Residential HVAC Sizing 2

[DRsquare]

I'm a building plans examiner in FL, and the energy code requires the capacity of the installed equipment to be no more than 115% of the load. Typical load calculations are ~80%SHR, while the capacity is always figured at 70% SHR. This gives a unit which has less sensible capacity than load, and way more latent capacity than needed.

Why then do I always "hear" that if you oversize a unit, you'll get a build up of moisture because the unit doesn't run long enough?

 

[Drazen]

0,8/0,7 comes to about 1.15, so if you have situation as described, you will still meet the code by satisfying both sensible and latent demands.

apparently, using smaller size unit is not "more efficient" if you sacrifice main purpose of your sizing. code should be met, but anything below code maximum is ok only if load coverage is ok.

moisture buildup occurs in on-off systems, where too large capacity causes short run-long idling cycles, and moisture buildup happens in that long idling which you cannot avoid.

that should not be an issue, however, with small oversizing.

 

[walkes]

Also note that some newer technology compressors will modulate to match the load. So slightly oversizing the unit may not have any bearing on the dehumidification capacity of the unit.

If the compressor is on-off and oversized for the load it will run, overcool the space, and shut off until the space temperature rises again. If the system can modulate to meet the load it will run continuously and the relative humidity in the space will be more even.

 

[317069]

what did you mean by " Typical load calculations are ~80%SHR, while the capacity is always figured at 70% SHR."
SHR is a barameter deppends on specific situation, there is no (typical and always)
back to your question, here is an example:
your sesible load is 80000 btu/hr
your latent load is 20000 btu/hr
total is 100000 btu/hr
if you choose 120000 btu/hr total unit with 100000 btu/hr sensible and 20000 btu/hr latent. and using a dry bulb thermostate to control this unit then the compressor will shut off after 48 minuit from running because he removed 80000 btu
(again this is an example, don't worry about numbers), and in this 48 minuts you machine will remove 16000 btu instead of 20000 btu, this 4000 btu difference will effect on confort.