Furnace sizing is where the gap between what homes have and what they need is widest in Sacramento. Our winters are mild. Many furnaces installed here were sized as if the house sat somewhere much colder, or simply matched whatever the builder used across the subdivision. Here is how to think about it.
How furnaces are rated
Furnaces are rated in BTUs per hour. You will see two numbers:
- Input BTUs, how much gas the furnace burns, for example 80,000
- Output BTUs, how much heat actually reaches the house, which depends on efficiency
An 80,000 BTU input furnace at 80 percent efficiency delivers about 64,000 BTUs of heat. The same input at 96 percent delivers about 76,800. When comparing furnaces, compare output, not input.
Why BTUs per square foot charts mislead
You will find charts recommending anywhere from 30 to 60 BTUs per square foot depending on climate zone. They are rough national averages. The number that matters is the heat your specific house loses on a cold night, and that depends on:
- Insulation in the attic, walls and floor
- Window area and glass type
- Air leakage around doors, windows and penetrations
- Duct location and leakage, especially ducts in an unconditioned attic
- The local winter design temperature
The industry method for this is an ACCA Manual J load calculation, the same one used for air conditioner sizing.
The Sacramento pattern: oversized furnaces
We regularly find 80,000 and 100,000 BTU furnaces in 1,600 to 2,000 square foot homes in Elk Grove, Natomas, Roseville and Rocklin. For our climate, many of those houses need considerably less. Oversizing a furnace causes the same problems as oversizing an air conditioner:
- Short cycling. It heats the thermostat’s room in minutes and shuts off. Frequent starts wear the igniter, inducer and blower. See furnace short cycling.
- Uneven heat. Short runs never warm the far rooms.
- Noise and drafts. A big furnace pushes more air than small ducts can handle quietly.
- Heat exchanger stress. Oversized furnaces on restrictive duct work can run hot, which is hard on the heat exchanger.
Airflow matters as much as BTUs
If a furnace also serves as the air handler for your air conditioner, its blower must move enough air for the cooling side. That is why furnace and AC sizing get decided together. A furnace that is right for heating but paired with the wrong blower capacity will not cool the house properly. Duct size limits this as well. Our article on duct sealing versus replacement covers the duct side.
Two stage and modulating furnaces
These run at low fire most of the time and step up only when needed. In a mild climate that softens the downside of a slightly oversized unit, because the furnace spends most of its time at its lower setting. They cost more up front, and in Sacramento the payback on premium efficiency is slow. Our best furnace brands guide works through the numbers.
Should you go heat pump instead?
If you are replacing both furnace and AC, it is worth pricing a heat pump. Sacramento’s mild winters suit heat pumps well. See heat pump versus furnace and what incentives are left in 2026.
How to check a furnace quote
- Ask for the heating load from a Manual J calculation.
- Compare the furnace’s output BTUs to that load, not its input.
- Ask whether the blower is sized for your air conditioner.
- Ask whether the ducts were checked for static pressure.
More in how to compare HVAC quotes.
What we do
We size heating and cooling together from a load calculation, measure static pressure, and very often recommend a smaller furnace than the one coming out. Replacement estimates are free, and the cost calculator gives a range in about a minute. See also when to replace a furnace and our heating services page.
Frequently asked questions
What size furnace do I need for a 2,000 square foot house?
It depends on insulation, windows, air leakage, duct location and climate. National charts give wide ranges, but many 2,000 square foot homes in Sacramento’s mild climate need less than the 80,000 to 100,000 BTU furnaces often installed. A load calculation gives the real number.
What is the difference between input and output BTUs?
Input is how much gas the furnace burns. Output is how much heat reaches the house, which is input multiplied by efficiency. Compare furnaces by output.
Is an oversized furnace bad?
Yes. It short cycles, heats rooms unevenly, can be noisy and puts extra wear on the igniter, inducer, blower and heat exchanger.
Should my furnace be sized with my air conditioner?
Yes, when the furnace blower also serves the AC. The blower has to move enough air for cooling, so heating and cooling sizing should be decided together.
Is a two stage furnace worth it in Sacramento?
It improves comfort and reduces short cycling, but the efficiency payback is slow in a mild climate. It can make sense if you plan to stay in the home for a long time.
How do I know if my furnace is too big?
It runs only a few minutes per cycle, the thermostat room overheats while other rooms stay cool, and it cycles many times an hour on moderate nights.