler, or a big water tank with a heat exchanger and circulator that would "act like" a high mass/volume boiler and then a smaller boiler could be attached to the heat exchanger, potentially with storage in that loop.

So, if/when the big old boiler gives out, I would replace it with a large insulated water tank of the same water volume, piped into the distro system and then treat that big water tank like the heat load of my new and modern boiler/storage system which would use the same pellet hopper and burner.

I managed to find the engineering manuals and stuff for my big old boiler on microfilm. Those "dead guys" really knew what they were doing. The efficiency of that boiler was measured at 70% burning coal. Basically they hooked it up to a big insulated barrel of water and put in a measured amount of coal of a known energy density and then burned it and measured the increase in water temp and then compared that to the "input btu" of the coal. They also did the same thing with wood and compared as well, wood got 50% efficiency which is pretty good for a non gassifier 100 years ago!

Guys who understand thermodynamics, combustion, flow rates etc... the way those dead guys did are mostly working as university profs and doing research these days. Back then, they were installing boilers!

From an efficiency standpoint, the amount of money I'd save on pellets would be less than $1000 a year if I went from 50% efficiency (I'm sure I will be better than that!!) to 85% efficiency (pellets are super cheap here). That makes for a loooong break even time on the cost of a new boiler, 2 storage tanks, piping, pumps, labour, etc etc. Of course, if the old boiler gives out then it's not a question of payback time anymore but as long as it's still functional, it comes down to payback time.

Switching from oil to pellets on the old boiler has a payback time of about 2-3 years, much better.