Tanks or Tankless

[dfischer]

As you may have noticed, I have a dual (really 50G) nat gas (40Kbtu) water heaters in series. They are 16 years old and are basically not working. I'll replace them both. But what with?

I'm tempted to run a 9GPM tankless (would meet my peak demand, I think), but I sorta fear the unknown. We've run everything under the sun here with the two 50's and I'm not too inclined to think we'll ever call for more than 9GPM. But.....

Q1) What do these do when I want to runs a bathroom sink of hot water? Will they even light on such a low demand?
Q2) Do you REALLY save money on these? Including maintenance and repair costs?
Q3) How long do they really last? I got 16 years out of some ho-hum box store units., and I'll have maybe $1,100 into replacing them. It tankless can't just sit there and run for that long AND save me at least $100 a year they aren't for me...
Q4) Can the GPM rating be trusted? We can assume my water temp it 57f, and that I have 40 psi. Will I be able to meet specs with that source water?
Q5) Sizing: What I run most things it seems like I'm taking mostly hot water. I assume my hot water demand is 2GPM per device. I'm not going to have more then 3 shower/baths running at one time, and while i may have up to three washers running at a time, I don't really expect them to ever be calling for water at the same time, not asking for much water. My feeling is 9gpm, it's it's real, should easily meet my need. Am I missing something?
Q6) What do these do when the demand exceeds supply? Do they lower flow or meet flow and not deliver called for temperature rise?

Thanks!

 

[Twowaxhack]

What type gas do you have ? Natural or propane

 

[dfischer]

Natural gas sir.

 

[Twowaxhack]

To run a shower at 3.0 gpm with an incoming cold temp of 55 and the tankless set at 120 requires a 77% hot to cold mix to hit a target shower temp of 105 degrees.

This requires 2.31 gpm of hot water per shower. For three showers simultaneously 6.9 gpm hot water volume Is required at 120.

At an incoming cold water temp of 55 degrees a 9.4 gpm Rinnai will not supply enough hot water for you.

It would be close . It could supply 6.5 At most.
But if you take hotter showers, the incoming water is colder, or the volume of your shower heads exceed 3,0 then its just not going to satisfy you.

 

[Twowaxhack]

I would do a large and a small tankless for your house or if I could talk you into it I’d install two 9.4 units. To keep them both the same model.
Or two 50 gallon tanks piped in parallel so they work as one unit.

 

[FishScreener]

Last I read on the GreenBuilding site: You can not recover the increased install and initial purchase costs over the life of the tankless heaters. Until they increased teh standards for tank type efficiencies, it was close. But, the increased insulation on the tank type made them considerably more efficient.

 

[Twowaxhack]

Last I read on the GreenBuilding site: You can not recover the increased install and initial purchase costs over the life of the tankless heaters. Until they increased teh standards for tank type efficiencies, it was close. But, the increased insulation on the tank type made them considerably more efficient.

That depends on how much hot water you need and how quick you need it for that to be true.,

I’d hate to be forced to use tanks if you just wanted to take 6 showers back to back.