thermostatic mixing valve TMV not working with new tankless combi boiler.

geofear · April 20

I have recently installed a new Navien tankless boiler, which is working fine.

I have in a back bathroom a TMV mixing valve which does not appear to work with the new boiler. I have just replaced the valve to no avail. It makes no difference how I adjust the valve, max, min, or anywhere in between, I get no hot water, AND the tankless shows no DHW flow thru the status screens. (only this faucet, which is after the mixing valve, behaves in this way). The cold and hot lines are not reversed.

It appears to be a flow issue thru the valve ???

Thank you for any thoughts you might have.

Big Ed_4 · April 20

It seems to be a bypass .. cold water bypassing the boiler .. Bad facet or shower body ,any connection upstream from the boiler will cause that issue.. Shut off the cold in that section and leave open the hot .See what happens , if it gets hot …bypass..

tcassano87 · April 21

what kind of mixing valve is it and how many GPM does it allow through? What does the flow look like its possible that it might be so minimal that it’s not picking it up. Especially when mixed with the cold water
have you tried removing the mixing valve and running the faucet without it?

pedmec · April 21

when you open the faucet the mixing valve is seving check how many gpms are flowing thru the water heater. There should be a menu parameter on the screen of the water heater to show you the GPM's. which model navien do you have?

pedmec · April 21

Navien needs a minimum 0.5 gpm flow to trigger ignition. it can be adjust a little lower

geofear · April 21

https://forum.heatinghelp.com/discussion/comment/1894762#Comment_1894762

Thank you for your comments. HOW can you lower it?

https://forum.heatinghelp.com/discussion/comment/1894761#Comment_1894761

It's a Navien ncb 240/110 h. It shows 0 gpm on the DMH status screen.

https://forum.heatinghelp.com/discussion/comment/1894750#Comment_1894750

I like your theory of a bypass, upstream. God knows how to find it.

Thanks again.

hot_rod · April 21

mo water comes out at all? Or it never gets warm ? Is the valve located at the tankless? What is the water pressure?

The tankless gas a minimum flow to fire, and also the mix valve has a minimum flow, which varies from brand to brand.

Generally .5 gpm on mix valves at the heater. Smaller point of use valves at the fixture will have lower flow rate minimums

Flow below .5 gpm will not cause the flow to stop, but temperature control can vary a bit at very low flow.

Run the water and see if the tankless is firing. If not you may need to remove the flow restrictor at the fixture Lav faucets and shower heads have restrictors