Practicality of energy recovery from HVAC systems?

So, if I understand correctly you are proposing to use a turbine in the exhaust vents in order to reclaim the energy from the flowing air.

However, in most cases the air is flowing due to forced flow, if you use a turbine to "suck" energy out the air then it tends to slow down. So then you would probably need to use bigger fans to push the air. So its a chicken and egg situation.

The main problem is that, as you increase the air speed, air flow tends to exhibit greater losses, so by the time you get from the fan to the turbine you would have lost more energy (compared to slower speeds). So IMHO, its going to be very difficult to harvest anything meaningful.

There are of course some exceptions. If for example you could create air flow without expending electrical or mechanical energy (see for example Trompe wall) and if you could dimension it in such a way that you would have an excess of air flow, then it would be possible to harvest "energy" for free.

Also, if you are talking about reclaiming other types of energy (not just kinetic energy of the air), as @SolarMike understood, then yes, it is more possible. However, more often that not it is not done. Actually, thinking it, and implementing it efficiently is a characteristic of a great (not just good) engineer.

Yes, this is possible.

Where I work, there are several very large fridges and freezers. The energy removed keeping them cool is pushed into a water tank that is the pre-heated water feed to the boiler, so the harder the freezers have to work means the water temperature gets raised before the boiler (fueled by gas) gets to raise the water temperature to the set value.

Energy recovery works best when it is a key component of the design. Solar Mikes example. From planning, design, operation and maintenance. Lose one of the four and it will fail.

But as an addon, it has limited success. NMech's issue with blocking air flow, which requires increasing airflow. You cannot get something for nothing.

You could add heat exchangers to smokestacks of factories and power plants to recover a portion of the heat going up the stacks. Ultimately, the viability of this approach depends on the amount of heat going up the stacks. Can heat recovery be justified via cost, energy efficiency, space, etc.

50% of the fuel burnt for ship propulsion is heat and up to 11% (5.6% of total) of this loss can be recovered by economizers and heat exchanger from gases going up the stacks. Here, the scale is sufficient to make energy recovery from stacks viable. 5.6% of 69.72MW is 3.8MW, which is more than enough to power most 2 stroke diesel driven cargo ships.

From MAN Waste Heat Recovery System (WHRS) for Reduction of Fuel Consumption, Emissions and EEDI

Heat can be used to generate steam for heating or power turbines and generate all of the electrical power for the ship, but the power system must be designed to allow operation of steam turbine generator and diesel generators. This is do-able but adds a level of complexity to the power system.

This is the extreme, not directly related to HVAC and ships have generators so recovered heat can be used, but it illustrates the need for energy recovery to be a component of the facility design and not an add-on. A holistic design approach.