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🌎 Climate Zone Math — Why a 27k in Florida Isn’t the Same as in Colorado

🧭 Introduction: Same System, Different Planet

When I first started recommending the MRCOOL 27,000 BTU 2-Zone DIY system (9k + 18k) to friends across the country, one question kept coming up:

“If it works for you in humid Florida, shouldn’t it be perfect for my cabin in Colorado?”

Short answer: nope.

Even though both homes might be the same square footage, the climate zone, altitude, and envelope conditions completely change what that 27k BTU system can actually deliver.

In Florida, your main enemy is humidity — your system must run longer to pull moisture out of sticky air.
In Colorado, it’s thin, dry air and elevation — your compressor and coils behave differently because air density drops roughly 3 % per 1,000 feet of altitude.

Today, I’ll break down the “climate math” I use when sizing or evaluating a system — and how to tweak your calculations so your 27k doesn’t become a 22k at 6,000 feet.

📊 Step 1: Climate Zones 101 — ASHRAE and DOE Speak Different Languages

If you’ve ever seen a map labelled “Climate Zones 1–8,” you’ve seen the DOE/ASHRAE climate classification used in the U.S. for building design and HVAC sizing.

Zone	Example States	Heating/Cooling Load Behaviour
1–2	Florida, Texas	High cooling, high humidity
3–4	Tennessee, Kansas	Balanced heating/cooling
5–6	Colorado, Illinois	Dominant heating load
7–8	Northern Plains, Alaska	Heavy heating, short cooling season

Each zone defines design temperatures — the average extreme highs/lows engineers use to calculate load.

Florida (Zone 2A): ~93°F outdoor design temp, ~78°F wet-bulb.
Colorado Front Range (Zone 5B): ~91°F dry-bulb, ~64°F wet-bulb — same temp, lower humidity.

Source: ASHRAE Climate Data Centre and Energy.gov Climate Zones Guide.

That’s the heart of the difference — same temperature, wildly different moisture content.

💧 Step 2: Humidity Load — Why Florida Feels Harder

Let’s talk latent vs. sensible heat.

Sensible load: the dry heat you feel and your thermostat measures (°F).
Latent load: the moisture your system has to remove from the air.

Florida’s “design dew point” can hit 75°F, while Colorado rarely breaks 55°F.
At 75°F dew point, each cubic foot of air contains nearly 2× the water vapour of dry western air.

Florida Example

If your 27k system in Florida has a SEER2 of 22.5 and a sensible heat ratio (SHR) of 0.72, that means:

72 % of its capacity goes to lowering the temperature
28 % goes to removing humidity

That 28 % latent load can easily consume 7,000–8,000 BTU/h of your total capacity — leaving less “dry cooling” for the thermostat.

In Colorado, that same system might run at an SHR closer to 0.90 (less latent demand), giving nearly 24k BTU/h of sensible cooling.

👉 Translation: a 27k in Florida “feels” like a 22k; a 27k in Colorado “feels” like a 25k.

🏔️ Step 3: Altitude & Air Density — The Invisible Derate

At sea level, air density is roughly 0.075 lb/ft³.
At 5,000 feet (Denver), it’s 0.063 lb/ft³ — about 16 % lower.

That matters because your system’s condenser and evaporator rely on the mass flow of air to exchange heat.
Less dense air = less mass per cubic foot = lower heat transfer.

Rough Rule

For every 1,000 ft of elevation, derate capacity by ~3 % and fan performance by ~1 %.

So for a 27,000 BTU system at 6,000 ft:
[
27,000 × (1 - 0.03×6) = 27,000 × 0.82 = 22,140\ \text{BTU/h effective}
]

Suddenly, your “27k” behaves like a 2-ton.

If you live above 5,000 ft, choose either:

A larger system (33–36k), or
Ensure ductless heads are properly zoned and don’t exceed the room load.

Reference: ASHRAE Handbook — Fundamentals, Ch. 14 (Altitude Corrections).

☀️ Step 4: Solar Gain — The Forgotten Factor

Colorado’s air may be dry, but the solar radiation is intense.
At 5,000–7,000 ft elevation, you receive roughly 10–12 % more UV and infrared energy.
That extra heat hits roofs and south-facing glass even in 75°F air.

Example:

A 200 ft² window in Florida (Zone 2A) might contribute 1,200 BTU/h solar gain midafternoon.
In Colorado’s thinner atmosphere, that same glass can reach 1,350–1,400 BTU/h — offsetting some of the cooling advantage from low humidity.

Good window films or Low-E glass drastically help here.
See: Efficient Windows Guide.

🧱 Step 5: Insulation and Envelope — Florida Keeps Air Out, Colorado Keeps Heat In

In Florida’s code (IECC Climate Zone 2A), wall R-value requirements are lower because the delta-T (temperature difference) between inside and outside isn’t huge, but vapour barriers are critical.
In Colorado (Zone 5B), insulation and air-sealing matter more than vapour retarders.

Component	Florida Code (2A)	Colorado Code (5B)
Ceiling	R-38	R-49
Wall	R-13	R-20 + R-5 continuous
Window SHGC	≤ 0.40	≤ 0.32
Duct Leakage	≤ 8 cfm/100 ft²	≤ 6 cfm/100 ft²

The tighter your envelope, the smaller your actual load.
That’s why my Florida 700-sq-ft living zone needed the full 18k head, while my buddy in Colorado used a 12k for a similar-sized great room.

📐 Step 6: The Math Mike Uses for Load Adjustment

I don’t do a full Manual J every time — but I use a simplified formula to check whether the manufacturer’s BTU chart fits my climate.

Base equation:
[
BTU = \text{Area} × \text{Load Factor} × \text{Climate Modifier} × \text{Altitude Factor}
]

Variable	Typical Values
Load Factor	25–35 BTU/ft²
Climate Modifier	1.15 (humid) / 0.95 (dry)
Altitude Factor	1 – 0.03 × (elevation / 1000)

Example A – Florida (Zone 2A)
Area = 700 ft², Load Factor = 35 BTU/ft², Climate = 1.15, Alt = 0
→ 700 × 35 × 1.15 = 28,175 BTU → 27 k perfect

Example B – Colorado (6,000 ft, Zone 5B)
700 × 30 × 0.95 × 0.82 = 16 ,353 BTU → 18 k more than enough

That’s why identical houses need different setups.

Reference: ACCA Manual J Overview.

🌡️ Step 7: Temperature Swing and Comfort Range

Dry-air climates allow slightly higher thermostat setpoints without feeling hot.
At 45 % RH, 78 °F feels about the same as 74 °F at 60 % RH (per ASHRAE 55 comfort chart).

So a Coloradan running 78 °F indoors can cut cooling load ~10 %, while a Floridian must maintain 74 °F to avoid clammy discomfort.

Reference: ASHRAE 55 – Thermal Environmental Conditions for Human Occupancy.

🔋 Step 8: Efficiency Ratings — SEER2 Lies by Omission

Every SEER2 number is derived from standardised conditions (95 °F outdoor, 80 °F dry-bulb, 67 °F wet-bulb).
But that’s a snapshot.

Florida’s true seasonal average ≈ is 88 °F with 70 °F WB → higher latent work.
Colorado’s ≈ 84 °F with 58 °F WB → lower latent, higher sensible efficiency.

That’s why your MRCOOL might score 22.5 SEER2 on paper but deliver 18–19 seasonal SEER in humid regions.

👉 Always look at SEER2 and Sensible Heat Ratio for a more realistic comparison.

🧊 Step 9: Heating Load Flip-Flop

In mixed or cold zones, your “cooling system” becomes a heater six months a year.
At altitude, air-source heat pumps must work against thin, cold air — and capacity drops sharply below 0 °F.

MRCOOL’s 5th Gen 27 k R-32 model lists:

Heating capacity: 28000 BTU at 47 °F
20,000 BTU at 17 °F
11 000 BTU at –5 °F

Colorado’s winter design temp ≈ is 4 °F, so supplemental heat (baseboard or gas) covers those dips.
Florida? You’ll rarely dip below 40 °F — the system loafs all winter.

🧠 Step 10: Real-World Data — Florida vs Colorado Side-by-Side

Metric	Florida (Zone 2A, Sea Level)	Colorado (Zone 5B, 6,000 ft)
Area	700 ft² + 250 ft² bedroom	Same
Outdoor design temp	93 °F DB / 78 °F WB	91 °F DB / 64 °F WB
Indoor setpoint	74 °F / 50 % RH	78 °F / 40 % RH
Latent share	28 %	10 %
Effective capacity	≈ 25000 BTU	≈ 22000 BTU (after altitude derate)
Monthly kWh (summer)	860	640
Comfort feedback	“Dry, comfy.”	“Cool, crisp.”

Takeaway: Florida’s humidity steals BTUs; Colorado’s thin air steals BTUs — just from different ends of the physics.

🔧 Step 11: How Pros Handle It — Manual S & Derate Tables

Professional installers consult Manual S (equipment selection) after Manual J.
It has correction tables for altitude, latent fractions, and outdoor design.
Most show ~3 % capacity loss per 1,000 ft above sea level and additional derates for high humidity.

ACCA and AHRI both emphasise these adjustments.

💡 Step 12: Smart Thermostat Tweaks by Region

Mike’s rule of thumb for setpoints:

Climate	Cooling	Heating	Fan Mode
Florida	74 °F Cool / Auto fan	68 °F Heat	Auto
Colorado	77 °F Cool / Low fan	70 °F Heat	Continuous Low

In dry air, running a continuous low-speed fan improves mixing and keeps even temperatures.
In humid air, use Auto so the coil stays cold long enough to dehumidify.

Reference: Energy.gov Thermostat Best Practices.

🧩 Step 13: Sizing Checklist — Florida vs Colorado

Factor	Florida (Zone 2A)	Colorado (Zone 5B)
Climate Modifier	× 1.15	× 0.95
Altitude Factor	1.00	0.82 @ 6 000 ft
Window Orientation	High solar gain	High elevation UV
Humidity Control	Priority #1	Minor
Heating Need	Low	High (backup needed)
Ideal System	27 k (9 k + 18 k)	24 k (9 k + 12 k) or add aux heat

🧰 Step 14: DIY Tips — Adapting MRCOOL Systems Across Climates

In humid zones:

Oversized slightly on BTUs (5–10 %), but set the fan to Auto.
Install condensate pumps with traps for heavy drainage.
Use ceiling fans to improve mixing without raising humidity.

In high altitude zones:

Add line-set insulation upgrade to preserve capacity.
Use line-set covers to minimise radiant heat pickup.
Verify voltage (altitude = lower density = different current draw).

In cold zones:

Enable Defrost Mode and ensure proper clearance for snow melt.
Add a base pan heater kit if your manufacturer offers it.

🧾 Step 15: The Economics of Getting It Right

Every 10 % sizing mistake can cost 8–12 % more in power per year.
A Florida homeowner who undersizes a 700 ft² zone might pay $120 more annually in runtime.
A Colorado homeowner who oversizes by the same amount pays $100 more in short-cycling inefficiency.

Getting climate math right saves you both cash and comfort.

🌎 Step 16: The Environmental Layer — Carbon by Region

According to the EPA Power Profiler, grid carbon intensity ranges from 400 g CO₂/kWh in Florida (natural gas heavy) to 250 g CO₂/kWh in Colorado (wind/solar mix).

So that same 220 kWh monthly savings from better sizing =

Florida: 88 kg CO₂ avoided
Colorado: 55 kg CO₂ avoided

🧠 Step 17: Lessons Learned (Mike’s Takeaways)

Same tonnage ≠ , same performance. Altitude, humidity, and solar gain rewrite your BTU balance.
Humidity eats tons. Florida’s 27 k behaves like 22 k sensible cooling.
Altitude derates. Every 1,000 ft = ≈ 3 % loss.
Comfort ≠ temperature. Relative humidity defines “feel.”
Don’t rely on one-size charts. Use at least a Manual J Lite or online calculator adjusted for your zip code.