Outside wood boilers

solarvic

Since this is a renewable energy forum, I am asking a couple questions about outside wood boilers. My brothers gas well will not run his boiler as of last week. He and I are doing research on different brands of outside wood boilers. My main question is about flat plate heat exchangers. Since his inside boiler is a closed pressurized system and the outside boiler is not pressurized that setup calls for a flat plate heat exchanger. The inside boiler is rated at about 145,0000 btu. I read somewhere you need a heat exchanger sized 150% the size of the inside pressurized boiler to transfer the heat from the outside wood boiler. The heat exchangers I was looking at said each plate is rated at 4000 btu. If that is correct I would need a 55 plate heat exchanger to meet that requirement. So my main question is would it be efficient to use a 80 plate exchanger? In Pa only phase 2 epa rated woodboilers are legal. Solarvic

waynefromnscanada

Re: Outside wood boilers

Hey solarvic, the ones I'm aware of here usually don't use a heat exchanger, perhaps they should. The bigger concerns among local owners appears to be the relatively short life of the outdoor furnace before they spring leaks (perhaps due to them not being sealed systems) the inefficiency resulting in much higher wood consumption than they were used to with their previous indoor wood furnace (perhaps due to the fact that heat escaping their indoor furnace helped to heat the house, while the outdoor unit is exposed to extreme cold and the winds strip away the slightest available heat), reliability problems with the ones using electronic controls, and for the neighbors - - - terrible smoke pollution. My neighbor has one just finishing it's second Winter, so far leaks are about the only problem they haven't had with it. I'm extremely thankful the wind is rarely in the exact direction to surround my house with acrid smoke, but when it is, I have to shut off the air exchanger, and even then, the smoke makes it's way inside. When the smoke bypasses my place, it sometimes descends on my neighbors homes like the smoke of a forest fire. I can only assume the ones around here are not Phase 2 EPA units, but regardless, it's amazing that for many years now Nova Scotia forbids installing or the selling of non-EPA wood stoves for homes, yet permits outdoor furnaces that emit such terrible pollution. And then there's the elderly couple who live in the road a ways. It's sad to see them having to keep a path shoveled from the house to the furnace, and having to go out in all weather, sunshine or blizzards, and struggle to push a wheelbarrow full of wood from the wood shed to the furnace. Sort of reminds me of the olden days before "indoor plumbing" when folks had leave their comfortable home in all kinds of weather to visit the outhouse and answer "the call of nature".
Just some things to consider.

Cariboocoot

Re: Outside wood boilers

From the other side of Canada where nearly every home is heated at least in part by wood .... I've seen exactly one of these outside boilers in our area. Think about that.

solarvic

Re: Outside wood boilers

Wayne, Now you know why Pennsylvania and a few other states require phase-2 certified outside wood boilers. The phase 2 are more efficient because they are designed to burn up the gases that previous models were spewing into the air. A phase-2 epa price is usually $2500.00 to $3000,00 more. Was even surprised that one model is built in California, the state that is in the forefront of pollution regulations. My brother is stuborn and will probably buy one of these furnaces. I will probably help him in the install. Solarvic

peakbagger

Re: Outside wood boilers

Swing on over the Hearth.com, they are a quite responsive bunch.

I realize that you will have tough time dissuading him but he will learn like many that OWBs even EPA phase 2 have a significant issue that they do not deal well with less than full output. When there is no demand for heat, the boiler has to go into low fire and still burn and in low fire mode is when they generate a large amount of pollution. The EPA solution is install fixed air ports that remain open all the time so the boiler always has enough air to burn cleanly and this means the boiler is going to crank out heat which has to go somewhere, so they overheat their house. Most folks just dont run them in those conditions, so they need standby heat for shoulder seasons. Also the new designs dont run well with less than dry wood. In PA, where oak is predominant, plan on drying the wood two to three years. Try to burn with wet wood and the boiler just wont perform. The right way to go is to incorporate thermal storage like a 1000 gallon insulated propane tank and only run the boiler on full once a day to heat the tank up and then run off the tank. If someone goes that far they install and indoor boiler with storage. The other alternative is GARN which is wood boiler built into a thermal storage tank. They are expensive and really seem to work only for folks with high heat demand like a farm or a community heating system.

The companies that sell OWBs are real good at making promises that they cant keep. Those pictures of folks burning green log length wood really has created some bad PR for OWBs. Sort of like small wind turbine manufacturers.

BB.

Re: Outside wood boilers

peakbagger wrote: »

The right way to go is to incorporate thermal storage like a 1000 gallon insulated propane tank and only run the boiler on full once a day to heat the tank up and then run off the tank.

Just to be clear--You are converting a "propane tank" into a hot water storage tank... Not heating the propane tank in -40C/F weather to keep the propane pressure up.

-Bill

solarvic

Re: Outside wood boilers

There are a lot of these outside wood boilers in my area and I know quite a few that have them. I agree that you need seasoned wood if you don,t want them to smoke. I had a central boiler at my place and it never smoked at all and it wasn,t epa certified. I sold it when I got my gas well!. Now I wish I would have kept it. In. brother has a natural gas fired boiler with a resivore in it. The outside boiler will circulate water thru a side arm heatexchanger to the waterheater than thru a flat plate heat exchanger and will go back to outside furnace to be reheated. The gas boiler inside will have a circulator pump to circulate the heated water from the flat plate heater to keep the water resovor in the inside boiler. The inside boiler would circulate the water to the wall heaters as before. Will still have propane to operate inside boiler for shoulder season or extreme cold if outside boiler can,t supply enough heat. My first question I asked was if you used a flatplate exchanger bigger then needed if it would be efficient and not cause any problems. No one answered that yet. I did see a wood boiler forum in my search and probably should join it and see what I can learn. solarvic

waynefromnscanada

Re: Outside wood boilers

solarvic wrote: »

You need seasoned wood if you don't want them to smoke. . solarvic

And that's one of the main problems in my area. As a selling tool, retailers are telling potential customers "Those things will burn anything, including wet wood".
So that's what a lot of owners do, get a tree cut down, block it up and stuff it in the boiler. And the firewood retailers push that idea as well, knowing the customer will burn a lot more wood if it's wet. Great for sales.

peakbagger

Re: Outside wood boilers

A response to BB, Yes the used propane tank is converted for thermal water storage. The reason folks use used propane tanks for thermal storage is that they were pressure vessels at one time so in theory will hold up to the 30 psig that normal boilers operate at. In some states, the "in theory" doesn't fly as once the tanks is altered with the appropriate new nozzles, they lose its pressure vessel certification unless the person altering the tank has an ASME R stamp to recertify the tank. If the state enforces ASME code (like Mass), the tank has to have an valid ASME certification. Most folks just install an altered tank and manage to sleep knowing that one day the tank could fail and hoping that their insurance company I willing to pay for the damages.

I personally for a few reasons have a 500 gallon unpressurized tank from American Solartechnics. I run the wood boiler once a day when its cold and then run the heat off the tank during the rest of the day. Two years since filling my heating oil tank and still about 40% volume left.

jebatty

Re: Outside wood boilers

Back to your original question on a plate hx. Btuh transfer capacity is all about Sides A and B input and output temperatures, approach temperature, gpm flow rates, and pressure drop. Then add in the circulator needed on Sides A and B to handle the pressure drop, both of the plate hx and of the plumbing on both sides of the plate hx. Plate hx manufacturers publish tables which help to determine the size needed: length, width, number of plates, size of ports. A good site to calculate the size of a plate hx to meet your needs: http://www.gea-phe.com/index.php?id=509&L=9

solarvic

Re: Outside wood boilers

Thanks for the flat plate chart. My brother bought the wood boiler today. So I guess I have a project helping him to install it. He already is cutting the wood so it can get seasoned for next fall. Solarvic

icarus

Re: Outside wood boilers

From what little I know of boilers (and it is little!) is inlet water temperature is critical for a cast iron boiler. Boiler temps below about 140F allow flue gas to condense in the boiler, leading to very short boiler life. This for a gas boiler, but is suspect it is similar for wood fired boilers as well. Modern modulating, condensing boilers get thier efficincy gains from running lower water temps, using the condesate to increase, but they use stainless steel heat exchangers instead of cast iron. My guess is the high failure rate if wood fire boilers is too low temps. Another issue is constantly introducing fresh water to the loop, which adds O2 which leads to rust.

Personally I am not a fan of wood boilers because they smoke way too much, largely because they burn large chunks of wood that are not well seasoned, and they are choked down too much of the time. Additionally, the complicated plumbing presents a Maintenace issue, as well as a free protection issue. Just more headache IMHO. I would go with a good EPA rated stove anyday given a choice.

Tony

niel

Re: Outside wood boilers

solarvic wrote: »

Thanks for the flat plate chart. My brother bought the wood boiler today. So I guess I have a project helping him to install it. He already is cutting the wood so it can get seasoned for next fall. Solarvic

solarvic,
a thought, why not go after the many fallen trees that are seasoned already? heck, some are still standing waiting to fall, but have been dead for some time already.

icarus

Re: Outside wood boilers

Wood truly doesn't season (dry properly) unless it is split. Do this experiment. Take two similar size rounds and weigh them fresh cut and mark them. Then split one into either quarters or halves. Store them as you would normally season your wood. Wait one year (or more) and reweigh the two pieces. You will discover that the split peice is very much lighter as a percentqge.

Then do the following, burn the two in a moderate fire, and after a bit, look at the butt end of each. You will most probably will notice a lot more water boiling out of the round than the splits. The problem with burning long (long length) rounds is that they are really not very dry, ergo they burn inefficiently and with lots of pollution.

Tony

waynefromnscanada

Re: Outside wood boilers

icarus wrote: »

From what little I know of boilers (and it is little!) is inlet water temperature is critical for a cast iron boiler. Boiler temps below about 140F allow flue gas to condense in the boiler, leading to very short boiler life. This for a gas boiler, but is suspect it is similar for wood fired boilers as well. Modern modulating, condensing boilers get their efficiency gains from running lower water temps, using the condensate to increase, but they use stainless steel heat exchangers instead of cast iron. My guess is the high failure rate if wood fire boilers is too low temps. Another issue is constantly introducing fresh water to the loop, which adds O2 which leads to rust.

Personally I am not a fan of wood boilers because they smoke way too much, largely because they burn large chunks of wood that are not well seasoned, and they are choked down too much of the time. Additionally, the complicated plumbing presents a Maintenance issue, as well as a free protection issue. Just more headache IMHO. I would go with a good EPA rated stove any day given a choice.

Tony

You obviously know more than most consumers about these things. The corrosion problem related to running lower than 140 degrees is also a long known issue with oil fired home furnaces. And yes, the unsealed water system is a real problem as well.
Re the large chunks of unseasoned wood, you're right there as well. Most people (around here virtually everyone with those outdoor boilers) do indeed burn wood that is either green, or still way too wet to burn properly, and this is encouraged by those selling both firewood and the boilers. How delighted consumers become when informed by manufacturers and retailers that these boilers "will burn anything, no need to season your firewood". I've tried to educate my neighbours of the need to properly season their firewood, but I may as well be talking to a wall, they've been convinced by others that it's not necessary with these boilers. They get their firewood cut in late Summer and delivered in 8 foot lengths, where it remains in a pile until just before the snow flies, when in a panic it gets blocked up in lengths that will fit the boiler. The part I enjoy is that due to the usual Winter wind direction, the massive clouds of smoke surround their own yard and buildings far more than anyone else's, including mine.

Cariboocoot

Re: Outside wood boilers

icarus wrote: »

Wood truly doesn't season (dry properly) unless it is split. Do this experiment. Take two similar size rounds and weigh them fresh cut and mark them. Then split one into either quarters or halves. Store them as you would normally season your wood. Wait one year (or more) and reweigh the two pieces. You will discover that the split peice is very much lighter as a percentqge.

Then do the following, burn the two in a moderate fire, and after a bit, look at the butt end of each. You will most probably will notice a lot more water boiling out of the round than the splits. The problem with burning long (long length) rounds is that they are really not very dry, ergo they burn inefficiently and with lots of pollution.

Tony

Yup. It's the same on this side of Canada where we have lots of dead trees standing thanks to an uncontrolled pine bark beetle epidemic.

What's more, standing dead is better than downed trees. Once they're down they start to pick up moisture and become sodden. Also if they've been standing dead too long they start to dry rot where they are.

It's not as easy as picking up a few dead trees and burning them. Even the type of wood and where it is makes a difference; our 'softwoods' (spruce, pine, fir) are harder than most places' due to the short growing season which makes the rings closer together like 'hardwoods'. Less creosote. This is fortuitous because most of it is softwood around here. And splitting the birch we get is best done with a machine because of the same reason; it's really hard!

solarvic

Re: Outside wood boilers

By law in Pennsylvania you can only buy a EPA certified Phase 2 outside wood boiler. The burning process is refined so they don,t smoke because of the higher burning temperatures. The gases that used to go up the chimney are burned in a gasification process that raises the temperature to 2000 F. most of the heat is extracted to where the exhaust temp is about 250 F. Owners of pre EPA outside wood burners are grandfathered to use as long as they last and are located at least 150 feet inside their property lines. I know of a person that owns one of the older ones that is only about 35 feet from his neighbors boundry and the niebor filed a complaint to the EPA. The EPA investigated and now he has to get rid of the outside wood burner and can,t resell it in Pennsylvania. EPA told him he can sell it to someone in Ohio as they havn,t adopted the EPA Phase-2 regulations yet. A wood stove dealer from Ohio can,t sell a Pennsylvania redident a non qualified EPA model. The non qualified EPA ones have a much lower selling price than the EPA qualified models. Some PA. residents buy the nonqualified ones from Ohio by having a friend tell the dealer they are buying the stove. I wouldn,t do that myself as the EPA is federal and I think you could get into a lot of trouble. If I ever decide to buy one I would want the EPA one because of no smoke and the efficiency is a lot higher. I have a lot of friends that have the older models and they all smoke some if they don,t use seasoned wood. Most people will tell you that an EPA one uses about half as much wood but it definatly has to be seasoned or it won,t burn right. Solarvic

LindaSwitch

Re: Outside wood boilers

Green Bay, Wi. (Yeah Packers We have a Classic Wood Boiler that we love. It is a little work but we love the heat. The drawback is we feel are concerned to leave the wood boiler in the winter to go on vacation. I am afraid the temp. will get too low and water will freeze. We were told that as long as the pump is circulating the water, we do not have to worry as teh water will get enough heat from the plenum of the furnace. What is everyone's experience with this? Hate to take a chance without hearing from everyone. Thanks

Mountain Don

Re: Outside wood boilers

Where I am the authorities / inspectors will not pass an installation without polypropylene antifreeze being used in the system. Same thing required for any hydronic system.

I don't understand how it can be claimed that heat is going to be picked up from the furnace plenum when there is no fire. That might work for a short time absence, but I would not want to push my luck on that.

ramloui

Re: Outside wood boilers

I think that what was meant to say was that some of the work done by a pump actually heats the fluid somewhat. So you would be safe as long as the heat generated through the pump is greater than the losses through the system.

The other issue revolves around the reliability of that pump. If the pump fails, then your system will freeze.

Don's suggestion of using a glycol based fluid makes the most sense.

706jim

Re: Outside wood boilers

Up in my part of the country, it can get REALLY cold; sometimes to -40 degrees. A friend has one of those outside boilers (and a recently added gas furnace; wonder why?) and he has told me that it can be downright dangerous to open up the firebox during extreme cold weather. The accumulated creosote lights up explosively and he has to be careful not to get burned.

Fortunately for him, he is located on a large acreage so that polluting the neighbour's air is not a problem.

One other friend has a closeby neighbour with one of these things and simply hates it. The guy burns anything he can get including old wood pallets and who knows what else and the smoke is really bad.

westbranch

Re: Outside wood boilers

The accumulated creosote

That is a sure sign he is not giving it enough air, dampering it down so far that it smoulders, instead of burning it hot.
Wouldn't want to be anywhere around his place!
I bet he is firing it 24/7 and loading it up twice a day to minimize the time he is outdoors...

waynefromnscanada

Re: Outside wood boilers

LindaSwitch wrote: »

Green Bay, Wi. (Yeah Packers We have a Classic Wood Boiler that we love. It is a little work but we love the heat. The drawback is we feel are concerned to leave the wood boiler in the winter to go on vacation. I am afraid the temp. will get too low and water will freeze. We were told that as long as the pump is circulating the water, we do not have to worry as teh water will get enough heat from the plenum of the furnace. What is everyone's experience with this? Hate to take a chance without hearing from everyone. Thanks

You are right to worry! Once the fire goes out, within a very few hours there will be no heat in the plenum or any other part of the outdoor furnace to warn the water. As long as the circulator pump runs ( that usually shuts down when the furnace cools off) the water could pick up some heat from inside the house - - until it too cools off, then the freezing begins. First ice crystals start sticking to the inside of the pipes if the water is moving, then build up, reducing flow. And so it goes. But by then the boiler parts will probably be, shall we say, "expanded" and no longer in the designed physical shape, but ready to extinguish the fire with a goodly spray or trickles of water when you return.

waynefromnscanada

Re: Outside wood boilers

706jim wrote: »

he has told me that it can be downright dangerous to open up the firebox during extreme cold weather. The accumulated creosote lights up explosively and he has to be careful not to get burned.

Sounds like he told you a rather tall tail. For the creosote to burn "explosively" it needs to be VERY hot to the point of releasing ignitable gas which would escape up the chimney. And if the fire is burning hot, creosote doesn't form in the first place, and any that had formed would burn away and go out with the smoke. More likely a rather fresh load of wood in a hot furnace releasing gasses that, if the air supply is reduced, will flash over when the door is opened, admitting oxygen into the firebox.

706jim

Re: Outside wood boilers

I'll ask him again when I see him next.

I never prodded him for this comment; he offered it unsolicited.

And he has run this system for several years, using the heat for his home and (in the summer) for his swimming pool.

westbranch

Re: Outside wood boilers

When you do mange to ask, ask about the species of wood and how long it had sat curing, covered etc.

solarvic

Re: Outside wood boilers

Hot water from the wood furnace circulates 24/7 thru a heat exchanger. The inside boiler furnace will circulate water thru the other side of the heat exchanger only when the thermostat calls for heat. You set the aquastat in the inside boiler lower than the aqustat in the outside boiler. The outside wood boiler has its own aqustat set at a higher temp so the the circulating water is hotter than aqustat setting in the inside wood boiler. The inside boiler will still circulate the hot water thru your registers without the furnace fireing up. The inside furnace will only fire up if the outside wood boiler water temp. drops below the setting on your aqustat in the inside boiler. So if you don,t fire up the outside boiler, the inside boiler will fire up to heat the house. Since the outside wood boilers circulating pump runs 24/7, it will pick up heat thru the heat exchanger from your inside boiler to keep the the water from freezing. If you have an air furnace the same principle applys. On the new EPA phase 2 outwood boilers you need quality seasoned wood and you won,t have creosote buildup. The person that has trouble with the creosote problem caused it himself because he is using junk unseasoned wood or he has a wood boiler with a bad design. I forgot to mention you should cut your wood a year ahead of time for it to be properly seasoned. Also, we didn,t put antifreeze in my brothers furnace as the BTU,S don,t transfer as efficiently as water only. If your electric was off long enough than you could have a freezing problem and your house could have the same freezing problems since your inside furnace wouldn,t run either. SolarVic