By Tony — the guy who’s fixed more “mysterious short-cycling” heaters than most people have owned heaters
🛠️ Introduction: When Your Heater Turns On and Off Like It’s Confused
Short-cycling is one of the most common issues in garages, workshops, barns, and outbuildings — and almost always one of the most misunderstood.
Customers will tell me:
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“The heater keeps turning on and off every few minutes.”
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“The room never gets warm.”
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“It shuts off before the garage heats up.”
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“It runs fine sometimes, then acts stupid.”
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“Must be a bad thermostat!”
Wrong.
**Short-cycling is almost never caused by the heater.
It’s caused by WHERE YOU PUT THE THERMOSTAT.**
And nobody talks about it.
This article breaks down exactly how thermostat placement destroys heater performance, causes rapid cycling, spikes propane usage, and makes people think their heater is undersized — when it’s really just suffering from a stupid thermostat location.
Reznor UDX 60,000 BTU Propane Unit Heater
📍 1. Thermostats Don’t Measure “Room Temperature.” They Measure the Air Around Them.
This is the first thing homeowners get wrong.
A thermostat isn’t magical.
It can’t sense the whole shop.
It isn’t averaging temperatures.
It isn’t “smart” just because it’s digital.
It measures one thing: the temperature of the air touching its sensor.
So if the air around the thermostat is:
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warmer than the room → it shuts off early
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colder than the room → it runs endlessly
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moving fast → temperature reading becomes unstable
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stagnant → temperature reading becomes slow and inaccurate
This is why thermostat placement controls everything your heater does.
🎯 2. Tony’s Top 8 Thermostat Location Mistakes (Every Installer Should Know)
Here are the placements I see that instantly cause short-cycling:
❌ 1. Too Close to the Heater Outlet
Warm air slams into the thermostat → it thinks the room is hot → shuts off heater early → short-cycle.
❌ 2. Too High on the Wall
Heat rises.
Even in a well-mixed shop, the upper air is warmer.
Thermostat reads “warm,” shuts off, but your floor is freezing.
❌ 3. In the Heater’s Line of Fire
If the thermostat can “see” the throw distance of the heater, it will ALWAYS short-cycle.
The warm jet of air tricks the thermostat every time.
❌ 4. Near Garage Doors
Cold air leaks → thermostat thinks room is freezing → heater runs nonstop.
Then door closes → thermostat overheats → short-cycle.
All in the same hour.
❌ 5. Near Exterior Walls or Windows
Cold spots confuse the sensor.
It cycles based on wall temperature, not room temperature.
❌ 6. On Metal Walls or Near Conduit
Metal radiates cold.
This tricks thermostats into thinking the room is cold.
❌ 7. Behind Shelving, Equipment, or Tools
Dead air pocket → thermostat senses “warm” earlier → shuts off heater even though room isn’t warm yet.
❌ 8. Next to Exhaust Fans, Dust Collectors, or Air Compressors
Airflow distorts temperature readings dramatically.
📏 3. Tony’s Ideal Thermostat Placement Rules (Never Fail)
Here’s how I place thermostats on EVERY professional install.
✔️ 1. 5 Feet Above the Floor (Comfort Height)
This height reads the average “human zone,” not the ceiling heat.
✔️ 2. Minimum 8–10 Feet Away From the Heater
Far enough to avoid jet-stream heat.
Close enough to reflect real working temperature.
✔️ 3. On an Interior Wall
Interior walls are stable and provide the best readings.
✔️ 4. Not Near Doors, Windows, or Drafts
Air leaks create temperature chaos.
✔️ 5. Not in Dead-Air Corners
Corners trap heat pockets.
Corners collect stagnation.
Corners lie.
✔️ 6. In the Middle Third of the Space
Not too close to heater, not too close to exterior wall.
✔️ 7. Away From Shelving, Lifts, Vehicles, or Tall Tools
You want open airflow.
Not restricted, not blocked.
✔️ 8. Avoid Electric Interference
Don’t mount near:
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compressors
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welders
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variable-speed drives
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big induction motors
These can cause EM interference in some digital thermostats.
🧊 4. Why Tall Ceilings Make Thermostat Placement Even More Critical
In tall garages and shops (12–20 ft):
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Heat piles up near the rafters
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Temperature at floor level can be 20–30°F colder
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Thermostat height becomes the entire game
If the thermostat is too high?
It shuts off early because it thinks the room is “warm enough.”
Meanwhile your floor is freezing.
If too low?
It runs forever because cold floor air never reaches set point.
For tall rooms, I always:
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use a 5-ft mounting height
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add a low-speed destratification fan
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angle heater downward 15–20°
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keep thermostat away from airflow paths
This combination stops 90% of cycling complaints.
🌬️ 5. Why Airflow Pattern Affects Thermostat Behavior More Than Temperature
Most people think thermostats measure heat.
They don’t.
They measure air temperature at a single spot, and air moves differently depending on:
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heater throw distance
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fan speed
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ceiling height
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circulation fans
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open doors
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placement of machinery
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insulation differences
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pressure zones
Any of those can make the thermostat “feel” a temperature that’s nothing like the real average.
Common airflow failures that lead to short-cycling:
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heater blowing directly at thermostat
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stagnant corner air warming above room temp
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cold “draft river” running along floor
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vertical temperature layering
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fans pushing warm air sideways into thermostat
Short-cycling isn’t random.
It’s airflow physics.
🔥 6. How Short-Cycling Damages Your Heater (and Wallet)
Short-cycling isn’t “just annoying.”
It kills heaters.
Here’s what it causes:
❌ Premature ignitor failure
More starts = shorter ignitor life.
❌ Heat exchanger stress
Metal expands/collapses repeatedly.
This causes cracks.
❌ High-limit trips
The furnace overheats because it short cycles before air mixes.
❌ Shortened motor life
Start-stop cycles kill blower motors.
❌ Increased fuel usage
Ignition is the least efficient part of burner operation.
❌ Cold floors and uneven heat
The heater never runs long enough to warm the space properly.
Fix the thermostat and all of these disappear.
🧪 7. Real-World Short-Cycle Diagnoses From Tony’s Logbook
Case 1 — Thermostat 3 ft From Heater Outlet
Heater: 60k BTU Reznor
Issue: Short-cycling every 3 minutes
Fix: Move thermostat to opposite interior wall
Result: Perfect 10–12 minute cycles
Case 2 — Thermostat Mounted 9 ft High
Room: 14-ft workshop
Issue: Room cold; thermostat happy
Fix: Lowered to 5 ft height
Result: Temp rise 16°F in first hour
Case 3 — Thermostat Above Tool Cabinet
Dead-air pocket trapped heat behind cabinet
Fix: Move thermostat 3 ft left
Result: Cycling normalized instantly
Case 4 — Thermostat Near Garage Door
Cold drafts kept thermostat reading low
Fix: Relocated mid-room
Result: 30% propane savings
Case 5 — Thermostat Facing Heater
Warm jet fooled thermostat
Fix: Rotate heater + move thermostat
Result: Eliminated false shutoffs
📘 8. Verified External Sources
These reputable sources confirm thermostat placement rules and short-cycling mechanics:
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Modine Hot Dawg Installation Manual (Mounting & Throw Charts)
https://modinehvac.com/ -
Reznor UDX Engineering Specifications
https://www.reznorhvac.com -
Building Science Corporation – Air Mixing & Comfort Control
https://buildingscience.com -
HVAC Ventilation & Throw Distance Principles (Titus HVAC)
https://www.titus-hvac.com/
These line up perfectly with Tony’s field rules.
📍 9. Tony’s Foolproof Thermostat Placement Checklist
Post this on your wall. Seriously.
✔️ 5 ft from floor
✔️ On an interior wall
✔️ Away from heater throw
✔️ 8–10 ft minimum from heater
✔️ Not near doors or windows
✔️ Not behind anything
✔️ Not in stagnant corners
✔️ Not near motors or fans
✔️ Not near ceiling
✔️ Away from heat sources (computers, welders, compressors)
Get these right and your heater will:
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stop short-cycling
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warm your room evenly
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run more efficiently
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last years longer
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consume less fuel
🚀 Conclusion: The Heater Isn’t Broken — The Thermostat Is in the Wrong Place
Short-cycling feels like a gas valve issue.
Or a pressure switch issue.
Or a bad thermostat.
Or a weak heater.
But in almost every case?
**Your thermostat is lying to your heater
because YOU put it in the wrong place.**
Once you move it:
✔️ heat stabilizes
✔️ cycles normalize
✔️ propane bills drop
✔️ comfort skyrockets
✔️ equipment lasts longer
It’s the cheapest fix in HVAC — and the most ignored.
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In the next topic we will know more about: The Garage Slab Heat Sink — Why Concrete Eats BTUs and How Tony Adjusts for It
