The ability to measure the delivered BTUs of an HVAC system has opened a new era in the HVAC industry. The amount of BTUs a system delivers into a building can be effectively calculated when system temperatures and airflows have been measured. Let’s take a look at the basic steps of BTU Measurement. NCI will feature live hands-on training sessions tomorrow and Friday at Mechanical Systems Week in the PerformanceTown Pavilion.
For decades we have been taught that the silver bullets to energy savings are high efficiency equipment and tight ducts. Once you master measuring live system BTU, it becomes clear that equipment and ducts are only pieces to the efficiency puzzle. If the system isn’t moving BTUs, efficiency will never be realized.
System Delivered BTUs
Let’s take a closer look at the definitive measurement of HVAC system efficiency…system delivered BTUs.
The simplest definition of a sensible BTU is that one BTU equals the amount of heat generated by completely burning a wooden kitchen match.
In the HVAC industry BTUs are always expressed as BTUs per hour. So an 80,000 BTU output furnace will deliver the heat equivalent to 80,000 wooden matches over a one hour time period from the equipment.
The shift in perception changed by measuring system delivered BTUs, is that all of the heat generated by a furnace rarely makes it into the envelope. Actually the average US heating and cooling system only delivers an average of 57% of the equipment rated BTUs into the home.
BTU Formulas
The foundation of BTU measurement stems from two timeless formulas; the Heating BTU formula and the Cooling BTU formula. They are different formulas but share similar ideas. Each takes the airflow and temperatures that have been measured in a live operating system and multiplies the volume of airflow times the temperature change times a BTU multiplier to find the delivered BTUs of a system.
Our opportunity is found in the difference between the equipment rated BTU and the delivered BTU of the system that actually makes it into your customer’s homes. Would you be surprised to find that many of the systems you worked on last week are only delivering 50% of the equipment rated BTUs into the homes they heat or cool?
Once you have the system airflow and the temperature change through the system, you can calculate the BTUs the system is delivering into the building.
Heating BTUs or Sensible BTUs
First, let’s look at the heating BTU or Sensible BTU formula:
Heating BTUs = CFM x ∆t x 1.08
• CFM = the fan CFM plotted on the fan tables, or measured at several points in the system.
• ∆t = the dry bulb temperature change through the system
• 1.08 = the heating BTU multiplier at sea level, or .075-lbs. of air per CFM x .24 (the specific heat of air) x 60 (minutes in an hour.) This factor will vary at higher altitudes and temperatures.
Heating BTU Example
Based on the system’s static pressure and fan speed, say the fan table shows the fan is delivering 1040 CFM, the temperature change from the air entering and exiting the heating equipment is 52.4°. What is the system delivered BTU through this heating equipment?
Answer: 1040 CFM x 52.4°x 1.08 = 58,856 Sensible BTUs
So, is 58,856 BTUs good or bad? It depends on the BTU rating of the furnace. If it’s a 100,000 BTU output furnace, you’re doing pretty poor. If it is a 60,000 BTU output you’re doing pretty well. This indicates the reality and fairness of measuring system delivered BTU.
Cooling BTUs
Let’s learn about how to calculate the BTU removal of a cooling system. Let’s start with the kind of temperature that needs to be measured to calculate cooling BTUs.
To calculate cooling BTUs you will measure wet bulb temperatures to measure the changes in heat and moisture in the conditioned air. The way we measure temperature for the cooling BTU formula requires enthalpy as a factor in the formula. We find enthalpy by measuring wet bulb temperature and then converting it to enthalpy. This can easily be done using one of the online enthalpy calculators, or a shortcut method is to use a Wet Bulb to Enthalpy Conversion Table.
An air conditioner removes two types of BTUs from the air that passes through it. It removes sensible BTUs and latent BTUs.
Sensible BTUs are the same kind of BTUs, or dry heat, that heating systems move. In a Direct Expansion (DX) cooling system, these BTUs are absorbed by the refrigerant at the indoor coil and carried outside and removed from the building by the condensing unit as dry heat blown out by the condenser fan.
Cooling systems also remove latent BTUs from the air that passes through it. When warm, moist air hits the indoor cooling coil, it’s forced to shed some of its moisture as it cools. This moisture, in the form of cold condensed water is removed from the building down the condensate drain. These are latent BTUs.
Now that we understand the two kinds of cooling BTUs, sensible and latent (together referred to as Total BTUs) let’s learn the Cooling or Total BTU formula:
Cooling BTUs = CFM x ∆h x 4.5
• CFM = The fan CFM plotted on the fan tables, or measured at several points in the system
• ∆h = the enthalpy change through the system (we get this number by converting wet bulb readings.)
• 4.5 = the cooling BTU multiplier at sea level, or .075-lbs. of air per CFM x 60 (minutes in an hour.) This factor will also change at higher altitudes. BTU multipliers will also change due to extreme air temperatures.
Cooling BTU Example
On a 3.5 cooling equipment, the fan table shows the fan is delivering 1340 CFM; the enthalpy change through the cooling system is 6.4 BTUs per pound of air. What’s the equipment delivered BTUs?
Answer: 1340 CFM x 6.4 Δh x 4.5 = 38,592 total BTUs
Pretty fair for a 3.5-ton system. On the other hand, really bad for a 5-ton system.
So that’s the basics of System Delivered BTU made simple. Sure, much more can be written about the subject and has been. We have filled books on the subject complete with very precise test procedures and instrument requirements and software that helps you gather this information and calculate BTUs in the field. But the challenge of this article is to find for yourself how well your home system is operating and then to begin to move this knowledge and skill with you into your everyday job.
Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free one-page Wet Bulb to Enthalpy Conversion Table, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.