Intermittent hot water on tankless heater

Plumbing Forums

Help Support Plumbing Forums:

bw nc

Member
Joined
Aug 31, 2021
Messages
10
Reaction score
2
Location
NC
We just moved into a house and there is a tankless hot water heater. We are experiencing the hot water disappearing intermittently in the Master Bath shower. The water turns cold for a minute or so and then returns. It only happens at this location. For what it is worth, I checked the shower cartridge and it seems fine.
 

bw nc

Member
Joined
Aug 31, 2021
Messages
10
Reaction score
2
Location
NC
We just moved into a house and there is a tankless hot water heater. We are experiencing the hot water disappearing intermittently in the Master Bath shower. The water turns cold for a minute or so and then returns. It only happens at this location. For what it is worth, I checked the shower cartridge and it seems fine.
FYI Rinnai R75LSi
 

bw nc

Member
Joined
Aug 31, 2021
Messages
10
Reaction score
2
Location
NC
Probably a bad cartridge or balancing spool
Thanks. I had a spare cartridge that I tried and it still went cold. When I look at the unit while the shower is running, I actually see the "in Use" light turn off. Whereas it stays on and functions normally when the other showers are in use.
 

Twowaxhack

Professional
Professional
Joined
Feb 9, 2021
Messages
9,225
Reaction score
4,053
Location
United states
Thanks. I had a spare cartridge that I tried and it still went cold. When I look at the unit while the shower is running, I actually see the "in Use" light turn off. Whereas it stays on and functions normally when the other showers are in use.

i have no idea, goodluck 👍
 

RS

Well-Known Member
Joined
Dec 25, 2019
Messages
460
Reaction score
161
Location
USA
Possibly that particular shower head has such low flow that it's below the minimum for the heater to keep heating? If someone would use hot water at another location that's on the same water heater it might increase the flow enough so the heater stays on. I have some understanding of how tankless water heaters work, and I don't think I ever want one!
 

Twowaxhack

Professional
Professional
Joined
Feb 9, 2021
Messages
9,225
Reaction score
4,053
Location
United states
Minimum activation flow rate .4 GPM

Minimum flow rate .26 gpm

So once it fires off at .4 gpm it can flow a minimum of .26 gpm. That’s a trickle.
 

bw nc

Member
Joined
Aug 31, 2021
Messages
10
Reaction score
2
Location
NC
Possibly that particular shower head has such low flow that it's below the minimum for the heater to keep heating? If someone would use hot water at another location that's on the same water heater it might increase the flow enough so the heater stays on. I have some understanding of how tankless water heaters work, and I don't think I ever want one!
thanks for the inf
 

bw nc

Member
Joined
Aug 31, 2021
Messages
10
Reaction score
2
Location
NC
interesting twist related to flow and pressure. When I turned on the hot water at the sink in the same bathroom and left it running during my shower, the water stayed warm in the shower...
 

Twowaxhack

Professional
Professional
Joined
Feb 9, 2021
Messages
9,225
Reaction score
4,053
Location
United states
interesting twist related to flow and pressure. When I turned on the hot water at the sink in the same bathroom and left it running during my shower, the water stayed warm in the shower...

Turn your water heater temp down some.

What temp is your water ?

Still doesn’t make much sense. .25 a minute minimum flow rate is not much water.......

Brainboard might be bad on that a rinnai. That or you’re not using much hot water when you shower.

No trouble codes ever thrown by the heater ?
 

bw nc

Member
Joined
Aug 31, 2021
Messages
10
Reaction score
2
Location
NC
Turn your water heater temp down some.

What temp is your water ?

Still doesn’t make much sense. .25 a minute minimum flow rate is not much water.......

Brainboard might be bad on that a rinnai. That or you’re not using much hot water when you shower.

No trouble codes ever thrown by the heater ?
temp is set at 120, did not see any error codes
 

Twowaxhack

Professional
Professional
Joined
Feb 9, 2021
Messages
9,225
Reaction score
4,053
Location
United states
temp is set at 120, did not see any error codes

So it doesn’t cut off if you use a lot of hot water, shower and lavatory running.

If nothings wrong with the master shower faucet then I would say the brainboard of the Rinnai may be defective.

I say that because I’d should only take .25gpm per minute to keep the Rinnai firing.

And it’s not throwing a code......

I suppose the flow sensor could be defective but usually it’ll throw a trouble code.

Turn off the cold water to the tankless water heater. Go to the lavatory faucet and open the hot water side only. Verify the hot water flow stops completely. If it doesn’t, that’s a problem.
 

bw nc

Member
Joined
Aug 31, 2021
Messages
10
Reaction score
2
Location
NC
So it doesn’t cut off if you use a lot of hot water, shower and lavatory running.

If nothings wrong with the master shower faucet then I would say the brainboard of the Rinnai may be defective.

I say that because I’d should only take .25gpm per minute to keep the Rinnai firing.

And it’s not throwing a code......

I suppose the flow sensor could be defective but usually it’ll throw a trouble code.

Turn off the cold water to the tankless water heater. Go to the lavatory faucet and open the hot water side only. Verify the hot water flow stops completely. If it doesn’t, that’s a problem.
I'll give it a try later on and let you know. Thanks for the info. Quite a little mystery, it's interesting playing detective, but my wife's not too thrilled when she has shampoo in her hair and it turns cold. :)
 

Twowaxhack

Professional
Professional
Joined
Feb 9, 2021
Messages
9,225
Reaction score
4,053
Location
United states
When it turns cold is there anything you can do to make it come back on.

Like stepping to the side and turning on all hot ?
Turning all the water off in the shower snd then back on ? Try that too
 

Surfer0063

Member
Joined
Sep 10, 2019
Messages
6
Reaction score
3
Location
Florida
Could be a cross-connection issue.

Do you have a second thermostatic valve in the shower?

I've had issue where the hot water flows through the valve fine but after a while, water starts to flow through another thermostatic valve instead and in turn starts pulling water from the cold side into the hot side system not allowing the required amount of flow through the tankless to operate the flow switch. Check the check valves in the thermostatic valve and replace.

If there's a recirculation pump, the water may be backflowing through the line creating a similar scenario above.
 

Twowaxhack

Professional
Professional
Joined
Feb 9, 2021
Messages
9,225
Reaction score
4,053
Location
United states
Could be a cross-connection issue.

Do you have a second thermostatic valve in the shower?

I've had issue where the hot water flows through the valve fine but after a while, water starts to flow through another thermostatic valve instead and in turn starts pulling water from the cold side into the hot side system not allowing the required amount of flow through the tankless to operate the flow switch. Check the check valves in the thermostatic valve and replace.

If there's a recirculation pump, the water may be backflowing through the line creating a similar scenario above.

I agree, I told him how to check for a cross connection. He claims nothings wrong with the shower valve.

The cold flow should also be reduced on a pressure balanced faucet if the hot flow is reduced enough to keep the Rinnai from firing. .25 GPM minimum once it’s up and running.

He claims to have replaced the cartridge. No idea what brand or type.

OP, please post a pic of your shower valve.
 

bw nc

Member
Joined
Aug 31, 2021
Messages
10
Reaction score
2
Location
NC
Had to go out of town. I will take a picture of the cartridge later this week. Thanks.
 

Latest posts

Top