Siamesed HVAC

I like the multi-speed compressor and multi- and variable-speed blowers for the level of efficiency they can wring out of an HVAC system. But if that single compressor goes kerflooey, where are you? You have a no-speed system.

As a handyman maintaining dozens of airconditioners, some of which serve gigantic mult-occupant spaces with no backups, I'm a big proponent of redundancy. If one goes, you can limp along with what remains. In our facility, we have a couple of truly enormous spaces that are served by air conditioner units that have within them more than one compressor, but share the refrigerant lines. I call this a multiplexed system; it can run at sharply reduced capacity on just one compressor. That is a very slow ramp up to uncomfortable temperatures. When all four fire up, everything cools right down in a hurry.

But this sort of thing isn't represented on the residential scale, and for the sake of uninterruptible service, maybe it shouldn't, at least not in every detail. I originally envisioned this as using multiple compressors but one refrigerant line, and heat pump reversing valves adapted to switch compressors in and out of the refrigerant loop as necessary, but that would be bad: a burned out compressor would poison and kill the other healthy compressors by contaminating the shared refrigerant; retaining some cooling capacity in spite of equipment failure is the whole point here.

My idea uses several small AC/R compressors, comparable to what is found in a window air conditioner or maybe even a refrigerator. Each separate compressor has its own refrigerant circuit, and each compressor and circuit is set up to operate at a slightly different temperature, so that as the conditioned space slowly overpowers a single cooling system, another joins up, and another until finally all the conditioning systems are working in concert. When the conditioned space is idle, a single unit would probably suffice to maintain the setback temperature. Delay timing circuits would prevent all the compressors and fans from firing up at once to prevent unpleasant demand spikes on the power system.

The concept lends itself nicely to a PTAC installation, using already-existing window HVAC compressors and their associated coil packs. Using longer refrigerant lines, it would just as easily translate to a split system with the air handler within the building envelope, and the noisy bits removed closer to the neighbors.

The concept would require a somewhat larger outdoor unit than what is typical, even for a PTAC system. It wouldn't be unnecessarily unattractive, being a long, low monolithic shape with cooling grids on the front and fans behind. It would have to be larger to provide space for the multiple condenser coils and fans, but would in general be quieter in operation, with the possible exception being when all units are running simultaneously.

One potential problem I have thought about: all the compressors and fans running simultaneously may create an unpleasant periodic harmonization.