Low Load Capabilities of the Coal Gun

Patrick,

I am new to the forum. I have searched for some info on this topic but have found limited information.

I am planning a boiler replacement project in about 2 years. I am planing now and starting preparations this winter, so when the time comes, minimal work will be required for the installation. I am seriously looking at switching from wood to coal for various reasons and am looking at a coal gun boiler.

I plan to heat the house, my domestic hot water and my workshop. The combination of these three loads places me at the AHS 260 size boiler. I plan to install the new boiler in the workshop and pipe heat to the house.

The workshop is a spuratic load, as I do not heat it when it is not occupied. The house and domestic hot water is about a 100 MBH load.
The AHS web site states that the thermoash control will maintain fire at low load conditions.

What is considered low load condtions? - Is there a "rule of thumb" of percentage of boiler capacity?

When do I have to worry about loss of fire from low load?

Do you have any data for idle and standby losses so I can do an economic analysis to see if there is a point that it would be more economical to switch back to a standalone domestic water heater?

Thanks in advance.

The following statement is taken from the AHS site: "With the Thermo Ash-Monitoring controlled grate, it is possible for the fire to remain lit for days during low usage periods without having to adjust the grate by hand as would be necessary in a timer-based grate control system."

I think the above statement says something different than: "The AHS web site states that the thermoash control will maintain fire at low load conditions."

I think it is possible to use the AHS during summer months with the standard timer control. I say this because it is my understanding that at least one member of the board is doing it. On the other hand, I don't think the thermoash control will always maintain a fire--it doesn't have the sensors and logic to react to a fire going out. The thermoash control as implemented on the AHS is limited to controlling ashing--it doesn't control when the forced draft comes on.

I would not use a S260 for your application. If you want an all coal solution I would use two S130 Coalguns. That way you can easily shut one boiler down when the demand is reduced. Another approach is to pair a propane or natural gas boiler with a S130. Slant Fin has a small two section cast iron boiler rated at 34,000 BTU. See http://www.slantfin.com/product-sentry.html. It's size would be idea as a load peaking boiler for a S130 and summertime heating of domestic hot water. Having natural gas or propane would also be a good fuel for an emergency generator.

Two size 130 coal guns would certainly be the optimum system but also very expensive installed solution. Oil and propane are my alternate back up fuels, but after combustion effiency calcs, both run in the $35 to $37 per million btu's range. Depending on where coal price settle in after this winter, I expect coal to run at $11 to $14 per million btu's. In my final system design, I anticipate a propane boiler in the house with a backup emergency. An oil boiler exists in the workshop and will be used for backup out there. I currently use wood as primary fuel and would be my cheapest source but is labor intensive. I need fewer hobbies, so I can pursue other interests. I have many years of HVAC design experience and system design is not my problem. I do not have experience with coal fire appliances however.

My problem is that I have house load at peak of 100 MBTUH and a workshop load at approx. 150 MBTUH peak. The workshop is used some nights for 2-3 hours and on weekends. Heat is turned completly off otherwise. This lopsided load presents the design challange.

I know as with any boiler, oversizing will result in loss system operating efficiency. The solution now becomes one of economics. How much do I spend on equipment verses how much higher cost fuel do I buy? I can figure this out if I enough info on the operation of the coal boiler I am looking at.

As far as the thermoash not firing the boiler, I understand this concept. It will shake the grate only if the grate ash gets cold, not bring on the draft fan. If the boiler does not have enough load, one of two things will happen: A. The boiler will overheat and require a dump zone or B. If the boiler is air tight enough, the fire will go out.

I have an option B that may turn out to be a better economic option. This would be installing a s130 with a large thermal storage tank that would charge during lighter load times such a nights and workdays when no one is home. However, the sizing and operation of thermal storage with large fluctuating loads can be somewhat tricky.

So, with all of that said, the bottom line is that I am trying to figure out coal operation and consumption during low loads and what is defined at a low load for a coal-gun boiler.

The S130, if located in your outbuilding, will provide a fair amount of heat to the building just from losses. I have mine in a 1600 square ft building and it maintains it at 50 degrees or highter during all but the coldest days without activating the heating system in the building.

I suspect your lowest cost approach would be the S130 and use existing systems for the few times of the year you hit peak loads.

A large water thermal storage would be a good idea. I'd be interesting in seeing your design trade off vs. cost analysis. Given that you have HVAC design experience I would suggest any water thermal water storage be designed so it could also be used for chilled water storage. I don't know where you are located but central A/C operating costs will likely become the next big energy cost. I have a gut feeling that ground coil heat sink chilled water systems will become the high end A/C system in the future. They will have higher capital costs but would have much lower operating costs especially if you get much lower night time electric rates. Obviously such a system would require a thermal water store. When I was still working my employer installed two 100 ton ice plants, which make ice at night and then used the chilled water for daytime cooling. We bought electric at raw rates since we had our own substation and the night time rate was almost give away prices.

Chilled water, Ice Storage, Geothermal, I have had experience with it all. As far as A/C, a window fan works for me especially with $0.14/kwh electric. However, we are getting a bit off topic.

What are people's experience with heating domestic hot water with the S-130 or S-260 during the summer? Do you shut them down and go to a backup water heater?

I used 85 gallons of propane in three months this summer for domestic hot water/cooking/clothes dryer. I use about 41 gallons of propane in the winter for cooking/clothes drying when I heat my domestic hot water with my wood boiler. I have a potential savings of 44 gallons of propane over three months. Last price I paid was $2.69 a gallon. This is $1.31 per day in propane cost for hot water.

So, one of my questions I am trying to answer is: Will I use less than $1.31 a day in coal to heat domestic hot water. If coal is $0.10 per pound ($200.00 per ton), will I use 13 pounds of coal or more per day for domestic and standby losses?

I've used my S130 to heat domestic hot water for a few summer months. Coal consumption is obviously lower than winter months but not as low as I expected. My expectation was based on my oil boiler usage and BTU to BTU fuel comparison plus a guess for the idle coal usage. I was using about 17-18 lbs a day. When we went on vacation I shut the coal boiler down and upon returning I'm just using oil. While it may be a little cheaper to heat with coal the convenience of oil is what I prefer. My installation has two many fixed piping losses to make domestic hot water heating only an efficient process.

I suspect that fixed piping losses will be a problem for me also due to a remote building installation so I am designing to maintain a propane fired domestic water heater for summer loads.

My main focus in my design will be running low heating loads plus domestice load at fringe seasons. I use oil as backup and heat for the fall/spring seasons. My goal is to elimnate my oil/propane consumption during these times.

How does the S-130 perform with say intermittant 10% loading on it? You might see this type of loading with weather we are currenting having where it is 70 deg. during the day and 50-55 deg. during the night. The house needs a little boost in the evening and morning to say comfortable.

steamup asked: "What are people's experience with heating domestic hot water with the S-130 or S-260 during the summer? Do you shut them down and go to a backup water heater?"

So far I have switched to oil for DHW once the weather warms to the point that I am airconditioning. I did an analysis of my oil consumption for DHW and for heating a small lap pool and it averaged .77 gal/day. My coal use during late spring with very little or no heat load suggested that my savings would be very small or non-existant to continue with coal. That analysis was done at $2.50/gal for oil and made no allocation for the extra work involved with coal. Location of the boiler in an outbuilding with resultant losses certainly contribute. I think a second contributor is the relatively large fire pot on the AHS-130. Considerably more coal is burning than in an EFM or Keystoker. The firepot on the AHS-260 is considerably larger than the 130.

The Bureau of Mines report on the A-A boiler has data on summertime usage. Since it's a similar boiler it might be helpful.

See: Bureau of Mines Report 4936 Axeman-Andersen Antratube Boiler

Looks carefully at the table or the Excel spreadsheet file for the data. It takes a bit of study to understand it.

Yanche,

If I read the table correctly it says that when burning pea it takes 12 lbs per day to keep a fire and 8 lbs per day for the domestic hot water use of the test house or a total of 20 lbs per day. While I haven't done a careful test those figures are certainly consistent with my limited observations.

Thank you Yanche for this link. I say this report some time ago and forgot complete about it.

Looking at the data and report, what is says and doesn't say gives me what I need to know for the AA130.

As with Bob's reply the AA130 has slightly greater usage than what you experienced. Since the AA in the report did not appear to have the insulated jacket and the s130 is supplied with one, your lower consumption experience on DHW seems to be right on what one could expect.

One thing the report did not mention was any problems with running on domestic water load only. It can be concluded that there were no significant issues with running this low load.

Data in this report raises another question I wish to ask the manufacturer about rated capacity. The report stated that the AA could achieve 130 buth output only with an increase in fan speed. I will start a new thread on this. I am still leaning towards a s260 but equipment cost may cause me to re-think.

steamup said: "One thing the report did not mention was any problems with running on domestic water load only. It can be concluded that there were no significant issues with running this low load."

My understanding is that the AA 130 includes a timer to maintain fire during low load periods. The AHS unit does not include a timer although I think one could be fitted without much difficulty.

I know it does directly apply to your short stove pipe but for some ideas on how to install it into a chimney thimble with a damper check my post here: 5" up to 6" flu pipe

Perhaps a 6 inch pipe concentric with your 5 inch pipe outside your shed would be the way to do it. The 6 inch would have the baro inside the shed. The 5 inch would stop short of the roof line just like in my baro installation. Let us know what finally works.

How about a heat dump solution? An Aquastat control that dumps heat to the pool whenever the water temp exceeds the regular Aquastat high limit set point. Or an injection pump that mixes the boiler water with pool return water to lower the temperature that gets pumped to the house. Too many pumps but it would work. Just trying ideas.

I agree with Bob as to the baro and manometer locations.