We do live in interesting times.

I know many certified refrigeration techs who would not touch this with a 10 meter pole (Canadian eh). I would not want to live next door with a DIY modified unit next to my house when it goes BANG – SSSSSSSSSSSSSSS! Right next to my HRV fresh air intake. You would be hearing from my Brother, who is a litigation lawyer.

I think someone has been reading how an advanced technology low ambient heat pump adds a plate heat exchanger to the outside unit to provide vapor injection to the compressor. But these advanced heat pumps, pioneered by Mitsubishi as “hyper heat” models, still have reversing valves, accumulators, electronic expansion valves and the ability to drive a variable speed compressor overspeed to maintain nominal btu output at low ambients. Plus a gas bypass circuit to defrost the outdoor coil. An amazing piece of value engineering for the residential and light commercial market, available from Mitsubishi for decades. However their patents may have expired. Today the advanced diyer can buy similar technology from several Chinese suppliers for 1/3 to 1/2 of the cost of a Mitsubishi hyper heat installed. A Chinese low ambient heat pump without the reliability and assurance of quality but anyone who could add a heat exchanger to an existing ac could do a complete install easily. And possibly have something that would work at the end of the day.

Extremely bad idea. Reversing valve, lots of copper plumbing, evpansion valve, defrost circuit board, defrost sensor. Most do it yourself as couldn’t do it because of the thousands of dollars of tools one would also need. But still the biggest reason not to do it….not engineered for efficiency.

M: Let’s start with the fact that it would almost certainly void the warranty on the air conditioner. If that’s not an issue, it seems that this “easy diy” modification would have to involve messing with the refrigerant, which isn’t legal if you don’t have the EPA 608 certification. That would certainly take it out of “easy diy” territory.

If this “pipe in a pipe heat exchanger which will be connected to a heat buffer” doesn’t require changing the refrigerant flows, you’re gonna have to explain to us how a system that moves heat from indoors to outdoors can do the opposite with this “easy diy” hack.

Look, here in Southern California electrification is pushed and incentivised. I do alot of system design. I also do in home sales as well as commercial proposals and project management.
I agree that a 100,000 btu furnace may be oversized. Definitely in Southern California. The efficiency of the unit matters such as 80%, 95% ETC. Where your btu output on said furnace would be the percentage of that
Other contractors in my area have had problems when swapping to heat pumps for these issues which are universal in the states: Ductwork and insulation. The homeowner complaints of high utility bills and taking very long to heat is the main issues.
Our number one item on a heat pump conversion is ductwork and insulation.
Have the right conversation.
A gas furnace requires less cfm to impact the air than cooling. With a heat pump you are using cooling cfms for heating. The temperature of the air is also not going to be as high with a heat pump.
Proper duct sizing and insulation should be the go to when deciding if the fit is correct as well if the homeowner will pursue implementing those recommendations if there system is inadequate.
Also, multi stage equipment and variable speed airflow need to be talked about. The least is a variable speed blower. That blower allows you to have proper cfm delivery, and adjustment to ducting afterward can be made on a prescription based approach.
I also recommend installing a ducted minisplit or multiple head system. Reason being is they are more targeted conditioning as well as greater design temps from 120 degrees down to 12. Traditional non variable speed heat pumps will drop capacity as it gets closer to 32 degrees and above the design temp for your area.
The technology though is getting there where Mitsubishi and Daikin wont habe that edge in design temp.

Randy: Multi-stage or variable capacity equipment is the way to go!