Why Your RTU Is Blowing Cold Air in January (And What That Clicking Sound Is Telling You)

By a Senior Commercial Service Technician | Climate Alignment HVAC/R

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It’s January in Denver. The thermostat is calling for heat. Your Trane Precedent or Carrier Weathermaster is running — you can hear it from the mechanical room — but the air coming out of the diffusers is cold. And now there’s a clicking sound from the rooftop that wasn’t there last week.

Tenants are calling. The building needs to stay habitable. And you’re about to climb onto a frozen roof at 5,280 feet to figure out what’s wrong.

Let’s get you the right answer before you go up there.

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What the Reversing Valve Actually Does

A heat pump RTU has one mechanical trick that separates it from a straight cooling unit: the reversing valve. It redirects refrigerant flow based on the season. In summer, it sends hot gas to the roof to reject heat outside. In winter, it flips and routes that hot gas indoors to heat the building.

When it gets stuck in the cooling position, your unit keeps running like it’s July — pulling heat out of the building and dumping it on the roof. In January. That’s not an inconvenience. In Colorado, occupied spaces have code-minimum temperature requirements, and a failed heating system creates real legal and operational exposure for facility managers.

The valve has two distinct failure points. Knowing which one you’re dealing with determines your entire response.

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Step One: Characterize the Click

Before you touch anything, stand near the unit and listen. The nature of that clicking sound is your first diagnostic data point.

• Slow, rhythmic click every 30–90 seconds — A pressure switch is tripping and resetting. The system is protecting itself from abnormal refrigerant pressures. The compressor may still be intact.
• Rapid chattering at the electrical panel — Likely contactor chatter caused by voltage drop. Check the supply voltage at the unit disconnect first. Low voltage on a 480V commercial unit sounds alarming, but it may be an upstream electrical issue, not a mechanical one.
• Single heavy clunk, then silence while the fan keeps running — A thermal overload trip inside the compressor. The motor couldn’t start under load, overheated, and cut itself off. Let it reset for 20–30 minutes minimum — but don’t restart until you’ve identified why it tripped.
• Grinding, rattling, or mechanical knock from inside the compressor housing — Stop. Do not restart the unit. That’s the sound of internal mechanical damage: broken reed valves, scroll separation, or a failed connecting rod. You’re past diagnostics and into compressor replacement territory.

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Step Two: Electrical Diagnosis — Is the Solenoid Coil Dead?

The reversing valve is controlled by a 24V solenoid coil mounted on top of the valve body. When the thermostat calls for heat, it sends 24V through the O or B terminal (Trane typically uses O; getting this backwards means the valve is energized in cooling and de-energized in heat — it’s own diagnostic trap worth checking first).

Run through this sequence before you condemn the valve:

1. Verify 24V is arriving at the solenoid terminals during an active heat call. No voltage means you have a controls or wiring issue upstream — not a valve problem.
2. Measure solenoid coil resistance. Pull the coil off the valve stem (it slides off without any brazing) and check resistance across the terminals. A healthy coil reads 8–20 ohms. An open circuit (OL) or dead short (0 ohms) means the coil has failed.
3. Replace the coil if it’s open. Solenoid coils are a stock part. Confirmed 24V signal plus an open coil is a 15-minute repair.

If the coil checks out — correct resistance, confirmed voltage — you’re not dealing with an electrical failure. You’re dealing with a mechanical one. And in Denver’s January, this is where it gets interesting.

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The Delta P Winter Paradox: Denver’s Hidden RTU Killer

This is the insight most troubleshooting guides skip — and it’s why experienced commercial techs get called back to jobs that were supposedly “already diagnosed.”

The reversing valve slide doesn’t move on its own. It’s shifted by refrigerant pressure differential — the difference between high-side and low-side pressure physically pushes a pilot valve that moves the slide assembly. Most valves need a minimum of 75–100 PSI pressure differential to shift reliably.

In July, that’s easy. Your high side runs 250–350 PSI, your low side runs 120–150 PSI. Plenty of delta P.

In January, at 5,280 feet, on a 10°F day with 25 mph wind across the condenser coils, those numbers collapse. Denver’s high altitude means air is already about 18% less dense than at sea level, which reduces heat transfer efficiency at the condenser even before wind chill enters the picture. On a hard, low-ambient day, you might see a high side of 150 PSI and a low side of 60 PSI. That’s 90 PSI of differential — right at the edge of what the valve needs. Any added resistance (debris in the bleed ports, a sticky slide from seasonal disuse, or minor sludge buildup) and the valve simply cannot shift.

The solenoid is working. The valve physically cannot move.

This is the Delta P Winter Paradox: the coldest day of the year is also the day your system is least capable of generating the pressure needed to switch into heat mode. It’s a physics problem, not a parts problem — and misdiagnosing it means replacing a valve that wasn’t broken.

The Cardboard Blocker Technique

This is a legitimate field diagnostic used by experienced commercial techs. Block a portion of the condenser coil face with cardboard or a low-ambient cover to restrict airflow across the coil. This artificially raises condensing pressure, increases your high-side pressure, and gives the valve the delta P it needs to shift.

Run the unit, watch your high-side gauge climb, and attempt the mode switch at the thermostat. If the valve shifts and the unit locks into heat mode, you’ve confirmed the diagnosis: low-ambient pressure differential failure, not a dead valve.

The permanent fix is a low-ambient kit — head pressure controls and cycling fan controls that maintain adequate condensing pressure regardless of outdoor temperature. This is exactly the kind of seasonal vulnerability that a commercial RTU maintenance plan catches before January arrives.

Denver Altitude Note: At 5,280 feet, refrigerant pressures, superheat targets, and subcooling specs all read differently than sea-level charts. Standard PT charts will give you inaccurate readings. Always use altitude-corrected pressure-temperature charts for the specific refrigerant in the system — R-410A and R-454B both behave differently at elevation.

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Can a Stuck Valve Damage the Compressor?

Yes — and this is why acting quickly matters.

When the valve is stuck in cooling mode, and the unit keeps running, the compressor is operating outside its intended winter parameters. Low suction pressures from cold ambient conditions push the compressor toward the edge of its operating envelope. More critically, if the unit gets cycled on and off repeatedly — a facility manager trying to force the valve to shift — each failed start attempt risks liquid refrigerant slugging back into the compressor.

Liquid doesn’t compress. Scroll compressors and reciprocating compressors both fail catastrophically when liquid refrigerant enters the compression chamber. That’s how a stuck reversing valve becomes a destroyed compressor.

If you’ve cycled the unit multiple times and the clicking has changed character — heavier, more mechanical, or now accompanied by a hard hum before the click — stop cycling it immediately and contact Climate Alignment for urgent heating restoration. Running the compressor again before a tech assesses it is a high-risk decision.

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Is It Possible to Free a Stuck Valve Without Replacing It?

Sometimes. The tap method — striking the valve body firmly with a rubber mallet while the unit is running and attempting to shift — can dislodge a slide stuck with debris or minor sludge. It’s not a permanent fix, but it works often enough to be a legitimate first attempt before condemning the valve.

If the valve has been stuck for an extended period, or if there’s visible damage to the valve body (dents, corrosion, or evidence of a previous brazing repair), it needs to come out. Reversing valve replacement is a nitrogen-purged brazing job: full refrigerant recovery, the old valve sweated out, the new one sweated in under nitrogen purge to prevent internal oxidation, followed by deep evacuation and recharge. On a 10-ton commercial RTU, factor in refrigerant recovery and recharge costs — R-410A and transitional A2L refrigerants like R-454B are priced differently, and with the EPA’s AIM Act phase-down ongoing, refrigerant availability and cost are variables worth discussing with your service provider.

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Your Diagnostic Checklist: What to Do Right Now

You’re on the roof, it’s freezing, and you need a sequence. Here it is — in order.

1. Listen first. Characterize the click before you open any panels. Grinding or mechanical knocking means you stop right there.
2. Check the supply voltage at the unit disconnect. Rule out an upstream electrical issue before diagnosing the unit itself.
3. Verify 24V at the solenoid terminals during an active heat call. No signal means a controls issue, not a valve issue.
4. Pull the solenoid coil and measure resistance. 8–20 ohms is healthy. OL or 0 ohms means replace the coil.
5. Check your system pressures. If delta P is below 75–100 PSI, you’re likely dealing with a low-ambient pressure issue, not a failed valve.
6. Try the cardboard blocker technique to artificially raise high-side pressure and attempt a mode shift.
7. Try the tap method if pressures are adequate, but the valve still won’t shift.
8. If none of the above works — or if compressor damage is suspected — stop and call a tech. You’ve done the right triage. Now let a professional take it from here.

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Next Steps: Turning This Diagnosis Into a Long-Term Fix

Getting the unit back into heat mode today is the immediate win. But if this failure happened once, the conditions that caused it haven’t changed — and without addressing the root cause, you’ll be back on this roof next January with the same problem.

Here’s what a complete resolution looks like:

If the failure was electrical (dead solenoid coil):
Coil replacement is a fast repair, but ask your tech to check the full 24V control circuit — thermostat wiring, O/B terminal configuration, and wire harness integrity at the Molex connectors. Coils don’t typically burn out in isolation; a voltage spike or intermittent short often precedes the failure.

If the failure was mechanical (Delta P / low ambient):
The unit needs a low-ambient kit installed before next winter. Head pressure controls and cycling condenser fan controls maintain adequate high-side pressure during cold-weather operation. This is a planned upgrade, not an emergency repair — which means it belongs in your preventive maintenance schedule before the next heating season starts.

If the compressor took damage:
A tech will need to assess amp draw against rated LRA, check for liquid slugging evidence (oil contamination, frost patterns on the suction line), and evaluate whether the compressor can be run safely. If scroll separation or internal valve damage occurred, compressor replacement is the path forward. Catching it now — before a full mechanical seizure — saves you the cost of secondary damage to the refrigerant circuit.

For multi-unit facilities:
If you’re managing a building with multiple RTUs of the same vintage, treat this failure as a fleet-wide signal. Units that share the same age, refrigerant type, and operating history tend to fail in clusters. A commercial maintenance agreement that includes low-ambient pressure checks and solenoid coil inspections every fall is the most cost-effective way to prevent the next emergency call.

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Conclusion

A reversing valve stuck in cooling mode in January isn’t a mystery — it’s a physics problem with a clear diagnostic path. Characterize the click, verify your 24V solenoid signal and coil resistance, then check your system pressures before you condemn the valve. In Denver’s cold ambient conditions at 5,280 feet, the Delta P paradox is the single most commonly missed diagnosis on commercial heat pump RTUs.

What you don’t want to do is keep cycling the unit and hope it resets. Every failed compressor start under these conditions is a gamble on an expensive component — and on a building full of people who need heat.

Need help with a stuck reversing valve or RTU heating failure? Contact Climate Alignment for 24/7 service.

Climate Alignment provides emergency commercial HVAC/R service across Metro Denver and surrounding areas — 24 hours a day, every day. We work on all major commercial RTU brands, including Trane Precedent, Carrier Weathermaster, and Lennox Landmark series. We show up prepared, diagnose fast, and give you a straight answer on what it takes to restore heat — no upselling, no guessing.

Call now or request a quote — we’re available 24/7 for commercial emergency response.

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