Are heat exhanger units beneficial for modern AC units ?

[ufstudent]

I am a college student doing a report on AC hot water heat recovery systems. Several AC professionals and alternative energy specialist tell me that in the 80's they recommended and installed many of these heat recovery units, but they no longer recommend them or install very many. They said that modern AC units make heat recovery less beneficial and can, in some cases, create problems with the AC systems. After looking into the question, I am having difficulty understanding what are the problems with the heat recovery systems as an add-on to modern AC units. Can some of you AC professionals out there please point out some of the specific problems as you understand them?

[mad fridgie]

That is a very big question, as the size range of AC equipment is large.
But if I take it you are talking about small to medium sized packages, then old systems were basically on/off systems and fairly robust refrigeration circuits. Modern systems now have variable refrigerant flow and some smarts in the controls to keep the system balanced, by adding third party heat recovery, there is a likelihood that the AC control system will deviate from the manufactures performance critiria.

[install monkey]

heat recovery units in cold weather pump cold air into the room causing less performance. to compensate this is electric duct heaters thus increased energy use.heat recovery units are more economical than just an inline fan so the best bet is to fit windows with trickle vents

[NoNickName]

Heat recovery poses no whatsoever threat to the chiller. It's a passive device that simply behaves as condenser or desuperheater. A proper set of control devices would account for the heat recovery and correct accordingly.
In some countries is becoming compulsory, e.g. in Switzerland heat recovery from MT and LT compressor packs in supermarkets is now resold to adjacent buildings and blocks for winter heating, or used in the very same heated space of the supermarket.
Heat recovery is not recommended on heat pumps, for obvious reasons.

[ufstudent]

Thank you for your answer. Im looking at units in the 2 to 5 ton range, single family home sizes for south Florida.

[mad fridgie]

If its a fixed speed machine, fairly basic, then not much of an issue, likley need a fan speed control on cond fan.

[mad fridgie]

Heat recovery poses no whatsoever threat to the chiller. It's a passive device that simply behaves as condenser or desuperheater. A proper set of control devices would account for the heat recovery and correct accordingly.
In some countries is becoming compulsory, e.g. in Switzerland heat recovery from MT and LT compressor packs in supermarkets is now resold to adjacent buildings and blocks for winter heating, or used in the very same heated space of the supermarket.
Heat recovery is not recommended on heat pumps, for obvious reasons.
If the small unit is complex (more than likley japanese), then care has to be taken how the controls are integrated, not something i would recommend without undertaking a quite a bit of research and practical testing. We are not talking about a design and build package.

[NoNickName]

No controls are needed in most cases, as long as primary main condenser is pressure-controlled.

[mad fridgie]

No controls are needed in most cases, as long as primary main condenser is pressure-controlled.

A lot of small modern AC units do not use pressure controls and are very critical charge.
And systen control is based up on multiple sensor control algorithms. Basic system yes simple fan speed control