(And Why That’s a Problem You’re Living With Every Day)
I recently visited a 1962 home in Croton on Hudson that had been beautifully renovated after a tree fell on it during a storm. The homeowners had done a lot right. They added blown-in cellulose insulation, rebuilt damaged exterior walls to modern code, and installed an Ecobee smart thermostat so they could actually see how their system was behaving.
And yet, something was still very wrong.
Their furnace was cycling on and off every five minutes. To stop the constant starting and stopping, the homeowner had programmed a 10-minute minimum run time into the thermostat.
That’s not normal. That’s a system fighting its own size.
When I ran a proper load calculation, the reason became clear.
The house needed about 29,000–30,000 BTUs of heating capacity.
The furnace that had been installed? 99,000 BTUs.
More than three times larger than necessary.
How “Bigger Is Better” Became the Default
In the HVAC industry, there’s been a long-standing — and damaging — belief that it’s safer to oversize equipment “just in case.” Contractors worried about cold-weather callbacks would pad their numbers by 20%, 30%, sometimes even 50%. Others skipped calculations entirely and simply replaced old equipment with the same size or bigger.
Once that system is installed, homeowners live with the consequences for 15–20 years, which is about how long HVAC equipment is expected to last.
What Oversizing Actually Does to a System
Short Cycling: Where the Damage Starts
When a heating or cooling system is too large, it reaches the thermostat setpoint too quickly and shuts off. But because it hasn’t run long enough to stabilize temperatures throughout the house, the space heats or cools back down almost immediately — and the system turns right back on.
That constant on-off behavior is called short cycling, and it’s one of the fastest ways to destroy equipment.
Every startup is the most stressful moment for a system. A properly sized system might cycle two or three times per hour. An oversized one can cycle ten to fifteen times per hour — putting several times more wear on components like the blower motor, igniter, and compressor.
It also wastes energy. Systems are least efficient during startup. If they’re constantly starting and stopping, they spend most of their life operating in their least efficient state.
Comfort Problems You Can Feel
Oversizing doesn’t just shorten equipment life — it affects how your home feels.
Instead of maintaining a steady temperature, the house swings. You might go from 68° to 74° and back again instead of sitting comfortably at 70°. Some rooms feel fine, others never quite do, because air isn’t circulating long enough to distribute evenly.
In summer, oversized air conditioning introduces another issue: humidity. Air conditioners remove moisture while they run. When a system is oversized, it cools the air quickly and shuts off before it can properly dehumidify. The result is a cold, clammy house that still feels uncomfortable.
What Proper Sizing Actually Looks Like
At Shelter Air, we perform Manual J load calculations on every home we work on. Not estimates. Not rules of thumb. A real, room-by-room analysis based on how the house is built and how it actually performs.
For this Westchester home, I used a 3D scanner to measure each room precisely. But square footage is only part of the story. A proper load calculation also considers:
- The age and construction of the home
- Insulation levels in walls, attic, and basement
- Window size, type, and orientation
- Air sealing and infiltration
- Occupancy patterns and internal heat gains
This homeowner had made meaningful improvements to the building envelope. Dense-pack cellulose was added to previously uninsulated walls. Two exterior walls were rebuilt to modern standards after the storm damage. Air sealing was improved throughout the house.
All of that reduced the home’s actual heating and cooling needs.
The equipment never adjusted.
The Real Cost of Oversizing
Oversizing costs you more in every direction.
Upfront, you pay for larger, more expensive equipment than you need. A 99,000-BTU furnace costs significantly more than a properly sized 30,000-BTU unit — and delivers worse comfort.
Then you keep paying. Higher utility bills from inefficient cycling. More frequent repairs. Shortened equipment lifespan. Ongoing comfort issues that never quite get resolved.
All of it is avoidable.
What We Recommended
For this home, we recommended replacing the oversized furnace with a properly sized heat pump system, reducing capacity from 99,000 BTUs to what the house actually needs.
That change would immediately improve comfort, reduce cycling, and lower operating costs. It would also allow the homeowners to take advantage of ConEd’s New York State incentives — up to $8,000 for decommissioning fossil-fuel systems and switching to electric heat pumps.
Right-sized equipment works the way it’s supposed to.
Questions You Should Ask Any Contractor
If you’re getting HVAC quotes, these questions matter:
- Will you perform a Manual J load calculation?
If the answer is “we don’t need to” or “we’ll just match what you have,” that’s a red flag. - What factors are included in your sizing?
Square footage alone isn’t enough. Insulation, windows, orientation, and air sealing should all be part of the conversation. - Can you show me the calculation and explain it?
A professional should be able to walk you through the reasoning. - What should my system’s runtime look like?
In moderate weather, a properly sized system typically runs 15–20 minutes per cycle. Five-minute cycles are a warning sign.
Why We Don’t Guess
At Shelter Air, we take a science-based approach to HVAC design. Every recommendation is backed by data specific to your home. With a background in chemistry and building science, we understand that HVAC systems don’t exist in isolation — they interact with your building envelope, indoor air quality, and how you actually live in your space.
If your system is short cycling, if your comfort is inconsistent, or if you’ve ever wondered whether your equipment is simply too big, a proper load calculation is the place to start.
It costs far less than years of inefficient operation and premature equipment replacement.
Your home doesn’t need “extra.”
It needs exactly the right size.
