Tankless water heater not working after mixing valve install

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Clifford82

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Interesting situation, not sure if it's coincidence or a fault of the setup, but one of my tankless water heaters quit working after I installed a mixing valve downstream.

THE SETUP: I have two Rheem 15a621 in series (yes, I know these are too small, it's a 1966 cabin and they were there when I bought it). They were both working fine, and then my inspector wanted me to install a mixing valve when I remodeled the bathroom in order to bring everything up to code. A few days after the valve install the second water heater stopped working. I installed this valve (https://www.cashacme.com/product/hg110-d/). So the setup goes:

COLD IN ----- RHEEM1 ------- RHEEM2 -------- VALVE -------MIXED OUT

THE SYMPTOMS: The second water heater has power, but it will usually not switch out of standby even when there's flow. If it happens to switch on it will then usually not switch off when the water turns off. So it seems to have trouble detecting changes in flow (resistant to turning on when there's flow, resistant to turn off when there's not). If it's stuck in the on position it will stay that way until it overheats and trips the circuit.

QUESTION #1: Does the presence of the mixing valve somehow cause the second water heater to misbehave, or is this coincidental? The valve seems to be working fine and there's normal water pressure coming out of it, so I don't think there's a backflow issue but who knows.

QUESTION#2: Any idea how to diagnose a flow sensor issue on this model (assuming that's what this is)? All the troubleshooting info I can find is for newer models with digital displays, this just has a knob and LEDs.

QUESTION#3: Assuming I can get the second water heater working again, would it make more sense to redo this circuit to have these two heaters in parallel instead of in series? I've been just using the one heater and it's providing sufficient temperature increase, so in theory the two in parallel should increase flow at that temperature increase, right?

Any help is appreciated, thanks!
 

Bannerman

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What is the temp of the water entering the mixing valve? A tankless requires sufficient GPM flow to activate the burner or heating element. If the water from the 1st WH is sufficient enough so the mixing valve is utilizing a low amount of hot water to achieve the desired water temp after the mixing valve, the resulting hot water flow rate maybe below that needed to activate the 2nd WH, particularly if the flow switch in the 2nd is less sensitive than the 1st.

Tankless units are commonly plumbed in parallel so as to evenly divide the flow rate through each. With 2 identical units, each will equally be responsible for 50% of the flow to fixtures (33% for 3 units, 25% for 4 etc.). This will then increase the total flow rate for the temperature rise required.

With gas tankless units plumbed in parallel, two or more will be usually interconnected with a communication cable so one will act as the Master to control the burner modulation and heat output for all units.
 

Clifford82

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Thanks for the replies; I haven't descaled these, so that's a good idea. From what I can read from reviews Rheem gets a bad rap because they don't release any service manuals or anything, so that's a no-go on wiring diagrams. I'll try cutting power to the first heater and see if the 2nd one behaves, if so maybe it's due to the low flow issue from the first heater being sufficient for the valve.
 
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