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Miscellaneous Engineering Data
Compare Gas tankless water heaters - Brands Bosch, Noritz, Rinnai, & Takagi
Compare gas tankless water heaters, examining the specs and features of Bosch, Noritz, Rinnai, and Takagi units.
I've made the table below to show what I think are the most important specifications for the tankless water heaters. These include the maximum Input BTUs, Minimum Flow Rate to turn on the heater, the energy factor, and the temperature rise at a given flow rate.
Comparing Maximum Input BTUs for Takagi, Rinnai, Noritz, and Bosch heaters
All four brands, Noritz, Bosch, Rinnai, and Takagi pretty similar sized units.
The maximum input BTUs basically tells you the size of the heater, giving you a way to quickly compare different models. Most all
tankless water heaters are around 80% efficient, so any two heaters that have similar input BTU's will have similar output capabilities. A water heater
with a higher efficiency will deliver a slightly higher flow for a given temperature rise and flow rate.
A BTU (British Thermal Unit) is the amount of heat required to heat 1 pound of water 1 degree. Water weighs about 8.3 pounds per gallon. Heating 1 gallon of water 100 degrees would require (( 8.3 pounds x 1 gallon x 100 degrees) / 80% efficiency ) = 1,037 BTUs.
Heating the water takes time of course, so if we wanted to heat that gallon of water in one minute the Btu consumption would be (1037 BTU x 60 minutes/hour)= 62,220 BTUs per hour.
To heat 2 gallons per minute 100 degrees will require roughly
124,000 BTUs per hour. 3 gallons per minute would be 189,000 BTUs.
Minimum Flow Rate
The minimum flow rate is important because that is how much flow you need through the heater to turn it on. The problems appear when you want a low-flow of hot water. If it takes 3/4 gallons per minute of flow to turn on the heater, and the output temperature of the heater is set for 120°, you have to mix colder water with the hot water to get the temperature you want.
Since you have to keep the flow rate high enough to keep the
heater turned on, and you have to mix it with cold, you simply cannot get a
small flow of hot water. To make matters worse, if you are trying to take
a shower, and you turn down the temperature enough for the heater to kick off,
you won't know it until that slug of cold water hits you a minute or so after
you've adjusted the valve, depending on how long it takes the water to travel
through the pipes to the fixture.
The energy factor (EF) tells what a water heater's overall energy efficiency is. It's based on the amount of hot water produced per unit of fuel consumed over a typical day.
Factors included in the energy factor are:
Comparing Temperature Rise & Flow Rate
The output flow rate is one of the most important factors since
it tells you exactly how much hot water the unit can produce. Bosch and Noritz give you the temperature rise at 77° and 75° which are close enough for
our comparison on their websites. Takagi provides graphs which are nice, so I used 75° for
Takagi. Rinnai only provides the flow rate at 35°, so to compare Rinnai flow
rates you can figure a little less than half of the given values.
Tankless Water Heater Costs compared
Where I could find them I listed the lowest prices I could find
online. They may therefore be incorrect. Keep in mind that
installing the heater can be very expensive. You may need special larger
stainless steel venting, and you may have to run a larger gas line as well.
If the location does not have a power outlet you might need to run an electrical
circuit since most gas tankless water heaters need power for their controls.
Gas Tankless Water Heater Comparison Table
* Commercial rated - Does not have an energy factor.
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