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Any other method than using ACCA Manual J load calculation is just a best guess. Below is a technical break down of how to size a heat pump. For a free home comfort check, please call the number at the bottom of the page.


Here is the technical explanation of how a heat pump is sized :


When sizing a heat pump for your home we begin by determining the required cooling capacity just as we would with any air conditioning system. Heat pumps are offered in the same tonnage increments as you expect from standard air conditioning so pick the proper tonnage based on the cooling requirements of your home.


Once you have determined the proper cooling capacity for your home, the next step is to determine the heating capacity of the heat pump you have selected. This information is available from the manufacturer's data sheet.


Note - The heating capacity will normally be very close to the cooling capacity in all heat pump systems.


How The Heating Capacity is Determined


A heat pumps' heating capacity is rated at a standard outside temperature of 47 degrees Fahrenheit. A rating method is required because the heat pump unit is placed outside and is subject to the varying temperatures it is exposed to. As the outside temperature declines, the system capacity will decline in a linear fashion because a heat pump extracts heat from the air and "pumps" it into your home. As a result, colder outside air contains less and less heat, resulting in reduced heating capacity. The standard 47-degree rating method simply means that we have a way to compare system capacity from
brand-to-brand.


Because of the reduction-in-capacity issue discussed above, we know that as it becomes colder outside the heat pump is producing less and less heat. This means that we need to supplement, or replace this reduction in capacity to keep our homes comfortable. This is accomplished with a "second stage" of heating in the form of a supplemental heating element. This element will bring the heating capacity back to the point where it can maintain an output of heat sufficient
to properly warm our homes.


Heat pump manufacturers' provide a psychometric chart on the heating capacity for all models they produce. The basic formula to determine heating capacity relative to the outside temperature is very similar for all systems. Note - The heating output capacity will be very similar to the cooling output in BTU's.

Our sizing example will use a Goodman 4 - TON CPLT048 outdoor heat pump matched with a variable speed air handler blower section, model AEPT060. This selection is arbitrary and used for explanation purposes only.


How to use the heating capacity graph : (NOT SHOWN IN THIS DOCUMENT)


This chart displays heating capacity on the vertical (upward) axis and outdoor temperature on the horizontal axis. Using our example 4 - TON Goodman heat pump we locate the standard rating temperature of 47 degrees on the horizontal axis and follow it straight up to the intersection of the heating capacity line and then view the corresponding point on the vertical (upward) axis. We can see that the result is about 46,000 BTU's. This result is our example systems' rated heating capacity. However, as you move to the right of 47 degrees on the horizontal axis, you can see that the capacity is reduced in a very linear fashion relative to the outside temperature.


For example, the next most commonly used informational rating point is 17 degrees Fahrenheit. Moving upward on the vertical axis from 17 degrees on the horizontal axis, we see that about 30,000 BTU's are produced at this temperature. This represents about a 35% drop in capacity as compared to the 46,000 BTU's, produced using the standard 47 degree rating.


Next, you will determine the heating design temperature for your area ( not yet active ) of the country. It is important to note that colder climate zones require larger capacity heating systems because of the increased load placed on the heating system. Simply put, the colder it is outside, the more heat you will require to maintain a comfortable temperature inside your home.


The average low is considered to be the average low temperature you would expect to experience just about every winter, not the coldest you would ever expect to see over a 5-year period for example. Florida has an average low of 35 degrees but can experience temperatures in the low 20's every 6 or 7 seven years.


Based on all that we know now about system design parameters, the next few steps follow :


1) Determine the heat loss of your home. This number represents the amount of heat your system will need to produce to offset heat lost through the windows, walls, etc. and keep your home comfortable in the winter. (We will use 58,000 btu's for this exercise.)


2) Determine the heating design temperature for your area ( not yet active ) of the country. ( We will use 27 degrees for this example )


3) Determine the capacity of the selected heat pump at 27 degrees outdoor design temperature, our example heating design temperature from step number 2 above. (We are assuming a 4-ton heat pump has been selected based on the cooling requirement of our example home). Simply select 27 degrees on the horizontal axis of our chart and move straight upward until you intersect the heating capacity line. Then move straight to the left over to the intersection on the vertical axis and you can see the outcome is about 32,000 BTU's. Remember, we want to know what the heating output of the selected heat pump will be for our example area with an average low of 27 degrees, our design temperature.


4) Subtract this capacity produced (32,000) from the heating capacity required for your home (58,000) to determine the supplemental BTU's required of the heating element. Remember, we need 58,000 BTU's to properly heat our example home but the heat pump is only providing 32,000 BTU's of that total at our design temperature of 27 degrees. We need to make up the shortfall with our supplemental heating element.


58,000 BTU's required
- 32,000 BTU's produced
26,000 BTU's shortfall (This is the BTU's required from our supplemental heating element.)


You could do all of this or call Jimmie Johnson Air & Heat for a free estimate at 336-566-8240 or visit us at www.jimmiejohnsonair.com