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Old 10-31-2021, 09:33 AM   #1
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Plumbing with only hot water.

Hello everyone,

I am currently in the process of installing the freshwater system in my build, and i have an idea that i'm going to implement and wanted some helpful criticism from you guys. My plan is to run only one supply line to all the fixtures connected to the outlet of the hot water heater (Girard rv tankless). The Girard has temperature control from a remote display, so the desired temperature would be obtained by setting the temperature with the remote display, no mixing required. This would eliminate the severe intermittent temperature fluctuations i've been reading about, by having full flow (or close to it) through the water heater. I would also have a recirculation line connected to the end of the furthest fixture (kitchen sink) with a solenoid valve to the freshwater tank, so hot water would be instantly available at every fixture with a push of a button, thus saving water. If cold water is desired, one would simply have to turn off the water heater at the remote display. I have everything set up right now so that if this plan goes horribly wrong, only a few changes will have to be made to turn the recirc line into a cold water supply line.

The specs of the freshwater system are:
100 gallon freshwater tank under the bed
1/2 PEX
Flojet 2.9 gpm, 50 psi
Accumulator on the discharge of the pump
Girard 2GWHAM tankless water heater

Does anyone see any flaws with my plan? Or have any ideas?

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Old 10-31-2021, 10:31 AM   #2
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It seems like a good plan to keep the flow rate high enough for the water heater to function. If the remote can be place conveniently near the shower and kitchen sink it would be a bonus.

Ted
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Old 10-31-2021, 11:09 AM   #3
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Your plan will work however it has the opposite problem of most systems-- that of having to wait for hot water to arrive at the spigot. With your plan you will have to wait for cold water while it displaces the hot water in the pipe.

Since you can still add a cold water line easily I'd recommend you do so along with adding a dedicated cold water pump and accumulator. It would be a cheap and simple upgrade and will eliminate the tepid water wait for both hot and cold water.
Jack
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Old 10-31-2021, 01:00 PM   #4
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I spent the week

Using a shower similar. Tank, pump, heater, shower. Only about 3 feet between shower head and heater. Took about 20 seconds to get warm water. Propane and this particular unit intended for outdoor use.

William
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Old 11-02-2021, 07:14 AM   #5
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My take on tempered water recirculation

Neat idea. I've got a similar idea that I'm fleshing out. Please excuse the poor line layout. I'm working on a better P&ID, but it's not done yet.

So in the picture, we've got fresh water, going through a check, a strainer, pump, accumulation tank and then to the water heater. Here's where I've added some things. At the water heater, I plan to use a thermostatic mixing valve to temper the hot water to a non-scalding but hot shower temp (105 F). I need to do this because of the way I'm heating my hot water has the capability to produce extremely hot temperatures (200 F+). I plan to run the tempered water to a pull chain valve in my shower to conserve water. But I was thinking I don't want to get blasted with the cold water sitting in the line each time I take a shower, so I'm thinking of adding a rollback line with a quarter turn ball valve and follow it with a heat actuated trap valve. This line would run from right before the pull chain valve, back to before the strainer.
The thought is that when I want to take a shower, I open that rollback valve, the pressure drop kicks on the pump which then circulates the water in the line and brings the shower up to the temp I've set without wasting any water. I can enjoy a cup of coffee, eat a burrito, etc... Once the shower is at temperature, the trap valve closes. I close the rollback, take my shower, and the system resets.
What do you think?
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Old 11-02-2021, 11:06 AM   #6
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You will need a pressure relief valve in case your 200 degrees jumps to 212 or more. What are your energy calcs for the 12v water heater? What material will you use for your lines?
Jack
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Old 11-02-2021, 11:59 AM   #7
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I am just guessing that 200+ is possible while driving, as it'll be tied to the coolant system. I'll probably pipe the relief valve from the water heater to the fresh water tank and I believe those are set at 210.

My plan is to use a 185 F switch on the DC side which will be a 600w coil run as a solar diversion load, and the Webasto also only heats to 176 F, so realistically that's the high side.

I had planned on pex with some copper for aesthetics by the shower. But that's on the tempered side. Do you have a suggestion?
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