What Size Tankless Water Heater Do I Need?

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Sizing a tankless water heater correctly is absolutely critical for ensuring it can meet your household’s hot water demands. With no storage tank, tankless units provide hot water solely based on their output capacity as heated water flows through.

Get the sizing wrong, and you’ll quickly run into frustrating issues. Undersize the tankless heater and you’ll deal with cold-water sandwiches – where the hot water flow sputters out whenever demand exceeds the unit’s heating ability. This leaves you stuck taking cold showers or unable to run multiple hot water sources simultaneously.

But oversizing is also problematic, representing wasted money. An oversized tankless has excessive heating capacity that winds up overheating water and venting that excess energy wastefully. This increases utility costs while providing no additional hot water benefit.

To optimize your tankless water heater investment, you need to accurately calculate the “Goldilocks” model size for your home – not too big and not too small, but just the right output capacity. In this guide, we’ll walk through all the key factors and calculations for choosing a perfectly sized tankless unit to ensure endless hot water for your household’s needs.

Understanding Hot Water Demand

The first crucial step in sizing a tankless water heater is calculating your household’s total potential hot water demand. This analysis provides the baseline maximum flow rate the tankless unit needs to be able to handle.

Calculating Total Hot Water Demand

Make a comprehensive list of every fixture, appliance, and outlet in your home that requires hot water. This includes:

  • Showers and bathtubs
  • Bathroom and kitchen sinks
  • Dishwashers and washing machines
  • Wetbars or basement utility sinks
  • Outdoor hose bibs or spigots used for hot water

For each device, locate the maximum flow rate either printed on the fixture itself or listed in the manufacturer’s specifications – measured in gallons per minute (GPM).

For example, a standard showerhead may have a 2.5 GPM flow rate, while bathroom sink faucets are typically 1-1.5 GPM. Look up the ratings for appliances like dishwashers and washing machines as well.

Once you have the GPM flow rates for each hot water point, simply add them all up to calculate the cumulative maximum total hot water demand for your entire household. While this total is just a theoretical maximum that likely never occurs, it provides the peak potential consumption rate to size the tankless system for.

Determining Peak Hour Demand

Of course, it’s highly improbable every single hot water source would actually be operating at maximum flow simultaneously. Your real-world usage is a fraction of that total. This is where calculating the “peak hour demand” comes in.

Most residential sizing for tankless water heaters assumes the peak demand equals around 60-70% of the maximum total flow rate. This accounts for typical usage patterns where maybe a couple showers and some sink faucets are running concurrently during peak times.

For example, if your home’s cumulative maximum total demand is 12 GPM across all fixtures, the peak hour demand sizing would be 0.6 x 12 GPM = 7.2 GPM for tank to cover 60% of maximum usage.

The peak hour percentage can vary between 50-70% based on your specific household size and bathroom count. Smaller homes may be sized closer to 60% while larger homes with more fixtures size for closer to 70% of maximum.

Considering Simultaneous Usage

While the peak hour calculation helps narrow your tankless sizing need, you should also think about your maximum potential simultaneous appliance usage. Does your family tend to have multiple showers going at the same time while also running the dishwasher, washing machine, and sink faucets?

If your household’s peak demand frequently involves several high-flow fixtures and appliances overlapping, you’ll need to size for the combined flow rates of those simultaneous uses. Noting your typical “worst case” scenarios helps ensure the tankless has enough output for those high consumption periods.

Factoring in Future Growth

Finally, it’s wise to build in a small buffer to your calculations by sizing the tankless water heater 10-15% larger than your current needs. This allows for future additions like extra bathrooms, hot tubs, or kitchen remodels that increase household demand.

While purchasing more capacity than currently required costs more upfront, it provides crucial flexibility. You don’t want to undersize and immediately face limited hot water as your family’s needs change or grow.

The key to finding the perfect tankless water heater size is thoroughly mapping out your maximum potential total demand by fixture, then adjusting for more realistic peak loading scenarios and long-term usage expectations. With an accurate picture of household need, you can select the properly-sized unit.

Temperature Rise Needs

Beyond just the flow rate calculations, there’s another huge factor that impacts what size tankless water heater you need – the temperature rise required to heat your home’s water supply to the desired level.

Basically, temperature rise refers to how many degrees the tankless unit needs to raise the temperature of the incoming groundwater before it reaches your showers, faucets, and appliances. It’s the difference between that initial cold water temperature and your desired hot water setting.

For example, let’s say the incoming water line to your home is a chilly 45°F. But you want piping hot 120°F water for luxurious, steamy showers. To get that 45°F water up to 120°F, your tankless heater needs to provide a 75°F temperature rise.

Measuring that incoming “StartingTemp” is crucial for sizing. If you’re on municipal water, just check with your utility company for the current average groundwater temperatures in your area. Well owners need to break out a basic thermometer and measure that cold water flow themselves when it’s coldest.

The goal is finding the lowest yearly groundwater temp, because that represents the maximum temperature rise you’ll need. Maybe it’s 40°F in winter but a higher 60°F during summer. You want to size the tankless heater for that deeper 80°F rise to cover year-round.

Speaking of desired finished temps, most households want tankless hot water output around 110-120°F. That’s the happy medium – hot enough for steamy showers and dishwashing, but not scalding. So if your groundwater is 45°F, a 115°F target means needing a 70°F temperature rise.

Here’s the key difference between gas and electric tankless models: Gas-fired units can typically provide much higher temperature rises in the 70-100°F range with some up to 110°F. But electric tankless heaters usually max out at only 50-60°F rise due to their heating mechanisms.

So if you need a really high rise based on cold groundwater, you’ll probably want to size for a gas model unless you’re ok with “lukewarm” output. But for less extreme rises below 60°F, electric could work while saving some space.

That’s the skinny on temperature rise – just one of those crucial tankless sizing factors you can’t ignore when choosing the right unit for your home’s needs. Combine it with your flow rate calculations, and you’re getting ultra-dialed!

Comparing Gas vs Electric Tankless

Now that we’ve covered calculating your household’s flow rate needs and temperature rise requirements, another major factor in sizing a tankless water heater is deciding between gas or electric models. While both fuel types can provide unlimited hot water on demand, they have some key differences impacting their sizing and installation.

Flow Rate and Heating Capabilities

In general, gas-fired tankless water heaters tend to have higher maximum flow rate capacities compared to electric models of a similar physical size. The gas burner combustion can superheat water flow to a greater level.

For example, a common gas condensing tankless rated for a 199,000 BTU input could potentially handle household flow rates up to 10 GPM with the right temperature rise conditions. Whereas an electric tankless limited to around 36 kW would likely max out closer to 7-8 GPM capability.

Maximum Temperature Rise Potential

We discussed earlier how gas tankless heaters also have an advantage in their ability to reach higher temperature rises, often 70-100°F compared to electric’s 50-60°F limit. This makes gas models better suited for colder climates or households needing very hot output water.

However, electric tankless can still work well for warm climates with more moderate groundwater temps below 60°F. They provide adequate hot water while having the benefits of no combustion gas venting requirements.

Gas Line and Venting Requirements

Speaking of venting, gas tankless installations require proper gas line sizing and a corrosion-resistant venting system to expel exhaust gases vertically through the roof or sidewall. This adds complexity and space requirements compared to electric models.

Not only must you have adequate space around the tankless unit for clearances, but also room to route the air intake and exhaust vent pipes – potentially through walls, attics, or ceilings. Improperly sized gas lines or ventilation can starve the burner of fuel/air and cause unsafe operation.

Electric Circuit and Voltage Needs

While electric tankless heaters have no venting needs, they do require significant electrical capacity compared to standard appliances. Most whole-home electric tankless units draw at least 150 Amps at 240 Volts upon startup, necessitating heavy gauge wiring and potentially a new electrical subpanel. Very large units could need 400A service or more.

Not having sufficient amperage and voltage capacity causes electric tankless heaters to reduce their maximum flow rate and heating output. So proper electrical service calculations and upgrades are critical for electric model sizing.

Operating Costs Comparisons

When it comes to monthly operating costs, gas tends to have a slight edge over electric for heating the same amount of hot water. Natural gas is generally cheaper than electricity in most markets. However, electric tankless avoids gas piping and combustion air venting costs.

Ideal Applications

For homes with existing gas lines, gas tankless is often the preferred choice as they have higher heating output capabilities and lower operating costs. They’re an ideal pairing for homeowners wanting to take advantage of affordable natural gas.

Electric tankless works best in warm climates where moderate groundwater temps eliminate the need for extremely high temperature rises. They make sense for all-electric homes or where adding new gas lines would be prohibitively expensive. Their static installation removes any risk of combustion gas hazards.

Both gas and electric tankless models come in a wide range of output capacities, making proper sizing crucial for each application. By understanding their relative flow rate abilities, venting/electrical requirements, operating costs, and ideal temperature rise situations, you can pick the most cost-effective option for endless hot water.

Climate Impact on Sizing

Your local climate plays a major role in determining the optimal tankless water heater size for your home. Factors like seasonal temperature swings, elevation, and water hardness all impact the heating load requirements. Let’s dive into the key climate considerations.

Seasonal Inlet Water Temp Fluctuations

We discussed earlier how calculating your home’s minimum incoming water temperature is critical for ensuring the tankless heater can achieve the needed temperature rise. But that minimum groundwater temp can fluctuate throughout the year, even in tropical Hawaii.

While inlet temperatures in most of the islands hover in the 70s year-round, homes in some higher elevation areas like Volcano on the Big Island can see inlet temps dip into the 50s during the winter months. Meanwhile, in the hottest summer it’s not uncommon for ground water to reach the 80s.

To ensure adequate heat output during cooler periods, you need to size the tankless water heater for that lowest inlet temperature rather than just considering the warmest times of year. Otherwise, you may experience reduced flow rates or cooler output during winter.

High Elevation Considerations

In addition to seasonal variables, homes at higher elevations must also factor in temperature rise impacts. Due to reduced atmospheric pressure, water has a lower boiling point and vaporizes more readily at higher altitudes.

This boiling point reduction means tankless heaters at high elevations can’t reach quite as high of outlet temperatures compared to units installed at sea level using the same BTU input. So a sizing allowance may be required for elevations above 2,500-4,000 feet depending on the manufacturer.

Effects of Hard vs Soft Water

Another climate-related factor is water hardness levels. Hard water with high mineral content like calcium and magnesium has a greater ability to coat a tankless heater’s heat exchanger with efficiency-robbing scale buildup over time.

In areas like Oahu’s leeward side with hard water conditions, you may need to upsize the tankless model slightly to compensate for that gradual loss of heat transfer efficiency. Or plan for more frequent deliming maintenance to remove scale buildup and restore heat output to rated levels.

By carefully considering climate impacts like inlet temperature fluctuations, elevation changes, and Hawaii’s sometimes hard water, you can properly size the tankless water heater for ample heating capacity year-round.

Installation Factors

Beyond just selecting the right tankless water heater size, there are several installation-related factors that impact sizing choices. Everything from activation flow rates and plumbing configurations to fuel supply and clearance requirements come into play.

Minimum Flow Rates to Activate

Different sized tankless units have varying minimum flow rate thresholds to fully activate their heating capabilities. Larger models may require 0.8 GPM or more to fire up, while smaller point-of-use heaters could activate with just 0.5 GPM flow.

If your plumbing setup has fixtures incapable of achieving that minimum flow, you’ll either need to upsize to a larger unit or install dedicated return loops to create higher recirculation rates.

Centralized vs Semi-Centralized

Speaking of plumbing configurations, are you installing a single, centralized tankless water heater? Or utilizing several “semi-centralized” units placed closer to dispersed hot water sources? This decision impacts sizing for each heater.

With centralized systems, the main unit handles the entire household’s demand, so sizing covers that cumulative maximum usage. But semi-centralized banks of smaller units allow sizing each one for more limited demands.

Space and Clearance Requirements

Tankless heaters require air intake and exhaust clearances for proper operation and servicing access. Manufacturers specify minimum indoor clearances of 6-12 inches from combustibles in all directions for gas models.

Outdoor installations obviously have different venting and clearance rules. But you’ll likely need working space around the unit for maintenance like air filter cleaning and flushing out sediment.

Gas Line and Venting Needs

For gas-fired tankless, you must have an adequate supply line with sufficient volume, pressure, and BTU delivery capabilities based on the unit’s maximum input rating. Line sizing requirements change for overall supply length and ground elevations.

Venting requirements cover exhaust termination clearances from doors, windows, overhangs, and air inlets based on local codes. Vertical roof venting distances increase for higher BTU models as well.

Electric Circuit Accommodations

On the electric tankless side, you need to accommodate awfully large circuit demands within panel capacities. A 36kW heater already needs 4x40A 240V circuits with #8AWG wire – let alone larger models!

Between heavy gauge wiring, new circuit accommodations, grounding, and potentially subpanel additions, the electrical service calculations are vital for avoiding nuisance breaker trips.

Recirculation for Endless Hot Water

If you absolutely need hot water instantly at every fixture with zero delay, you’ll likely need a recirculation system with dedicated hot water return line. This allows the tankless heater to continually circulate hot water rather than reheating it with each use.

Choosing the right plumbing configuration, accommodating adequate space, venting, gas/electrical supplies, and potential recirculation all factor into sizing. Don’t overlook these aspects which can make-or-break on-demand performance.

Avoiding Sizing Mistakes

Even with all the calculations and factors covered so far, it’s still easy to make sizing mistakes when selecting a tankless water heater. Be mindful of these common pitfalls that can lead to under-sizing or over-sizing issues:

Not Accounting for Maximum Potential Demand

While sizing for realistic peak demands rather than theoretical maximums makes sense, don’t underestimate those high-usage scenarios. Ensure you accurately map out your household’s potential for multiple, overlapping hot water demands like multiple showers and appliances running simultaneously.

Underestimating Temperature Rise Needs

Thoroughly research your home’s minimum incoming water temperature, especially if on well water with seasonal fluctuations. Size based on achieving the temperature rise during the coldest winter conditions, not just mild seasons. Understand gas vs electric temperature rise capabilities.

Failing to Consider Down-Stream Pipe Sizing

Even with a properly sized tankless heater, downstream pipe diameter and lengths can restrict hot water flow if under-sized. Account for pressure drop and reduced flow rates through unsafe pipe sizes. Upsize distribution lines as needed.

Improper Fuel Type for Climate and Demand

Don’t install an electric tankless heater in a cold climate requiring an extreme 80°F+ temperature rise it can’t provide. Conversely, don’t oversize with gas if electrical demands are low. Match the fuel source to your home’s temperature rise and peak load.

Not Allowing Buffer for Future Growth

Try to size 10-15% higher than current household demands to absorb future additions like renovations or hot tubs. A little buffer provides flexibility, while under-sizing leaves no room for growth.

Mistakes in Flow Rate Measurements

Carefully measure individual fixture flow rates rather than guesstimating or using rough approximations. Small measurement errors get compounded into much larger water heating requirement miscalculations.

By avoiding these common sizing mistakes regarding demand, temperature rise, pipe sizing, fuel source, future growth, and fixture measurements, you’ll ensure your tankless installation provides adequate hot water for the long haul.

Pro Sizing vs DIY

While this guide provides an in-depth look at all the factors for sizing a tankless water heater, the calculations and considerations can still be complex for DIY homeowners. That’s why many choose to utilize professional sizing consultations rather than trying to crunch the numbers themselves.

The benefits of professional sizing start with expertise that accounts for factors homeowners may easily overlook. Installers have access to precise calculation tools and charts developed specifically for various tankless water heater models. Their experience allows them to accurately gauge temperature rise needs, probable usage demands, and ideal fuel types for each situation.

Pros can also make educated recommendations on upsizing beyond stated household needs to build in capacity buffers for future growth. Their insights protect against undersizing mistakes that lead to disappointing hot water shortages down the road.

While some companies charge separately for sizing consultations, the better HVAC contractors include it as part of their upfront quote process. This allows a detailed analysis and calculation of your home’s projected hot water needs before presenting you with recommended tankless model and installation options.

Whether you attempt DIY sizing or not, the goal should be installing a properly sized tankless water heater from the start. This investment avoids the headaches and added costs of replacing an undersized or oversized unit down the road.

Never Run Out of Hot Water Again with Proper Tankless Sizing

Whether you’re upgrading an outdated tank water heater or going tankless in a new home construction, sizing the system correctly is absolutely vital. With their instant, on-demand heating capabilities, tankless units can truly provide endless hot water for your whole household’s needs. But only if they’re sized properly from the start.

This guide walked you through all the key considerations like calculating peak hour hot water demands, determining temperature rise requirements based on climate, understanding fuel type differences, and even installation factors that impact sizing choices. We covered common mistakes to avoid so you don’t inadvertently undersize or oversize.

While crunching all those numbers yourself is certainly possible, many homeowners opt for the professional expertise of our experienced team. At Steve’s Plumbing and AC Service, we offer free in-home sizing consultations utilizing industry calculation tools. Our decades of experience ensure no stone is left unturned in properly measuring and sizing the ideal tankless system for your Hawaii home.

Our team doesn’t just help pick the perfect sized unit either – we handle the full installation process including gas line and venting, electrical service accommodations, and even recirculation systems if needed. Our Infinite Hot Water System design approach leaves no cold water sandwich in sight.

So if you’re ready to upgrade to a new tankless water heater fitted specifically for your home’s hot water demands, get sized up correctly from the start. Contact us at Steve’s Plumbing and AC Service today at (808) 563-4054 to schedule your free sizing consultation and quote. With our sizing precision, you’ll enjoy endless hot water showers, sink fill-ups, and appliance operation for years to come.

What Our Clients Say

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