Making sure your heat pump system is sized correctly
Heat pump systems are generally the best heating and cooling solution when it comes to efficiency and energy savings, but a significant amount of detail and planning must go into the sizing process in order for them to operate properly. The burden is on the contractor to make sure your equipment is sized correctly, but some still rely on guesswork — which can lead to much higher costs than necessary! Knowing the basics of how a system should be properly sized can help you make more informed decisions when it comes to choosing an installer and comparing quotes.
A paradigm shift
Heat pump technology has improved dramatically in the last decade, and the effectiveness of air-source heat pump systems in previously unsuitable climates is now an accepted fact. Their versatility, compact design and unparalleled efficiency have pushed them to the forefront of the residential and commercial HVAC industry — both as a product in high demand and a key component of state-sponsored emissions reduction programs. Still, the emphasis on heat pumps as a “go-to” option for HVAC (at least here in New York State) is a very recent development. The way air-source heat pumps operate introduces a number of design considerations that many installers of fossil fuel systems may need to adjust to. Because thermal comfort is the most important outcome that contractors strive for, they are often incentivized to be generous with the equipment they recommend, which in turn leads to installing equipment that is much larger than necessary. However, oversizing systems inevitably leads to negative outcomes:
Short cycling
This is the most commonly known problem with oversized systems, and occurs because an oversized system is blasting warm or cold air and heating or cooling too quickly rather than gradually. The temperature swings prompt your HVAC system to start up and shut down in rapid succession.
If left unchecked, short cycling can cause significant damage to your HVAC system. The constant starting and stopping adds strain to the compressor, blower fan motor, and other components, accelerating wear and tear.
Uncomfortable temperatures
Short cycling also leads to greater fluctuations in temperature, making spaces feel too hot or too cold.
Poor dehumidification
An appropriately sized HVAC system in cooling mode should dehumidify the air in your home, but an oversized system will not stay on long enough to complete the dehumidification process.
Higher costs
Your HVAC system is often your home’s largest energy-consuming appliance, so you want it to be working efficiently — but an oversized system that is short cycling will be operating at a significantly lower efficiency level, which can raise your energy bills. Short cycling also puts excess wear on your equipment, leading to more frequent breakdowns and repairs and even shorten its lifespan.
There’s already a lot to consider when upgrading your HVAC system, and no one wants the stress of dealing with the aforementioned issues. So, how should your contractor make sure your new system is sized correctly?
It is common practice to replace an old heating system with a new system of the same size. This is a good approach assuming that the original system was sized appropriately and that the building itself and building standards have not changed a great deal over the lifetime of the system. However, furnaces and boilers can remain in service for multiple decades and a home will often go through substantial changes (planned or otherwise) over the years. The largest air source heat pump systems are capable of 50-60% of the heat output of the largest residential furnaces and boilers. A sales representative without a great deal of system design experience might assume that a furnace rated at 100,000 BTU/hr reflects the actual demand for heat in the building and conclude that an air-source heat pump may not be appropriate for the building. That may in fact be the case, but the rated capacity of existing equipment does not necessarily indicate much of anything about the actual needs of the building. If you are a considering heat pumps for your home or business, it is important to understand a few things about how a system should be selected and whether or not a contractor you are working with is making well-considered recommendations.
Best-case scenario system sizing
When selecting equipment for a customer, the most important consideration is comfort. A system is optimized for comfort when it is appropriately sized to meet the building load requirements, or the peak heat loss and heat gain of a building. This calculation should be as accurate as possible as it will inform the system size necessary to efficiently meet heating and cooling demand. The most widely used method for determining residential load is the national ANSI-recognized standard, Manual J. This set of calculations yields a reliable estimate expressed in BTUs (British thermal units) of how much heat must be added to a space to maintain a fixed indoor temperature assuming a fixed outdoor temperature (typically 5° F outside, 70° F inside). The dimensions of the walls, entryways, windows etc., the material these components are made of, and the rate of air-leakage in the house all affect the Manual J outcome. Your HVAC contractor should be able to make a fairly accurate assessment of most of these factors in a typical site visit, but for the best results a homeowner can request a residential energy audit.
The residential energy audit will provide your contractor with a more comprehensive look at your home’s energy use. It will pinpoint where energy is being wasted, point out any health or safety issues, and identify opportunities for energy savings. In New York State, all homeowners of one-to-four family homes can get a no-cost energy audit conducted by a NYSERDA participating residential auditor.
Our process
While every contractor’s approach to a job will vary slightly, these are the steps we take with new customers to ensure sizing accuracy:
We start every project with a free site visit. During this visit we use 3D modeling software to capture a scan of the building, which will show us accurate dimensions, layout, and the number of windows and doors. We also use this time to gather more information and discuss equipment options with the homeowner. We may ask if the building has ductwork currently installed that is suitable for a heat pump system, where in the home heating and cooling is most needed, or if the customer would like to keep a backup heat source for the most extreme scenarios.
The information gathered during the site visit (and from the residential energy audit if one was completed) is then used to recreate the building in a drawing-based software, which will run the Manual J calculations and produce the heating load in BTUs. This is the number we use to select accurately sized equipment and calculate the New York State Clean Heat rebate amount.
We prepare a free quote for the customer, or multiple quotes if they wish to compare different options and pricing. The equipment and its size in BTUs, the necessary ancillary components required for installation, how the system will be zoned, and the rebate deduction (if applicable) are all included in the quote.
Every building and home is different, so there’s no one-size-fits-all solution when it comes to heating and cooling. We firmly believe that HVAC contractors should be designing systems, not just selling them!
If you’re planning to make the change to a heat pump system, discussing how your heating system will be sized with your contractor is a great way to make sure you’re getting the best value for money and preventing problems down the line.
For a custom-designed system to keep your home or business feeling amazing year-round, get in touch with our team and get a free estimate!
