How To Determine Tankless Water Heater Size

Tankless water heaters are rated by the maximum temperature rise possible at a given menstruation charge per unit. Therefore, to size a demand water heater, y'all need to determine the menstruation rate and the temperature rise you'll demand for its application (whole firm or a remote application, such as just a bathroom) in your home. It is of import to annotation that you should never endeavour and save coin by undersizing your tankless water heater.

Footstep 1:

Determine the maximum number of devices you want to run and their total flow charge per unit. Then, add together upwardly their menstruation rates (gallons per minute). This is the desired catamenia rate you'll want for the demand water heater.

For example, allow's say you lot expect to simultaneously run a hot water faucet with a menstruum rate of 0.75 gallons per minute and a shower caput with a flow rate of 2.6 gallons per minute. The flow rate through the demand water heater would need to be at least three.26 gallons per minute. To reduce flow rates, install low-flow water fixtures.

Footstep 2:

Determine required temperature rise. To make up one's mind temperature rise, subtract the incoming h2o temperature from the desired output temperature. Unless you know otherwise, presume that the incoming water temperature is 50°F (By using a low temperature assumption you ensure that you will not undersize your tankless unit.

If you lot live in a warm climate your water temperature will likely be much higher.) For most uses, yous'll want your water heated to around 105–115°. In this example, yous'd need a demand h2o heater that produces a temperature rise of 55°.

Step three:

Sizing Example: An average shower will be between 104–106° and uses ii.6 gallons of water. Assuming your h2o temperature is 40° coming into your habitation, and you desire to produce enough hot water to run 2 showers at the same time, what temperature rise would y'all need to produce to accomplish this?

Answer: You'll need to raise the incoming water temperature from 40 degrees to 105. Yous'll need to be able to heat at least 5.ii gallons of water. So yous'll demand a tankless water heater that tin produce at least a 60 caste rise in temperature at 5.2 gallons per infinitesimal.

Flow Rates

How much hot water do you demand at one time? Do yous need to run 2 showers at the aforementioned time or perhaps a shower and a couple sinks? The chart below shows the range of water usage range and average water temperatures for diverse fixtures. We suggest using 2.5 gpm for a shower and ane.0 gpm for a bathroom as a reference point in determining your total simultaneous water needs.

Fixture	Average Catamenia GPM	Average Temp.
Tub	iv.0 GPM	102°F
Shower	2.v – 3.0 GPM	104°F
Washing Machine	ii.0 GPM	120°F
Dishwasher	one.5 GPM	110°F
Kitchen Sink	one.5 GPM	110°F

For example, if you are running two showers at the aforementioned time, you will demand 5 gallons of hot h2o per minute from the tankless water heater. If you lot were running a shower and the washing machine at the aforementioned fourth dimension, you would need four.5 gallons per minute from the water heater. In either of these situations, you will want to size the unit that meets or exceeds how much hot water you demand at the aforementioned fourth dimension.

Notation: tankless water heaters are intended to oestrus potable (drinking) water merely and the inlet water to a tankless unit of measurement should not be pre-heated.

Other Sizing Notes

Gas tankless water heaters are able to produce a larger temperature rise per gpm than electrical models. Nearly demand water heaters are rated for a variety of inlet temperatures. Typically, a 70°F water temperature rise is possible at a menstruation charge per unit of five gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric water heaters. Faster catamenia rates or cooler inlet temperatures can sometimes reduce the water temperature at the most afar faucet.

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Plagued by a chronic instance of curiosity, Jeff Flowers is just a dude that annoys everyone around him with his loquacious goofiness. From beer to habitation living, Jeff is just trying to hack his mode through life and write a few notes virtually it along the way. You can follow his ramblings here, or heed to him complain about Austin traffic on Twitter at @Bukowsky.

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