With the Inflation Reduction Act (IRA) rebates and other energy efficiency incentives out there now, it’s a great time to start using a heat pump to heat your home. If you’re currently heating with a furnace and have an air conditioner as part of the system, you have an easy way to switch to a heat pump when you’re ready to replace the AC. Let me introduce you to what’s called a dual fuel heating system (or dual fuel heat pump).
The furnace and air conditioner goes dual fuel
The standard heating and air conditioning system used in many places is a furnace for heating and air conditioner for cooling. Because they use the same air duct system to distribute the heating and cooling, they’re connected. In the photo above and the one just below, you can see them on the left side.
The furnace provides the heating and also the air movement. That’s where the blower that moves the air is located. The indoor part of the air conditioner is the box sitting on top of the furnace. In cooling mode, the blower in the furnace runs but the heating part of the furnace doesn’t operate. The air conditioner also has an outdoor unit, where the compressor and outdoor coil live.
That box sitting above the coil, though, could be part of a heat pump. In either case, it’s the indoor part of a split system AC or heat pump and usually includes the indoor coil and the metering device. The other part of the heat pumping action occurs outdoors. (See my article on how a heat pump gets heat from cold air for more details.)
When it’s time to replace the air conditioner…
Back in 2009, I lived in a condo and had to replace the 25 year old air conditioner. We had a furnace for heating, but I wanted the ability to provide at least some of my heat with a heat pump. (I was already advocating for electrification then but not as much as I do now.) When the air conditioner was dying, I replaced it with a heat pump. I think the additional cost was only $300.
Although the cost difference is higher now, you still have the opportunity to switch to a heat pump when it comes time to replace the air conditioner. And if your AC is more than ten years old, you should be planning for that day. More than 15 years old? It may well be time to make the switch. The IRA and other incentives probably will pay the difference in cost between and AC and heat pump. Depending on where you live, those incentives might even cover more than the AC/heat pump cost difference.
After the switch, you’ll have a dual fuel heat pump. It will use the heat pump in milder weather and switch to the furnace when the outdoor temperature drops to a certain point.
When you switch from an air conditioner to a heat pump and keep your furnace, you now have two heating systems. You still have the furnace, but the heat pump will probably do most of your heating. Above a certain temperature, the heat pump does all the heating. Below that temperature, the heat pump turns off and the furnace takes over.
Ideally, that transition should be at or even a bit below the thermal balance point. Now, the balance point—because I know you’re wondering—is the temperature where an air coupled heat pump produces just enough heating to meet the heating load of the house.
At outdoor temperatures above the balance point, the heat pump produces more than enough heat and runs by itself. At outdoor temperatures below the balance point, the heat pump can’t heat the house sufficiently. In a straight heat pump system, that usually means there’s some type of auxiliary heat that kicks in and makes up the difference while the heat pump keeps running. In a dual fuel system, though, the heat pump turns off at the setpoint and the furnace comes on.
Pretty simple concept, right? Another way to handle the transition is to set it up so that the system switches from heat pump to furnace at the economic balance point. As it gets colder outdoors, an air coupled heat pump not only loses capacity but it also drops in efficiency. The economic balance point uses the costs of gas and electricity to switch over when it becomes more economical to heat with the furnace.
Are you ready to switch to a heat pump?
If you need a new air conditioner but your furnace is still in good shape, this is the easy way to switch to a heat pump. The resulting dual fuel heat pump can give you the best of both worlds. It’s a step toward electrification without relying on expensive electric resistance auxiliary heat on those really cold days.
And if you’re brave or foolhardy enough, you could even try locking out the furnace completely so the system runs in heat pump only mode. That’s essentially what I’ve done with my current home, although I have no furnace or auxiliary heat. Here’s how we did in the arctic blast last December. But make sure you know what your heating load is before doing this.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to our newsletter and follow him on LinkedIn.
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