When upgrading your home’s hot water system, choosing the right size tankless water heater is as important as selecting a quality brand. An undersized system will struggle to meet your household’s needs, while an oversized one can waste energy and increase upfront costs.
Understanding how tankless water heaters work, the factors that impact sizing, and the importance of professional installation will ensure you enjoy endless hot water without sacrificing efficiency or comfort.
Whether you’re replacing an older unit or installing one for the first time, making the right choice from the start will pay off for years to come.
So, why does sizing matter?
Consistent hot water supply
A tankless water heater must be able to meet your household’s peak hot water demand. An undersized unit won’t keep up when multiple fixtures — like a shower, dishwasher, and washing machine — run simultaneously. As a result, you get inconsistent temperatures or even a complete loss of hot water mid-use.
Energy efficiency and cost savings
The right-sized system operates at its optimal performance range, using only the energy necessary to heat the water you need. An oversized unit may use more energy than necessary, while an undersized one can overwork itself, leading to inefficiency and higher utility bills.
Appliance longevity
A properly sized heater experiences less strain during operation, which helps extend its lifespan. Undersized units often run at maximum output for extended periods, accelerating wear and tear, while oversized systems may short-cycle, leading to unnecessary mechanical stress.
Comfort and convenience
Sizing isn’t merely about function — it’s also about comfort. With the right capacity, you’ll avoid temperature fluctuations, slow recovery times, or scheduling around hot water availability.
Compliance with manufacturer guidelines
Most manufacturers provide flow rate and temperature rise specifications for different household sizes. Correct sizing ensures your system meets warranty requirements and avoids installation issues.
How to calculate the right size
1. Determine your peak hot water demand (flow rate)
Start by figuring out the maximum amount of hot water your household uses at one time. We measure this in gallons per minute (GPM). For example:
- Shower: ~2.0–2.5 GPM
- Kitchen faucet: ~1.0–1.5 GPM
- Dishwasher: ~1.5–2.0 GPM
- Washing machine: ~2.0–2.5 GPM
Add up the GPM of all fixtures you might use simultaneously. For instance, running a shower (2.5 GPM) and dishwasher (1.5 GPM) at the same time means your heater needs to supply 4.0 GPM.
2. Factor in temperature rise
Tankless water heaters must heat incoming water to your desired temperature. “Temperature rise” is the difference between your incoming water temperature and your preferred hot water output.
Measure or estimate your incoming water temperature (often 40–50°F in colder climates, 50–60°F in warmer ones).
Subtract it from your target temperature (typically 120°F for most homes).
For example, if your incoming water is 50°F and you want 120°F, you need a 70°F temperature rise.
3. Match your needs to the manufacturer’s specifications
Manufacturers list GPM ratings for their units at specific temperature rises. Find a unit that can handle your required GPM at your needed temperature rise. For instance, a heater might supply 5.0 GPM at a 70°F rise — enough for two showers and a sink running simultaneously.
4. Consider household size and usage habits
- Small households (1–2 people) often need 3–5 GPM units.
- Medium households (3–4 people) usually require 5–7 GPM.
- Large households (5+ people) may need 7+ GPM or even multiple units.
Also, factor in whether your household uses hot water for multiple tasks at once or is more staggered throughout the day.
5. Consult a professional for accuracy
While you can estimate your needs, a plumbing professional can measure your exact demand, test incoming water temperature, and recommend the ideal unit. This ensures you get an efficient, reliable, and long-lasting system.
When to upgrade your water heater size
Knowing when to upgrade to a larger heater is key to keeping your household comfortable and avoiding frustrating hot water shortages.
One clear sign is if your current system struggles to keep up with peak demand, like when showers turn lukewarm because multiple taps or appliances are running at the same time. This often happens when your household grows, whether from adding family members, roommates, or frequent guests.
Another reason to upgrade is if you’ve recently remodeled or expanded your home. Adding bathrooms, a larger kitchen, a laundry room, or hot water–using features like a spa tub can significantly increase your demand. If your water heater wasn’t sized with these additions in mind, it may not be able to deliver the necessary flow rate.
Also, consider an upgrade if your climate demands more heating power than your system can provide. In colder areas, incoming water temperatures are lower, requiring a higher capacity unit to maintain the same comfort level.
Finally, if your unit is older and nearing the end of its lifespan, replacing it with a properly sized, more efficient model can improve performance while lowering long-term operating costs.
Why professional sizing and installation matter
Choosing the right size tankless water heater is about more than comfort. It’s also about efficiency, safety, and protecting your investment.
A plumber — a professional water heater installer — has the expertise to calculate your household’s peak hot water demand and match it with the correct unit capacity. They factor in details like the number of fixtures, your climate’s incoming water temperature, and your family’s usage patterns — things that online calculators or guesswork easily overlook.
Proper installation is as important as correct sizing. An improperly installed heater can suffer from performance issues, reduced lifespan, or even dangerous gas or water leaks.
Professional water heater installers ensure the water heater is connected to the right gas, electric, and venting systems, pressure-tested for leaks, and set to operate at safe, efficient temperatures.
By relying on experts, you get the right system for your needs and ensure it’s set up for optimal performance from day one. This reduces the risk of costly repairs, improves energy efficiency, and keeps your household supplied with reliable hot water for years to come.
Upgrade your tankless water heater with Salisbury Plumbing
A properly sized and professionally installed tankless water heater can transform your home’s hot water experience — delivering daily efficiency, reliability, and comfort. By considering your household’s needs and relying on expert guidance, you’ll ensure you get the perfect system for your lifestyle.
When it’s time to upgrade, turn to Salisbury Plumbing for all your tankless hot water needs and every other plumbing project on your list. Our team of professional water heater installers has the expertise to size, install, and maintain your system so it runs flawlessly for years. Don’t leave your comfort to chance.
Contact Salisbury Plumbing today and enjoy the peace of mind that comes with expert service.
