This is a conversation about gas furnaces, so if you have an all-electric heat pump or some other heat source then this article is not for you, but please check back soon since we will be writing about other forms of heating in the coming weeks.
Types of gas furnaces
There are two primary categories of gas furnaces. First is an open combustion or 80 percent efficient furnace. The second is a sealed combustion or condensing furnace which is typically 92–98 percent efficient. In each of these categories we have three standard options:
What’s the difference between a single speed and a multi-stage furnace?
On a gas furnace, a single stage has one level of heat — all the way on with full fire. If we compare that to a gas stove that you cook with then we rarely want full fire. It’s usually better to cook with a medium amount of heat for longer periods, and heating your home works in much the same way.
Having a lower heat running longer will allow your home to heat up more evenly, eliminating hot and cold spots. Often cold spots in a home are the result of your heating system being too large and overheating the area with the thermostat shutting down the heating system before the rest of the home was comfortable.
According to building code, professional HVAC installers should size your furnace to heat your home to 70 degrees when the outside temperature is 13 degrees. So a single speed furnace can be too large any time the weather is warmer than 20 degrees — which is 80 percent of our heating season here in Kentucky. Having a furnace that is too large 80 percent of the time does not make sense—that’s why we recommend multi-stage furnaces.
How does variable speed mean in HVAC?
Variable speed refers to the fan system in the furnace. This is the blower that pushes air through the ductwork into your home in both heating and air conditioning season. A variable speed fan can speed up and slow down to match the different stages of the heating or cooling season.
Most modern cars have variable speed fans on their heating and cooling system and most of us probably find it more comfortable to keep our car’s fan setting on the lowest speed possible. Don’t you think your home should at least be as comfortable as your car?
Variable speed systems also improve the efficiency of your air conditioner which allows the air to move through the cooling process slower. This helps to dehumidify the home in the cooling season and is important since every 5 percent increase in humidity affects your perception of temperature by 3 degrees, so lower humidity in cooling season will keep you more comfortable.
One of the biggest advantages of variable speed HVAC systems is that when the system is not heating or cooling it can slow down and run the fan at half of its normal speed to circulate air. This also allows it to be constantly filtering air through your the air filter resulting in less dust, fewer allergens and overall better indoor air quality. Last but not least, variable speed furnaces are the quietest furnace option on the market. So if your heating and cooling system is too loud switching to a variable speed system is a must.
Which is the difference between open combustion and sealed combustion furnaces?
Open combustion furnaces have been the norm since we first started using forced air gas in the 1930s. Open combustion furnaces rely on the air in the room around them to fuel the combustion process with the needed oxygen to mix with the gas and ultimately to create fire. There are several negatives to this because the byproduct of this combustion process is carbon monoxide. Open combustion furnaces have more opportunity to create a carbon monoxide health hazard.
Closed combustion furnaces, on the other hand, typically have two white PVC pipes. One draws its own fresh air in from outside and then vents its exhaust outside through the other pipe. This closed combustion furnace process is the safest on the market and it also creates a more efficient furnace.
Open combustion furnaces lose about 20 percent of the heat you pay for through the exhaust pipe. A closed combustion system can lose as little as 2 percent. Because of this high efficiency gain, many gas utility companies will rebate as much as $450 for the upgrade to a closed combustion furnace, which typically pays for as much as half of the cost of the upgrade.
Are there situations where a closed combustion furnace is not recommended?
There are two situations that come up regularly where we do not recommend a closed combustion furnace. When switching from an open combustion furnace to a closed combustion furnace, new PVC vent pipes to the outside have to be installed. Sometimes it is not practical to run new pipes — for instance, if you have a heavily finished basement since most homeowners do not want us cutting drywall to install a new furnace.
The other situation when a closed combustion furnace is not recommended is when the furnace is in an attic. Since a closed combustion furnace is so efficient, the exhaust gases condense and create enough moisture that they can freeze. So when attic temperatures drop below freezing these pipes could freeze and burst, causing a lot of damage.
You said you might have to cut drywall to change my furnace. Is getting a new furnace a messy process?
Usually it is not. In most cases, we can change out your furnace in less than a day and have your heat back on before your house has time to get cold. Almost all of our furnace replacements are completed with no other work needed in the house and we usually leave the area around the furnace cleaner than we found it when we started the job.