Tankless water heater- no code, not firing

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BPhillips

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Hello all,
Brand new to this forum and hoping for a little guidance.
The set up:
I have two tankless Rudds 199,000 btu plumbed in parallel (both inlets to one manifold and both outlets to another manifold).
They have been working great for a few years without any issues at all. As of yesterday, one stopped igniting- the unit turns on, remote display comes on and works as it should. When water is normally turned on (via circulation pump) one comes on and one does not. No P1 error for low flow. The kicker is that if I use a transfer pump to circulate, it comes on like it should (more volume, more pressure).

I have been reading about water solenoids and turbines and while I am very mechanically inclined, have not worked on a tankless heater. I was going to OHM out the water sensor but don’t know what range I’m looking for.

Any direction or assistance would be great!
Thank you,
Brian
 

BPhillips

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Have you checked in incoming air filters? If those are blocked it tends to shut down the unit.
Not sure where they would be... since the units are exterior units, I believe the combustion air just comes in at the base and out the exhaust on the front of the unit. I can post a pic if you could point me in the right direction...
Also was thinking... I did use some shop air to gently blow from the hot side bleeder through the inlet side (was thinking something was blocking the inlet) - not sure if it would have blown out a solenoid or something. Inlet water filters are clean too.
 

BPhillips

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Reach4

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Can you valve off the heater that is firing? Maybe that unit has been providing most of the flow.
 

BPhillips

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Can you valve off the heater that is firing? Maybe that unit has been providing most of the flow.

yep, I tried that thinking maybe too much water was going through the working one and not enough through the problematic one... didn’t get a P1 error or an ignite.... I’ve been running the working one for a few days by itself with no issues but no luck on the “bad” one. It’s as if it doesn’t see that water is flowing through it at all. I could understand low flow or even another error code but it’s just asleep no matter what :)
B
 

Reach4

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It’s as if it doesn’t see that water is flowing through it at all. I could understand low flow or even another error code but it’s just asleep no matter what :)
And yet it responds to the recirc pump flow. Mysterious.
 

MKS

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If the units are identical you can check resistance on the same parts and compare. Or swap parts like ignition coils, as I have done on cars.
I assume you flush/clean them on a regular basis.
 
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