Sizing gas tankless water heater

2 years ago

We're about to move into a new house, and one of the first things we'd like to do is replace the ancient, space-sucking gas tank water heater with a gas tankless. I'm basically between the Rinnai RL75iN and the RU160iN but am at a loss trying to figure out correct sizing--despite reading countless articles about flow rates, increasing water temps, etc. For context, we have 2 showers and are a small household (2 adults, 1 infant) in San Francisco (water temps ~57-62F). Since we haven't moved in yet it's a total guess as to GPMs for the various fixtures. The goal is to have the heater for 20 years so I also want to make sure future needs (i.e., more kids) are covered, while not overspending. Thanks in advance!

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Comments (4)

PRO
Charles Ross Homes
2 years ago
The two models you are considering are different with respect to capacity, operating efficiency and installation requirements.

We had a Rinnai with a similar capacity to the RL75iN for a family of four with 2.5 baths. The unit was mounted on the home's exterior and we get about two months of cold weather in the winter. We never had an issue with it.

Two considerations when changing to tankless: 1.) you'll need to ensure adequate gas service to your home to support the heater plus whatever other gas appliances you may have, and 2.) I think a 20-year life expectancy is pretty optimistic. I got 12 years out of mine without performing the recommended annual maintenance which I was very happy with. I replaced it with the same model.
dadoes
2 years ago
My tankless (electric) is 17yo.
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Fred derf fred: I too was set on going tankless - till i saw the upcharge. It is coming to about $5,750 more to go with (4) gas tankless (Rinnai 94LS 9.4gpm top of the line) compared to State gas water heaters - as spec'd in the drawings of our house. I did research and found out that gas tankless do have an EF (efficiency rating) of .82, however GAS (i believe you compared electric tank) tank heaters commonly have .62, and a few have .68 efficiency. This results in savings of tankless is about $250 per year, for all 4 units combined. So it would take me more than 20 years to recoup the cost difference. This was based on a formula as provided by the dept. of energy. Additionally, State, who sells both, has a 'calculator' on their website that corraborated the results. Also, please note that your existing ELECTRIC tank heater was very OLD. New, gas models are much more efficient so I do not think it is a fair comparison. I am no expert, but these are my findings. I would greatly appreciate any other comments ASAP as we are about to make the decision on going tank or tankless
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Jake The Wonderdog
2 years ago
last modified: 2 years ago
So here's a tip on sizing;

1. you want a condensing unit that vents with PVC.

2. You want a 200k btu/h so you don't have to worry about sizing. There's no reason to do smaller.

You should know this isn't an exact replacement. You need the gas supply to the house, and to the water heater. I had to run a new 1" gas line from my meter to the water heater in my house.

Also, tankless take considerably longer to get hot. Make sure you don't have a long run from the heater or you will wait forever.

Go in with your eyes open.

That said, thetankless that I like are the Rheem. They have some of the lowest flow rates to activate and to keep running.
PRO
Charles Ross Homes
2 years ago
Good point about the wait for hot water at the use point since it's only heated on demand with a tankless hot water heater. The two Rinnai units you are considering are recirculation compatible so you can install a recirculation pump and minimize the waiting time, albeit at the expense of more on/off heating and the energy required by the pump. Some manufacturers offer "tankless" units with an onboard pump and buffer tank built in which simplifies installation. You'll need to install an additional line for recirculation if you go that route.

Sizing gas tankless water heater

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Charles Ross Homes

dadoes

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