Losing hot water

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Ole Dodge

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I have a Naiven tankless water heater that does a fantastic job! However, if you are in the shower and another hot water faucet is turned on, the water goes cold. We have 70+ psi pressure and 3/4” PVC and cove piping. Any thoughts?
 

Diehard

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Does it go cold at both the shower and other hot water faucet?
Which model/size unit is it?
I assume the shower flow would be 1.6 gpm and the faucet at 2 gpm max.
 

Ole Dodge

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Thank you Diehard! It is Naiven model NPE-240A NG. It has a built in recirculating pump. It is 199,900 BTU/Hr. It flows 8.7 gpm at 45F temperature rise. To answer your question, you can turn on both and they will be hot, but if you are in the shower and turn the other one on the temperature drops quickly (and uncomfortably). I have considered installing a temperature sensing cartridge in the shower.
 

Diehard

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Even at 67°F(36°C) Temp Rise, that unit has a rating of 5.6 GPM. So it would appear that the units size should not be the problem.

Although the shower cartridge could very well be the source of the problem, I would be inclined to try a combination of other fixtures running together, without the shower running, before replacing the shower cartridge.
 

Jeff Handy

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If the shower cartridge is anti scald pressure balanced type, it has probably gone bad.
 

breplum

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There is a check valve built in to the NPE240 series, inside the unit near the bottom.
I have solved the exact problem you are experiencing countless times by having Navien send out a free replacement check valve (if you are in warranty, otherwise just buy one from a supplier). It is extremely easy to replace.
There is a o-ring on those check valves that with just a little age, can cause it to hang up and cause the symptoms.
 

Diehard

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Which line was the check protecting from back flow?

Do you happen to know how it caused the water to turn cold only when the 2 fixtures were in use together? Would be interesting knowing what was going on.
 
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