Brand new tankless still not providing hot water to tub

[DDShap]

Hi all! I'm new to the forum. I'd be grateful for any help or advice on my issue.

The wife and I bought a new house in July. Small house, only 1400 sq ft and 2 bathrooms. After a couple of months, my wife noticed that the tub in the guest bathroom was no longer producing hot water. We'd get hot water for about 15 seconds, then it would run cold. It wasn't a problem anywhere else in the house. My friend who is a plumber thought it might be because our current tankless water heater is located on the opposite side of the house from that bathroom, and that combined with the size of it and age (we weren't sure how old it was_ suggested getting a new and larger tankless water heater. We bought one yesterday (it's a Rinnai RL75e) and after he installed it, it still hasn't fixed the problem. He's baffled as to why this is happening.

The odd thing is that even if the distance was a problem, the other water sources in that bathroom are not having an issue. The sink and shower both run the hot water fine, but when having it come out the bottom into the tub, the water still does as before, going hot for 15 or so seconds, then just producing cold water. We ran some troubleshooting and noticed that when switching the water in the bathroom from the shower to the tub, the heater literally turns off after a few seconds. What could the reason possibly be for this, and how might we fix it?

 

[Jadnashua]

Some tub/shower valves can create a cross-over internal to themselves. Especially if the seals dry out. NOt all designs are susceptible to this effect. It may be that all you need is a new cartridge and then, maybe once a month or so, turn the water on/off in there to work it a bit. If it ends up drawing too much cold, the hot flow may not be enough to keep the tankless on via it's flow sensor.

Do you know what brand it is? One of the pros will probably recognize whether it is one of those that this can happen to.

 

[Sponsor]

 

[DDShap]

Some tub/shower valves can create a cross-over internal to themselves. Especially if the seals dry out. NOt all designs are susceptible to this effect. It may be that all you need is a new cartridge and then, maybe once a month or so, turn the water on/off in there to work it a bit. If it ends up drawing too much cold, the hot flow may not be enough to keep the tankless on via it's flow sensor.

Do you know what brand it is? One of the pros will probably recognize whether it is one of those that this can happen to.

It's a Rinnai RL75e. If what you're referring to is a mixing valve, it was actually the first thing my plumber did to try to fix the problem was to replace mine with a new one. It was only until after that didn't work that he suggested getting a new tankless water heater. I do not yet have a circulation pump. Could that possibly fix the problem?

 

[Reach4]

If what you're referring to is a mixing valve, it was actually the first thing my plumber did to try to fix the problem was to replace mine with a new one.

Is that mixing valve only serving that hot tub? Feel the output of the mixing valve once the tub stops getting the expected hot water. Is that hot? Feel the hot input to the mixing valve. Is that hot?

 

[Jadnashua]

A cross-over can occur in some vanity faucets, too...so, check that out.

An easy thing to try is to shut the hot or cold off to the vanity at the wall and see if the tub then functions normally. Shutting one of those valves will stop a cross-over, if it's there. A new tub/shower valve should not be defective, but anything's possible.

By either listening or viewing the status indicators, you can usually tell if the tankless system is actually running. Have someone turn the tub valve on while watching the tankless and note what it's actually doing.