Power Vent or Tankless?

[KFreerksen]

Hello,

I currently have a 40 gallon Power Vented Natural Gas Water Heater installed in my house. It is about 15 years old and is starting to go out. I would like to replace it but would like advice on what product to get. 2 years ago we replaced all the water lines with pex, and ran hot water to a manifold with a line from the manifold to each fixture. I can get an equivalent power vented water heater for $800 but I am also considering buying a tankless water heater for a similar price. I was looking at a Rinnai 9.8 gpm 199,000 btu tankless, but I have some concerns.
1) I only have 1 bathroom but our water here runs pretty cold, typically around 45-55 degrees so would need a considerable increase lowering the flow rate. Will we have issues triggering hot water due to low flow? I can't really rerun the lines again to accommodate a recirculating system.
2) Current WH is only 46,000 BTUs and has a 1/2" gas line. there is a 3/4" line ran to the area that splits to two 1/2" lines to supply the WH and Furnace. Will a 199,000 BTU tankless water heater need a new dedicated 3/4" line rant to it or can I use the existing line for both furnace and tankless water heater?

Any input from someone with more experience with tankless water heaters would be greatly appreciated.

 

[CT18]

You would need your gas demand calculated for you. I would go back to power vent unless you have a large garden or hot tub. I am a plumber and would not have a tankless ever. Like you say your flow will be knocked down significantly to get the correct temp rise. My parents were 10 day no hot water waiting on warranty part to come in. I have seen leaves block the exhaust causing problems, snow drifts and critter nest.

 

[fixitron]

Tankless is more efficient than a tank due to standby heat loss to the air around a tank, but the tankless loses efficiency when it runs for short draws. For example, many people turn on hot and cold water to wash their hands. Assuming enough flow to start the tankless, it takes about 10 seconds longer for the tankless to go through its startup routine and start heating water, plus any delay for the time to get to the faucet. The tankless will run for a very short time, being inefficient and then losing the heat left over in the tankless heater. They are much more complex than tank type heaters and require more annual maintenance for the gas side and for de-scaling. Tankless are much more sensitive to harsh water. Most tankless require a minimum of 3/4 GPM flow to start, while some require as little as 1/2 GPM. Where tankless shines is with a lot of demand, such as large households and second homes where larger groups come to recreate for the weekends, or large jetted tubs. Tankless do require a much larger gas line.