Electric Heat Troubleshooting, Service, and Math Class
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Heat strips are one form of electric heat. It would stand to reason that taking a bit of heat strip off wouldn’t hurt anything, but the electric heat would be less effective. However, when you factor Ohm’s law into the equation, you’d be taking off part of the heat strip and reducing the resistance, meaning that the amps will increase. Heat strips also burn out when they touch another piece of metal, like the casing around it, as the amp draw becomes much higher.
Electric heat has some benefits, including its consistent BTU output (3.41 BTUs per watt); regardless of the outdoor temperature, you can expect the same BTU output all the time. It is also reliable and usually easy to work on. However, it is inefficient. Electric heat is the baseline of COP (coefficient of performance), and a pool heat pump that has a COP of 3 is three times more efficient than just the electric heat.
Electric heat has other issues, including odors or smoke when it first starts after a long time of inactivity. (It’s easy to burn off the heat strips on a PM, especially if you jumper W to R or use the thermostat to burn off the heat strips.) Electric heat also causes high bills when it runs too much and causes stress on the electrical service. Electric heat elements also draw very high current, so you have to manage your wire sizing and electrical connections. You can also encounter mistakes with blower interlock; you don’t want to run the heat strips without the blower running.
In a 4.6-kilowatt (5kW) heater, you would divide 4,600 by the unit’s rated voltage to get the amps of electric heat. The wattage decreases when the voltage decreases because of Watt’s law. When you put more voltage (electrical pressure) on the circuit, you get more current. So, the total watts would decrease if volts or amps were to decrease. If you divide the watts by voltage, you can find out your amps. To find the BTU output, take the watts (volts x amps) and multiply them by 3.41. You can also use Ohm’s law to find the resistance. Take the voltage and divide it by the amps to get the resistance (ohms). The voltage is a moving target, but resistance stays constant for the most part.
The data tag should tell you the proper wire size to feed the air handler; look for the MCA (minimum circuit ampacity) to help you make your decision. Keep in mind that 5-kW heaters are not necessarily 5 kilowatts.
When it comes to terminals, you have W, W1, W2, W3, and E. W is just heat; it is usually just for a single stage of heat. W1 means first-stage heat; it indicates the compressor with the reversing valve de-energized. W2 means second-stage heat and can refer to electric heat. W3 is the next stage of heat, and E is just for emergency heat. We used to have to jumper Y1 and W1 and a few other terminals together on older thermostats; they perform similar functions, but the reversing valve would distinguish heating from cooling.
When testing heat strips, you can usually (but not always) expect 5kW heaters to draw 20 amps and 10kW to draw 40 amps but still 20 amps per strip. However, the way you read the amps can change based on the voltage (230v vs. 208v). Commercial buildings often have 208 volts supplied to them because they are hooked up to two of three power legs.
Heat strips also have thermal limits; therm-o-discs can be found on the high and low sides for extra protection. These discs also have temperature ratings on them. You can tell if the thermal limits are open by ohming them. If they are open, you’ll measure OL or infinite ohms. If they are closed, the ohms will be low.
To control the electric heat, a unit will have either a contactor or a sequencing relay. Sequencers have contacts that close at different times. You don’t want to use a 90-340 relay, but we sometimes use those for a fan interlock.
When using a 90-340 for the fan interlock, you must set it up correctly as not to melt the relay; the current from the heat strips should not go through the relay at all. To avoid confusion as much as possible, follow the wiring diagram and think of the relay as upside down.
We can also stage heat strips if we have more than one. Staging makes the unit draw lower inrush current and use less electric heat (the less you use, the better).
When testing heat strips, you can use three methods: measure the heat strip amps, test on and test off, and force the system into defrost to test the heat strip amps.
Heat pumps with supplementary heat may have special code considerations when wiring to the outdoor thermostat, which you must keep in mind.