How to Size my Heating/Air Conditioning Gas Pack

A gas pack is an HVAC system that contains the gas furnace and condensing unit in one housing – a gas packaged unit.  This differs from split systems where the condensing unit or heat pump is outside the house and the gas furnace or air handler is inside.

When considering a  gas pack for your home or commercial building, you’ll want to know what size you need.  Having a properly sized gas pack is essential.  One that has too little capacity won’t get the job done; one that is too large will waste energy and may fail due to short-cycling, a problem that is very hard on the equipment.

Correctly Sizing a Heating/Air Conditioning Gas Pack

The best way to get a precise size for your HVAC system is to have an HVAC professional complete a thorough analysis of your home’s needs.  This is called a load calculation.  It takes into consideration the climate where you live, the size of your home and its layout, the number of windows you have and their type/quality, the amount of insulation in the home, and even more.

The climate is, of course, one of the biggest factors.  Take 2 identical homes and place one in Miami and the other in Minneapolis.  The Miami home will require a gas pack with a higher capacity air conditioner and a lower capacity furnace.  The opposite will be true in Minneapolis, where the larger furnace is essential.

BTU’s Required in Your Region

Btu stands for British Thermal Units and is a measurement of how much heat a furnace can produce per hour.  Residential gas furnaces range from about 40,000 Btu to about 150,000 Btu.  Now, divide the country into 5 zones from south to north, with zone 1 including Texas and parts of Arizona, along with the coastal states.  You can find these zone maps online easily.  Locate your zone.  Here is a rough, but fairly accurate way of predicting the Btu capacity of the gas furnace you’ll need.  We list the zones and give a number of Btu needed per square foot.

Zone 1: 35 Btu / sq. ft.

Zone 2: 40 Btu / sq. ft.

Zone 3: 45 Btu / sq. ft.

Zone 4: 50 Btu / sq. ft.

Zone 5: 60 Btu / sq. ft.

By this standard, an 1,800 square foot home in Miami (Zone 1) will require a gas pack with a 63,000 Btu furnace.  The exact same home in Minneapolis will require a 108,000 Btu furnace approximately – almost twice the size!  Find your home and do the math.

One other factor should be considered.  Furnaces have different efficiencies, which is a measurement of how much of the produced heat is used or wasted.

Our figures consider 100% efficiency, which isn’t offered in any furnace.  There are 80% furnaces which we call standard efficiency.  Medium- and high-efficiency furnaces range from 90% to 98% efficient.  The Minneapolis home needs 108,000 Btu. If the owner chooses a 110,000 Btu furnace that is 98% efficient, we would multiply 110,000 by .98 and get 107,800 usable Btu, which is right on the money.  If the owner selects an 80% efficient model, and goes with the 110,000 Btu size, the usable Btu (110,000x.80) will only be 88,000 Btu, since 22,000 Btu will be lost up the chimney.  Therefore, the owner would need to install a 137,500 Btu furnace with 80% efficiency to get 110,000 usable Btu.  Factor efficiency of the unit you choose into the equation or you may end up chilly in winter for years to come!

 

Cooling Capacity Required in Your Region

For cooling, the rule of thumb for homes in the middle of the country, or zone 3, is that you need roughly one ton of cooling for every 500 square feet.  As the climate gets warmer, reduce he square feet by 50 per zone, and do the opposite going north.  That gives these approximate values in terms of cooling tons per square feet.

Zone 1: One ton per 400 square feet; Zone 2: One ton per 450 square feet; Zone 3: One ton per 500 square feet; Zone 4: One ton per 550 square feet; Zone 5: One ton per 600 square feet.

Our Miami home would require a 4.5 ton air conditioner.  The Minneapolis home would need a 3 ton air conditioner.

Conclusion

Properly sizing your gas pack heating and air conditioner unit is an important part of the efficiency, performance and longevity you’ll get from it. This guide offers basics to point you in the right direction.  Having a qualified contractor give you a precise estimate after doing a complete load calculation is always  a good idea.

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