Marry Tankless and Tank

Users who are viewing this thread

J.H. Aull

New Member
Messages
1
Reaction score
0
Points
1
Location
North Carolina
I've got two 40 gal tanks in a series providing my hot water with recirculating pump and non-dedicated return line. First one in the series has failed and we've been "surviving" off the one.

I'm thinking of the Navien 240 tankless to feed my tank (no sense in ditching a good tank). Since the Navien has it's own pump and buffer tank, I would run a dedicated return line from my farthest draw back to the Navien and ditch my under sink valves and separate pump. I do have a 5 psi natural gas line feeding the house and on municipal water supply in NC.

Anyone see any issues with this or have recommendations? Thanks so much for your thoughts.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
What would be the advantage of the 199,000 BTU/hr Navien?

Using an standard gas fired tank as a buffer tank for a big-burner condensing tankless creates a big step down in efficiency compared to tankless-only, since tank still has a very high standby loss. If you were using a highly insulated electric tank for the buffer that would not be the case. Gas tank hot water heaters convect room air through the center tank heat exchanger, taking heat out of the water and putting it into the room (or the flue) when the burner is not running. By putting the heat into a lossy tank you're going from potentially 90%+ net efficiency down to less than 70%.

Your system also has significant efficiency losses due to the recirculation runs, but that's a separate issue.

If you're going to spend Navien 240 kind of money on a hot water heater, and all-stainless modulating condensing 50 gallon tank like the smallest HTP Phoenix Light Duty (light duty only as compared to commercial hot water heaters), is probably a better solution. It has 2x the burner output of a typical 40 gallon tank, none of the flow restrictions of a tankless, but comparable efficiency, and it'll last at least as long. It's more money up front, but if you actually need a 40 gallon buffer to make the Navien work well for you the tank would need to be replaced within the decade anyway (hopefully with a well insulated buffer tank, and not a lossy gas-burner.)
 

Master Plumber Mark

Sensitivity trainer and plumber of mens souls
Messages
5,533
Reaction score
354
Points
83
Location
indianapolis indiana - land of the free, home of
Website
www.weilhammerplumbing.com
LOL....Dana you are a hoot.... you crack me up.......
you make this sound like you are installing something on the space shuttle

Lets factor in the stand-by heat loss of a 75 gal heater compaired to a tankless....
that is great information to confuse someone with.......

you do realize if you install a normal 75 gallon gas heater
and simply install a blanket on the unit it basically is all a wash

I hope the guy knocks himself out attempting to re-invent the wheel
and he might even save himself as much as 5 bucks a month taking your advice..
and .thats priceless......
:p:p:D:D:):rolleyes:..
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
I've built stuff that went up on the space shuttle, but it wasn't a hot water heating system, for sure! :)

Note that it was predicated on "If you're going to spend Navien 240 kind of money on a hot water heater..." . Spending money on the tankless doesn't really buy him anything, which is the point.

Belaboring what is clearly too fine a point, what means "... if you install a normal 75 gallon gas heater and simply install a blanket on the unit it basically is all a wash" in 'merican dialect? All a wash compared to what?

A modulating condensing stainless tank HW will have about 2x the lifespan of standard 75 gallon tank, so you'd have to do some present-value accounting over the lifecycle to come up with the actual cost differences, not just the marginal monthly cost of the fuel (which would also require some fuel price inflation sensitivity analysis.) But if you can show the math on how you derive $5/month, please do! :rolleyes:
 

Master Plumber Mark

Sensitivity trainer and plumber of mens souls
Messages
5,533
Reaction score
354
Points
83
Location
indianapolis indiana - land of the free, home of
Website
www.weilhammerplumbing.com
which would also require some fuel price inflation sensitivity analysis.)
But if you can show the math on how you derive $5/month, please do! :rolleyes:

Dana....
You are a hoot ...... lets discuss some " fuel price inflation sensitivity analysis..." . OHHH BOY.. everybody is sensitive to fuel price inflation....
now thats a hot topic to get this thread to come alive......

Dana...we could do some chart graphs and equate the delta pi vs the average length of time they take a shower.. LOL.. we will also have to factor in when they have company,,,, ect ect.....

In good old mid-western dialect Dana
I am basically doing what is called "spit-balling" ....
that is like throwing a number at the
wall and see if it sticks......
perhaps I could be off as much as 10 dollars a month
but I bet I will be somewhere in the "ballpark"
Do you get it ....thats called ballparking

Dana when you factor in the cost of maintaince and upkeep with any tankless water you already know its a wash over a simple 75 gallon heater per year....

You know it, I know it and everybody knows it but if you got to hide behind all the fancy words and bs.....ok fine....

Now get out the old "pro-tractor" and start drawing me some pretty circles and graphs would you....
get out that old slide ruler and your calculator and throw some fancy numbers at me .

We got to get that "fuel price inflation sensitivity analysis..." nailed down real good......... . OHHH BOY.. LOL.....:p:p:p:D:D;)...


YEEEEE-----HAWWWWWWW.....Dana........

..
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
At least somebody appreciates my humor on this site! ;)
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks