What Size Tankless Water Heater Do You Need? Complete Sizing Guide for Denver Homes

Choosing the right tankless water heater is all about matching the unit’s capacity to your home’s hot-water demand. When the sizing is right, you get powerful, stable hot water even in Denver’s cold winters. When it’s wrong, you end up with temperature drops, low flow, and a system that never feels “strong enough.”

This in-depth guide covers:

• What GPM means and how to calculate it
• How Denver’s winter temperatures affect sizing
• BTU and kW formulas (explained simply)
• Sizing examples for small, medium, and large homes
• Gas vs. electric tankless capacity differences
• High-altitude considerations
• Maintenance and long-term reliability
• When to choose one large unit vs. multiple smaller units

If you want accurate in-home measurements, Comfy Cave Heating & Air provides free sizing estimates, full installation, and financing for all tankless systems.

How to Determine the Right Size Tankless Water Heater (Step-by-Step)

Tankless sizing requires two key numbers:

1. Peak flow rate (GPM) — How many gallons of hot water your home uses at once
2. Temperature rise (ΔT) — How much heating the system must do to reach your preferred hot-water temperature

Once you have those numbers, you can calculate the required BTU (for gas) or kW (for electric). Here’s how to do it properly.

1. What Is GPM and How Do You Calculate Peak Hot Water Demand?

GPM (gallons per minute) tells you how much hot water your fixtures use while running.
To size a tankless unit, you must add up the GPM of all fixtures likely to run simultaneously.

Typical GPM by Fixture (Denver Homes)

How to calculate your peak GPM

1. List the fixtures commonly used at the same time.
2. Add the GPM from the table above.
3. If unsure, measure flow using the “one-gallon bucket test.”

Example peak demand

• 2 showers (2.0 GPM each) → 4.0 GPM
• 1 kitchen sink (1.5 GPM) → 1.5 GPM
  Total = 5.5 GPM

This is your peak demand, and the tankless system must support it during Denver’s coldest months.

2. How Does Temperature Rise Impact Tankless Sizing?

Temperature rise = desired hot water temp (120°F) − incoming water temp

In Denver:

• Winter inlet water can be as low as 40°F
• Summer inlet water is typically 55–60°F

Since winter is the worst-case scenario, always size for the coldest season.

Typical Denver temperature rise

If inlet water is 40°F and target is 120°F:

ΔT = 80°F

A higher temperature rise reduces a unit’s maximum GPM.
This is why people who buy based on “summer performance” often experience cold showers in winter.

3. How to Convert GPM + ΔT Into BTU (Gas) or kW (Electric)

Gas tankless BTU formula

BTU/hr = GPM × ΔT × 500

After calculating this “water side” BTU, divide by the unit’s efficiency (typically 0.80–0.95) to find the necessary input BTU/hr.

Electric tankless kW formula

kW = BTU/hr ÷ 3,412

Worked example

Using the earlier example:

• Peak GPM = 5.5
• ΔT = 80°F

Water-side BTU = 5.5 × 80 × 500 = 220,000 BTU/hr
Assume 85% efficiency:

Input BTU needed ≈ 259,000 BTU/hr

So you’d need:

• A high-BTU gas tankless unit, or
• Two medium-sized units in parallel, or
• For electric: a large kW installation with panel upgrades

4. Sizing Examples for Small, Medium, and Large Denver Homes

Small homes / apartments (1 bathroom)

Peak GPM: 1.5–3.0
Recommended:

• Small gas tankless unit
• Compact electric unit (if ΔT is low)

Medium homes (2–3 bathrooms)

Peak GPM: 4–7
Recommended:

• Mid- to high-capacity gas tankless unit
• Possibly two electric units depending on panel capacity

Large homes (3–5 bathrooms)

Peak GPM: 7–10+
Recommended:

• Large 180,000–250,000 BTU gas unit
• Parallel tankless units
• Hybrid tankless + buffer tank for peak loads

For complex setups, on-site analysis is strongly recommended.

5. Gas vs. Electric Tankless Sizing: Key Differences

Gas and electric units handle load very differently.

Gas tankless systems (best for most Denver homes)

• Higher GPM capacity
• Perform well with large ΔT
• Need proper venting
• May need gas-line upgrades
• May require altitude adjustments

Electric tankless systems

• No venting needed
• Simpler installation
• Limited GPM at high ΔT
• Often require breaker panel upgrades

Capacity comparison

6. Denver-Specific Factors That Affect Sizing

Denver’s conditions make tankless sizing more challenging than in warmer states:

Altitude (5,280 ft)

• Reduces oxygen → reduces combustion efficiency
• Some gas units must be derated
• Always choose high-altitude-approved models

Winter temps (cold inlet water)

• Raises temperature rise
• Reduces a unit’s rated GPM by 20–40%
• Proper sizing must consider winter, not summer

Water hardness

• Causes scale buildup
• Reduces heat exchanger efficiency
• Requires descaling every 6–12 months

Permit requirements

• Gas, venting, and electrical work must be inspected
• Comfy Cave handles all Denver Metro permits

7. When to Choose One Large Unit vs. Multiple Smaller Units

One large gas tankless makes sense when:

• The home has 3–4 bathrooms
• Gas-line capacity is sufficient
• Venting options exist

Two smaller units in parallel work best when:

• The home has 4+ bathrooms
• Hot water demand is unpredictable
• You want redundancy (if one fails, the other runs)

Hybrid setups are ideal for:

• Homes with very high peak demand
• Long-distance plumbing runs
• Multi-family or light commercial use

Comfy Cave can help you compare all three solutions on-site.

8. Common Sizing Mistakes to Avoid

Many homeowners (and even some contractors) make these errors:

Mistake 1: Sizing based on summer inlet temperatures

→ Causes winter performance failure.

Mistake 2: Ignoring simultaneous usage

→ Two showers can exceed a small unit’s capacity instantly.

Mistake 3: Choosing electric without checking panel capacity

→ Often requires expensive upgrades (200–400 amp service).

Mistake 4: Underestimating future demand

→ Families grow, schedules change, and loads increase.

Mistake 5: Forgetting maintenance in hard-water areas

→ Scale reduces effective GPM and shortens lifespan.

Avoiding these mistakes ensures your system works reliably year-round.

9. How Comfy Cave Heating & Air Helps Denver Homeowners Size Tankless Systems Accurately

Comfy Cave offers:

• On-site GPM testing
• Winter inlet temperature measurements
• Gas pressure + electric capacity checks
• High-altitude setup adjustments
• Full installation + maintenance plans
• Financing for medium and large projects

For Tankless Water Heater Services in Denver, visit:
https://comfycave.com/water-heaters/

For broader HVAC help, explore:
HVAC Services in Denver — https://comfycave.com

Conclusion

Sizing a tankless water heater correctly ensures steady, comfortable hot water all year — especially during Denver’s coldest months. By calculating your peak GPM, temperature rise, and fuel-type requirements, you can choose the right gas or electric system with confidence.

If you want professional help choosing the perfect size, Comfy Cave Heating & Air provides free estimates, accurate in-home measurements, and expert installation across the Denver Metro area.