LeftieBiker wrote: ↑Tue Nov 16, 2021 5:48 am
The problem is that my housemate isn't going to go for a $9k project in which we have people working inside the house for days, taking apart half of the furnace (which we will be needing any day now) and exceeding their estimate by thousands of dollars. The only ways I can get a heat pump this year are either by doing what I propose above, or by doing another mini-split install - this time with 3 or 4 air handlers. I barely managed the last one with one
air handler, and I'm much more feeble now. It's looking like I'll just be hooking up the portable heat pump in the dining room, and running that very loud piece of...technology when it won't disturb us. I really hate that we're still burning fuel oil (she is terrified of gas furnaces) in 2021, as the world begins to burn.
I'm not really an expert (just play one here) and had to learn as I went on my project, but I really do think you would need a new furnace and A coil and outside unit. But you should be able to keep your old lines, though its possible pulling bigger or new properly sized lines would be more efficient. And they would need vacuuming and refrigerant weighed in.
Excluding some really crazy janky stuff, a new forced air furnace (you want NG? Or keep Oil?) would likely be needed, and that acts as the blower. If I recall correctly, when you have the furnace acting as the blower, the A coil must go above (up stream) AND they can't both run at the same time in heating mode, because the furnace is too hot, and trying to run the furnace and the A Coil (in heat pump mode) is too hot for the coil. Which is a bit disappointing because the coil still produces heat, just diminishing returns as it gets colder outside. With an all electric setup you CAN run both the heat pump and the emergency heat - but that is because first the heat pump heats the air through the coil (which is BEFORE the blower) and then the warm air is then heated more by the electric heat strip kit. So that is an important difference - in your setup you PUSH the air through the coil. In mine, the air is pulled through the coil first. Just something to remember.
But can you technically keep your old oil furnace, replace the A coil and outside condenser? I think so. Most furnaces have a blower speed setting and you might need to adjust it. A new oil furnace on its own is probably 2-3k? I actually looked at this route for myself - replacing my NG furnace, adding the A coil on top and condenser outside. But it added substantial cost because now I had to get a new furnace AND the A-coil and condenser kit. Plus the DIY pre-charged line kits were only compatible with the AC ONLY A coil/condenser kit (or the full heat pump kit, but no NG furnace), and I wanted a heat pump. Add in that I realized when the NG furnace was running the heat pump was forced off, and I decided to go all electric. We don't get that cold anyway, so the heat strip will likely never run. I installed it basically as a backup in case my heat pump ever dies. That all sounds a bit confusing, sorry. In other words the DIY kits (when I got it) was AC and furnace, or Heatpump only. But if you don't want to go DIY, there must be a A-coil and Heatpump setup using your old furnace as the blower.
Not sure any of this helped, but good luck!