Gas furnace switch to heat pump installation?

Question

I currently have a gas furnace/AC combo, but would eventually like to replace the entire system with an air source heat pump. I imagine this won't be a small or simple project, so I'm curious what all is involved. I'm in a condo building with one unit above me with my current AC condenser on the rooftop.

My current understanding of what would be needed:

• Run new thermostat wire with at least 8 connectors
• Run new coils or lines or whatever to the new heat pump on the roof
• Since I already have an electrical connection to my AC, would not need an additional breaker
• The furnace itself would be gone but I'd still need an air handler
• The venting from the furnace could be completely removed or just plugged up inside the wall

I'm sure I'm missing things but how close am I?

One HVAC company quoted me ~$18k for the whole thing, and that was after relentlessly trying to convince me that I should just stick with my gas furnace. Is that quote reasonable or totally nuts?

Thank you in advance!

AC condenser on the roof

Connections coming out of the floor on the roof

Box on the roof for 240V connection to the AC condenser, and the current breaker

Gas furnace in the utility closet

Thermostat wiring. The left wire bundle with 2 wires goes up to the roof and into the AC condenser.

Coil and wires going up to the roof. I think they're just hitching a ride with the ducts for some part of the journey, but the rest is unknown.

Answer 1

The newest heat pumps work fine at low temperature

Just make sure you get one as there's also a lot of obsolete crud out there on the market and your local traditional HVAC installer will help you find it. (sorry, that's mean).

Understand what you actually have now: a stack

You have a traditional forced-air gas furnace. That consists of a "stack" of appliances, some of which are optional bolt-ons: (bottom to top)

• A gas-y burny thing that makes a lot of heat
• An Air Handler aka a big blower and some ducting which shoves air through the stack. The burny thing commands the blower to run when it's hot enough. However, other machines in the stack can also request air handling even if the burner is off. We'll use that in a bit.
• An Air Filter you should probably change from time to time.
• A large network of Ducts to shove conditioned air around the house.
• Some air return passages to bring air back to the furnace/air handler so we aren't shoving good air outside.

Now, on top of that, you have optional things that can also go in the stack.

• Ultraviolet lights for killing bugs
• Better air filters.
• A humidifier.
• A Freon-Air heat exchanger for use by air conditioners and heat pumps.

And every house in America has that last one, don't they? :)

Your air conditioning works by using the air handler and duct system nominally owned by the gas furnace, to carry the chilled air around the house. Without it, all you'd have is an enormous ice cube around the heat exchanger.

Delete the furnace, delete the air handler.

Your proposal is to delete the furnace and all those silly ducts. *Oh man, I am with you - at the lodge we have a building that is 2 large rooms, and the downstairs was greatly degraded by someone installing a gas furnace and a "duct-o-pus", and the drop ceiling to hide that makes the room dismal.

The air handler is part of the furnace.

So if you delete the furnace, you delete the air handler and now you have no way to move heat around the house. And likewise, if you delete the ducts, you now have to figure a new way to transport heat around the house.

Now I'll grant you: traditional HVAC guys are traditional, and want to sell 70% gas furnaces and 6 SEER A/C units forever and don't like change. So they are conservative thinkers. But they're not wrong. Retaining the furnace and ducts is the cheapest way to resolve heat distribution. In my case I have 2 large rooms so 2 mini-split heads will be fine. But say you are in the snow belt and your bathroom has a duct coming into it. Go with wall-units off a mini-split, are you saying with a straight face you plan to put a unit in the bathroom? If not, then what if there's a pipe in an exterior wall, letting the bathroom get cold results in it freezing? Whoops. A/C contractors don't want to be involved in surprises like that.

It's OK to go all-heat-pump and let the gas furnace just sit there being an air handler.

That will do several things for you.

• You get to arbitrage between cost of gas and electricity, running the cheapest one this season/month. E.G. during a cold snap, when the heat pump's COP descends below say 2, you may find it more economic to run the gas. European gas is priced by the kWH which makes these comparisons super easy. Smarter thermostats can set that up as "emergency heat" because they know the outside temperature, because they are on your Internet asking the Weather Service the temperature at the nearest airport. Although an integrated system may simply ask the outside unit.
• it's there if your heat pump fails - turning it from "OMG need a repairman RIGHT NOW before the pipes freeze" to "OK, get on the list to have that fixed".
• on the off-chance you have a super-chill that is limiting heat pump performance to "not as warm as we'd like", you have options.

Answer 2

In general terms what you describe sounds accurate. You would also need condensate management, which could likely reuse whatever is set up for your current A/C. The power may actually run directly to the air handler and then from there to the outdoor unit, so there's going to be some wiring changes. Also if going all electric you will want some heat strips so that things don't blow cold during a defrost cycle, requiring additional wiring and capacity in the panel. It's not strictly required in a temperate climate, as high quality modern heat pumps can work well down to freezing or a bit below. I still suggest it because it's good to have as a backup in case your heat pump fails.

I had a similar quoting experience when doing the swap from a 25 year old furnace to a heat pump with electric backup. Two of the contractors I brought in wanted there to be a gas furnace as a backup. The third one was onboard with my all-electric plan. Quotes were $16k to $18k for high end units. The contractor I went with had the $18k quote. This was before COVID so prices have no doubt gone up since then.

In the climate here (Seattle area) skipping the furnace was the right move. In the last 12 months, the heat strips have spent about 44 hours running. The cost of having them installed was trivial compared to the cost for a backup gas furnace. The electricity for them cost about $50 across the year. Given that gas isn't free, there would have effectively never been a payback period on doing gas backup instead. The lifetime additional cost of backup electricity over natural gas will be under $1k, much cheaper than the added cost for a gas furnace and probably even cheaper than integrating your existing furnace with a new heat pump.

If you are in a climate that spends a significant amount of time below freezing, or a place with expensive electricity, the math could be different enough to suggest a natural gas backup. Personally I had a previous experience living in a home with electric only and knew that it could work well in the climate here, so I was comfortable skipping the gas. Bills and comfort since then have validated the choice: the new variable speed air handler is generally quieter, the heating bills are the same or lower despite the high cost of electricity compared to gas by the kW/BTU, and comfort is better. If you can't find a contractor that has had good experience installing heat pump only systems, that's probably your answer.

Also don't skip over mini splits as a possibility. Nowadays they can work down to very cold temperatures. They are efficient, quiet, and flexible to install.