Friday, December 4, 2009

Mechanical Equipment Sizing

We remember miserably cold February nights and sweltering hot August days for years, but the reality is that they are rare.  Of course, we want to be comfortable in offices, homes, and stores, so we want plenty of A/C and heating.  But it is important to size heating and cooling systems so they keep us comfortable most of the time, not all the time.

If we size systems so that we are always comfortable, the system is expensive to build, expensive to operate and will perform poorly on normal days.   Looking at weather data for Northern Virginia, for example, we can see that it is hardly ever hotter than 92F and hardly ever colder than 12F.  The chart below shows hours per year at different temperatures for typical business weekdays and for nights and weekends.   This data is for a typical year, as defined by the US NOAA weather service, and obtained from NREL.

Other cities are different, but the patterns are similar.  Boston hardly ever gets hotter than 87F.  And Miami hardly ever gets colder than 50F.

Oversized units cost more in construction, and the costs cascade through the building.   A commercial roof top unit that is 20% larger than necessary weighs more, takes up more space, requires larger ducts, larger wire, larger electric service and costs far more to buy and install.   In use, it burns power and fuel, driving up your electric bill.   In exchange for that extra expense, you may be comfortable on the hottest days, but otherwise the A/C removes less humidity than a properly sized unit, making it feel clammy indoors.

If you manage a number of buildings, pick an old one, or pick a space that is underperforming financially, and experiment.   Modernize, disconnect or replace equipment.  Monitor systems, weather and bills, and document the results.  If you have tenants, get their help and publicize efforts, so everyone can pitch in, and everyone gets credit for the changes.  Once you modify one site, the rest will be easy, but you need to engineer it carefully to avoid problems.

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