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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.
Maximum Input BTUs
The maximum input BTUs basically tells you the size of the heater, giving you a way to quickly compare different models. Most all tankless 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.
Calculating BTUs
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 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.
Energy Factor
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:
Recovery efficiency – how efficiently the heat from the energy source is transferred to the water
Standby losses – the percentage of heat loss per hour from the stored water compared to the heat content of the water (water heaters with storage tanks)
Cycling losses – the loss of heat as the water circulates through a water heater tank, and/or inlet and outlet pipes.
The higher the energy factor, the more efficient the water heater is. However, higher energy factor values don't always mean lower annual operating costs, especially when you compare fuel sources.
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
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
Brand/Model Max Min. EF GPM@ Price
BTU GPM Temp Rise
Bosch
Bosch 2700ES 199,000 0.65 0.82 4.2 77° F $1,100
Bosch 2400ES 175,000 0.65 0.82 3.8 77° F $1,050
Bosch 1600H 117,000 0.6 0.69 2.4 77° F $649
Noritz
Noritz N-0931M 250,000 0.5 N/A 5.6 75° F $2,399
Noritz 841 199,900 0.5 0.91 5.0 75° F $2,399
Noritz N-0751M 199,900 0.5 0.83 4.5 75° F $1,299
Noritz N-0631S 185,000 0.5 0.83 3.8 75° F $1,029
Noritz 531 140,000 0.5 0.82 3.1 75° F $829
Rinnai
Rinnai R98LSi 237,000 0.9 N/A* 9.8 35° F
Rinnai R50LSi 150,000 0.6 0.82 5.0 35° F $679
Rinnai R75LSi 180,000 0.6 0.82 7.5 35° F $975
Rinnai R94LSi 190,000 0.6 0.82 9.4 35° F
Takagi
Takagi T-K Jr. 140,000 0.5 0.81 2.5 75° F $599
Takagi T-K3 199,000 0.5 0.84 3.6 75° F $800
Takagi T-H1 199,000 0.5 0.92 4.8 75° F
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