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Proper and accurate heat pump sizing is not easy and requires decent knowledge about the system and HVAC and the right tools. In order to answer the question “how big of a heat pump do I need” or “what size heat pump do I need for XXX sq. ft.” let’s take a look at some useful tips and suggestions from the experts.
As the climate in North America varies from one extreme (freezing temperature in northern areas) to extremely hot summers in southern parts, more variables should be taken into account if you want properly to size the heat pump.
And you have an option; either to contact a professional for the free quotes and sizing estimation or consult the sizing guides found online and DIY. But you have to be careful and avoid what most of so-called expert do; they make calculations based on the home floor space (area) or the old HVAC system.
Either one approach you choose, the proper heat pump sizing should provide an acceptable indoor comfort: temperature and humidity; energy efficiency and savings.
As the air-to-air heat pump is capable to provide both heating and cooling, it often happens, especially in central and northern parts of North America that a heat pump is sized to provide most of the heat required by a house, because the heating load is larger than cooling loads. In this case, the heat pump will be too large for cooling, and that will result in lower performance and lack of dehumidification during the summer months.
In humid climates if the unit is not sized properly, short cycling occurs, which leads to lower humidity control, comfort and due to its often stops and starts, the unit can be exposed to frequent failures and shorter lifespan. This is happening when the unit is oversized, which is very common to see, because the sales reps or contractors are trying to avoid the hassle after the sales and installation.
An oversized system costs more, the efficiency is reduced, energy cost is higher and the comfort is not as expected. The heat pump will reach the set temperature too quickly and shut off often, resulting in temperature fluctuation and poor humidity control. The system works harder, especially a fan, to distribute the air when the duct leakage is higher.
An undersized unit will work at its maximum capacity for as long as it is needed, in order to meet the set temperature, or even non-stop if it cannot keep up with the thermostat demand, leading to discomfort and potential failure of the compressor.
The goal is to reach the temperature quickly enough for greater comfort and less stress, and easily maintain the set temperature, so your heat pump uses less energy, keeps the efficiency high while maintaining the comfort you expect.
The first steps when selecting the air-source heat pump is to determine the correct cooling and heating capacity, which are usually similar and can be found in the unit’s specs.
The recommended way for sizing the heat pump, such as the air-to-air type, in North America, is using the Manual J and Manual S procedure, which are recommended by the Air Conditioning Contractor’s Association of America (ACCA). In Canada, to determine the heating and cooling loads, use a sizing method CSA-F280-M90 ‘‘Determining the Required Capacity of Residential Space Heating and Cooling Appliances.’'
Manual J is used to determine the heating and cooling loads (capacities) based on the house structure, size, number of doors and windows and the type, insulation, duct leakage, winter and summer conditions and other factors. There are also computer programs which are used with the Manual J to simplify calculations. For the indoor temperature in the cooling mode, Manual J recommends the temperature of 75 F, while for heating mode 70 F, but it depends on the user preference.
Manual S procedure is used to select the right equipment such as the duct work for the proper air flow, air handlers, electric resistance heat, thermostats… This is also important because some elements are more some less advanced. For example, variable speed is better than a single speed because it dehumidifies better and with less energy used.
Sizing a heat pump can be tricky and not as simple as we homeowners think. Bigger is not always better. As per the advice of professionals the system should not be sized to provide all the heat required by a house, it is better to meet the heating loads between 80 % and 90% and provide not more than 125% of the cooling load – as suggested by Manual S. The balance point should be between 32 F and 23 F. It is even better to choose a two-stage heat pump as it can meet the heating and cooling loads when on low speed while the high heating loads can be met when the unit is on high speed. This estimation will give satisfactory results such as the improved performance and comfort and reduced energy use, costs and maintenance.
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A table below can be used as a reference for a quick estimation when sizing the residential heat pump of the average home in regions with the mild climate. The heating and cooling capacity of the heat pump is expressed in tons where one ton equals 12,000 Btu/h.
Depends on the region, the supplemental heating might be needed (usually when the balance point is below 30 F). For example, for the mild climates use the 5 kW heating elements for the heat pump of 1.5-2 tons or 10 Kw for up to 5.0 tons heat pumps.
A link to ACCA-approved software for the Manual J residential load calculation.
Again, sizing a heat pump is not easy. It must be sized correctly in order to get all the benefits it offers and installed properly or it won’t operate adequately. This is why the sizing methods from above are recommended – they include the safety factors that guarantee the comfort for a long time.
Recommendation is to hire an experienced HVAC contractor who will take the variables into account, explain and inform you what to expect, how to operate and maintain, service and what are the warranty terms. But before that collect several quotes and compare the estimations and check out the customer reviews.
Keep in mind that the estimations of the heating/cooling requirements should be based on the heat loss / gain calculations, with the ductwork, electrical, plumbing and heating system checked and not on the some rule-of-thumb techniques. You don’t need to spend more money than what is needed.