1.5:1 ratio for evaporators & condensers

Eugene_Silberstein

Thanks for the input Constantin... Always a pleasure to see/hear/read you!

Ross_7

1.5:1 ratio for evaporators & condensers

Prof. Eugene,
What are the advantages of upsizing the evaporator in an a/c split system? i.e., using a 2-ton evap. matching it with a 1.5 ton condensing unit. I can't seem to locate a "black & white" answer. I would think lower amp draw, and head pressure for the condensing unit. For the evaporator, it would de-humidify, and cool more efficiently. As opposed, to match the system with a 1:1 ratio. I remember my ref. instructor mentioning this, but I don't have an exact reference. TIA
Ross

Eugene_Silberstein

No Black and White

Ross,

The reason you can't find a black and white answer is because there are many variables in play here, including the actual heat gain on the space compared to the capacity of the evaporator.

On of tha main goals of oversizing the evaporator is to allow for longer run cycles and increased dehumidification. With a larger coil, the suction pressure will be higher as the coil will be absorbing more heat. The problem with this is that, if the "oversized" coil is too large, short cycling can result.

Oversizing the evaporator can actually lead to increased head pressure as the condenser must now reject more heat from the system due to the increased "heat absorbing" capacity of the evaporator.

Back in the day (mid 80's) oversizing the evaporator coil was a very common practice and mismatcvhed systems were more commonly encountered than "properly matched" systems.

With all other things being equal, an oversized evaporator coil will result in a reduced compression ratio, increasing the efficiency of the system.

This brings us to our present day, higher efficiency systems that have larger condenser coils and evaporator coils. The larger condenser coil lowers the head pressure, while the larger evaporator coil raises the suction pressure. This combination lowers the compression ratio and increases the system's effiicency.

Ross_7

Dated Practices?

Gosh, I'm starting to feel kinda "dated" now! :-)
Usually, I would just install 1.5 to 1. 2-ton coil with a 1.5 ton condensing unit, and leave the 2-ton fixed orifice in place. Never had any problem with any of the installs. Well, thanks for the insight, Prof. Eugene. I appreciate it!
Ross

Eugene_Silberstein

Dated?

Are we being dated? Damn my back hurts!

Constantin

Lennox XC21 series is ARI rated for it...

... they have published specifications and everything else in their detailed manuals / brochures on the units. IIRC, the net result was higher sensible heat, lower latent heat removal, and a higher SEER by about 0.5 points for a one ton difference between the condenser and the air handler.

I did it for two reasons, one that my contractor wanted to oversize the units ridiculously, and because the air handlers were already installed before I wised up and performed my own manual J calculation.

So, while I have 5 tons of condenser capacity outside, the air handlers inside are rated for 7 tons. For all of June, we have hit stage 2 for less than 1 hour on one of the two condensers... and that one hour was in conditions beyond the design temp per ASHRAE (91F vs. 95F). The other condenser has yet to hit stage 2 this year.

Looks like the 4.5 tons I calculated were closer to the mark... it could become interesting to downsize the respective fan motors in the air handlers... because of how undersized the duct work is relative to the air handlers. But that is a different story, for another day.

Constantin

No, thank you, for all that you share here...

Cheers! C.