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Old 12-31-2020, 10:13 AM   #1
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Which pump for tankless heater?

So we pulled the trigger on a Precision Temp RV-550 NSP EC tankless water heater. Now we need to figure out how to feed it.

I understand that fluctuations in flow may cause issues. We're considering either a variable speed pump, or a standard pump + accumulator (I guess option 3 could be a variable speed pump + accumulator, though this seems redundant). Does anyone have any advice that may help us decide. One factor that has us leaning towards the variable is noise, as the pump will likely be mounted under our bed.

FYI while I had initially planned on creative plumbing to make navy showers possible, after examining our water use & limiting our output volume, I don't think they'll be necessary. So we won't be trickling water or doing the navy thing.

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Old 12-31-2020, 11:25 AM   #2
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Looks like a good heater--especially the bottom exhaust. However, contrary to their instructions, I'd still recommend a recirculating circuit on the system to save water. The real reason they want you to leave the water running during your entire shower is that the heater shuts down when the water ceases to flow resulting in a blast of cold water when the tap is again turned on. This of course makes their heater "seem" less than perfect. Using the recirculation shunt will allow your heater to stay on without running unnecessary amounts of water down the drain.

To get an idea of how much water a normal shower uses, time the next shower you take then let the shower run into a 5 gal bucket (or two) for the same amount of time. That will give you an idea how large your water tank will need to be and may convince you that navy showers aren't so bad.

As to the accumulator, its real purpose is to keep the pump from turning on and off like a machine gun when the water flow is slow. It doesn't reduce the total run time of the pump but rather smooths out its cycles. Originally I didn't have an accumulator but once I installed one I can't imagine not having one.
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Old 12-31-2020, 11:46 AM   #3
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All understood. Many thanks!
Going to heed this advice & implement a re-circulation system after all.
This is another one of those things that I don't want to have to redo if it doesn't work out. Switching pumps is easy, but re-plumbing later would not be.


Regarding the accumulator, would you consider it + a standard pump to be a livable solution in a small bus? Or is a VS worth the squeeze? I keep reading of people complaining about noise. Assuming you soft mount the thing, is the noise really that bad through what - in our case - would be at least a couple thicknesses of 3/4" ply + a mattress? If one of us are using water, it's likely both of us is awake. Is it really so annoying?
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Old 12-31-2020, 12:13 PM   #4
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I have no experience with VS pumps so I looked into them. It seems that their advantage is that with 5 chambers and a flow related motor speed that they wouldn't need an accumulator. That said, any running pump motor is still going to be noisy. I really don't like pump noise but I think the old school pump and accumulator, once soft mounted and enclosed in a sound attenuating case remains tolerable and most cost effective.
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Old 12-31-2020, 12:25 PM   #5
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My buddy just got a tankless heater at his cabin. The problem I see with it is having enough flow to kick it on, so low flow devices like the bidet sprayer I recommended might not have enough flow to make it turn on.

I’m curious how people have implemented a recirculating circuit
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Old 12-31-2020, 12:38 PM   #6
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I would envision a diverter valve on the hot water outlet side of the heater back to the fresh water tank
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Old 12-31-2020, 12:52 PM   #7
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Quote:
Originally Posted by Danjo View Post
I’m curious how people have implemented a recirculating circuit

I'm researching Jack's solution now so I can shamelessly rip it off. Shhhh. Don't want to let him know!


https://www.skoolie.net/forums/f50/r...ter-31472.html
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Old 12-31-2020, 12:58 PM   #8
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Rwnielsen------Exactly!
Jack

The small white box with the red and yellow wires is the solenoid valve.

.


I put a stainless steel bead chain in the path of the water where it entered the top of the tank--it stops the tinkle-torium sound of the returning water.

.
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Old 12-31-2020, 01:49 PM   #9
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Awesome. Thanks for this. And love the chain! Maybe you were the wrong person to ask about noise. I just need to tune my tinnitus to the proper frequency to block that stuff out

Something I don't understand from the literature related to the issue at hand:

"Shutting off the shower with the shower head button wastes water. Each time this is done, the shower head “trickles”, filling the hot water line with cold water. This cold water has to be purged from the line each time the shower head is turned back on. Tests have shown that this will not save water and sometimes uses more water than leaving the shower run continuously".

Obviously this is one scenario where a recirculation circuit would solve the problem. What I don't understand is why this is a problem in the first place. Why is it assumed shutting off the water at the outlet will result in a trickle? Why doesn't it just stop immediately? Are they assuming you're using leaky fixtures? I've got to be missing something obvious here.


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Old 12-31-2020, 07:58 PM   #10
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I installed 2 tankless water heaters in our conversion, one for the kitchen sink and another for the bathroom, that also serves as my darkroom. This was to keep all lines very short and inside/warm. On the kitchen sink I just connected it up nothing special. Works good for washing dishes. That did not work for the shower. Hot-cold, Hot-cold, Hot-cold, Hot-cold hell no! This is how I fixed it. I installed a 2.5 gal under sink electric tank type water heater plumbed after the tankless unit and have a hand valve that bypasses to the water holding tank. I use the pump and bypass valve to load the 2.5 gal tank with hot water. From there the hot water goes to a water tempering valve hot side that I removed from a 1 hour photo lab as they decommissioned it. The tempering valve goes to the hot water valve in the shower. My wife likes her shower at 106 and I like mine at 98-100. Once set it is very stable. These valves were supposed to hold water @ 100 plus or minus ½ degree for color film. The only valve you turn on is the hot. The cold is just used to rinse the shower. If connected to mains power I can run the electric with the propane for back up and take long showers. It is on a separate on off switch. My wife thinks it is to complicated and makes me set it up. Takes 15-30 seconds, but cold water never hits you. In the future I intend to re-plumb it so I can have it either before or after the propane heater. These instant water heaters are very sensitive to incoming water temperature.
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Old 12-31-2020, 07:59 PM   #11
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I should add insulate the lines.
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Old 12-31-2020, 08:19 PM   #12
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That is a great improvement to the simple recirculating system!

Bad person! Now I'll be champing at the bit to do the same.

Jack
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Old 01-02-2021, 12:51 PM   #13
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That's VERY cool, S2mikon! Great idea.
I'm going to have to mull this over a bit.
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