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Outdoor wood Boiler vs Outdoor forced Air furnace Options
#1 Posted : Tuesday, February 17, 2009 12:34:36 AM
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I guess since I own a wood boiler I may be able to help you decide.First let me describe where I live and what I heat so you can compare.I do not sell them,I do not advocate either one or the other,I have no exp with forced hot air wood heaters other than the fact that I own a propane forced hot air furnace.I own a 24 x 26 foot 1200 square foot Cape style home.I heat my domestic hot water in winter with the boiler.I live in southwestern NH with USDA Zone 5 winters.A hard winter will  consume 12 cords or more between the first week in September to the last week in May.Thats our heating season here,but that is also with domestic hot water.I compare that with 12- 275 gallon fillups of heating oil and they are the same.My wood boiler heats the equivalent of 12 -275 gallon fillups of heating oil per annum,(3300 gals.)which is about right for my heat load here + hot water.The only motor involved is the 1/15 hp Taco circpump(about the equivalent of a 60 watt lite bulb)in the back of the boiler.It runs all the time.Okay,the biggest difference between a forced air wood furnace and a outdoor wood boiler is the fact that the wood boiler does not stay 'on' all the time.Thats why they use less wood than the forced air furnace.the wood furnace is basically a woodstove with a blower in it.When the house calls for heat the air blows across the constantly running woodstove and heats your air.The wood boiler does'nt work that way.What it does is heat up water in the  water jacket surrounding the firebox to 190 degrees.Once the water temperature is at 190 degrees a damper shuts the fire 'off' or in other words shuts off its air supply.The fire is still there and its  hot but its not burning because there is no air.When the house calls for heat and draws off the heat stored in the water in the water jacket the water temperature drops,activating the damper,opening the air supply up and restarting the fire.This is why a wood boiler will use far less wood than a hot air wood furnace.On a cold day (-20 mornings)I go out twice and my wife once,unless I fill it up,then I only need to do it once.During spring and fall once a days' fine..sometimes I'll let it go out and draw off the heat in the stored water all day then start a fire before bedtime.Hope this gives you an idea on how they work...

#2 Posted : Tuesday, February 17, 2009 7:39:03 AM
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Around here the house insurance company's consider a forced air wood stove to be the same as a in-building wood stove.

This means that the main reason for using a outdoor wood stove is removed.

You also have to think about the whole system and your future plans. Installing a in floor water heating system is a step towards a ground source heat pump.

#3 Posted : Wednesday, February 25, 2009 2:17:26 AM
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Posts: 134,494

I agree with LaserBill, think about your future uses.  With an outdoor boiler, you will be heating water (or glycol).  With hot water, you can warm your house and heat your domestic water as well, all with one system.  You might even find an ammonia chiller that can cool your house in the summer.  To do all this properly will require a fair amount of expertise, as well as some money.

If you opt for the forced air heating, all you will get is a warm house.  Your domestic hot water will still require gas (or electricity).

I have two natural gas-fired boilers, one to heat my house, and one to provide domestic hot water.  The domestic-use boiler will also be routed through a heat exchanger to warm my hot tub.  Bear in mind that I am connected to a natural gas well, so my gas expense is zero.

The boiler that warms my house has a small circulating pump on it that moves heated water thru a heat exchanger that is installed in the air handler inside the house.  I measured the current draw the other day while the furnace was running.  The fan and the circulating pump drew something like 2 amps.   If my math is correct, it costs me about $.08 to run my furnace for 1 hour - so the electrical cost to operate a circulating pump is nearly nothing.

The reason I have two boilers is because I didn't plan for future uses (like LaserBill suggested).  I still could have accomplished everything with one boiler, but I found a (second) good used commercial water heater (boiler) that made things more simple.

I noticed on the Denver craigslist that there are two indirect water heaters for sale for $300 or so, which seems reasonable to me, considering they are $1,000 new.  Also, if you know what you need and are not in a big hurry, you can find inexpensive valves, aquastats, etc, on ebay.

Good luck to you.


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#4 Posted : Wednesday, February 25, 2009 2:17:26 AM
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Posts: 134,494
First I know almost nothing about these.  The things I do know are with a Boiler it cost twice as much as a Forced Air furnace.  My hot water is a instant unit so I don't need a Boiler for hot water.
I have been reading posts here on different questions about boilers. It sounds like a lot more energy with extra motors and such.  I have been told by Forced Air installers that there is only one motor for circulating the hot air.
I have been informed that with the Forced Air that you fill those 3x a day instead of two.  They also said that is not true about only twice a day.  Keep in mind this is what I have been told.  With the Forced Air there is no digging.  It is placed close to the house and connected into your duct work.  I don't know how much cord wood people go through with theirs or how much a Forced Air one would use.
I sure could use some help in deciding which to get.  Both are a lot of money.
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