Short answer: the right heat pump size depends on your home, not its square footage alone. Insulation, windows, sun exposure, ductwork, and how you live in the house all change how much heating and cooling it needs. We size the system to your home during a free in-home estimate, not to the label on your old furnace or AC.
How heat pump size is measured
Heat pumps are sized in tons. One ton is 12,000 BTUs of heating or cooling per hour. A 2-ton system moves 24,000 BTUs per hour, a 3-ton moves 36,000, and so on.
Tons say nothing about how big the unit looks. They describe how much heat the system can move. The goal is to match that number to what your house actually loses in winter and gains in summer.
Why you can't just go by square footage
Two 1,800-square-foot homes in the same neighborhood can need very different systems. Here is what changes the answer:
- Insulation. A remodeled home with a well-insulated attic holds heat far better than an original one with thin or settled insulation.
- Windows. Older single-pane windows lose heat in winter and let in sun in summer. Newer double-pane windows make a big difference.
- Sun exposure. A west-facing living room in Santa Rosa or Windsor heats up in the afternoon. A shaded Mill Valley home with big trees behaves very differently.
- Ducts. Leaky or undersized ducts waste a lot of what the system makes. A great heat pump on bad ducts still feels like a bad system.
- Layout. Open floor plans, vaulted ceilings, additions, and upstairs rooms all change how air moves.
- How you live. Home office, big family, or a house that's empty all day? We ask.
What happens if the system is too big or too small
Bigger is not safer. Both mistakes cause problems.
| Too small | Too big | |
|---|---|---|
| Comfort | Struggles on the hottest and coldest days | Heats or cools too fast, then shuts off |
| Humidity | Fine, but runs constantly | Doesn't run long enough to pull moisture from the air |
| Noise and wear | Works hard all the time | Frequent starting and stopping |
| Bills | Higher than they should be | Higher than they should be |
An oversized system is one of the most common problems we see in older North Bay homes, because the old unit was often sized by rule of thumb or simply copied from the one before it. That is also why we don't just match the old unit's label. The house may have been upgraded since, or the old unit may never have been right.
How Green Core sizes your system
Sizing is part of the free in-home estimate. We:
- Walk the house and look at insulation, windows, and the rooms that are always too hot or too cold.
- Inspect your ducts, attic or crawlspace, and electrical panel.
- Ask what you want fixed, whether that's a noisy AC, high bills, or an upstairs that never cools.
- Recommend a size and give you clear written options, priced to your home.
If your ducts are the real problem, we'll tell you. Sometimes duct work improvements make a bigger difference than a bigger system. If a part of the house has no ducts, such as an addition or ADU, ductless mini splits can be sized room by room.
Inverter systems and sizing
Modern inverter heat pumps adjust their output instead of running at full blast or off. That gives you more forgiving performance across our mild winters and summer heat spikes. It does not mean sizing stops mattering. A correctly sized heat pump with a variable-speed compressor gives the best comfort, quietest operation, and lowest running cost.
Common questions
Can I just replace my old unit with the same size?
You can, but it's not always the right call. Homes change, and the original unit may have been too big or too small. We check the house first, then recommend a size.
Does a bigger heat pump heat my house faster?
Not in a way you'll like. An oversized system tends to cycle on and off, which can leave rooms uneven and the air humid in summer. A properly sized system runs longer at a lower, steadier output.
Will adding insulation or new windows change the size I need?
Yes, often downward. If you plan to improve the house soon, tell us before we quote so we can size for the home you'll have.
Do I need a different size for a mini split?
Mini splits are sized per room or zone. We look at each space you want to heat and cool and match an indoor unit to it.
Not sure what size heat pump your home needs? Call Green Core at (707) 988-5858 or request a free estimate.