Tankless hot water heaters are not as simple and straight forward in their operation as you might think. What you need to know before you buy one!
It’s not easy to choose a new tankless water heater. I’m going to compare four different brands of gas tankless water heaters, examining the specs, warranties and features of Bosch, Reehm, 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.
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 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.
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 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 deg, 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. 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 deg and 75 deg which are close enough for our comparison on their websites. Takagi provides graphs which are nice, so I used 75 deg for Takagi. Rinnai only provides the flow rate at 35 deg, 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 BTU |
Min. GPM |
EF | GPM@ Temp Rise |
Price |
Bosch |
77 deg F | ||||
| Bosch 2700ES | 199,000 | 0.65 | 0.82 | 4.2 | $1,100 |
| Bosch 2400ES | 175,000 | 0.65 | 0.82 | 3.8 | $1,050 |
| Bosch 1600H | 117,000 | 0.6 | 0.69 | 2.4 | $649 |
Noritz |
75 deg F | ||||
| Noritz N-0931M | 250,000 | 0.5 | N/A | 5.6 | $2,399 |
| Noritz 841 | 199,900 | 0.5 | 0.91 | 5.0 | $2,399 |
| Noritz N-0751M | 199,900 | 0.5 | 0.83 | 4.5 | $1,299 |
| Noritz N-0631S | 185,000 | 0.5 | 0.83 | 3.8 | $1,029 |
| Noritz 531 | 140,000 | 0.5 | 0.82 | 3.1 | $829 |
Rinnai |
35 deg F | ||||
| Rinnai R98LSi | 237,000 | 0.9 | N/A* | 9.8 | |
| Rinnai R50LSi | 150,000 | 0.6 | 0.82 | 5.0 | $679 |
| Rinnai R75LSi | 180,000 | 0.6 | .082 | 7.5 | $975 |
| Rinnai R94LSi | 190,000 | 0.6 | 0.82 | 9.4 | |
Takagi |
75 deg F | ||||
| T-K Jr. | 140,000 | 0.5 | 0.81 | 2.5 | $599 |
| T-K3 | 199,000 | 0.5 | 0.84 | 3.6 | $800 |
| T-H1 | 199,000 | 0.5 | 0.92 | 4.8 |
* Commercial rated – Does not have an energy factor.