Posts made in May 2011

The Heat Efficiency of Wood Burning Stoves vs. Gas Stoves

Ever wonder about the overall efficiency of a wood burning stove as opposed to a gas stove? There are many differences between the two types of stoves but understanding the distinction in combustion and heating efficiency is the most helpful towards making a decision as to which stove is beneficial for you and your family’s needs.

Combustion vs. Heat Transfer Efficiency

wood burning stove

Combustion Efficiency vs. Heat Transfer Efficiency

First off, defining combustion efficiency and heating transfer efficiency is important to understand the rest of this discussion. Combustion efficiency is the percentage of fuel that is actually turned into heat instead of ash, vapor, steam, etc. This form of efficiency is a measure of the percentage of your fuel expenses it takes to create heat in your stove, while taking into account the amount of waste that is created from this fuel usage.

Heat transfer efficiency is how much of the heat generated actually radiates throughout the rooms in your home. This is essential to take note of because there’s a distinct difference between the fuel creating heat within your stove and that actual heat being expelled throughout your home from the stove. There’s normally a lot more heat created within the stove then is available to heat your home.

Wood Burning Stoves Heating Efficiency

The fuel that generates heat within a wood burning stove is obviously firewood. Wood burning stoves are often less efficient when it comes to an equal balance of fuel to heat release because the outside of these types of stoves don’t typically have an efficient heat exchanger. A heat exchanger is a piece of the equipment on the stove that helps transfer heat from the stove to the rest of the room. Heat exchangers on wood burning stoves don’t have the surface area for the proper extraction and distribution of heat relative to the amount of fuel used to generate the heat. Wood stoves are much more combustion efficient then they are heat transfer efficient.

However, wood stoves still remain efficient heating options because of the fact they are in the area of your home that you want heated. Unlike a wood stove insert or a fireplace, a wood burning stove is often near the center of the room and is exterior from a wall. Therefore, the stove generates heat closer to you and your family. Lastly, wood burning stoves are especially worth purchasing, despite their heat transfer inefficiencies, if you don’t have to pay for the wood to fuel them. If you have access to your own firewood, then the heat transfer inefficiencies aren’t costing you the same downsides as they would if you’re buying firewood.

Gas Stove Heat Efficiency

Gas stoves are powered by gas via the piping in your home, much like a conventional cooking oven. Gas utility services are often more of an expense than firewood to fuel a stove. Yet, the overall combustion and heat transfer efficiency of a gas stove is much higher than that of a wood stove. It all comes down to pricing.

Heat exchangers in gas stoves help prevent escaping heat from your stove and help better distribute it throughout your home. They are typically more viable within a gas stove then in a wood stove, therefore less heat escapes from the stove making a gas stove more efficient in both respects.

Many gas stoves have high levels of insulation and tight fitting door hinges that a wood stove does not. This severely increases the efficiency of a gas stove over a wood stove because they are able to retain higher levels of heat for longer periods of time.

In the end, both gas and wood stoves have high levels of heat and combustion efficiency. Gas stoves are typically more reliable in terms of efficiency then wood stoves, but often at a higher price.

How Many Things Can You Put On One Chimney? Or Multiple Appliance Venting

Let’s start with a few definitions.

chimney flue ventingAppliances include fireplaces, woodstoves, furnaces, boilers, pellet stoves, hot water heaters, etc. They’re all individual appliances.

A chimney is a structure that has one or more flues in it.

A flue is simply the chimney passageway that vents the fumes from whatever is attached to it. (A flue is not the same as a damper either; a damper is something that can block the flue.)is simply the chimney passageway that vents the fumes from whatever is attached to it. (A flue is not the same as a damper either; a damper is something that can block the flue.)

How many appliances can you have per chimney?

And even though the question always comes across as “how many on one chimney?” let’s make sure to discuss “how many on one flue?” The answer to the question is: “It Depends”.

The rules are found in various NFPA standards and in the IRC (International Residential Code.) This article is general in nature but for those who want to drill down into the details, most of the information can be found in IRC chapters 10, 13, 18 and 24.

Solid fuel burning appliances.

Solid fuel includes coal or corn or cherry pits, but for most of us that means cord wood or pellets. The rule here is easy and clear.

IRC M1801.12 (PDF Here) Multiple solid fuel prohibited. A solid-fuel-burning appliance or fireplace shall not connect to a chimney passageway venting another appliance.

In other words, only one appliance per flue, period. It goes without saying, I hope, that gas or oil appliances cannot be vented into a flue which also vents a solid fuel appliance. EVERY SOLID FUEL APPLANCE GETS ITS OWN VENT!

How about hooking up a woodstove into an existing masonry fireplace flue?

That’s OK as long as:

  • The fireplace has been blocked off. Remember, only one appliance per flue!
  • The liner for the woodstove has to be properly sized, which generally means the same size as the collar-size coming from the appliance.
  • Make sure the chimney is clear of combustible materials before inserting the smaller liner.

Gas and Oil Appliance Venting

Gas fireplaces are factory-built systems. The manufacturer’s listing and instructions will preclude attaching any other appliances to it.

Multiple gas or oil furnaces or boilers, as well as hot water heaters, can be vented into one flue. There are a few rules to mention:

  • The rules apply to listed appliances. While I have never seen an unlisted gas or oil furnace in my life, if you have one, you are referred back to the rules for solid fuel burning appliances- one per flue.
  • If venting two or more appliances on the same flue, you have to know the flue can handle it, as determined but the BTU input and other factors.
  • Both or all appliances have to be on the same floor. So, no furnaces in the basement or room heaters on the second level of your home.
  • The connectors for the appliances have to be offset. They can’t come into the flue at the same height, and especially never directly across from each other.
  • The smaller of the two connectors go into the flue above the larger one (usually meaning the hot water heater).
  • As a general rule, don’t mix “natural draft” appliances and “fan assisted” appliances on the same flue. This rule is more complicated than this, but if this is your case, be sure you refer to the manufacturer’s instructions. Call an HVAC company and make them show you to your satisfaction it’s right. Don’t take anyone’s word for it, see it in writing.

The NFPA 54 (Gas) and the NFPA 31 (Oil) show diagrams in great detail, and cover sizing the connectors as well (connectors are the smoke pipes that carry the fumes from the appliance to the chimney flue.)

Read another helpful article by the American Society of Home Inspectors.