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Building Science

Manual J Doesn’t Tell You Equipment Capacity

The amount of heating and cooling your house needs isn’t the only factor in determining the size of your heating and cooling equipment

Use ACCA's Manual J to calculate heating and cooling loads. A Manual J calculation is the first step to sizing heating and cooling equipment.
Image Credit: Energy Vanguard

Here’s a little conundrum for you. To get the right amount of heating and cooling to each room in your home, you need a load calculation. Rules of thumb don’t work. But if you do a load calculation, the result isn’t the size of air conditioner, heat pump, furnace, or boiler you need. It’s only the first step to sizing your system.

Do you know why? Let’s take a look.

Heating with combustion

This one’s easy. Let’s say the heating load for your home (in those annoying imperial units) is 50,000 BTU per hour. That means you have to install a furnace or boiler that can provide 50 kBTU/hr of heat to the home under design conditions. If the unit you install is 80% efficient, you need to install one that has a capacity of 62.5 kBTU/hr or higher. If you install a 96% efficient system, you need one with a capacity of 52 kBTU/hr.

This brings up the issue of input versus output capacity. HVAC pros normally talk about combustion appliances in terms of input capacity. The load calculation tells you what your output capacity needs to be.

The nice thing about sizing combustion appliances is that the capacity generally doesn’t change with changing outdoor conditions. When you burn a therm of natural gas, a gallon of oil, or a pound of coal, the amount of heat given off depends only on the fuel, not what the outdoor temperature is.

But there’s a little caveat to that rule. The output capacity of a combustion appliance does depend on elevation. If you’re burning natural gas in Aspen, Colorado, which is at 8,000 feet of elevation, your output capacity is lower because the density of air is lower.

Heating with a heat pump

If you’re…

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