Stoves, The Basics...

Stoves are enclosed appliances that burn a fuel to produce heat. This heat can be used for cooking, heating the home, or both but we will only look at heating your home here. In the early days, the word was used to describe a unit that heated a single room. For that reason, in the hvac industry, the term "furnace" is used to describe a heating appliance that is connected to a duct system and is designed to heat the entire home while the word "stove" is used to describe a heating unit that is not connected to a duct system.

There are many types of units and they are classified according to the type of fuel that they burn. We will begin by looking at units that burn wood (logs) to produce the heat.

  Wood Burning Stoves...

The basic wood fired unit consists of a heat chamber, ash pan, combustion air inlet, and an exhaust pipe that is referred to as a flue. The principle of operation is pretty simple.

A small fire is started manually and then fuel such as wood, coal, etc. is added. The unit draws in air from the room and uses it to burn the fuel. 

The exhaust gases (smoke) are then drawn through the flue to the outside of the home. As the fuel is burned, ashes are produced and they fall through a grate where they are collected in the ash pan. These stoves were usually made of cast iron which was very good at absorbing heat and then radiating it out into the room.

One of the disadvantages of these basic units was that they required manually adjusting the combustion air to control the heat output. This problem was overcome by adding a component called a combustion or draft blower. This blower is used to control the amount of air that is supplied to the fire which in turn controls the amount of heat produced. The earliest forms were a simple single speed fan that was either on or off. Modern units incorporate variable speed motors into the combustion blower. When this technology is combined with digital sensors and controls, these motors allow for more accurate control of the heat output.

Another problem with the old basic units was that they were not very efficient. This lack of efficiency not only resulted in more wood used but also put a lot of pollutants into the air. It was found that the pollution could be reduced by burning the smoke from the exhaust but it required a relatively high temperature of at least 1000 degrees fahrenheit.

In the United States, the Environmental Protection Agency (EPA) has implemented a certification program for wood stoves to comply with the provisions of the Clean Air Act.

This certification process places limits on the emissions from these units. For non-catalytic units, the standard is 7.5 grams/hour while for catalytic units the standard for certification is 4.1 grams/hour.

**NOTE** Some cities and states have adopted more stringent emission limits.

This problem is overcome by adding a component called a catalytic combustor. This is a ceramic honeycomb shaped device that is made out of a special type of metal. The exhaust gases from the unit are forced through this combustor where the special metal combines with the smoke and lowers the temperature at which it burns to about 500 degrees. This device increases the efficiency of the stoves and decreases their pollution but they require periodic replacement.

Another way to solve these problems is to replace the catalytic combustor with a process called gasification or dual combustion. In these units, a second heating chamber is added and combustion air is injected into this chamber. The temperature is high enough to burn the flue gases.

The modern units also incorporate a fan system to transfer the heat from the unit to the home more efficiently. These blowers can be controlled through a temperature control or by a manual switch. This has allowed the units to effectively heat larger areas as long as they are not divided into separate rooms.

Some units also surround the firebox with a ceramic or soapstone exterior. This has an advantage in that the heat is readily absorbed and then slowly released into the room over time. Another advantage is these units look more like an elegant piece of furniture than a heating appliance.

Before purchasing a unit, you can check out our brand ratings page where we use our service experience to review the brands.

 Pellet Stoves...

Another common type of unit is referred to as pellet stoves. These units are designed to burn wood pellets and/or grains such as corn, wheat, rye, etc. Wood pellets and shelled corn have about the same heat content (8000 btu/lb) but the corn tends to produce more ash. Not all wood pellets are the same so they are separated into grades. In general, premium pellets produce <1% ash while standard pellets produce <3% ash.

The operation of wood pellet units begins when either an onboard control or a remote thermostat senses the need for heating. An auger is started and runs for a preprogrammed period of time. This takes the pellets from the storage hopper and puts them into the actual burn chamber which is called a burn pot. The auger is then normally shut off and an electric igniter is supplied power. The ignitor builds up heat and starts to glow while a combustion fan supplies air to the unit. After a few minutes, the pellets are ignited by the heat from the ignitor. The auger is then restarted and continues to supply fuel to the burn pot at a predetermined rate. The units also typically have a convection fan that can be controlled either manually or automatically. This fan circulates air from the room around the stove where it picks up heat and then is blown into the room.

Before purchasing one of these units, you can check our brand reviews where we use our service experience to help you to get the most value for your money.

Although the amount of maintenance that each unit needs depends on the design of the unit, all pellet stoves require some routine service to keep them operating safely and efficiently. Most of the time, this is easy and can be performed by the owner. For assistance, you can check out our maintenance guide.

 Gas Stoves...

A gas fired unit can be a good option for heating when you do not want to have the mess or hassle of using wood. Most of these units combine a gas burner with ceramic logs and ash beds or ceramic beds of coal. This gives you the look and feel of a wood or coal fire with the convenience of gas.

Most of these units have a pilot light. This is a small flame that is constantly burning to ignite the main burner when heat is desired. This pilot is normally lit at the beginning of the heating season and left on until the end of the season. Modern units use a piezo ignitor which causes a spark to ignite the gas for the pilot.

On units with a millivolt control system, this pilot light produces heat that is directed to the end of a thermopile generator. This is a device that turns the heat from the pilot light into an electrical voltage. This occurs due to a process where two dissimilar metals generate a voltage at their junction when they are heated. This voltage is then used to control a gas valve that supplies the gas to the main burner. This control system has the advantage that the units can still operate during a power outage.

Some gas stoves use electricity from the home for their control system. They have a small control transformer that reduces the homes' power to low voltage for the gas valve and other controls. The main disadvantage of this control system is that, during a power outage, the stove will not operate unless it is equipped with a battery backup system.

Most of the units that are available can be controlled both manually or through a thermostat. To control the units manually, there is normally a simple on/off switch mounted on the unit. With a thermostat control, which can be either mounted in the unit, on the wall of the room, or incorporated into a remote control, the operation of the switch is based on the temperature of the room. When the switch is turned on, electricity from the thermopile or control transformer is supplied to the gas valve solenoid. This causes the valve to open, and gas to be allowed to flow to the burner. Here the gas is combined with the proper amount of air (called combustion air) and is ignited. When the gas burns, exhaust gases are produced and directed out of the home through a vent (flue) system.

There are generally two types of vent systems conventional vent and direct vent. Conventional vent systems (commonly called B vent systems) have a single metal pipe that transfers the exhaust gases to the outside of the home. These systems use air from the inside of the home as combustion air. This is a less efficient way of venting the units because the air that is used for combustion must then be replaced with air from outside the home. The air that replaces the combustion air must then be heated. A direct vent (balanced flue) system uses a concentric vent which is actually a pipe inside of a pipe. The combustion air for the unit is pulled in from the outside of the home through one of the pipes while the exhaust gases are removed through the other. This provides more efficient operation as well as enhanced safety.

On some units, the heat that is produced by burning the gas is transformed to a cast iron shell or other material such as soapstone. These materials store a lot of heat and then slowly release it through radiant heating. These units are good for providing a constant amount of heat over longer periods of time due to the time it takes for the materials to absorb the heat.

On many units, a small fan is added to the stove. This convection fan is usually controlled by a temperature sensor. When a set temperature is reached, a switch closes and the fan is started. Air from the room is then pulled into the unit and circulated over the firebox/heat exchanger where it absorbs some of the heat produced by the burning gas. The air is then blown out of the unit and back into the room. These units are good for providing quicker heating.

You can shop for gas stoves with a pro by using our brand ratings page.

   Thermostoves...

A thermostove or boiler stove is a combination of a boiler and a stove. They release heat into the room like a regular stove but also transfer a large part of the heat to water that is circulated in a heat exchanger. This water is then pumped to a storage tank and to the radiators that are connected to the home's central heating system. Some of the units can also be connected to provide the domestic hot water for the home.

Before you buy one, you can compare models on our thermostove ratings page.

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