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Revised:
February 6th, 2023
You might assume when you’re buying a furnace that anything that provides central heating will do the job. Right?
Well, yes. It will provide heat. But it turns out that furnace sizes range from one-and-a-half tons to five, where one ton equals 12,000 British Thermal Units (BTUs). One BTU is the amount of heat necessary to heat a pound of water by one degree. (This is why you’ll hear BTUs used with air conditioners as well; it’s the amount of heat the system can remove.)
And where does the right size come from? It depends on the contractor.
Some HVAC companies simply check the square footage of your home and work off of that. Others look at your existing furnace’s size and make the exact same recommendation.
Is either of those estimates accurate? Maybe, but both numbers are a guess. And a guess is not good enough.
The sad reality is that, according to the U.S. Department of Energy, 70-90% of HVAC systems have at least one performance-compromising fault incurred at installation; half of those are due to sizing. And the wrong furnace sizing will create an uncomfortable environment as well as shorten the furnace’s lifespan.
How do you avoid this? Read on.
Figuring Out the Right Size Furnace for Your Home the Right Way
On every single estimate, we at Fire & Ice Heating and Air Conditioning do what’s called a Manual J load calculation. It is recommended practice by both the Air Condition Contractors of America (ACCA) and the U.S. Department of Energy.
A Manual J takes into account the square footage of windows, the type and number of doors, which way your home is facing (a home facing south will receive more of the sunlight’s warmth), square footage of walls and ceiling, the year the house was built, did the owners build an addition, the type of insulation, the number of occupants, and more.
This will determine the actual heating load for your home. Depending on what type of furnace you’re going to go with, we’ll do the math to make sure the furnace we recommended is sized correctly for a zero-degree Fahrenheit day. That’s what we size for in Ohio.
Once you know how many BTUs we need to heat your home effectively, you can calculate the tonnage easily.
Why Don’t All HVAC Contractors Do a Manual J Load Calculation?
A Manual J load calculation takes time. If you do a thorough one, it takes about 10 to 15 minutes. It does take some information-gathering from the homeowner. For instance, we ask what type of insulation is in the house, how many occupants live there full-time, and whether they have built an addition.
It’s not hard, by any means. A lazy HVAC contractor might be trying to get to the next sales job as soon as possible.
What Happens When We Recommend a Furnace That’s Smaller Than What the House Has in it?
Do we get pushback from customers when we tell them that they don’t need that big a unit now?
Yes. Especially for the homeowners who have been in the house for 20 years or longer. They tell me that they didn’t have any issues before, so why change? And won’t a smaller furnace struggle to heat the entire house?
We have to assure them that we took the time to do this correctly.
This is especially true if the homeowner wants to go from a standard-efficiency model to a high-efficiency one. (A standard-efficiency furnace can run at 80% at best, meaning that it converts 80 cents out of every dollar into heat. You’re only getting 80% of the BTUs of heat. The other 20% is waste and is vented outside of the house. A high-efficiency rating is 90% or better. Most of the ones we install are 96% or higher.)
Let’s do the math.
If you have a 100,000 BTU 80% furnace, you can go to an 80,000 BTU high-efficiency furnace with no loss in comfort. 80,000 BTUs in a high-efficiency furnace nets you about 75,000 BTUs of heat. So you’re getting almost the exact same size output as with the larger unit.
And since high-efficiency furnaces are much better at producing heat than standard-efficiency ones, they don’t have to run as long, which will help you save on your energy bill.
What Happens If the Furnace Is Too Small?
If it’s too small, your undersized furnace will run a lot more than it should. It’s going to try and fail to match the thermostat’s set point. On very cold days, it might run all day and not succeed in warming the house.
Your gas or electricity bill will go up because it’s running more than it needs to.
It’s also going to put a huge strain on your furnace. Those moving parts can last 15 to 20 years if they’re taken care of. Ten years of being overworked almost guarantees you’ll be shopping for a new heat source sooner than that.
If it is undersized, there’s not much homeowners can do to help out the HVAC system. To send heat to cold rooms, they would need to cut off some supply vents, so now you’re left with a room or two that’s not receiving any heat. We definitely do not recommend doing that.
The only positive you receive from a too-small furnace versus one that’s too big is that a longer runtime is better for indoor air quality (IAQ). Because the furnace air will take in more air, it can do a better job of capturing dust and airborne bacteria. Viruses such as COVID thrive in low-humidity air. The furnace filter can trap that better.
If it’s a little too big, it will achieve the set temperature a little too quickly (more on this later) and turn off too soon. If it runs longer, you’ll have better air filtration and better IAQ. It’s going to do a good job of mixing air because it is running.
Anytime you suggest to a customer that the existing furnace is too big, and that they should buy a unit that’s smaller, that’s a little scary at first. We hear, “What if I don’t have enough heat?”
We take the time to tell them why we’re suggesting this. We explain the load calculation. We go over the benefits of the new furnace and how much heat it can produce.
What Happens If the Furnace Is Too Big?
When it’s sized too big, it’s going to reach the set temperature too quickly. It’s called “short cycling.”
If your heating system is operating correctly, it should cycle about 2 to 3 times per hour. It’s called a “cycle count.” A cycle is complete when you’re moving new air and replacing old air completely. Rooms farthest away from the furnace will be especially affected. It requires 4-7 cycles per hour to get the same level of comfort upstairs as downstairs.
The colder it gets, the longer the run cycle will be, and the greater the number of cycles.
With an oversized furnace, it will start up, run for 5-10 minutes, then it will shut off for 5-10 minutes. Then it will start back up. The only room that will get warmed properly will be where the thermostat is. The room that’s 50 feet from the system had no opportunity to move any air at all. The heated air is just now getting to that room when the furnace shuts off.
Because not as much air is being moved, that leaves stagnant air that is dusty and has more impurities.
It’s also going to cause more wear and tear because the constant starting and stopping are hard on the system.
Related: How Much Does a New Furnace Replacement Cost?
Related: The Complete Guide to Home Furnaces
Buying the Correct Size Furnace in Columbus, Ohio
It would be nice if, when you’re investing in a new HVAC system, the process were simple. It would be even better if you could count on the salesperson to do their job thoroughly.
Every time we size a system for a customer, and see that their current unit isn’t sized correctly, we shake our heads. There’s no reason not to do a proper load calculation. It doesn’t even take long.
Here at Fire & Ice, we strive to be upfront with our homeowners. That’s why we always do an honest assessment. You deserve it.
If you’re in the market for a furnace, we’d love to be the one you call. Fill out your zip code in the graphic below to see if you’re in our service area. We hope to hear from you soon.
