Dual Heat Pumps w VS and common return Not Twinned

[rjp]

I have dual York heat pumps, two VS air handlers with a common return. They are not twinned and they are not the same size (one is 3 ton the other 4 ton). Based on other posts, I see that the returns must be separated.

So far I have not had blower failures but I wonder what failure mechanisms are likely with this setup? Could it impact the heating capacity of one of the units or is it primarily an issue of one blower possibly rotating in reverse due to the other unit's suction?

[BaldLoonie]

Variable speed air handlers should never be on a common duct system. They'll fight each other trying to maintain airflow against varying static pressures. Anything with common ducts should be matched units and twinned so blowers start together. Very likely that if one blower starts first the other will rotate backwards then start backwards.

[rjp]

Appreciate the quick response. Besides potentiallly running backwards, are there any previously identified issues that have happened with common return on two VS air handlers/heat pumps? Will the VS motors run slower if it does not run backward? Starve the airflow on one unit?

[dash]

As baldie sad "They'll fight each other trying to maintain airflow against varying static pressures. "


This can cause early motor failures.

[beenthere]

Last thing you want, is to have to be replacing those VS motors right after their out of part warranty.

[rjp]

Thanks. Don't want to replace the motors; not because of warranty as much as extended downtime while parts are shipped in.