Understanding Tankless Water Heater Sizing

How Do Tankless Water Heaters Work?

Alright, let’s break it down. Unlike old-school water heaters that keep a big tank of hot water ready at all times, tankless water heaters only heat up the water when you actually need it. This makes them way more energy-efficient, and you don’t have to stress about running out of hot water—unless, of course, you get a unit that’s too small for your needs. So, getting the right size is pretty crucial.

Important Things to Consider When Choosing

There are two main things you need to think about when figuring out the right size tankless water heater: the flow rate and the temperature rise.

First, let’s talk about the flow rate, which is measured in gallons per minute (GPM). This is all about how much hot water you’ll need at the same time. Imagine you’ve got someone in the shower, someone else washing dishes, and the washing machine running. You need to add up all the hot water these activities require. For instance, a typical shower uses about 2.5 GPM, and a dishwasher might use 1.5 GPM. If you add those together, you’ll get a sense of the total flow rate your heater needs to handle.

Next up is the temperature rise. This just means the difference between how hot the water is when it comes into your house and how hot you want it to be when it comes out of the tap. For example, if your water comes in at 50°F and you want it at 120°F, the temperature rise is 70°F. Basically, the bigger the difference, the harder your heater has to work.

Don’t forget to think about your climate, too. If you live in a colder area, your incoming water temperature will be lower, which means your heater will need to work harder to get the water up to the right temperature. So, people in colder climates often need a bigger unit than those in warmer areas.

Calculating the Right Size

Now, let’s walk through how to figure out the right size tankless water heater for your place.

Step 1: Identify Peak Hot Water Demand

Start by thinking about the busiest time in your house when it comes to hot water use. Maybe that’s when you’ve got a couple of showers going, the dishwasher running, and someone doing laundry. Each of these uses a certain amount of hot water, and you need to know the flow rate for each. A shower, for example, might use 2.5 GPM, and a washing machine could use 2.0 GPM. Add these up to get the total flow rate you’ll need during peak times.

Step 2: Calculate Total GPM

Once you’ve got the flow rates for all the things that might be happening at once, add them together. This will give you the total GPM that your tankless water heater needs to deliver.

Step 3: Determine Temperature Rise

Next, figure out the temperature rise. Subtract the temperature of the water coming into your house from the temperature you want it to be. So, if your incoming water is 50°F and you want it at 120°F, you’re looking at a temperature rise of 70°F.

Example Calculation

Let’s say you live somewhere pretty chilly where the incoming water is around 40°F. You plan on running two showers and the washing machine at the same time. Here’s how you’d figure out the total GPM:

• Each shower uses about 2.5 GPM.
• The washing machine uses 2.0 GPM.

That gives you a total of 7.0 GPM.

Next, you need to calculate the temperature rise. If you want your water to be 120°F and it’s coming in at 40°F, that’s a rise of 80°F. So, you’d need a tankless water heater that can provide 7.0 GPM at an 80°F temperature rise.

Best Tankless Water Heater Size Calculators

To make your life easier, there are some great online calculators that can help you figure out the right size tankless water heater. These tools ask for details like your desired GPM, temperature rise, and sometimes even your location to give you the best recommendation.

• Rheem’s Tankless Water Heater Sizing Tool is super easy to use and works for both homes and businesses. You just plug in your numbers, and it does the rest, even if you’re considering using more than one unit.
• Noritz’s Tankless Water Heater Sizing Calculator is another solid option. It guides you through everything, from figuring out your flow rate to picking the right unit based on your specific needs. It’s especially handy if you’re in a colder area and need something powerful.
• Bosch’s Tankless Water Heater Sizing Calculator is a straightforward tool that quickly tells you what size heater you need based on your flow rate and temperature rise. If you’re looking for a quick answer, this is a great place to start.

Using these calculators is a breeze. Just enter your peak flow rate, the temperature rise you need, and your local water temperature, and you’ll get a solid recommendation for what size tankless water heater you should buy.

Common Pitfalls to Avoid When Choosing

There are a few common pitfalls people run into when sizing a tankless water heater.

One mistake is misjudging your flow rate. It’s easy to either overestimate and end up with a heater that’s too big and expensive, or underestimate and find yourself with not enough hot water. To avoid this, really think about how many things will be using hot water at once.

Another issue is not considering the impact of your climate. If you live in a colder area and don’t account for the lower incoming water temperature, you might end up with a heater that just can’t keep up. Make sure you know your local water temperature and factor it into your calculations.

Lastly, don’t forget to consider how many people will be using hot water at the same time. In a busy household, multiple showers, dishwashing, and laundry might all be happening at once, so make sure your heater can handle the load.

Choosing the Right Tankless Water Heater for You

When it comes to picking out your tankless water heater, brand reputation is something you shouldn’t overlook. Companies like Rheem, Noritz, and Bosch have been around for a while and offer solid products with good warranties. A good warranty is like a safety net—if anything goes wrong, you’re covered without having to spend a lot out of pocket.

Also, keep an eye on energy efficiency ratings. Heaters with high Energy Factor (EF) ratings are more efficient, which means they’ll save you money on your energy bills over time. Even though these models might cost more upfront, they usually pay off in the long run.

If you’re feeling unsure about any part of this, it’s a good idea to consult with a professional. A certified plumber or HVAC technician can help you figure out the right size and make sure the installation is done right. It’s worth it to get expert advice if you’re not 100% confident in doing it yourself.

Conclusion

Getting the size right for your tankless water heater is key to making sure you always have hot water when you need it, without wasting energy. By understanding how to calculate flow rates, temperature rise, and considering the climate you live in, you can make an informed decision that suits your home’s needs. Those online calculators we talked about can really simplify the process, giving you accurate recommendations based on your specific situation. And remember, if you’re ever in doubt, reaching out to a professional is a smart move.

FAQs

What happens if my tankless water heater is too small?

If your tankless water heater is too small, it won’t be able to keep up with your household’s hot water demands, especially if multiple fixtures are in use at the same time. This can lead to lukewarm or even cold water when you need it most.

Can I use one tankless water heater for my entire house?

Absolutely, you can use one tankless water heater for your entire house, as long as it’s sized correctly to handle the total flow rate of all your fixtures and appliances.

How do I find the incoming water temperature?

You can estimate the incoming water temperature based on where you live and the time of year. Some online calculators also provide this information based on your zip code.

Do tankless water heaters work well in cold weather?

Yes, tankless water heaters work well in cold climates, but it’s important to size them correctly to account for the lower incoming water temperature. This typically means opting for a more powerful unit that can handle the higher temperature rise.

How often should I service my tankless water heater?

It’s a good idea to service your tankless water heater once a year. Regular maintenance keeps it running efficiently and helps extend its lifespan.