Installing a second tankless as a backup plan, wondering what the most efficient way is

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drewskie

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Currently using a 15-year-old Bosch aquastar tankless that I am sure is on its last leg. I plan on replacing the current tankless soon, but I would also like to install a second tankless, which I have already purchased. It's a Rinnai value model.
I I've been reading a few forums, but I am still confused on the best way to install the second tankless. I would prefer to have it only as a backup and not have it run while the Bosch is running, so I understand that a shut off valve is the best way to do that.
What is the proper way to plumb the second unit? I will be having a handyman install it, but he is very familiar with plumbing so he knows what he's doing, but I'm just wondering how to actually plumb the unit so that it is easy to turn on if needed.

I read a comment on a separate plumbing forum that stated it's a bad idea to have the second tank sitting because it can build up bacteria, but I'm not sure if that's the case if there is not water running through it constantly.
 

Bannerman

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it can build up bacteria, but I'm not sure if that's the case if there is not water running through it constantly.
Water sitting for an extended time within the unit is exactly the problem. Stagnant water will become a breading bed for bacteria.

With regular preventative maintenance including descaling, tankless units will usually have a longer lifespan compared to a tank type WH. Unless there has been a history of issues or there are some obvious problems with the Bosch unit, it will likely continue operating for another decade.
 
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drewskie

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Thank you. So if I do install the second tankless, and don't have water running through it until I turn the valve, would it be safe to use?

As for the Bosch, it's somewhere in the neighborhood of 15 to 18 years old and I have never once done maintenance on it including descaling. I called about three or four local plumbers to see if they can "rebuild" it and every single one of them told me to replace it and don't bother trying to fix anything. It's also very difficult to find anyone that works on Bosch.
 

Fitter30

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This will not be cheap. 8 valves 4 for Bosch 2 shut off, two vent and drain one for bypass, 2 for Rinnia ,one for bypass. Don't forget flue ,fresh air, gas piping and wiring.
 

John Gayewski

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If you want to do this the proper way its to cascade two units. Their internal logic alternates them or runs them both as needed. It's also not a handyman job. Handymen can't cut into piping or change plumbing around for customers.
 

John Gayewski

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John Gayewski

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If you did two in this fashion you could actually turn one off and run one at a time, but this is how it needs to be done. The fact that your handyman doesn't know this is more evidence that a handy man shouldn't be performing this work. Nor should a homeowner. Pretty sure (almost everywhere in the us) it's illegal for a handyman without a plumbing license to do plumbing.
 

Bannerman

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Cascaded systems typically utilize identical WH units, or at least will utilize compatible, non-identical models from the same manufacturer.

Will a Bosch even communicate with a Rinnai, or vice versa?


Pretty sure (almost everywhere in the us) it's illegal for a handyman without a plumbing license to do plumbing.
While plumbing alterations is a concern, a greater concern is gas supply and venting connections by anyone that is unqualified.
 

John Gayewski

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Cascaded systems typically utilize identical WH units, or at least will utilize compatible, non-identical models from the same manufacturer.

Will a Bosch even communicate with a Rinnai, or vice versa?



While plumbing alterations is a concern, a greater concern is gas supply and venting connections by anyone that is unqualified.
No no. I'm (uneffectively) trying to say if he wants two water heaters he needs to cascade them and get the same compatible units. The plan to install one in parallel with an old unit and wait for the old one to break isn't a good plan.
 

drewskie

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So, I'm assuming it's not a very common thing to install to water heaters? It seems like many other people would want to have a backup plan in case one dies out. Couldn't I essentially just install them as two separate units with a bypass valve?
 

John Gayewski

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So, I'm assuming it's not a very common thing to install to water heaters? It seems like many other people would want to have a backup plan in case one dies out. Couldn't I essentially just install them as two separate units with a bypass valve?
The two water heaters that are meant to be with one another IS the backup plan. Cascading units can have one shut off if need be.

Adding another tankless is gonna be more work to add it seperated from the other unit rather than cascading.
 
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