Lennox HVAC FAQ: Heat Exchanger Warranty, Thermostat Not Working, and AC Compressor Testing

I've done emergency HVAC dispatch for the last 7 years—200+ after-hours calls. More times than I can count, a 'major failure' turned out to be something simple. These are the six Lennox questions I answer most often, with the honest, no-sugarcoat version.

Does the Lennox heat exchanger warranty cover labor?

Short answer: no. The Lennox limited warranty covers the heat exchanger part itself, but it does not cover labor. Labor coverage depends on the dealer and your exact warranty status.

In January 2024, I had a homeowner with a cracked heat exchanger in a 10-year-old Lennox furnace. The part was free under warranty. The labor, gaskets, and minor materials ran about $680. He was upset until I walked through the numbers—parts covered, labor not. That's how most manufacturer warranties work.

Before you pay for anything, check two things:

• Is the unit registered? Lennox's full warranty usually requires registration within 90 days of installation. Unregistered systems get a shorter coverage window.
• Is the quote actually a warranty claim? I've seen quotes with a 'heat exchanger' line item when the homeowner was under warranty and the part should have been $0.

Look up the model and serial number on Lennox's warranty lookup page, or ask a dealer to check it while you're on the phone. It takes two minutes, and it can save you thousands.

Why is my Lennox thermostat not working?

Before you call an emergency tech, run through this list. I've been at someone's door at 2 a.m. for dead batteries. I charged for it, and I still felt silly.

1. If the screen is blank, replace the batteries. Even a hardwired thermostat has backup batteries.
2. Check the furnace power switch. It looks like a regular switch on the side of the furnace. If it's off, the thermostat has no 24V power.
3. Push the furnace door in all the way. A safety switch kills power when the panel is open.
4. Change the air filter. A clogged filter can cause the system to lock out, which makes the thermostat seem dead.
5. Pull the thermostat off the wall, wait 30 seconds, and snap it back on. That resets the electronics.

If the screen is on but says 'heat on' and nothing is happening, that's a different problem—likely no 24V at the control board or a bad contactor. But if the thermostat is completely dead, start with batteries. In my experience, that fixes about half of no-display calls.

Can I replace baseboard heaters and a boiler with a Lennox heat pump?

This question comes up with older houses that have baseboard heat. The answer is usually yes, but it depends on what kind of baseboard you have.

If your baseboard heater is hydronic—hot water from a boiler—a Lennox ductless heat pump can replace it without ductwork. Each room gets a wall-mounted head, and you get cooling too. That removes the boiler maintenance and gives you a backup heat source in a way a single boiler can't.

If your baseboards are electric, the math is different. Electric baseboard heaters are cheap to install but expensive to run. A heat pump will usually be cheaper to operate, but the upfront cost is higher.

In my area, a three-zone ductless install runs somewhere between $8,000 and $18,000. That's a big number. But don't compare only upfront quotes. Compare 10 years of operating costs, especially if you're replacing electric baseboard. The higher-efficiency unit often wins on total cost, even when the sticker price makes you wince.

How to test an AC compressor safely

I'll give you the ohm test we use in the field, but first the safety part: shut off power at the disconnect and the breaker. The capacitor stores a lethal charge even with power off. Discharge it with a purpose-built bleed resistor or an insulated-handle screwdriver, then verify with a voltmeter.

Set your multimeter to ohms and find the three terminal labels on the compressor: Common, Run, Start.

• Common to Run: usually 1–5 ohms.
• Common to Start: usually 3–8 ohms.
• Start to Run: should equal the sum of the first two readings.

If you get OL (open) on any winding, the compressor is electrically bad. If you get zero, it's shorted. And if Start to Run doesn't match the sum, you likely have an internal problem.

Then check to ground. Put one probe on a compressor terminal and the other on a clean copper line or ground lug. It shouldn't show continuity. If it beeps, the winding is shorted to ground, and that usually means replacement.

This static test only checks electrical windings. If the compressor reads good but won't start, suspect a bad capacitor, stuck contactor, or overload. Don't condemn the compressor until you've checked those.

Should I buy the cheap aftermarket compressor or the Lennox factory part?

This is where people save $150 and lose $1,100. I've seen it happen multiple times.

In June 2024, a customer chose a non-OEM compressor to save about $200. It locked up 14 months later. The labor, refrigerant, and second compressor cost more than the factory unit would have in the first place. Predictable, but painful.

I'm not saying every aftermarket compressor is garbage. But a compressor is the most stressed part of the system. Tolerances matter, refrigerant charge matters, and warranty matters. A Lennox-branded replacement is designed around that system's operating range. A generic part might be fine—or it might cost you twice.

My rule of thumb: get the OEM price, get the aftermarket price, subtract the difference, then add one realistic emergency service call to the aftermarket number. If it fails, that's your risk. In most cases, the OEM becomes the cheaper choice over the life of the system.

What do I do if my Lennox heat exchanger is cracked?

Turn off the furnace and call a professional. A cracked heat exchanger can leak carbon monoxide. That's not a DIY project, and it's not something to leave for tomorrow.

If a tech says the heat exchanger is bad, ask for proof. A proper inspection includes removing the blower and doing a combustion analysis. Then check the warranty status. As I said, the part may be covered while the labor isn't.

If the heat exchanger is under warranty, a repair might cost $700 to $1,000 in labor. If it's not, a new furnace might be a better use of money than a $1,800 replacement part.

Don't let anyone rush you into a new furnace before the warranty claim is checked. Last winter, a customer got a 'we can replace it for $5,800' pitch. The heat exchanger was under warranty. We replaced it for $750 in labor, and that furnace is still running today. Verify the warranty first, then decide.