Introduction to Wood Pellet Stove Ratings for Efficiency
Wood pellet stoves burn small pellets produced from sawmill residues that would normally be discarded. There are many wood pellet stove models available, having various heat outputs and efficiencies suitable for domestic hot water and central heating purposes.
Here we shall have a quick look at the operation of a pellet burning stove and we will examine the wood pellet stove ratings based on their efficiency.
We begin then with an overview of a typical wood pellet burning stove used to produce heat for a hot air heating system.
Operation of a Typical Wood Pellet Stove
The wood pellets are normally stored in a hopper close to the location of the stove. I saw one last year, where the agent for the stove had put the storage hopper into a small cupboard beside the stove.
The wood pellets are then transferred from the supply hopper to the stove hopper via a circular pipe. As this is a manual feed, the only noise is an occasional tinkle of pellets being supplied to the stove hopper.
The pellets are fed to the stove furnace along with combustion air delivered by a small fan. The pellets are ignited automatically and burn with a virtually smokeless bright blue flame at around 250°F, the residue falling into an ash hopper below the stove furnace.
The hot gases from the furnace are passed through a heat exchanger. The air then passes through the exchanger heating it before being circulated by another fan to the rooms of the house. This is done through two sets of ducting, one supplying the hot air and the other returning the cooler air back to the stove to be reheated; this is known as a hot air system.
The ash hopper is usually only emptied once or twice a month, as very little residue is produced. This ash can be used on the flower or vegetable garden; provided the pellets are processed from a known wood source (check with pellet supplier for this information).
The pellet stove can run a hot air system (as per drawing), or normal central heating radiators. Again, the one I saw was being used to heat the domestic water and run six radiators.
I have included a layout drawing of a typical wood pellet stove below. (Please click on image to enlarge).
Price of Wood Pellets
Wood pellets are about 1/4" diameter by 3/4" long.
Typically, wood pellets cost $3-$6 per 40lb bag and In an efficient stove, the 40lb bag will produce less than a cup full of ash.
It is cheaper to buy wood pellets by the ton, at an average cost of $190 / ton
An image of wood pellets is shown below.
Wood Pellet Stove Ratings for Efficiency
When purchasing a wood pellet stove, you should ensure that it meets the Environmental Protection Agency Standards. You should also look for the Efficiency Range and Stove heat output given in BTU’s.
I have set some parameters for the selection of efficient wood pellet stoves.
These include the following:
1. Efficiency Range - should be above 70% efficient.
2. EPA Certified - must not produce more than 7.5 grams of smoke per hour; however most wood pellet stoves are exempt from this section of the regulations as their emissions are much lower than this figure.
3. Heat Output - should be in the range of 50,000 BTU’s/hr - 65000 BTU’s/hr capacity for a typical three-bedroom family home.
4. Automatic Control - essential for efficient combustion of the pellets and will help you to save from adjusting the pellets feed and air mixtures to suit temperature requirements.
5. Automatic Pellet Igniting - automatic ignition is also essential for convenience.
6. Automatic Safety Cut-out - this is mandatory by EPA regulations. It is a probe that is fitted to the flue that cuts out the boiler when overheated.
7. Stove Hopper Capacity - the hopper should be capable of containing enough pellets to operate the stove for 24 hours.
8. Tax Credits - ensure the stove qualifies for US tax credit; normally a pellet stove of over 70% efficiency qualifies for this.
From these parameters, I have produced the table below. (Please click on the image to enlarge the table).
From the above table, all three meet the required parameters, with the Extraflame Lucrezia having the highest wood pellet stove ratings for efficiency at 95%.