I could build one for my house for <$1000 and <2 weeks work. I could do all the design work for one which could be manufactured at scale in <6 months and <$500 per unit (although I have no idea how much extra there would be for regulatory and similar work in this domain, and the NREs for getting a production line would be quite a bit extra too).
Short story is that a heat exchanger would add resistance to airflow, but that could be trivially countered with ... a fan. I could design a control system for setting fan speed to make it look like an unobstructed passage for <$30, but more likely than not, it'd be sufficient to just have a powerful enough fan to have more airflow than an unobstructed flue.
Disclaimer: I've done work in an adjacent field, and I'm confident about all of the above working. I can give you the parts list almost off-hand if you want a longer story, right down to the pressure sensor I'd use if I needed a control system. That said, I have close to zero background in civil engineering, regulatory requirements around things like this, and similar. My knowledge of thermodynamics and thermal engineering is also quite good, but my knowledge of HVAC specifically could fit on the head of a pin. I give the background so that you can decide whether or not to believe me. :)
Sounds like a great startup
I think it's probably worth doing, but I have a list a dozen long of better startups. The key risks around something like this as a startup are:
1) Regulatory and compliance. A lot of the reason things like this aren't adopted is here. I can point you to a dozen ways to build better homes which don't meet code which is written around things like inches between studs rather than structural integrity.
2) Dissemination. In order for something like this to be successful: (a) people need to know about it (b) installers need to know how it works. Most people have no idea what a heat exchanger is.
3) Fundraising. Building things is expensive, so this would be a raise in at least the single-digit millions to start an efficient production line. No one will fund you unless you've solved (1) and (2).
To give an idea of how hard consumer ignorance is:
I was working on a random home project. I could find just one appropriate heat exchanger being sold in my local hardware store, with Panasonic's brand power behind it. The hardware store itself doesn't know what it is:
https://www.homedepot.com/p/Panasonic-WhisperComfort-40-20-2...
Home Depot thinks it's a bathroom exhaust fan. It's an ERV, which is a super-fancy sort of exchanger which also exchanges / captures humidity. The manual says not to use this in a bathroom, kitchen, or similar since it literally doesn't work for blowing out humid air.... However, that's how Home Depot markets it.
I can ship the best product in the world, and no one will buy it if they don't know about it or understand it.
Pulling together a real business plan means thinking through problems like that. I can build this thing would be easy, at least for a good engineer. That doesn't mean one can make money doing so.
