Do you need a panel upgrade for a heat pump?

Why older houses run out of capacity

Houses built before about 1990 often have a 100-amp electrical service. That was plenty when the furnace burned gas and nobody charged a car in the garage. Every step toward going electric adds a large load that runs for hours at a time, and those loads add up faster than most people expect. Once a house is fully electric, it usually needs more than 150 amps.

What each appliance adds

The heat pump's backup heat is the load people underestimate most. A cold-climate heat pump works efficiently well below freezing, but most installs include electric resistance backup for the coldest hours. That backup is a big load, and the calculation has to include it even if it rarely runs.

How to know for sure

To see what service you have now, look at the main breaker in your panel. It's usually marked with its rating. If it says 100, that's your service size, though the panel rating and the actual service wires can differ, which is one more reason to have it checked properly.

Ways to avoid a service upgrade

Upgrading the service is disruptive and involves the utility. A calculation that shows you're over capacity doesn't automatically mean you need a new service. Try these first.

1. 1.

   Add a load management device

   Codes increasingly accept devices that stop two big loads from running at the same time, often an EV charger and one other appliance. Both get installed, only one draws power at a time, and the calculated load drops.

2. 2.

   Use a smart panel or circuit-level controls

   These watch your usage and switch off loads when needed, which can keep a 100A service working through several upgrades. They cost more than a simple interlock and much less than a new service.

3. 3.

   Pick equipment that draws less

   A heat pump water heater instead of a standard electric one, a heat pump dryer instead of a vented electric one, a car charger set to a lower amperage. Charging overnight rarely needs the full rate.

4. 4.

   Cut back the backup heat

   Sizing the heat pump properly and limiting the electric backup removes the single biggest load. Some people keep their gas furnace as the backup, which removes it entirely.

5. 5.

   Upgrade the service if you still need to

   That means a new panel, meter base, service wires and utility connection. Budget time for the utility's schedule, which is often the longest part.

Getting the heat pump itself right

Heat pumps are less forgiving of a bad install than the equipment they replace. An oversized gas furnace wastes some fuel. An oversized heat pump turns on and off constantly, doesn't pull humidity out properly, and costs more to run than one sized correctly.

• Ask for a proper load calculation. A room-by-room heat loss and gain calculation, such as Manual J, based on your actual walls, insulation, windows and which way the house faces. Sizing from the floor area or from the old furnace just carries forward whatever was wrong before.
• Check the ducts. Heat pumps move more air at a lower temperature than a furnace. Ducts sized for a furnace are often too small, and you'll notice it as noise and weak airflow in the rooms farthest from the unit.
• Decide on the backup heat on purpose. Electric backup is simple, and it's the biggest load in your calculation. Keeping the gas furnace as backup avoids that load, but it means keeping a gas appliance.
• Seal and insulate first if you can. Every improvement to the house shrinks the heat pump you need, which lowers the purchase price and the electrical load. Doing it the other way round means buying a bigger unit than the house will end up needing.
• Plan for defrosting. The outdoor unit drips water when it defrosts. Mount it on a stand above the snow line, where the meltwater won't freeze on a walkway.

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