glentre
Well-known member
There have been a number of threads here in the General Garage Discussion forum recently regarding sizing HVAC units for garages. All seem to be sizing their equipment on rule of thumb ideas, generally on a sq ft per ton basis which is useful only to get in the ballpark of what size unit may be required. There is no substitute for doing an actual load calculation on the garage. Even though two buildings may be about the same size, there may be a huge difference in geographical/climate location, insulation and building materials used plus the intended use (how often the big doors will be opened for example).
HVAC units are manufactured to remove a fixed ratio of sensible heat (temperature) and latent heat (moisture), usually in the 75 to 85 % range. Many think oversizing their unit is a safe decision since this will make sure they have a system large enough to keep them cool. The result is the oversized unit will quickly meet the sensible (temperature) load and then shut off before taking care of the latent (moisture) load because that is inherent in its design. This gives you a satisfactorily cool garage temperature-wise but with high humidity levels. Complicating the problem is the unit (being too large for the load) will cycle off and on throughout the day which results in air moving through an already wet evaporator coil and further increasing the humidity level in the space. This is why spaces with high latent (moisture) loads (think dance or night clubs) need to purposely oversize their units in order to be able to remove the humidity to comfortable levels and then use reheat coils in the ductwork to raise the room temperatures so the occupants don't feel too cold.
Sizing the unit much too small is not good either because it will be incapable of cooling the space to desirable temperatures, although it will be better from a moisture removing standpoint than an oversized unit and will be more economical.
The best way is to size the unit based on accurate load calculations and an understanding of how the garage is going to be used. Many use architects to design their buildings because they are not knowledgeable enough themselves but they feel selecting a HVAC unit can be based on general rules of thumb. Load and use calculations are complicated. If budget allows, a HVAC engineer is preferred. Or, a HVAC contractor who knows how to do these calculations would also suffice. Even a DIY owner can look to the internet for load calculation information. The point is, selecting a unit correctly is not an easy thing to do and should be well thought out before going ahead with this important decision.
Glen
HVAC units are manufactured to remove a fixed ratio of sensible heat (temperature) and latent heat (moisture), usually in the 75 to 85 % range. Many think oversizing their unit is a safe decision since this will make sure they have a system large enough to keep them cool. The result is the oversized unit will quickly meet the sensible (temperature) load and then shut off before taking care of the latent (moisture) load because that is inherent in its design. This gives you a satisfactorily cool garage temperature-wise but with high humidity levels. Complicating the problem is the unit (being too large for the load) will cycle off and on throughout the day which results in air moving through an already wet evaporator coil and further increasing the humidity level in the space. This is why spaces with high latent (moisture) loads (think dance or night clubs) need to purposely oversize their units in order to be able to remove the humidity to comfortable levels and then use reheat coils in the ductwork to raise the room temperatures so the occupants don't feel too cold.
Sizing the unit much too small is not good either because it will be incapable of cooling the space to desirable temperatures, although it will be better from a moisture removing standpoint than an oversized unit and will be more economical.
The best way is to size the unit based on accurate load calculations and an understanding of how the garage is going to be used. Many use architects to design their buildings because they are not knowledgeable enough themselves but they feel selecting a HVAC unit can be based on general rules of thumb. Load and use calculations are complicated. If budget allows, a HVAC engineer is preferred. Or, a HVAC contractor who knows how to do these calculations would also suffice. Even a DIY owner can look to the internet for load calculation information. The point is, selecting a unit correctly is not an easy thing to do and should be well thought out before going ahead with this important decision.
Glen
