This past weekend I was thinking about my furnace. Not whether it is going to work or how long it is going to last (I’m on an annual maintenance plan, so I don’t worry about that). But I was wondering if it truly does keep me as comfortable as it could. Â
Nowadays, there are multiple types of furnaces, 80% efficient, 90%-98% efficient, single-stage, two-stage, modulating, variable speed, etc. There are a lot of options, but if you don’t know the differences it’s hard to know what would work best for your home and budget.
What Furnace Efficiency Rating Means
The percentage is related to efficiency; the higher the number, the more efficient the unit is, the lower the number, the less efficient. Ninety percent of furnaces are more efficient than 80% of furnaces because they are able to recycle the unused heat and re-circulate it back inside a secondary heat exchanger, thus getting more heat for the amount of gas that is put in. The exhaust flue must be switched from metal to PVC when you go from an 80% efficient unit to a 90%+ efficient model.
The PVC in 90% furnaces is necessary because the high-efficiency furnaces actually have a lot lower exhaust temperature than the 80% models. This lower exhaust temperature creates condensation and will rust out the metal flue system; therefore, PVC is the material of choice for high-efficiency furnaces.
How to Identify Furnace Type
Not sure what type of furnace you have? Look at the exhaust coming out of it; if its metal – your furnace is 80% efficient or lower, if it’s made out of PVC then your furnace is a high-efficiency unit that is 90% or greater (assuming the correct material was installed by the installing contractor)
Another main difference in models of furnaces is the number of stages the equipment has. A single-stage furnace is simple, it’s either on or off. A two-stage furnace is a little more complex; it has a low-fire (first stage) and high-fire (second stage). A modulating furnace is even more complex and can modulate the flow of gas and air depending on what the demand is; basically, it has a speed between 30% and 100% output.
Everything you need to know about multi-stage furnaces
The reason to have a multi-stage furnace? They are more comfortable and more efficient. The efficiency is gained because, in lower stages, the furnace does not use as much gas, air, or electricity as in the higher stages. They are more comfortable because the house has fewer temperature swings, and the furnace can maintain a temperature much closer to the set point. To give you an example, I will get a little technical but keep it simple.
How they operate in your home
For example, your home requires 100,000 BTU of heat in order to stay 75 degrees inside on a 3-degree day. What if it’s 40 degrees outside and not 3 degrees? Do you need all 100,000 BTU? No! You may only need 50,000 BTU when it’s 40 degrees outside.
– With the single-stage unit, it will turn on and give your home 100,000 BTU of heat (remember it’s only on or off). So, the single-stage unit will give your home “the blanket effect” where it turns on to heat, heats very quickly, and shuts back off (the furnace is short cycling because it has too much heat for that particular day, but you still may need that much heat when its colder out). Kind of like when you get cold in the middle of the night, wake up and put the blanket on, then wake back up an hour later to take the blanket back off because now you are too hot. That is no way to be comfortable in your home, in fact, some people with single-stage furnaces wake up sweating a lot.
– With the two-stage furnace, the furnace will be able to run two different amounts of heat. Approximately 60,000 BTU in the first stage and 100,000 BTU of heat in the second stage. Now on that 40-degree day, the furnace will be able to run first-stage with 60,000 BTU and no longer short-cycle (giving you too much heat for the conditions), but if the temperature dips to 3 degrees outside and the home needs more heat, it will ramp up to the second stage and provide the 100,000 BTU it needs on cold days. Two-stage equipment is great for people who feel their home is always too hot or cold.
– With the modulating unit, it can simply output however much heat it needs to in order to bring the space to a comfortable temperature within a reasonable amount of time. Unlike single-stage or two-stage furnaces, the modulating furnace can change between multiple different amounts of heat. Basically, anywhere between 30,000 BTU to 100,000 BTU and all the numbers in between. So, if you set your thermostat on 75, the house should stay 75 with little fluctuation because the furnace can run less gas/air to heat slower (allowing a more even temperature overall) and ramp the heat up as it needs to. No more hot/cold spots, the blanket effect, or crazy temperature swings. Modulating furnaces are really nice when it comes to keeping your house at a stable temperature and heating the home evenly.
I know there is a lot to it, and we can go even more in depth but I don’t want to overwhelm you in today’s post. If you would like to learn more about the different types of furnaces, how they work, and which furnaces may be best for your home, we would be happy to help at Stay Cool Climate Control. Feel free to reach out by text, email, phone, etc., for more information or for one of our team members to come out for an in-house consultation. We would love for Stay Cool Climate Control in O Fallon, Missouri, to be your trusted HVAC advisor!
Stay Cool, Stay Comfortable – Jeremy W