Measure Heat Strip Performance

Since the cold has set in, we've received many calls and e-mails from contractors and technicians wresting with electric heat systems. The two main concerns are: How to determine and rate electric heater performance; and how to figure the right amount of airflow to safely increase or decrease supply air temperatures. Let’s take a closer look.

Heat Strip BTU
Most electric heat questions have been related to the heat strip capacity. It seems that if electric heat is 100% efficient, they should easily all produce 100% measured performance, right? As we have all learned, there are plenty of reasons why systems don’t work as they should, and electric heat is no exception.

Equipment nameplates often list only the Kilowatt (Kw) rating of the heating elements or Heat Strips. Rarely is the rated BTU listed. Ideally, to check the rated BTU, go to the manufacturer’s engineering data found in the installation packet. This is the best source, but sometimes this information isn’t available.

The quick formula to convert KW to BTU is easy: simply multiply the heat strip KW times the constant of 3413. For example, take a 9.8 Kw heater and multiply the Kw by 3413 to find the expected BTU of 33,447 BTU.

This is the short formula, however, and if your testing proves the BTU delivery is substantially off, you may want to do additional testing and take a closer look.

The most typical problem we find is that the actual voltage may be different than the manufacturer’s voltage rating for the electric heater. If the manufacture built the heat strips and rated them to operate at 240 volts, and the actual voltage in the field is only 230 Volts, the BTU will only be 92% of its rated output. If the actual voltage measures 220 volts, the BTU will be 84% of capacity. And at 208 volts, you’ll find the BTU at 75% of the published rating. Should the line voltage be only 120 volts, delivered BTU will drop to 50% of the assumed BTU output. So check voltage at the heat strip.

The Kilowatt Formula
Often verifying actual field conditions is the only way to assure your BTU rating is accurate. This will require the measurement of actual amperage and voltage at the heating element. Then plug your measurement into Kilowatts formula to find actual Kw.

Here’s the formula: Volts x Amps / 1000 = Heat Strip Kw. Measure actual voltage and actual amperage at the heat strips, and then apply the formula.
Next, multiply the measured Kw by 3413 to get true heat strip BTU. Then compare the true Kw BTU to the equipment or system delivered BTU to find field efficiency.

Here’s a table that you can use to help you determine the change in Kw or BTU if the actual field voltage is different that the manufacturer’s specified voltage. The table also lists the reduction in Amp draw if the field voltage is reduced.

Electric Heater Voltage Variations

If you find a BTU delivery problem on electric heaters in the field, one solution may be to check voltage to assure the field voltage matched the manufacturer’s rated voltage and apply the formula and table above.

Airflow and Temperature
Sometimes we receive complaints from our customers that they become chilled by supply register air temperatures during the winter months when electric or auxiliary heat pump heat kicks in. They would like the discharge air temperature increased to improve comfort.

One of the best solutions to this problem is to reduce airflow to increase the supply air temperature. Less airflow over the heat strips requires each cubic foot of air to carry more BTU of heat off the heat strips. More BTU per CFM equals warmer supply air.

One caveat: Care must be taken not to reduce airflow to the point where there’s not enough air over the heaters to remove the heat from them. If this happens, the elements may overheat and damage may occur, thermal protection will kick in or fire may result. Are you scared? We hope do. Check the manufacturer’s specs and be certain to stay within the published specifications.

Calculate Required Airflow
For comfort purposes, it seems most people around the industry agree that a minimum electric heat supply air discharge temperature of 100F is desirable.

You can calculate the how much airflow is needed once you have determined the BTU output of the electric heater and you have decided on the desired discharge air temperature. Remember register air discharge temperature and equipment air discharge temperature are usually a few degrees different, so keep an eye on heat strip temperature change as much as equipment temperature change.

First, take the desired supply air temperature and subtract typical entering air temperature. This will give you the temperature change or t needed over the heat strips.

The calculation to determine required airflow is to divide the rated or actual Electric Heater BTU into the desired temperature rise. The answer is the required airflow.

Here’s an example:

Or Required Airflow = 1130 CFM

Check your calculated airflow against the manufacturer’s engineering data to be sure that the required airflow will deliver the temperature rise that falls within the manufacturer’s specifications.

Then adjust airflow to the required CFM. This can be done by reading the installation instructions and setting dipswitches. Or by changing the fan speed tap. In a belt drive unit you’ll need to change the pulley size to increase or decrease fan speed as needed. Remember the airflow to each register and grille will adjust proportionally, so normally there isn’t a need to rebalance the system.

After adjusting airflow, measure actual airflow and temperature rise through the equipment and through the system. While you’re at it, multiply CFM x t x 1.08 to find the delivered BTU.

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 one page procedure describing how to test heat strip performance, 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.