Heat Pump Sizing - Factor 1: Traditional Method vs. Manual J

There is a lot of mixed information online when it comes to heating with heat pumps in cold climates. New England and Midwest face some of the coldest weather in the country, and many people are hesitant to install heat pumps because they worry that the system won’t keep up. The effectiveness of your system during the coldest days of the year is dependant on a few major factors, and in this article we will discuss the first factor, equipment sizing.

Equipment Sizing - Traditional Method vs. Manual J

Sizing HVAC equipment is a calculation, and is more scientific than many tradtional HVAC companies acknowledge. Every house has it’s own requirements and the load calculation needs to be tailored to each house individually. The results of a Manual J can vary drastically from the traditional method, and it is highly recommended that you have a Manual J done prior to installing a new system, especially when installing a heat pump system.

Traditional Method

The traditional way to size a system is by taking the square footage of the home and multiplying it by a quantity of BTUs assigned to the age of the house. The BTUs assigned is generally between 20 BTUs to 40 BTUs per square foot, so a 1000 square foot house will get a heating load of 20,000 BTUs to 40,000 BTUs. This is a huge range and can lead to discomfort or frequent short cycling for both furnaces and heat pump systems.

In the past, the traditional method has worked decently well because of the technological ability of older furnaces. A single-stage furnace turns on, brings the house to the setpoint temperature on the thermostat, and turns off. Each time the house dips below the temperature you set on the thermostat, the furnace turns on and you feel a blast of warm air. Because the furnace has one job to do, turn on and heat then turn off, an oversized furnace poses less risk during extreme cold and during the shoulder seasons.

Modern heat pump systems are designed to run continuously at a low speed, consistently providing heat or ac. When you look at a heat pump systems spec sheet or submittal you will usually see the heating capacity at 47 degrees, 17 degrees, 5 degress, and for cold-climate heat pumps, 0 degrees, -5 degrees, and the lowest rated temperature which is often between -13 degrees to -22 degrees.

You will also see the systems minimum capacity towards the top of the spec sheet, which may be a line item, or it is listed in the operating range. The Northeast and Midwest have a climate that ranges from negative temperatures in the winter, to the high 90s in the summer. This is where the traditional sizing method can fail. The traditional method doesn’t take minimum and maximum capacities into consideration, and doesn’t provide evidence that the heat pump system will operate properly throughout the year.

Manual J

The Manual J takes each component of your house into consideration and creates a heating and cooling load for each room, ensuring that the load requirements from constuction differences within a house are measured and applied to correct areas. You will frequently hear in the energy efficiency world that a house operates as a system, and this is true. Every house is a cohesive environment, but a house built in 1950 that has an addition from 2015 will have differing load requirements between sections, so the system is more complex than many think.

When doing a Manual J, you want to have as much information as possible including exterior wall, garage wall, floor, ceiling, and slab insulation, window values, types, and age, whole-home leakage, orientation, and so on. For example, the load of a finished basement with an insulated garage wall could require a 9,000 BTU wall unit, but if the garage wall is uninsulated it may need to be a 12,000 BTU wall unit. This is a huge difference when it is 0 degrees outside and a Manual J could have prevented this problem.

The Manual J is a malleable report, allowing you to see the heating and cooling load of an existing home, and the heating and cooling load after completing efficiency upgrades that you are considering. All ACCA Aproved Manual J softwares allow you change every input in the program, and this is important for homes going through a major renovation and homes that are adding insulation. I have seen existing homes that have a heating load of around 60,000 BTUs drop down into the mid 30,000 BTUs because of the upgrade work they are getting done. Installing a 60,000 BTU system into a home like this would be a big mistake and is completely avoidable.

Conclusion

The traditional sizing method worked well in the past for many homes, and there is some sense to the method and why it was done. The problem with the traditional sizing method today is that HVAC equipment has been through drastic advancements, and these advancements complicate equipment sizing.

Although the advancements complicate equipment sizing, these advancements are a really good thing. Modern HVAC systems are variable speed, modulating, or multi-stage. This means that heat pump systems, furnaces, and AC condensers have the ability to operate within different capacity and power ranges, saving in usage costs, and create more comfort and consistency within a home.

The only way to ensure the equipment you were quoted is right for your home is to make sure the contractor did a Manual J, or to get a Manual J done for yourself. New HVAC equipment is expensive and you shouldn’t have to worry if you are making the right choice. There are too many variables from home to home to size a system with generalized numbers and calculations, and the only way to know that the equipment is sized properly is with a Manual J.