Sensible Heat Ratio (SHR)

Every piece of air conditioning equipment can move a certain amount of heat BTUs (British Thermal Units) at set conditions. In most cases, during the cooling mode, a portion of those BTUs will go toward changing the temperature of the air, and a part will go toward changing vapor water in the air into water that collects on the evaporator and then drains out.

The BTUs that go toward changing the TEMPERATURE of the air are called SENSIBLE, and the ones that go toward removing water from the air are called LATENT. The percentage of the capacity that goes toward sensible cooling at a given set of conditions for a given piece of equipment or space is called SENSIBLE HEAT RATIO (SHR). So, a system that has an SHR 0f 0.70 and 30,000 total BTUs of capacity at a set of conditions would produce 21,000 BTUs of sensible cooling and 9,000 BTUs of latent removal because 30,000 x 0.7 = 21,000, and the rest 30,000 x 0.3 = 9,000.

Higher SHR (closer to 1.0) = More change in temperature and less humidity removed

Lower SHR = less change in temperature and more humidity removed

In the HVAC industry, there is a set of standard conditions used to compare one piece of equipment to another. When a system has an SHR rating listed, it would often be at AHRI conditions unless the specs state otherwise.

When doing a load calculation, a good designer will calculate and consider the internal and external latent and sensible loads. They will then match up with equipment accordingly based not only on one set of design conditions but also on the range of seasonal and occupant conditions that the structure is likely to experience based on design and climate. By following ACCA (Manual J & S) and ASHRAE (62.2 & 62.1, for example) standards, a designer will have guidelines to follow. These guidelines include matching the space SHR to a piece of equipment that will make a good match at similar conditions. It does often need some digging into manufacturers' specs to interpret this data for the equipment.

In the example above from a Lennox unit, you can see that the SHR is listed and highly variable based on outdoor temperature, airflow setting, and indoor wet-bulb and dry-bulb temperatures. In this example, you would need to multiply the total capacity x SHR to calculate the actual sensible and latent capacity.

This example from Carrier has no SHR listed. Instead, it lists the specific sensible and total capacities. You can easily calculate the SHR by dividing the sensible capacity by the total capacity, and the latent is simply the sensible subtracted from the total.

The cool thing is that this understanding can help both designers and commissioning technicians to match equipment properly and even make further adjustments using airflow to get a near-perfect match, which leads to lower power consumption, less short cycling, and better humidity control.

—Bryan

5 responses to “Sensible Heat Ratio (SHR)”

  1. Bryan,

    Another interesting thing you can do with this information is to determine the approximate target temperature split under any load condition. There are some additional footnotes on that chart likely saying the return air conditions are at 80 degrees at each of the respective wet-bulb temperatures.

    To do so, find the sensible capacity at any set of conditions, for example at 95 degrees outdoor air and 1400 CFM, the sensible capacity is:

    At 72 wb 25,010 BTUH
    At 67 wb 31,730 BTUH
    At 63 wb 37,360 BTUH
    At 57 wb 37,930 BTUH

    Using the sensible heat formula, BTUH = 1.08 x CFM x Delta T
    Delta T = BTUH /(1.08 x CFM)

    So…..
    Delta T = 25, 010/(1.08 x 1400)
    or 16.6

    Delta T = 31,370/(1.08 x 1400)
    or 20.74

    Delta T = 37,360/(1.08 x 1400)
    or 24.70

    Delta T = 37,930/(1.08 x 1400)
    or 25.08

    So you can see also that the target temperature split has a lot also to do with the return air and outdoor air conditions.

    • Hey Jim I’ve been using a chart called TEET for a couple years. It does not use outdoor temp in the formula. Just return web bulb and dry bulb. Is the TEET chart not very accurate?

  2. Good stuff Jim!

    The indoor wet bulb plays such a huge role in how the unit operates that is pains me to see how little this value plays in our industry although it is definitely getting better. At about 70F wb (~65% RH) the sensible capacity drops off quickly due to the rapidly increasing moisture reduction (Condensation) occurring at the coil. This keeps the coil temp from getting lower as well, especially if a TXV is being utilized. Just to show how much of a geek I am, you can use the chart to determine how much moisture the unit pulls from the air (It’s dehumidification capacity). At 57 the dew point of the air is ~ 43F which is less than the coil temperature. At 67F wb, the latent capacity is 10 630 btu/hr. which when divided by 970btu/lb. (heat of vaporization/condensing) gives about 11lbs/hr. of water removal. Bumping up the wb to 72F and the unit goes into serious dehumidification mode even with a fixed blower and compressor. Water removal is 21340btuhr./970btulb leaving us with 22 lbs./hr. of water removed. As long as there is sufficient outside dry bulb load driving the need for cooling (If a dry bulb t-stat is used), the AC will remove moisture based on the indoor latent load (Wet Bulb).

    Did anyone notice that even though the total capacity of the system increased substantially during high wet bulb loads, the total energy draw of the system actually went down?

  3. Jim, I’ve been using a chart called TEET for a few years. It does not have the outdoor temperature. Just uses return air dry bulb and return air wet bulb to determine the what the approximate supply air temp should be. Is that chart flawed? Or is it close? I’d be happy to send you a copy of the chart.

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