Draft control?

[boondockin]

Hello,

I have spent the past couple of weeks in my garage fabricating a new wood boiler for my house. I have the boiler built but need some help with the controls. I currently have a gas boiler installed that simply runs with 3 zone pumps and no valves. I want to run these pumps into the wood boiler, exit the wood boiler into the gas boiler and onto the heating system. Attached is a Crayola CAD drawing of the basic boiler layout. As you can see, the cold water comes in the bottom tubes and exits through the upper tubes as hot water. The draft air comes in a 6” pipe on the back of the boiler, down the back, under the coal bed, and is delivered via jetted tubes about 7.5” above the coal bed. The entire firebox is lined in brick up to the water jackets. I want the water temperature in the boiler to control the actuation of the damper, i.e. the temp reaches 200 degrees and the damper closes and at 160 degrees the damper opens. Can you recommend economical (eBay is OK) controls for accomplishing this? Also, I am going to run a line to a domestic water heat exchanger. Does this require it’s own circulatory pump to function effectively? If so, how is this pump actuated?

Thank you in advance for your help!

Tom

 

[KeithO]

Tom: Is this for a cabin ? Most insurance companies will not insure the property if you have an unlisted appliance.

To the operation of your boiler: If you are going to continue to get along with your neighbors, and be happy about the wood consumption, here is the best sugestion: Don't use a damper style regulator at all. Get yourself some water storage (knowing how much water requires knowing how much wood you can load in the boiler, what the combustion efficiency is). Being very optimistic one could assume 50%, you may approach that if the stove design is perfect and dialed in and you run the fire "flat out" as long as it will burn. Bearin mind that this is exactly how the gas boiler is designed. Turning the gas on and of is obviously easier than starting a wood fire, but thats a detail...

So not knowing much about your boiler dimensionally or really being able to comment on the design except to say that you have to provide for heated air for a good clean secondary burn close to the top of the firebox, as well as something that will trap heat to keep the secondary burn "lit off". For an example of this concept look at this article (broken link removed to http://www.woodheat.org/technology/outbobpen2.htm)

Here is a basic thumbsuck: I'll assume you will be burning seasoned white oak, which has a recoverable BTU value of 4484 btu/lb. The "full heat value" is listed at closer to 7000btu/lb but because of the moisture content, a lot of that heat goes toward vaporizing that water. I'll also assume you have a firebox that will take a 3 cu ft load of wood, which with white oak is 47.2lb/cuft thus 142lbs for a full load (hope you are fit ?). Now turning to your home, of which I also know nothing, I'll assume you have a heat load of 60 000 btu/hr (which is a pretty high heat load - so either a good size house well insulated or a badly insulated small place). That is a nice location right up by Lake superior BTW, but I bet those westerlies come through there fast and furious with heavy lake effect snows in winter ?

So, with the above numbers, 3 cu ft firebox, 142lbs of white oak, assuming 50% efficiency of your boiler, 60kbtu/hr heat load, the wood in the firebox should deliver 635k btu of heat, of which 50% goes out the chimney, leaving 317k btu to heat the house. What that suggests is that the load of wood would last a bit less than 5 1/2 hours before needing a reload. You would be going through a LOT of wood at that rate and from the article I posted the link to, the damper type boilers have efficiencies of around 24% ! You should have no problem setting up the boiler to achieve a nice steady burn at those kind of rates, just like a wood stove.

If the numbers look a little excessive, it may be that you have a smaller well insulated house and your heat load may only be 30k btu/hr. That would translate into close to 11 hour burn times per load which would be pretty good. I heated most of my 1300sqft floor plan with my corn stove last winter and I estimate it was cranking out something of the order of 20k btu/hr. I have a fairly well insulated house and our climate is nowhere near as severe as yours. I am also well protected from the North wind in my location.

If you have an insulated water storage tank of a thousand gal, it allows you to run the boiler more "wide open" and you will get a hotter cleaner more efficient burn. The boiler burn time will be shorter, but the house can run off the stored hot water in between burns. The water temperature will gradually drop, which is a more user friendly "mode" that constantly needing the boiler running and having a fast drop in temperature if the boiler goes off line for any reason. You will need a seperate circulation pump between the storage tank and the boiler which is thermostatically controlled. Below a certain temp the pump is off to avoid heat loss through the tubes in the boiler. Once you fire the boiler up, the pump needs to start running immediately (possibly with a timer to latch it on for a bit longer than it takes to heat up) Once the water temperature comes up, the thermostat latches the pump on and it stays on until the fire has burnt out and the water temp drops below the low threshold again. The "loop" circulation pumps can be controlled thermostatically by zone and they circulate only water from the water storage tank, not directly from the boiler. You didn't mention where the boiler was going to be installed (inside or outside) but inside is recomended for least heat loss and freeze protection in case the boiler actually goes down for any reason.

So depending on how your boiler is designed / sized and what its possible combustion efficiency will be, you have an idea of what may be possible.

Keith

 

[Gooserider]

I would reverse the flow direction in the water jacket - you want a "counterflow" setup so that the hottest water is closest to the flames where it can "peak" it's heat level, with the cold water entering the coldest part of the firebox so that it can pick up whatever heat is in that area before going to the hotter regions. This is the same way that nature builds any sort of exchange system - look at a fish's gills for a perfect example.

Otherwise I'd second the comments on the value of heat storage.

Gooserider

 

[boondockin]

Keith,

This is for my home not cabin. I contacted my insurance carrier before I began fabrication. They are a local insurer thus understand how things work up here. Once the installation is complete, they will write me a letter stating that the boiler has no effect on my coverage.

Regarding the boiler itself, this project is on a limited budget. I forget my total natural gas consumption last year but it equated to roughly 10 cord of hardwood @ 50% efficiency. Needless to say, the $1900 in gas severely limited my available funds. Originally, I planned to use a boiler that my father built 25 years ago but ran into some issues with the critters that moved into the water jackets. Even more of an issue was the “debris” they left behind (gag!). When I came across some “scrap” steel I decided that I would reuse the fire brick and give it a whirl. The materials that I had available and my limited knowledge of boiler design are what drove the design. Installed, I should have less than $350 invested in the boiler.

Heating Requirements:

I have a 2000 sq. ft. –ish home. It is 2 stories and has blown insulation throughout. The downside is that the windows are single pane and the Westerlies from Superior do put a chill through them. The boiler is to be installed in the basement which gives me flue about 25’ above ground. I hope this height will keep the exhaust from bothering my neighbors. Besides, the outdoor boiler that the guy down the block installed last year still has them distracted!

Boiler Design:

If I were to fill the fire box within 2.5” of the top of the door opening, I have 7 ft^3 available volume (17.5”W x 31.5”D x 22”H). As far as dampening the fire goes, I had planned running the damper (naturally aspirated) on the intake side or possibly installing a blower (super-charged) on the intake with the exhaust side being free-flowing. I was presuming that the fire brick would help maintain the firebox temperature. I used 2.5X4.5X9” bricks. The floor is brick. The walls are covered with brick which stop 3” shy of the exchange tubes. In total, about 70 bricks were used. Also, the exterior of the firebox will be wrapped in 3” of fiberglass with a sheet metal cover.

Water Storage:

I do not have a storage tank yet but planned on using a 40-gallon hot water tank with a 50-ft copper coil inside for my domestic water exchange. This could be considered as storage I suppose. I can’t imagine what I would use for a 1,000 gal storage container. I have a 250 gal fuel oil drum that could be useful if properly insulated.

For fuel I have about 13 cord of semi-seasoned mix: White Ash, Yellow Birch, Soft Maple, Hard Maple. As a typical Yooper, I still have a few cord that needs to be cut/split.

I appreciate the link. That is a very interesting article. I think my current design limits me from adding the “re-burning chamber” as he did. Any additional comments/recommendation are encouraged.


Gooserider,

What you said about the direction of flow makes good sense although I don’t know how a fish’s gills work. You may have to elaborate on that one. 

 

[Gooserider]

Explanation of fish gill type "counterflow" exchange... This is greatly simplified, but the basic principles apply to any kind of "exchange" situation.


As most of us know, a fish's gills serve the same function as our lungs - they act as the exchange point where CO2 in the blood is exchanged for O2 dissolved in the water. Water goes in through the fish's mouth, and out through the gill flaps. What is important is the direction of the blood flow.

There are basically two "choices" - the blood could flow in the same direction as the water, from the front of the fish towards the back, or in the opposite direction, from the rear towards the front. Which direction makes a big difference - to understand why it is important to look at the physics of the exchange process.

The exchange is largely driven by the relative difference in the amounts of each gas present in the two fluids - the gas will move from the higher concentration area to the lower, it will not go in the opposite direction. Thus the blood enters with a lot of CO2 and little O2, and leaves with a lot of O2 and little CO2. The water enters with a lot of O2 and leaves with a lot of CO2.

For example sake, I will assume that both gasses will exchange at similar rates in opposite directions, so the total of the two will always add up to 100, (so we only need to track one gas for each, as the other will be doing the opposite) and that there are 10 measurement points that we can see how much of each gas is present. In between each point, 1/4 of the difference will be exchanged. (in round numbers)

If the flows are both in the same direction, what we see will look like this...

Exchange #....1....2....3....4....5....6....7....8....9....10
Blood 02 %....0...25..37..44..47..49..50...50..50...50
Water O2 %..100.75..63..56..53..51..50..50...50....50

Notice, you rapidly get to a balance at about 50%, and there is little exchange going on for most of the distance.

With a counterflow, the blood enters at the end of the gill flap, where most of the exchange has taken place, so it is at 100% CO2 exchanging with 90% CO2 so it gets about 2.5% exchange, at the next step the 97.5% blood encounters water at a lower percentage, so it exchanges more, and so on until at the entry point, the blood that has exchanged all the way through is encountering water at 100%, I'm not sure how to make the math work to get the right numbers but it works something like this... keep in mind that the blood is moving in the OPPOSITE direction of the water...

Exchange #....1....2....3....4....5....6....7....8....9....10
Blood C02 %.00..10..20..30..40...50..60..70...80...90
Water O2 %..100.90..80..70..60..50..40..30...20....10

(forgive ugly formatting)

The exchange is never actually 100%, but because there is a difference all the way through, it is much higher. I suspect if you do some Googling for it, you will find better examples.

Gooserider

 

[KeithO]

Tom:
Based on the numbers I gave you, a cord of seasoned wood would last just over 6 days. For a 6 month season with the same assumed heat demand, you would need close to 29 cords of seasoned wood. If your wood is green, the delivered heat would be much lower and to compensate you would have to burn at a faster total rate, in other words, using some of the wood to do nothing more than dry out the next load. Your consumption may go up to 45 cords. I don't think this is an attractive prospect for you. Even in the UP, wood is worth something, and it has to be cut, split, stacked, hauled to the furnace and ashes disposed of. That rate of wood consumption sounds like hard labor.

I think your firebox is too big. I would reline all of the sides you currently have firebrick in with more firebrick, to reduce the loading space for wood. I would also look at adding a "good" insulator (ceramic board) between the steel firebox and the outer layer of firebrick to keep the heat in as much as possible. Firebrick is good for storing heat, but a poor insulator compared to the "right stuff". If you can keep that firebrick hot, it will go a long way towards a clean burn. Have you considered the reliability of your boiler ? If you have a problem, are you going to run the propane water heater to avoid the house freezing ? Also, don't forget that you want as few exchagers as possible in the system when it is running. Heat exchngers only work when a constant temperature differential exists on both sides.

Regarding the house: is it 2 stories with a basement ( 3 levels ?) and is it 1000sq ft per level ? If so, it is indeed a big house to heat in that climate. How many people living in the house ? If only 1 or 2, would it not be better to think of space heating and reduce the temperature in the house to primarily offer freeze protection ?