Sizing a Whole-Home Tankless Water Heater in Denver-Boulder

May 17, 2022

Old Faithful Geyser When it comes to replacing or upgrading an existing water heater, many homeowners take the time to learn about the tankless option, as opposed to replacing an existing tank with pretty much the same. A number of our customers choose whole-house tankless gas water heaters for a variety of reasons. Higher efficiency, their compact, wall-hung design, and longer life expectancies are among the reasons we hear when a family ‘goes tankless.’ When a new, whole-home tankless heater is properly sized and matched with the number of occupants, hot water demand, and usage habits, customer satisfaction levels with these products is consistently high.

So, how do we go about sizing a tankless gas water heater for your home? Without getting too buried in the technical weeds, in this blog post we’d like present some of the crucial factors that come into play when determining the right size tankless water heater for families in the Denver-Boulder area. You’ll see that it’s not as simple as just consulting a manufacturer or online supplier’s website. Our area is not the same as everywhere else in the country!

Homeowner Factors

When providing a quote for a whole-house tankless water there are a number of important factors that our plumbing team takes into account that focus on the occupants of the home and the home itself. These considerations include the number of people living there, their water usage habits and daily routines, as well as the number and types of plumbing fixtures in the home. For example, how many plumbing appliances are normally operated at the same time? On another home plumbing front, is there an exceptionally large bathtub in the master suite that needs to be taken into account in sizing, so that tub can be filled rapidly enough? Our plumbing team takes great care in learning as much as we can about the home and occupants, so we can come up with the right match for their unique circumstances.

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Ratings & Terms That Relate to Sizing Your Tankless Water Heater

Before we discuss what makes the Boulder-Denver area unique, let’s define in layman’s terms some of key terms and phrases that frequently enter our conversation with homeowners considering these products:

Incoming Water Temperature

In the Denver-Boulder area during the winter months, the temperature of our municipal water supply and residential water wells can drop significantly. At times, the water delivered to our homes could be much colder than the typical 50 degree Fahrenheit (F) range used in many sizing calculations. This difference in incoming water temperature must be taken into account in order to provide a unit that will do the job that’s expected.

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Output Water Temperature

Most gas tankless heaters allow you to set a specific output temperature for the hot water being distributed from the unit to your home. Our team routinely sets the output water temperature for each tankless unit that we install at a conservative but safe for all occupants 120 degrees F. It’s not a difficult procedure to adjust the temperature, and some homeowners choose to adjust them higher, as desired.

Temperature Rise

Measured in degrees Fahrenheit here in the United States, the temperature rise is the increase in water temperature when cold domestic water makes its single pass through the tankless unit’s heat exchanger on its way to where hot water is needed in the home. Under normal operating conditions, a temperature rise of approximately 90 degrees F is required to achieve the typical operating temperature of domestic hot water, about 120 to 140 degrees F.

  • Rule of Thumb: The higher the desired output water temperature, and as a result the higher the temperature rise needed, the lower the flow rate that the unit will be able to provide at that temperature (i.e. how much hot water, at the higher temp., will you receive at the tap). We admit this can be a bit confusing, at first. Read on about flow rates below.

Flow Rate

The flow rate is the amount (or volume) of hot water that a tankless heater can supply at the desired output temperature. This very important rating is measured in gallons per minute, or GPM, at a given temperature rise. The flow rate number directly relates to how much hot water at the desired temperature will be at your disposal, and therefore how many hot water-consuming plumbing appliances can be operated simultaneously (think dishes, laundry, showers, etc.). In the case of the large soak tub mentioned above, it translates to how quickly you’ll be able to fill that tub with steamy hot water.

High flow ratings at high temperature rises for tankless equipment equate to having more hot water available, when needed, and are a big reason that the BTU gas ratings of our most popular tankless heaters have been known to surprise some of our customers. Smaller units that we have available simply do not meet the whole-home demands of many families we speak with.

BTU Gas Rating of Tankless Heaters

The gas input rating of a tankless heater is measured in BTUs (British Thermal Units) per hour, and is one of the primary factors in determining how many people and fixtures a whole-home tankless unit can serve at one time. The higher the number, the more hot water you’ll have available. As alluded to above, fairly large size units are needed to satisfy the needs of an entire house and family. However, it’s noteworthy that most tankless water heaters have what is known as a modulating gas valve (just like variable-capacity furnaces and boilers), which automatically adjusts gas consumption upward and downward, as needed, to achieve the desired output temperature and volume of hot water needed. As hot water demand increases, the unit automatically increases gas consumption and heat output to match those needs; as demand decreases, it does the reverse. So, even though the BTU rating of the unit may be fairly high by traditional water heater standards, the modulating gas valve eliminates wasted gas.

In summary, the key to accurate tankless water heater sizing involves determining the right BTU size unit that will be able to supply the desired hot water temperature in volumes that will serve the needs of the family. This takes into account the incoming cold water temperature to the unit, the desired hot water output temperature, and the temperature rise and flow ratings needed to accomplish this goal.

Our Denver-Boulder Elevation and Accurate Tankless Heater Sizing!

Tankless Water Heater InstallationFor Denver-Boulder area homeowners, however, the above summary isn’t the complete story. Just as altitude plays an important role in properly sizing gas furnaces and boilers, it’s also critical for accurately sizing whole-house tankless gas water heaters in the Mile High area. In a nutshell, due to there being less oxygen in the air the higher up we go, the heating capacity of gas-fired unit is less than what the spec sheets say. Specifically, input and output ratings of gas-fired equipment are derated, or reduced, by 4% for every 1,000 feet above sea level. That’s a lot less heat available up here, compared to the coasts or other low-lying areas.

Given that Denver is located just under 5,300 feet above sea level and Boulder just over 5,300 feet elevation, the heat output for a tankless system (or any gas appliance) in our area is, in round numbers, 20% less than it would be at sea level (5 thousand feet elevation x .04 derate per thousand feet = .20, or 20% less heat produced).

An Important takeaway for Boulder-Denver area homeowners: The websites you may look at online usually do not take elevation above sea level into account in their charts and rule-of-thumb formulas for equipment sizing and performance expectations. Be careful whose advice you take. Trust experts like our team at Save Home Heat, who are very familiar with tankless install parameters in our region.

How Do Efficiency Ratings Affect Tankless Water Heater Sizing?

For the sake of this discussion, while the UEF efficiency ratings of tankless water heaters are certainly an important consideration, we are going to defer for now to a future blog post that will dive deeper into these ratings for tankless waters. Suffice to say that the efficiency ratings of the whole-home tankless gas water heaters our team installs are significantly higher than tank-type water heaters, and these ratings are another factor involved in accurate sizing. In general, the UEF rating of a tankless unit that we recommend does not usually affect the size of the unit specified – but of course it will affect operating costs.

Common Sizes of Tankless Gas Water Heaters in Denver-Boulder

Rinnai Tankless Water HeatersFor a sense of perspective when shopping for a whole-house tankless water heater in the Denver-Boulder area, one of the most popular products that our team installs is the Rinnai model RU199iN, which Rinnai classifies as a Super High-Efficiency unit. The RU199in has a gas heating input rating of 199,000 BTUs per hour, which is similar in size to some of the whole-home heating boilers that we install. Our team has found that for many homes and families that we visit, this size unit is often the best match for their needs.

The Rinnai RU series of tankless heaters range in size from 130,000 BTU to the 199,000 BTU, which is similar to other tankless water heaters in their product line. Here are some of the other reasons our team recommends the Rinnai RU199iN tankless water heater:

  • Established manufacturer with an excellent track record and strong factory support in our area
  • Very high efficiency (.93 UEF rating), significantly higher than standard tank type heaters
  • Modulating gas valve with a very broad range, can adjust gas consumption to as low as just 8% of full rating
  • Compact, wall-hung unit, great for conserving valuable floorspace
  • Versatile venting, including sealed combustion and direct vent options
  • Wi-Fi capable
  • ENERGY STAR certified

Multiple Tankless Units For High Demand Homes

Be aware that tankless water heaters can be linked together, providing even greater hot water availability, when needed, so you don’t need to be limited by how much hot water a single unit can supply – and don’t worry, a multi-unit tankless system will not waste gas needlessly. In some cases, our team may instead recommend using an auxiliary storage tank in tandem with a single unit to increase hot water availability. As always, our plumbing and heating team assesses each situation separately to come up with what we feel is the best hot water solution for each customer we visit.

Whole-Home Tankless Water Heater Expertise in Denver-Boulder

For high levels of expertise sizing, installing, and servicing whole-house tankless gas water heaters in the Boulder-Denver area, please reach out to our team today for friendly, low-pressure assistance. If we feel that a tankless heater doesn’t make sense for your specific circumstances, we are well equipped to discuss the full range of other hot water options available. Our goal is to help you find the best fit for you and your home. You can count on Save Home Heat to supply you with ample amounts of information and to answer all your questions fully and patiently, so you can make a purchasing decision that you’ll feel good about for many years to come.

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