Can One Unit Heater Cover a Shop and Office? Zoning and Sizing for Mixed Spaces

You’ve got a big, open workshop — tools everywhere, maybe a lift or a few workbenches — and right next to it, a small office where you handle paperwork, invoices, and that never-ending pile of coffee cups.

Now you’re planning to install a heater. The big question:

Can one unit heater really handle both spaces?

It’s a fair question. I get calls about this all the time from shop owners, DIYers, and even homeowners who turned their garage into a combo workshop and office. They all want to save money and keep things simple — one unit, one thermostat, one install.

The truth? Sometimes one heater can do it. But sometimes you end up with one toasty shop and an office that still feels like an icebox.

👉 Reznor UDX 60,000 BTU Propane Unit Heater — the workhorse I recommend most often. Sealed combustion, compact footprint, and solid efficiency for mixed-space garages.

Let’s break down why it happens, how to size correctly, and what zoning tricks can actually make one heater cover two spaces efficiently.

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🧱 1. The Problem with “One Heater for All” Thinking

I’ve walked into dozens of garages and shops where people tried to heat everything with a single unit. On paper, it makes sense — if your total square footage matches the heater’s BTU output, it should all balance out, right?

Not quite.

Your shop and office are two totally different environments. They might share a wall, but they don’t share airflow, insulation levels, or heat retention.

The result:

• The shop stays comfortable (because the heater’s blowing right into it).

• The office stays cold (because heat doesn’t travel through walls or around corners).

Tony’s take:

“Heat doesn’t bend — it goes where you point it. If your office isn’t in the heater’s line of sight, it’s probably out of luck.”

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⚙️ 2. Why Shop and Office Spaces Heat Differently

Let’s talk about the physics of it.

🧰 Workshops are “heat hungry”

Workshops are usually:

• Larger in volume

• Poorly insulated

• Filled with metal tools and cold surfaces

• Exposed to open doors or garage bays

When you open a garage door in January, you lose thousands of BTUs in seconds. The heater has to recover quickly to maintain a comfortable baseline temperature — usually around 55–60°F for working conditions.

🪑 Offices, on the other hand…

Your office likely has:

• Lower ceilings

• Carpet or flooring that retains heat

• Insulated walls

• Less air leakage

And you probably want it to be 10–15°F warmer than the shop — around 68–72°F.

Now imagine trying to maintain that 12°F difference using one thermostat mounted in the middle of the shop. By the time your office hits 70°F, your shop could feel like a sauna.

Here’s the takeaway:
Even though the spaces are side-by-side, their thermal loads are completely different.

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🧮 3. How to Size One Unit for Two Spaces

If you still want to try a single unit, it all starts with correct sizing.

The Base Formula

BTUs = (Square Footage × 50) × Climate Factor

Then, adjust for insulation and air leakage.

(From the U.S. Department of Energy Climate Zone Map)

Example:

Shop: 800 sq. ft.
Office: 200 sq. ft.
Total: 1,000 sq. ft. in Michigan (cold zone ×1.25)

→ 1,000 × 50 × 1.25 = 62,500 BTUs needed.

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🧰 4. Airflow — The Real Secret to Heating Two Spaces

A heater doesn’t just need to make heat; it needs to move it.

Without airflow, your office will sit there like an island — isolated from the warm air in your shop.

🔹 Tip 1: Placement Matters

Mount the unit near the shared wall and aim it diagonally across the shop, not directly at the door.
If the office is off to the side, angle the louvers slightly toward the doorway.

If you have a dividing wall, cut a small transfer vent (6x12” or larger) near the top and another near the bottom to allow warm air to circulate and cool air to return.

Tony’s line:

“If your heater can’t ‘see’ the office, make sure the air can.”

🔹 Tip 2: Use Fans to Balance Temperatures

A low-speed ceiling fan or small wall fan works wonders for pushing warm air toward cooler zones.

• Mount the fan high on the shared wall.

• Run it on a low, continuous setting.

• Reverse it if necessary to pull air instead of push.

That single fan can cut temperature differences between rooms from 15°F down to 3–5°F.

🔹 Tip 3: Leave Doors Open (or Add Louvers)

If your office door stays closed most of the day, you’re creating a thermal barrier. Warm air can’t enter, and cold air can’t escape.

A simple fix:
Install a door louver vent or leave the door cracked slightly.

If you must close the door for noise or privacy, add a small return vent near the floor connected to the shop air.

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🧱 5. When One Heater Just Won’t Cut It

Sometimes, no amount of airflow will balance your temperatures. You’re dealing with separate air zones — each with its own load, insulation, and usage pattern.

Here’s how to know when one heater isn’t enough:

Tony’s insight:

“If your shop and office differ by more than 10°F for an hour straight, it’s time to zone. Otherwise, you’re just feeding the meter.”

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🧮 6. Understanding Zoning — One System, Two Comfort Levels

Zoning doesn’t necessarily mean buying two full systems. It just means controlling where the heat goes.

Here are your main options:

🔹 Option 1: Airflow Zoning (Cheapest)

Use your existing heater and redistribute air.

• Add vents or grilles through the wall.

• Use a quiet inline duct fan or booster fan.

• Balance with a return vent low on the opposite wall.

This works best when the office is small (less than 25% of total area).

🔹 Option 2: Secondary Heat Source

Keep your main shop heater for the large space, and add a small electric heater or panel in the office.

• Use independent thermostats.

• Costs less than oversizing the main heater.

🔹 Option 3: Ducted Unit Heater

Some gas or propane units (like certain Reznor models) can duct short runs to adjacent rooms.

• Great for shared-wall offices.

• More complex install, but very efficient.

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🧱 7. Climate and Insulation Play Huge Roles

The colder your climate, the harder it becomes to heat both spaces evenly from one source.

Also, check insulation levels:

• Walls: R-13 minimum

• Ceilings: R-19 or better

• Garage doors: R-8 insulated panels

A small investment in insulation can reduce your BTU requirement by 15–25%.
See the EPA’s Weatherization and Insulation Guide for detailed R-values.

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⚙️ 8. The Oversizing Trap

I see this mistake constantly — someone realizes their office is cold and thinks, “I’ll just buy a bigger heater.”

Bad idea.

Oversizing a unit leads to:

• Short cycling (unit turns on and off too fast)

• Higher propane or gas use

• Uneven heat distribution

• Shorter equipment life

If you need to heat two different environments, increasing BTUs won’t fix poor airflow or bad zoning.

Tony’s warning:

“Oversizing is like fixing a leaky roof by buying a bigger bucket — it doesn’t stop the problem, it just fills faster.”

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💡 9. The Right Way to Balance Two Spaces

Here’s my go-to checklist whenever a customer wants one heater for a shop and office combo:

✅ Step 1: Find Total BTU Load

Add the square footage of both areas and multiply by 50 × your climate factor.

✅ Step 2: Place the Heater Strategically

Mount it near the shared wall and point airflow toward both zones if possible.

✅ Step 3: Add Air Circulation

Use ceiling fans, wall fans, or passive vents to equalize temperatures.

✅ Step 4: Separate Thermostats (Optional)

Even with one heater, you can use a dual-sensor thermostat that averages readings from both rooms.

✅ Step 5: Seal and Insulate

If the office isn’t sealed properly, all the heat you push there will just leak out.

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🧰 10. Real-World Example — Michigan Auto Shop

Let me give you a case from last winter.

An 800 sq. ft. auto shop with a 200 sq. ft. attached office. They installed a single Reznor UDX 60k propane heater in the shop. The office was freezing.

We checked everything:

• The shared wall was insulated.

• The door stayed closed 90% of the time.

• The ceiling in the shop was 12 feet high — hot air was trapped.

Fix:

1. Installed a ceiling fan to push warm air down.

2. Cut two 8x12 vents through the wall (one high, one low).

3. Added a small desk heater in the office for backup.

Result:

• Temperature difference dropped from 12°F to 3°F.

• Propane use went down 10%.

• Employee satisfaction: “My coffee doesn’t freeze anymore.”

Sometimes, small airflow adjustments make all the difference.

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🔋 11. The Cost of Doing It Right

Adding vents or fans costs next to nothing compared to another heater.

So for a few hundred bucks, you can often make a single heater feel like a full dual-zone setup.

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🧱 12. Product Recommendations

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🧯 13. Safety First — Venting and Air Quality

If you’re heating two rooms from one source, pay extra attention to ventilation.

• Use sealed-combustion units (like Reznor UDX or Hot Dawg HD) to prevent exhaust mixing.

• Do not use unvented heaters in any office or shared airspace.

• Always install carbon monoxide detectors in both rooms.

From the U.S. Consumer Product Safety Commission:

“Even low levels of carbon monoxide can cause symptoms in as little as two hours.”

Tony’s advice:

“If your heater burns fuel, treat fresh air like another piece of equipment — you can’t run safely without it.”

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🧠 14. Tony’s Rule of Thumb

Here’s my simple formula from the field:

If your office is less than 25% of total square footage, one heater can handle both.

If it’s more than 25%, or you close the door all day, treat it as a separate zone.

It’s that straightforward.

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✅ 15. Tony’s Final Advice — Control the Heat, Don’t Fight It

Heating a shop and office with one unit isn’t impossible — it just requires planning, placement, and airflow.

If you:

• Mount your heater strategically,

• Use fans or vents to balance air, and

• Keep both rooms insulated…

You can absolutely get consistent comfort from a single unit.

But if your spaces are sealed off, unevenly insulated, or used differently, don’t waste money trying to overpower physics — create two zones instead.

“One heater can heat two rooms — if the air can move between them. If it can’t, you’re just burning fuel to warm a wall.”

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In the next topic we will know more about: How to Use a BTU Calculator (Without Getting Misled by Generic Numbers)