Installing a Lennox Side Discharge Heat Pump: The 7-Step Checklist I Wish I Had in 2019

If you're a contractor or commercial building owner about to put in a Lennox side discharge heat pump, this checklist is for you. I've been handling HVAC replacement and installation orders for about seven years now, and I've personally made—and documented—enough mistakes to fill a small binder. Roughly $4,000 in wasted budget, if I'm being honest. In 2019, I installed my first side discharge unit without checking half of the things on this list. The callback cost me two days and a chunk of my credibility with the client.

So here's the checklist I now run before every single Lennox side discharge job. It's based on my experience with maybe 40 residential and light commercial setups. If you're doing a massive custom commercial project, your specifics might differ, but the core steps hold up.

What This Checklist Covers

This is for Lennox side discharge heat pump installations—the SL25KPV or XP20 style units that sit closer to the building and pull air from the side. I've also included the stuff that trips up most first-timers: filter sizing, sensor issues, and the whole heat pump vs air conditioner conversation that comes up at the end of every quote.

Step 1: Verify the Side Discharge Clearance Before Anything Else

Side discharge units are great because they don't need as much clearance as traditional upflow condensers. But "less clearance" isn't "no clearance." I've seen installers tuck these things right against a wall and wonder why the high-pressure switch kept tripping.

Lennox typically calls for a minimum of 12 inches from the discharge side to any obstruction. Walk around the pad. Look at the soffit. Look at the neighbor's fence. Take a photo. Check it twice. On my first side discharge install, I left 10 inches because the client really wanted the unit tucked away. It ran—but it screamed. High head pressure, noisy operation, and a service call two weeks later. The 2 extra inches would've taken me five minutes.

• Minimum 12 inches on the discharge side (check the specific IOM for your model)
• Check overhead obstructions—some of these units need additional top clearance
• Leave enough room on the filter access side. More on that below.

Step 2: Order the Right 20x25x1 Air Filter—Don't Guess

The 20x25x1 air filter size is probably the most common filter size I order, especially when a Lennox furnace is paired with a heat pump. But here's the thing about nominal vs actual dimensions: a filter labeled 20x25x1 often actually measures about 19.5 x 24.5 x 0.75. That's normal. The problem is when you or the homeowner grab a "20x25" filter that's actually a 20x25 for a different brand's cabinet. The fit gets sloppy, air bypasses the filter, and the coil gets dirty. Then the heat pump performance drops, and the homeowner blames the equipment. It wasn't the equipment. It was the filter.

What I do now: I physically measure the filter slot on every job. I also check the MERV rating. For a heat pump system, I usually recommend MERV 8 for standard homes, and MERV 11 if the homeowner has allergies or you're installing an air cleaner cabinet. Just don't go MERV 13+ unless you've verified the blower can handle the static pressure. That's a whole other lesson.

I once ordered 20x25x1 filters for a Lennox system, and the client texted me a photo because the filter was too small. It turned out their previous tech had been installing the wrong size for years. We measured the actual slot, ordered the right size, and their system's static pressure dropped noticeably. The filter cost $12. The lesson cost me a bit of embarrassment.

Step 3: Understand the Heat Pump vs Air Conditioner Conversation

This is the part where clients ask, "Should we just go with a straight AC instead of a heat pump?" and you need to give them a real answer, not a textbook one.

The headline difference is straightforward: a heat pump is an air conditioner that can reverse the refrigerant flow and provide heating. But the decision gets more nuanced when you're pairing a side discharge unit with a furnace. You've got a few paths:

• Straight AC + gas furnace: Cheaper upfront. Good if gas is cheap in your area and summers are brutal.
• Heat pump + gas furnace (hybrid): The heat pump handles mild weather heating, the furnace kicks in for cold snaps and defrost. This is where I see the most satisfied customers.
• Heat pump with electric backup: Common in places with mild winters or where gas isn't available. Watch the operating cost if you're in a cold climate.

I tend to recommend the hybrid setup for most of my customers in the mid-Atlantic region. The side discharge units like the SL25KPV can work down to some pretty low outdoor temps, but the furnace backup means the homeowner never loses sleep over a cold snap. I had a customer in 2022 who cheaped out and skipped the heat pump, went straight AC. Then a January freeze hit their area and their electric backup strips couldn't keep up. They called me, and I had to explain that the heat pump wasn't an add-on I could just bolt on. That was a $4,000 conversation.

Step 4: Check the Oil Pressure Sensor—Before You Charge the System

Here's a weird one that cost me a full day in September 2023. The oil pressure sensor on some Lennox compressors is a known point of failure. I know, "oil pressure sensor" sounds like something out of an engine, not a heat pump. But these scroll compressors have oil pressure switches that can trip if the sensor is faulty or if there's a pressure differential problem.

What tripped me up was installing a brand new compressor, evacuating the line set, charging it to spec, and then watching it trip on oil pressure fault within 20 minutes. I checked the pressures. I checked the wiring. I finally called Lennox tech support, and the diagnosis was a faulty oil pressure sensor straight from the factory. I had to recover the charge, swap the sensor, evacuate again, and recharge. The part was maybe $60. The labor—that was the expensive part—was mine, and it was a tough conversation with the client.

Now, I test the oil pressure sensor before I do the full evacuation and charge. It's a 10-minute check: apply power to the compressor, watch that the oil pressure switch closes, and verify the sensor's reading on the service tool. If it looks off—replacing it before charging is way cheaper.

Here's something vendors won't tell you: sensor failures are common enough that Lennox tech support will walk you through it, but they're not going to proactively tell you to pre-test it. It's a "known issue" that's not officially a recall. If you're installing a remanufactured or new compressor, take the 10 minutes.

Step 5: Verify the Thermostat and Control Wiring

This is the step that guys skip because they've "done a hundred thermostats." But side discharge heat pumps with variable speed compressors are not your grandfather's thermostat. The wiring is different. You've got communicating systems here. If you're using a Lennox iComfort Wi-Fi thermostat, the wiring is proprietary. It's not a standard 24V thermostat, and trusting a basic thermostat to control a variable speed compressor is a recipe for the system running like a single-stage unit and the client complaining about uneven temperatures.

Take a photo of the existing wiring before you pull the old thermostat. Check for any low-voltage shorts before connecting the new thermostat. I burned out a transformer once because a wire touched the cabinet—$80 part, but the theory was embarrassing. That's a story for another time.

Key point: Not every Lennox side discharge heat pump requires a communicating thermostat. The XP20 for example, can work with a standard two-stage thermostat. But if you're installing an SL25KPV, you're leaving money on the table—and performance on the floor—if you don't use the communicating setup.

Step 6: Use the Right Lineset and Insulation

Almost every evaporator coil leak I've seen is because the installer didn't insulate the suction line properly or used oversized lineset when running to the condenser.

Look up the manufacturer's requirements for the specific model. Most Lennox air conditioners and heat pumps specify a particular lineset diameter. I had a job where a previous contractor used a 3/8" line where a 1/2" was needed on a heat pump—because the old AC used a 3/8" line. The result was a lower capacity and a compressor that struggled. The change cut the running time by a noticeable margin and probably extended the compressor's life.

Also, don't use the old lineset without flushing it if you're replacing a system that burned out a compressor. The acid in the old oil will kill the new compressor. That's a basic one, but I've seen it done. I once caught 47 potential errors using this checklist in the past 18 months—that compressor failure is one of them.

Step 7: Get the Warranty Registration Done Right

Lennox's compressor warranty is a selling point, but the registration is not optional. A lot of homeowners and even contractors assume that the warranty is automatic. The compressor warranty on the higher-end Lennox units is usually 10 years, and the parts warranty is typically 10 years or 5 years, depending on the model. But if it's not registered within 90 days (this is the standard limit, but verify the specific paperwork), you're likely looking at a reduced warranty period.

I had a client call me after their compressor failed, and they said, "Lennox said the unit wasn't registered. Can you do something?" I felt bad, because I was the one who installed it. The homeowner didn't know. I didn't follow up. The compressor was 8 years old and only had a 1-year labor warranty; it was a $1,800 repair. It turned out that the unit was still under the standard 5-year parts warranty, which was better than nothing, but not what they were told at the time of sale.

Now, I register the product on the spot, right after the install. I take a photo of the serial number, log it in my system, and email the homeowner a confirmation. This takes 15 minutes and saves a potential headache years later. "The manufacturer won't tell you this, but registering the unit is what turns a 'standard warranty' into a 'good warranty'."

The Mistake Most People Make

The biggest error I see is that contractors treat a side discharge heat pump install like a straight AC swap. Yes, the refrigerant is the same. Yes, a lot of the electrical work is similar. But there are these three quiet differences: the lineset size, the thermostat wiring, and the oil pressure sensor. If you miss any of those, you'll probably still get the system running, but you'll get a callback within a year. And a callback is the most expensive mistake you can make.

My Experience and Its Limits

My experience is based on about 40 Lennox side discharge installs, mostly residential and light commercial. If you're working with a massive rooftop commercial system, your experience might differ. I've only worked with US-spec units, so I can't speak to how these principles apply to international versions with different refrigerant requirements.

Bottom Line

Run the checklist in order. Measure the clearance. Measure the filter slot. Have the heat pump vs AC conversation with real numbers. Test the oil pressure sensor. Wire the thermostat correctly. Use the right lineset. Register the warranty. And if you're not sure about something, take the 10 minutes to call the manufacturer's tech line. It's way cheaper than a callback.

Personally, I think the Lennox side discharge heat pumps are some of the more reliable units on the market when installed correctly. I've had good luck with the quiet operation and the SEER ratings that actually show up in a homeowner's utility bill. But I've also seen what happens when a rushed installation turns a great machine into a service headache.

Take it from someone who learned this the hard way.