Finding Equipment Rated Capacity

In the HVAC industry, system efficiency ratings are creating quite a stir as the long time energy saving staple of “high efficiency” equipment is beginning to be questioned.

The government and utilities departments are finally realizing that sealing ducts can often reduce delivered capacity and actually increase energy usage.

As ducts are sealed without additional duct capacity added to the system, static pressure increases and system airflow diminishes. If a contractor then fails to adjust refrigerant charge or combustion, the heat transfer declines rapidly and system capacity reduces.

The notion of rating an HVAC system based on the delivered BTU was nearly impossible a decade ago because the technology and instrumentation wasn’t available. Back then essential test procedures could only be duplicated in the laboratory.

Design and Actual Performance
Rating a system’s performance is based on two key system values. First, we measure system airflow and temperatures and calculate the BTU the system is delivering. Second, we check the manufacturer’s engineering data and determine the equipment’s rated capacity based on current outdoor conditions.

We won’t dwell on the testing procedures for the system right now, but let’s take a look at the challenges we face with deciphering the rated BTU of equipment under a wide variety of live field conditions. To keep this article to a reasonable length, we’ll hold off on geothermal and hydronic systems for now. Study the published engineering data to discover the system’s performance variables.

Gas and Oil Heating
Oil, natural gas and propane burners are easy to deal with. Typically the rated output of the appliance is printed right on the equipment nameplate. Be sure to use the rated output, not the rated input. Input is based on the heat content of the volume of fuel burned. The output is the manufacturer’s stated value of the heat the equipment can deliver under ideal conditions.

Heat Pumps in Heating Mode
Heat pump BTU output is a moving target. The colder it gets outdoors, the lower the heat transfer capacity of the equipment. It’s just physics. However, for your convenience each manufacturer publishes specifications for each piece of equipment that will help you determine what that equipment’s capacity is rated for under any outdoor condition.

Just for fun, here’s an array of rated BTU capacities for the same piece of equipment under different outdoor conditions:

Nominal Tons Outdoor Temperature Rated BTU Capacity
3 Tons (36,000 BTU) 70 Degrees 44,500 BTU (A 4 Ton?)
3 Tons (36,000 BTU) 50 Degrees 34,800 BTU (A 3 Ton?)
3 Tone (36,000 BTU) 30 Degrees 23,400 BTU (A 2 Ton?)
3 Tons (36,000 BTU) 0 Degrees 13,700 BTU (A 1 Ton?)

Now, that’s interesting. Every time we dig deeper into the manufacturer’s engineering data, it seems there’s another fresh perspective that broadens our understanding.

Get in the habit of collecting manufacturer’s tables containing this essential engineering data to help you peg rated equipment BTU capacities under field conditions.

National Comfort Institute (NCI) has created generic tables for you to use in a pinch in the field when manufacturer’s data isn’t available to you. Since manufacturer’s data is always more accurate, be sure to note on your report if you used generic performance data.

Electric Heating
There are two levels of determining the rated BTU output of electric heat forced air systems. The quick method is to multiply the electric heat Kilowatt rating listed on the Manufacturer’s nameplate data by the BTU constant for electric heat of 3413. This method is recommended for the HeatMaxx or Score level one testing. This is a quick test method often used by service techs to take a fast snapshot of a system’s performance. The 3413 number comes from 3.413 BTU per watt.

When we use this number, we assume a voltage of 240. If your final test numbers lead you to believe that the heat strip BTU delivery is questionable, you should measure the actual heat strip voltage x measured amperage x a measured power factor to more accurately identify electric heat capacity.

More and more technicians are using this advanced method of testing electric heat capacity to increase their rating accuracy. Experience is a great teacher.

Air Conditioning
Just like the heating capacity of heat pumps, cooling capacity is also a moving target. The fact that most areas of the country have summer design conditions that are no more than 30 degrees warmer than indoor temperatures, the swing isn’t quite so drastic as our heat pump numbers.

Let’s look at a cooling performance table similar to the heat pump heating table. You’ll see we can also get more BTUs at cooler outdoor temperatures and less BTUs at warmer outdoor temperatures.

Nominal Tons Outdoor Temperature Rated BTU Capacity
3 Tons (36,000 BTU) 105 Degrees 30,200 BTU (A 2.5 Ton?)
3 Tons (36,000 BTU) 95Degrees 33,600 BTU (A 2.75 Ton?)
3 Tone (36,000 BTU) 85 Degrees 36,000 BTU (A 3 Ton)
3 Tons (36,000 BTU) 75 Degrees 38,000 BTU (A 3.25 Ton?)

Advertised cooling system capacity is based on AHRI standards of 95 degrees outdoor with a 67 degree wet bulb and 80 degree dry bulb return air temperature, at 400 CFM per ton.

You’ll find under these conditions most equipment capacity runs about 11,200 BTU per ton. At 85-degree outdoor temperature we get our 12,000 BTU per ton, and at 75 degrees outdoor we get a wonderful 12,700 BTU per ton, while at 105 degrees we’re down around 10,000 BTU per ton.

Start collecting manufacturer’s cooling performance tables whenever you can. We’re hearing that more customers have home office copy machines that you can use when you find well-preserved data in the attic or basement. We also have a Generic Cooling Performance Tables available on the NCI website.

Altitude Correction
Finally one last bit of advice, if you’re someone that enjoys the benefits of the high open spaces, altitude plays a key role in both heating and cooling equipment capacity.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a free NCI Generic Cooling Capacity table to help you determine rated output this summer, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.