Key Takeaways
- Resistance testing before installation establishes a baseline. Measuring ohms across each element before power-up catches open elements that would otherwise trigger callbacks within weeks.
- Know your parallel configurations. A 10kW kit is usually two 5kW elements in parallel. Each element reads 10-12 ohms individually; the total kit reads ~5.8 ohms at the main lugs. If the total reads ~11.5 ohms, one element is open.
- Sequencers hide element failures. A single open element in a multi-stage heat kit may not trigger obvious symptoms until the coldest night of the year, when the customer calls at 2 AM.
- Pre-install testing takes 5 minutes. Post-callback diagnosis, truck roll, and customer frustration take considerably longer.
Heating season means heat kit installs. A quick resistance test before power-up verifies what’s actually in the box. Takes five minutes. Most techs don’t bother.
Understanding Heat Strip Resistance
Electric resistance heat is straightforward: current flows through nichrome wire elements, generating heat. The same resistance principles that apply to motor windings apply here.
Most heat kits contain multiple elements wired in parallel. A 10kW kit typically has two 5kW elements. This matters for testing:
Individual Element Resistance (5kW coils at 240V): Each standard 5kW element reads approximately 10-12 ohms.
Total Kit Resistance at Main Lugs:
| Heat Kit Rating | Total Resistance | Configuration |
|---|---|---|
| 5 kW | 11.5 Ω | Single element |
| 10 kW | 5.8 Ω | Two 5kW in parallel |
| 15 kW | 3.8 Ω | Three 5kW in parallel |
| 20 kW | 2.9 Ω | Four 5kW in parallel |
An element reading OL (open line) is obviously failed. If a 10kW kit reads ~11.5 ohms at the main lugs instead of 5.8 ohms, one of the two parallel elements is open. You’re only getting half your rated capacity. The same diagnostic principles covered in our general guide to HVAC troubleshooting apply here: verify before condemning.
The Pre-Installation Protocol
Before mounting the heat kit in the air handler, verify each element:
- Disconnect all power. Lock out the disconnect. Verify zero voltage before touching anything.
- Set your meter to ohms. The 200Ω range works for most heat strips.
- Measure each element individually. Disconnect the factory wiring at the element terminals. Each 5kW element should read 10-12 ohms. An OL reading means that element is open and the kit should not be installed.
- Verify total kit resistance. With elements wired in parallel at the main lugs, a 10kW kit should read approximately 5.8 ohms. If it reads ~11.5 ohms instead, one element is open.
- Check element to ground. One lead on an element terminal, the other on the heat kit chassis. You should read OL (infinite resistance). Any measurable resistance indicates a ground fault.
This same verification process applies when troubleshooting motor windings or checking capacitors under load. The principle is identical: verify the load before energizing it.
Why Sequencers Complicate Diagnosis
Heat strips don’t energize all at once. Sequencers stage the elements to prevent massive inrush current. A typical 15kW heat kit might have three 5kW stages.
Here’s the problem: if one element in a three-stage system is open, the system still produces two-thirds of its rated heat. The homeowner may not notice during mild weather. But when outdoor temps drop and auxiliary heat kicks in fully, the missing 5kW becomes obvious. By then, the install was weeks ago, and the callback costs you $250-400.
The same logic applies to avoiding premature compressor failure. Verify components before energizing, and you avoid the expensive callbacks later.
Pre-install resistance testing catches this before you leave the job.
Building the Habit
The resistance check needs to become automatic. Train crews to include it in the standard install checklist:
- Document the readings. Write the measured resistance for each element on the installation paperwork. This protects you if a failure occurs later.
- Flag out-of-spec readings immediately. Don’t install questionable components hoping they’ll work.
- Verify after wiring. A quick recheck after completing electrical connections confirms you didn’t create a problem during installation.
Know Before You Go: Heat kit sizing depends on the home's electrical service and heating load. Before rolling to a heat pump install, look up the property details on Property.com to verify square footage, electrical panel capacity, and permit history. Showing up with a 15kW kit for a home that can only support 10kW creates problems before you even open the box.
Wrapping It Up
Heat strips remain the most common auxiliary heat solution for heat pump systems. Cold climate heat pumps reduce reliance on auxiliary heat, but don’t eliminate it entirely.
As heat pump adoption accelerates, contractors who build verification into standard procedures will avoid the callbacks that catch competitors off guard. Five minutes with a meter beats a truck roll every time.
Additional Sources
- National Comfort Institute. “Measure Heat Strip Performance.” NCI Technical Procedures. 2023.
- ASHRAE Handbook: HVAC Systems and Equipment. “Electric Heating.” American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2020.
- Air Conditioning Contractors of America (ACCA). “Quality Installation Standards and Callback Prevention.” 2024.
