How do you size a commercial heat pump?

Commercial HVAC / July 22,

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Did you inherit a legacy HVAC system during your last office move? Or is your current HVAC system subject to frequent temperature control issues and breakdowns? If so, it may be time for an upgrade. Selecting the right-size HVAC unit is the first step in preempting future performance issues. But how do you size a commercial HVAC system correctly?

How to properly size a commercial HVAC unit involves basic math skills and some knowledge of relevant HVAC principles. Today we&#;ll review which factors weigh into sizing your HVAC system, the calculations required to do so, and why correct sizing is crucial to the long-term functionality of your system.

How To Estimate the Commercial HVAC Size You Need

Size matters! Determining your HVAC sizing is critical for energy efficiency, performance, and controlling installation costs. When it comes to HVAC sizing, remember the Goldilocks rule: not too small and not too large. &#;Just right&#; is the goal of creating a comfortable environment.

HVAC systems are measured in tons or tonnage. This is not a reference to the system&#;s weight, but to how many Btus (British thermal units) a system can produce.

In short, a Btu is the amount of energy required to lower or raise the temperature of a pound of water by one degree Fahrenheit. Therefore, determining your unit&#;s tonnage involves calculating how many Btus are needed to generate the amount of energy required for heating and cooling.

Size Your Commercial HVAC System in Five Steps

1. Calculate the square footage of the space you want to cool. Note: If your ceilings are higher than eight feet tall, the following calculations may need to be adjusted.
2. Divide the square footage area by 500.
3. Multiply the result from Step 2 by 12,000.
4. Add 380 Btu for each building occupant, plus 1,200 Btu for each kitchen and 1,000 Btu for each window in the space.
5. Convert the result from Step 4 to tons by dividing it by 12,000. This final number will indicate the HVAC system size you will need for your building.

For example:

• If you have a 4,500 square foot office space, divide 4,500 by 500 to get 9.
• Multiply 9 by 12,000 to get 108,000.
• If 12 people work there, add 4,560 (12 x 380 Btu).
• If the space has eight windows, add 8,000 (8 x 1,000 Btu).
• If the space has one kitchen, add 1,200 (1 x 1,200 Btu).
• Divide 121,760 by 12,000 to get a 10 ton HVAC system.

Other HVAC Sizing Considerations

The Air Conditioning Contractors of America (ACCA) has published numerous manuals detailing the sizing and design of air conditioning systems. For example, one manual, Manual N, instructs that there are four considerations in determining the correct HVAC equipment for any commercial building:

• Application: Is the space an office, restaurant, grocery store, or retail outlet?
• Building Type: Is the space a single-story building, multi-story building, a warehouse, or another building type?
• Square Footage: What is the size of the space to be heated or cooled?
• HVAC Equipment Type: Does the building use gas or electric heat? Will it be a water-cooled or an air-cooled system?

The ACCA&#;s Manual N also considers building construction materials, including:

• What are the inside and outside walls made of?
• What is the insulation level between walls?
• What are the roof and floors made of?
• What kind of windows does the building have?
• How many people will work in the space?
• What types of electronics, lights, and plants are present, and how many of each?

What&#;s Load Got To Do With It?

In addition to these calculations and considerations, commercial HVAC system sizing also depends on the loads generated in the building. The &#;load&#; is the amount of heat your cooling system must remove (or the amount of cold your heating system must remove) to maintain a constant temperature.

Loads are divided into external loads and internal loads, and you should consider both when sizing your HVAC system.

• External loads result from weather conditions that bring heat and cold directly into the interiors, from weatherization, and as a result of the building&#;s design.
• Internal loads result from internal factors like people, lighting, equipment, and fresh air.

Load is balanced with your HVAC system&#;s capacity, which is the amount of cooling or heating a system can produce at maximum effort.

Why is it Important to Choose the Correct HVAC Size?

The size of a commercial HVAC system directly affects its cost, performance, and maintenance issues. Therefore, it&#;s vitally important that you choose the correct HVAC size when installing a new heating, ventilation, and cooling system to keep people comfortable and protect your investment.

What Happens When Your HVAC Is Too Large?

An HVAC unit that&#;s too large for your space can cause poor air quality and excess humidity, leading to mold generation, asthma risks, and general discomfort. Oversized HVAC units also contribute to frequent maintenance calls, energy waste, increased wear and tear, and higher installation costs, not to mention higher initial unit costs.

What Happens When Your HVAC Is Too Small?

Conversely, HVAC units that are too small obviously can&#;t cool the space efficiently. Higher energy costs, increased wear and tear, and slow cooling result from the compressor being pushed past its limits.

HVAC Estimation DOs and DON&#;Ts

• DO ask about ductwork and components that will provide the most efficient setup.
• DON&#;T assume you&#;ll be replacing an older HVAC unit with the same size unit. New energy efficiencies can mean you could get by with a smaller system.
• DO ask a licensed professional if you&#;re unsure about what you need.
• DON&#;T assume bigger -- or smaller -- is better. Every situation is different.

Need Help With Commercial HVAC? Call FASTECH

Whether you&#;re replacing an old unit or starting anew, a commercial HVAC unit is a significant business investment that should be taken seriously. If you&#;re not confident sizing your commercial HVAC system, you&#;re not alone.

The HVAC technicians at FASTECH are here to help. We provide expert commercial HVAC system installation, maintenance, and upgrades that save money and extend the lifespan of your HVAC system.

FASTECH is the leading provider of end-to-end commercial HVAC and refrigeration services. Over the last 25+ years, our expert team has provided cooling and refrigeration services for thousands of clients across nearly every industry, including retail, restaurants, pharmacies, service stations, and more.

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The Manual J calculation is the most thorough, reliable method for determining a home's heating and cooling load, developed by the ACCA. Here's a partial list of what goes into the equation:

• The size of your home, including square footage and ceiling heights. Generally, bigger homes need bigger heat pumps&#;but not always.

• The layout of your home. Heat rises, so homes that sprawl horizontally tend to have a higher load than multi-story homes on a smaller footprint.

  For more Commercial Air Source Heat Pumpinformation, please contact us. We will provide professional answers.

• Windows. More and larger windows increase your load because they leak heat faster than walls. Single-pane windows do, too&#;they have a lower U value than double-pane windows. And the more sunlight that your windows let in, the higher your cooling load&#;but the lower your heating load.

• Insulation. More or higher-quality insulation generally means your home has a higher R-value, which measures heat retention. Higher R values in your walls and attic or roof lead to lower loads&#;sometimes substantially so. Modern insulation can easily shave a half-ton or more off the load compared to older standards.

• Air leakage (aka air infiltration). Ideally, you'll get a blower-door test, which tells you how much air leaks through all the little gaps and cracks around your home. But most calculations rely on estimates that are pre-built into the Manual J procedure (more on those later), and each step up in "tightness" can reduce the building's load by a few thousand Btu.

• Local climate. On the 1% coldest days in your town, what's the temperature? What about on the hottest days? And the most humid? Your load&#;and the way you should pick a heat pump model&#;is based on these extremes rather than averages.

• Ductwork (for ducted heat pumps&#;not mini-splits). Duct placement matters; your load will be higher if they're in an unfinished basement or attic. Better duct insulation and air sealing will bring the load down, among other benefits. (Then there's the separate issue of duct sizing and whether they can handle the airflow your system needs to keep every room comfortable.)

• Body heat and heat-generating appliances. The more people in your house, the more body heat they'll give off (typically a few hundred Btu/h per person at rest). Appliances and electronics warm up your home, too.

• Preferred temperature. Every degree you turn the thermostat up in the winter or down in the summer can change the load by a few hundred Btu.

Small tweaks to the insulation values, air infiltration scores, and window types can add up to a difference of more than 1 ton in your building's load. If you miscalculate those factors, your estimate will be way off. (This should start to illustrate why simple shortcuts are a waste of your time.)

Most installers won't do a "perfect" Manual J calculation. For example, they can't precisely measure insulation thickness or duct sizing when it's hidden behind drywall. And blower-door tests are rare because they're relatively time-consuming (and time is money).

This doesn't necessarily mean that they're cutting corners, either. Real-world experience counts for a lot, and after a few years in the field, installers might have a pretty good idea of how buildings tend to perform in their home region.

Installers might not even perform an "official" Manual J calculation, though it's tough to understand why: At least some of the approved software makes it pretty easy to get a decent estimate even without precise data. For example, most programs let you enter either a precise air-infiltration score or pick a qualitative score from a drop-down menu.

Can you just look at the capacity of your existing air conditioner, furnace, or boiler and use that as your target for a heat pump? The experts we've talked to don't recommend it. According to experts we've talked to, HVAC equipment tends to be massively oversized because most installers haven't traditionally taken many measurements. (It's also not as big of a deal when furnaces or boilers are oversized because the price doesn't increase much with the capacity.) And if you're in a climate with cold winters, the cooling capacity of your central AC probably won't translate to match your heating needs.

Manual J is not an exact science

Like any design discipline, HVAC is part art, and there's rarely one right answer to heat pump sizing. High-quality installers can come to slightly different conclusions, and that's normal&#;within a certain range, anyway. And even the ACCA (the authors of Manual J) acknowledge that there's some wiggle room: You'll still be comfortable even with equipment that's 10 percent undersized or up to 30 percent oversized (with a variable-speed compressor; fixed-speed systems don't have quite as much wiggle room).

Let's say that an installer runs a complete, rigorous Manual J calculation based on perfect information about a single-family house and finds that the load is 2.6 tons. Heat pumps only come in half-ton increments&#;the closest options are either 2.5 tons or 3 tons.

So is it best to round up to 3 tons to be safe? Well&#;maybe. Some so-called 3-ton systems actually work more like 2.9-ton systems, so that might be a good fit. Certain 3-ton systems might struggle to dehumidify a house on a muggy but relatively mild day, while other 3-ton systems will have no problem with that. And in some situations, some pros could recommend a 2.5-ton heat pump since Manual J (arguably) overestimates heating loads.

Capacity can change with the weather&#;sometimes

On top of that, capacity changes with the weather, so apples-to-apples comparisons can be tough. A heat pump's advertised size is based on its heating capacity at a very mild 47 degrees. But heat pump performance changes with the weather, and many lose their heating ability as temperatures drop below freezing. (It's a myth that heat pumps can't work in cold weather, but many models will indeed struggle.)

For example, a 3-ton single-stage heat pump that's built for a climate like Miami might act more like a 1.5-ton heat pump on a chilly winter night in New York City and a one heat pump during a cold snap in Chicago&#;just when your heating load would be at its greatest.

On the other hand, a 3-ton "cold climate" heat pump with a variable speed compressor can work like a 3-ton system all winter in NYC and Chicago. (NEEP maintains a database of heat pumps that can maintain at least some of their heating capacities in cold climates.)

Another complication: Loads of heat pumps fall somewhere in between that "warm climate" and "cold climate" distinction, and so do many parts of the US, including cities like Charlotte, Atlanta, and Dallas. They can drop to 20 degrees Fahrenheit, which is tough for a super-basic to handle. But an expensive cold-climate system would be overkill. (Ditto for Miami, from the previous example.) The top-line capacity specs aren't useful here, so installers must look deeper.

It's entirely possible in an in-between situation like this that you'll get one quote for a so-called 3-ton system and another for a 3.5-ton system. But when the temperature drops to the expected local low of 17 degrees, they both perform like 2.8-ton heat pumps&#;which might be exactly what both installers were aiming for.

Those installers must also consider how those same heat pumps will handle cooling and dehumidification. It gets a little bit complicated in climates with cold winters, moderately hot summers, and stretches with significant humidity. We won't peel back that onion here. But it's one more example of how great installers need to thread the needle and find smart compromises&#;and why that might lead to sizing inconsistencies between quotes.

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