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Unread 02-20-2020, 04:40 PM   #16
jadnashua
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Depending on the brand and model of tankless system you have, some of them are designed so that they can be ganged to gain higher throughput. That should eliminate your flow restriction. The controllers are interconnected so they know what is going on and can adjust. It will start out with one, and add more units on as needed, up to the max available, if it's not sufficient.

I'd use 1" pex or cpvc up to the shower as a minimum. That would be equivalent to about a 3/4" copper supply line. The advantage of pex in this situation is that you wouldn't need any fittings, just enough clearance to fish the thing through the wall. Both copper and cpvc would require fittings, and each of those creates friction, reducing the dynamic pressure and decreasing the available volume. Dynamic pressure is while using water and that will vary. Static pressure is without any flow and would be the same if the line was 1/4" or 3", but the available volume goes up as the diameter does...remember there's the radius squared factor in the area calculation, so a little bit results in a non-linear change.
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Unread 02-20-2020, 04:55 PM   #17
speed51133
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Thanks, yes, my tankless unit does allow for it to be gang banged in series with a common controller. The control board and cable is only 100 bucks. It would just cost more than adding the electric unit. I am not sure my gas service can handle another unit of the same kind (Rinnai RU98i). These things are gas hogs. I also have a gas furnace, fireplace and gas cooking. I remember this being kind of an issue when the house was built and required a larger service line that the utility company wanted to run. Also, I am not sure if they can share vents, I doubt it, but I am running out of venting room The combination of the furnace, tankless water heater, and radiant heat boiler all have huge intake and exhaust lines clogging up the area.

You know, I bet i could just cap the CPVC feeds in the shower and leave it all be. This would maintain the integrity of the system for all the other fixtures on the floor and below. I could then run a dedicated pex run of hot and cold (maybe even just hot, as that is what is mostly used). With long drill bits I may be able to do it without opening up walls.
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Unread 02-20-2020, 08:34 PM   #18
makethatkerdistick
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Not sure if it's relevant for your situation but most piping also has a derating factor when dealing with hot water. Let's say you have a certain flow speed of x feet per second inside a certain tubing, then that should be reduced considerably when hot water is piped through it. The hotter, the slower the maximum fps should be. For copper one can look these recommendations up at the Copper Development Association's website. I know that generally PEX tolerates higher fps than copper but don't know about how the derating is calculated. This is to prevent premature wear of the tubing. It's a bit akin to the derating of electrical circuits.
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Unread 02-21-2020, 09:33 AM   #19
speed51133
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Thanks, I am confirming with Rinnai that adding a second heater will attain my goal. I presume I can just double the output, but that might not be the case.

Turns out I have a delta temperature of 60 to 65F. I also want 10GPM. Looking at the tables, each heater can do 6.5 to 7GPM at that temperature delta.

Then I need to confirm with the utility company that my gas service can support the load.

Then I need to figure out with Rinnai if I can combine the two heaters with one intake/exhasut run. The literature shows I can, but I am not sure if I can just tap into the existing run....

The heater sells for $1600 new but I found an open box buy on ebay for $1100. I would probably install it myself.
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Unread 02-21-2020, 11:55 AM   #20
jadnashua
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FWIW, for copper, the maximum velocity on hot is 5fps, and 8fps on cold. Exceed that and you can get erosion on the pipes, excessive dynamic pressure loss, and water flow noises. Pex is higher, don't remember the specific numbers, but the ID is smaller, so you end up with about the same volume IF you haven't run it like copper and have lots of fittings along the pathway.

Plumbing code doesn't like dead end water supply runs. So, while you could leave the existing stuff there in the walls, you should find where it branches off and cap it there versus at the final end in the shower.

IF your tankless system can be augmented with a second (or more!) units, you need to follow the manufacturer's instructions very carefully.

THere should be a tag or marking on your gas meter that indicates the maximum it can handle. If yours needs more, you could get out easy if the pipe sizes are okay with just a meter change. If you have to increase the whole supply run, it can get a lot more expensive. Some places might offer the option of a higher pressure output from the meter, but then, you'd need auxiliary pressure reduction valves at all of your gas fired devices or at least on their respective branches.
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Unread 02-21-2020, 12:54 PM   #21
speed51133
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thanks, tip noted. I was planning on verifying with the utility provider.

I would love to avoid a dead ended run. I need to see if the plumber remembers how he ran the lines....

Annoys me that I thought about all this when the house was designed and built, but I just did not insist enough. I was assured what I had spec'd out was already overkill for what I wanted.....I just did not know enough.

Had similar discussions with the electrician regarding amperage on circuits and actually made him rip out installed wiring as I had already told him what I wanted....
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