Why It is Necessary to Install the Right Size HVAC System: Heating and Cooling Load vs. Equipment Size
Some of the information from this blog comes from: https://www.greenbuildingadvisor.com/article/manual-j-doesnt-tell-you-equipment-capacity
Equipment Capacity Is Not Determined by a Manual J
When it comes to picking the right size of your heating and cooling systems, it is not just a matter of how many BTUs of heat your home requires.
Selecting the right size for your HVAC system is based on a more comprehensive understanding of the heating and cooling loads as compared to the heating and cooling capacities of the equipment. This is important for efficiency, comfort, and durability.
Heating with Combustion Systems
First, the types of heating systems that use combustion – with natural gas, oil or even coal. For instance, if your house has a heating load of 50,000 BTUs per hour. On this setting, you need a furnace or boiler that can produce at least 50,000 BTUs of heat per hour.
However, there is one factor that you have not taken into consideration in this calculation: efficiency. If your combustion system is 80% efficient, then you will need a unit with a higher input capacity; precisely 62.5 kBTU/hr. For instance, if you go for a more efficient system, say 96%, then the required capacity is reduced to 52 kBTU/hr.
While it may seem straightforward, there’s a key distinction between input and output capacity in the HVAC world. The load calculation tells you how much heat your home needs (the output), but professionals typically talk about combustion appliances in terms of their input capacity.
One of the advantages of combustion heating is that the output is quite constant regardless of the conditions prevailing outside. The heat produced depends on the fuel used and not on the outside temperature. However, there is one exception: the elevation. For instance, in areas like Aspen in Colorado where the elevation is 8,000 feet, the low air pressure reduces the system’s output capacity.
Heating with Heat Pumps
Now, let’s turn to heat pumps, which are used for both heating and cooling. Heat pumps are specified by the capacity at certain outdoor temperatures, 47°F and 17°F. The indoor temperature during testing is set at 70°F. This information, along with the manufacturer’s specifications, can be used to determine the appropriate heat pump for your climate.
But there’s a problem: the balance point. The heating capacity of an air-source heat pump decreases as the outdoor temperature drops. The balance point is the temperature at which the heat pump’s heating capacity is equal to the heating load of the house. Below this temperature, the heat pump may not be able to supply enough heat and backup heating will be required.
A traditional form of backup heat is electric resistance heat, also referred to as strip heat. Although effective, the use of strip heat is accompanied by a significant energy efficiency penalty. For this reason, it should be used as little as possible. To do this, you will have to make sure that your heating and cooling needs are well balanced. This is because most homes are mismatched between the two systems; you may either have a bigger cooling system than you need or require more supplemental heat.
Cooling with Heat Pumps and Air Conditioners
This brings us to the cooling side of things, which is often the most confusing part of HVAC sizing. The cooling load of your home is determined by the Manual J calculation, which determines how much cooling your home needs. Nevertheless, there is one catch: when it comes to air conditioners and heat pumps, cooling is not only the total cooling capacity but also the cooling of sensible (temperature) and latent (humidity) loads.
Here’s where things can get confusing. For instance, you might find that your home has a total cooling load of 18,632 BTU/hr. This total consists of a sensible load of 14,833 BTU/hr and a latent load of 3,799 BTU/hr. If you simply ask for a system that meets the total load, you might end up with an air conditioner that controls the latent load while failing to provide sufficient sensible cooling. For instance, an air conditioner that has an 18,632 BTU/hr rating may only provide 13,042 BTU/hr of sensible cooling, or 12% less than the required amount.
To guarantee that both sensible and latent cooling demands are fulfilled, you may need a bigger unit, which can carry up to about 23,600 BTU/hr in total. Besides that, there are other modifications that a professional HVAC designer will make based on specific conditions since the equipment’s rated capacity is tested under conditions different from those present in your home.
Factors Affecting Cooling Capacity
It is important to note that the conditions for testing HVAC products are fixed but you may be in different conditions. The Air Conditioning, Heating, and Refrigeration Institute (AHRI) evaluates the cooling products at 95°F outdoor temperature. But in places with high outdoor design temperature like Phoenix, the cooling capacity will be reduced. For instance, in Atlanta where the outdoor design temperature is 92°F, you may get slightly higher capacity. Conditions inside the house also play a role.
At the time of testing, AHRI simulates homes to be kept at 80°F with 51% relative humidity. However, people usually set their thermostats at 75°F with 50% relative humidity. To achieve these real-life settings, more cooling capacity may be required.
The Difference Between Load and Capacity
The major concept that follows from this is the difference between load and capacity, two terms that are often used interchangeably. A Manual J load calculation will determine how much heat or cooling your home requires, but it will not state how large the system that is required to meet that load is. Capacity is the nominal output of the HVAC equipment and refers to its capability to supply the required amount of heat or cooling as required to meet the load.
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Remember, a Manual J does not determine equipment capacity.
Now on the other hand, a Manual S is a guideline developed by the Air Conditioning Contractors of America (ACCA) for choosing the proper heating and cooling equipment for a building according to its particular load requirements. It prevents HVAC systems from being either too large or too small by taking into account climate, equipment performance data, and indoor comfort needs. Manual S is used in conjunction with Manual J (load calculations) to maximize system efficiency and performance.
In summary, it is necessary to take into consideration not only the heating and cooling demands of the structure but also the manner in which the chosen equipment will function in practical conditions. It is important to ensure that you work with a qualified HVAC professional when sizing and operating your system to not only enhance comfort but also minimize energy consumption and load on the equipment. With this knowledge, you can make sure that your HVAC system is the right size for both your heating and cooling needs. Properly sized systems are vital for achieving energy efficiency, comfort, and durability.