Connecting new tankless heater with old hot water tank?


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Old 02-22-14, 10:35 PM
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Connecting new tankless heater with old hot water tank?

I currently have two (50gal) Hot Water Heater Tanks (Rheem PowerVent #21VP50-1) with their outflow connected, which together have been heating the water in our house pretty well for the past 20-25 yrs. (I have 3 full bathrooms and 7 people in my household.) One of these tanks is now beyond repair, and I ordered a tankless system (Rheem RTGH-95DVLN) to replace it. But I still have one functioning 50gal tank unit, and I'm wondering whether there is some way to install the new (tankless) unit that will enable me to benefit as much as possible from the efficiency of the tankless unit, while keeping the old tank unit connected as a backup.
Does it make sense to use the old 50gal as a "storage tank" connected to the tankless unit?
Would it make sense to merge the outflow from the tank and the tankless unit, and then close the valve on the tank unit most of the way?
If I shut off the tank unit entirely, can I expect the condition of the tank to corrode or deteriorate more rapidly, and would it require frequent flushing to maintain it?

Would appreciate any recommendations.

Thank you
 
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Old 02-23-14, 06:35 AM
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Hello...

IMO I would get a small electric as a buffer tank..

Best way is as described here..

http://r.search.yahoo.com/r/_ylt=A0L...hbDecJan08.pdf
 
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Old 02-23-14, 08:00 AM
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Thanks. I had actually just read that same article (after I did a search), and I understand that's probably my best option. But I'm not sure if I want spend the extra money at this point. Also, I am trying to figure out whether there is any efficient way to continue to use my remaining 50gal gas tank in some way that could benefit the efficiency of the tankless unit that I'm installing, and to be available as a backup unit.

thanks again
 
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Old 02-23-14, 08:08 AM
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Theres a lot of variables that are not stated.

Is the tankless hooked up?
How is it vented?
How many BTU's is it?
Where did or are you going to tap into for the gas?

Whats the make and model of the unit?

You may find a tankless heater will save you 0 and actually cost more to use/own... Plus the inconvenience of h/c sandwiching l, longer wait times, higher maintenance costs...
 
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Old 02-23-14, 12:39 PM
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The tankless unit is direct vent, 11,000-199,000 BTU (Here are the manufacturer's specs for the RTGH-95DVLN: http://www.rheem.com/documents/prest...ies-spec-sheet
It is replacing the 50gal tank unit that I am replacing, so it will connect to the same gas line. My question is how to utilize my second 50gal tank unit as a backup, and in conjunction with the new tankless unit that I plan on installing. The 50gal tank unit is a Rheem PowerVent #21VP50-1

Thanks again for your help.
 
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Old 02-23-14, 01:05 PM
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A problem I see is that most tankless units require much more gas flow than a typical gas WH.
 
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Old 02-23-14, 02:24 PM
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I know my gas line is 3/4 inch, but I'm not sure what is involved in hooking up the gas line to the tankless unit, since I'm having someone do the installation for me. But I would like to figure out whether there is any efficient way to continue to use my remaining 50gal tank unit in conjunction with the new tankless unit, and if it makes sense to connect them together somehow.

thx
 
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Old 02-23-14, 03:13 PM
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but I'm not sure what is involved in hooking up the gas line to the tankless unit, since I'm having someone do the installation for me.
I assume he should know that you most likely cant hook up the 199k btu tankless to the existing gas line then?...

But I would like to figure out whether there is any efficient way to continue to use my remaining 50gal tank unit in conjunction with the new tankless unit, and if it makes sense to connect them together somehow.
The installer should be able to tell you.
 
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Old 02-23-14, 09:26 PM
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I'm not sure I understand the issue. The specs on the tankless unit say "3/4 inch gas connection" and the gas line running to the old tank says 3/4. Aren't these these same then? Is the gas connection size in the specs referring to ID or OD?
 
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Old 02-23-14, 09:59 PM
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The gas line, since its attached to the rest of the home, can probably only support that water heater.. That water heater is 40k btu.. Support meaning supply enough gas to it.

The tankless is 199k btu..

If your whole gas line from the meter to the heater was not designed for a 199k btu appliance in that spot...

Gas piping is designed by volume, not pressure.


If this is true that your gas line needs to be reconfigured, and then if you install the unit anyway, and when it fires it will cause say your furnace to starve for gas... Or other appliance...Then you have draft , carbon, and CO issues...

Thats how you wake up dead...
 
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Old 02-23-14, 10:11 PM
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A simple fix without any real hassle is to run a dedicated line from the gas meter to the unit... You need to tell me the distance and I can tell you the size pipe you need. You can run trac pipe if you like...

I'm having someone do the installation for me.
Is this guy a pro? He should have told you all this...

Oh and please for your saftey please take out a permit...


One last thing that you may not be happy with if this is true... Since you had two 50's I would take it you have a giant tub/jacuzzi to fill??? Well those faucets are in upwards of 9 GPM plus..If they were not then it would take a long time to fill..

With that said that tankless heater just is not going to have the GPM capacity to fill that tub quickly.

In fact may take an 90 minutes or so to fill.
 
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Old 02-24-14, 06:54 AM
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A simple fix without any real hassle is to
The only simple fix is to go back with another 50 gallon water heater. Even with a new dedicated 3/4" line from the meter, there is no guarantee the meter can supply the volume of gas needed and may need to be upgraded. I suppose if the installer is a pro he has already brought this issue up too.
 
 

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