Harman SF-360 Trident wood/coal boiler

Does anyone have one of these boilers,I was thinking about purchasing one. I have a little over 3000 sq. ft. to heat. Existing boiler is oil fired hot water baseboard. I want to tie into existing boiler,but I want to heat with this Harman ONLY. I was wondering if it would do the job? It claims to heat 4000 sq. ft. It holds 42 gallons of water. I have a add on wood boiler presently,that holds 50 gallons of water. It does NOT satisfy my needs currently. I don't know its BTU rating because it was homemade. Though it is homemade, it works well. I'm wondering if the newer technology of the Harman would recover quicker, with less water. Any Ideas would be a help!

I wouldn't worry about the difference in water volume, I was stunned when I found out the volume in an A-A boiler. IIRC its about 22 gallons. The Harman you speak of is rated at 180,000 BTUs, I would compare that to the numbers on your existing oil unit, it should have a plate with gross and net BTU numbers. If the oil is the same or less than the Harman, you should be able to heat the house and domestic water with no trouble with that unit. I would think the oil burner you have is in the 140-160,000 BTU range.

PS, I checked their website, looks like a nice boiler.

What exactly is a A-A boiler? I don't have a clue.

It is a stoker that has been around since the mid '40s and consider by some (me included) to be the best. They are the Rolls Royce of anthracite burners and are priced accordingly.


http://axeman-anderson.com/pdf/anthratube.pdf

Thanks for the info coaledsweat. I need some advice from someone who has experience in handfired boiler operation and design. I'm wondering how a handfired boiler idles when there is no call for heat? And how does it prevent overheating besides the temp./ press. switch? Whats the best transfere of heat in boiler, water tubes passing through stove above the fire? Or serpentine conduction on the sides and top of stove? Or any other idea's? I wanted to have a idea before I went shopping for boiler. I want a dual fuel due to the fact that I have a good wood supply. Any ideas ?

Hello tigboy, since I've designed and built my own wood/coal boiler I'll offer you my opinions and experiences.

First, when you put burning wood into the equation, you limit your options for an efficient waterjacket/heat exchanger design. The reason is that the waterjacket/ heat exchanger design for a wood stove must be simple with easily accessable surfaces, with cleanout doors and access panels. Wood smoke condenses on surfaces cooler than 250-300*, leaving hard, crusty creosote behind. You must be able to scrape the creosote off the inside of the boiler to keep the boiler's efficiency up. The creosote insulates the waterjacket from the heat of the fire.

With a coal only boiler, the by-products of coal are heat and fly ash. Fly ash will fall off or can be vacuumed off the waterjacket/heat exchanger surfaces in a coal boiler. In an EFM or Gentleman Janitor, Keystoker, Harman and other coal boilers, there are many surfaces, mostly vertical that the hot coal-heated air pass over and between exchanging heat into the waterjacket. These many surfaces are inaccesable to scrapers that would be needed to remove creosote, but these surfaces can be cleaned with just a brush, or stiff wire and a vacuum. The fly ash just falls off. The efficiency stays high.

So for wood burning, the waterjacket/heat exchanger design must allow scraping and removal of the crusty creosote. I made my 'Big Bertha' boiler with all flat internal surfaces, including tubes above the fire, all accessable with a small garden hoe and scraper, all sweepable and accessable with a shop vacuum. Extra cleanout doors, and the entire boiler made of Stainless Steel to offset the effects of the corrosion from creosote deposits.

I have a lot more surface area inside my boiler than usually found in wood burners, but much less compared to the average coal boiler this size. So my 'Big Bertha' is a very good wood burner, but not so good at recovering heat from coal. I also made it have a huge water capcity to absorb temperature and fire overshoots.

The control of a hand feed boiler is based on water temp and an aquastat, the aquastat controls a combustion blower and often a moveable flapper over the air intake for intake air control. But, once you have a large wood or coal fire burning, you cannot very easily slow it down to an idle. Shutting off the air will still take many minutes to reduce the heat output of the fire. So overshooting the target water temp is planned on and common.

A typical scenario that results in an overshoot would be the morning ritual: people wake up, thermostats are moved to warmer temps, either automaticly or by hand, and people start to take showers. All these put a large BTU load on the boiler, so the combustion fan runs, the fire gets really going, creating lots of heat to heat the house and domestic water. But suddenly, everyone gets in their cars and heads off to work and school. The thermostats are turned down, no more hot water is being used. The big hot fire is roaring along, putting 140,000BTU per hour into the water, but the water is no longer circulating, the water soon gets too hot, and the fire is STILL not back at an idle..

So a boiler needs a 'heat dump' circuit, one that can be automaticly turned on to 'dump' heat when the boiler threatens to overheat and boil. Or a boiler needs to be set to run at say 150*, and have large enough water capacity to absorb the heat and not boil.

The introduction of oil and gas burners that could instantly remove all heat from a boiler made the above scenario no longer a problem.

So a big hand feed boiler has a problem it has a big fire to make lots of heat, but the instalation must be done so that if it overshoots the water temp significantly it has a way to dump the excess heat. Or it must be able to store it in excess water. The problem with excess water is that it takes lots of time to heat this water up to a funcional temperature once it drops below target temp.

Stoker wood boilers can almost instantly reduce the heat output by stopping the feed of fresh coal, and stopping the combustion blower, the fire idles down very fast, almost eliminating overshoots. But you can't burn wood in a stoker boiler.

My fingers are getting tired, I hope I've answered some of your questions, and not created too many more.

Greg L

Thanks LsFarm, you've been a great help. I have been experimenting, I made a boiler out of my Harman mk III. I too like to tinker with stuff.Being a metal fabricator/welder by trade,farmer by destiny,I love trying to create solutions to outrageous heating/fuel prices. My converted Harman works well, but I think the coal/ heat transfere you spoke of is my problem. I really want to heat my whole house with a dual fuel system. One problem I have may be my setup. I elected to put my boiler in the attached 2-car garage.The garage is insulated, but the 16 ft. overhead door is not. The garage stays so warm from the radiant heat,we spend alot more time out there.(Maybe a living room in future). My Harman is not insulated and the water jackets are somewhat large according to some. My oil boiler is rated at 130,000 BTU. I think the Harman is just a little small. By the way,does your homemade BB heat the whole house? If you did it again, what would you change?? It's been great talking to you and coaledsweat. I may not succeed in my quest to build a dual fuel but I will keep going back to the drawing board. Hope to hear some more ideas. P.S. If I knew the SF-360 Trident would satisfy my needs, I really just wanted to copy the design,It's interior design looks alot like the Harman SF 250 dual fuel.

Hi tigboy, yes the 'Big Bertha' did heat the whole house, but because of it's water capacity, it there was a significant BTU load put on it, and the water temp dropped, it took a long time to recover the temperature.

If I were to do it over again, I'd make the heat exchanger like an EFM or Gentleman Janitor. and forget the wood. Or possibly add additional cleanouts so I could clean out the creosote from the smaller narrower passageways designed for coal burning.

I made a 4" water jacket all around the boiler, and it holds ~200-300 gallons of water, too complicated to calculate. This water capacity is good for wood, but not for coal. My AA boiler which has a fast recovery, roughly 15 minutes instead of hours, is only ~30 gallons.. If I was to make another boiler I'd make the water jacket and heat exchangers only 2" thick so the water can't develope laminar flow inside, and I'd make it much less water capacity for coal. With wood, this 'new' design may have problems with hot spots and localized boiling.



Take care, Greg L

Question,what is laminar flow?
I am beginning to see that I am not going to get the best of both worlds,wood / coal boiler efficiency. It's either one or the other.If that is the case then I will pursue the coal.
Why is less water more practical for coal? I'm guessing it heats faster with a thinner water jacket,but how does this improve recovery? I think I'm lost!! HELP

tigboy wrote:Question,what is laminar flow?

If you ever watched cigarrette smoke rise , it takes two forms. When laminar, it rises straight up very quickly in a narrow column. The trails of smoke are in a narrow, laminated form. When turbulent, it rises very slowly, tumbling and rolling in a random and misshapen path.

Laminer flow takes less area and produces much higher gas speeds.

Hello, Lsfarm
I've bin reading alot in the forum and have seen the term ('heat dump' circuit) used a few times. How would you control this? If you want it to go on at 200*?

Laminar flow occurs when a fluid flows in parallel layers, with no disruption between the layers. It can be a liquid or a gas.

If you could have an ideal boiler you would want one that could produce hot water instantly and could be turned on and off with a switch. Such a boiler would use the fuel most efficiently and have no standby losses. The closest to this ideal would be and electric on-demand domestic hot water water heater. With solid fuel boilers the temperature of the combustion gases are low compared to liquid or gas fuels. This means the surface area and hence the water volume are larger. Greater water volume slows the change to demand, increasing or decreasing. A large volume also increases standby losses. Whether it matters depends. If the standby loss also heats you house it doesn't reduce the operating efficiency. If however your boiler is outside the heated space it's a loss. If you are heating your domestic hot water in summer time with coal and the boiler in space you are air conditioning you are in effect air conditioning the boiler losses. Some argue a large boiler water capacity is desirable because it a heat reservoir. But boilers are generally poorly insulated. In my opinion if you want a heat reservoir use a well insulated tank.

Boy, I sure got alot to learn yet. I think this forum is great. My home made Harman works pretty good,Of course there is always room for improvement.I have this idea that I don't know if it will work. By the way Yanche, I do use all the radient heat off my boiler,it heats my garage.I guess that is somewhat efficient. Anyway I was thinking about adding a 50-80 gal. electric hot water tank to my setup. I was just going to use the tank as a resevoir for hot water,not domestic. I wasn't going to energize it, just use the insulated tank. My thought was I could heat it at night when the thermostats were down and have a head start for the morning. Two problems are: A) Does it sound feasible??
B) I'm not sure how to plumb it in if it would work??
I had ideas of making a stand and keeping it above the boiler for possible thermal siphon. Anybody think I'm crazy?
Thanks for all the imput.

The laminar flow I refer to is in the too thick water jacket and cross tubes in my boiler. The water flowing next to the very hot steel wall or tube wall moves slowly, and acts like an insulating layer between the steel and the center core of faster moving water that doesn't pick up enough heat.

If the water jacket was thinner, there would be more turbulence in the water flow and better heat transfer from the steel wall to the water.

Greg L