Spastic Water heater

Moved in to a new, fancy building. When I take a shower, hot water comes out for 3 or 4 minutes then turns cold. Then hot again after 3 or 4 minutes. Also, the water pressure slows down when the water gets colder.

The heater fires up fine. Thermostat is set near the top.

Anyone have any ideas? I have had many of these instant heaters over the years in Korea and Malaysia but never had any issues so no clue.


Found a link to the product.

Says something about good for 20 minutes. Hmmm…

Sounds to me like the fall in water pressure could be causing the heater to switch off. Most heaters won’t kick in unless there’s a certain minimum flow rate.

How is your water supply fed into the apartment? Is there a booster pump on the roof? Could be that that’s failing. Just a thought. I suppose you’ll have to ask the maintenance drones about it, if it’s a managed building.

Something like that happened to us last fall. We had turned the water temp way down for summer so that the water was warm at best. Then in the fall, when the temp went down at night (shower time) but the infrastructure and walls stayed warm due to heat stored during the sunny day, the water would turn hot, then absolutely cold, and finally after 3-4 minutes it would gradually warm up again.

Solution was to simply turn the temp up in the fall so that warm water was closer in temp to hot water. That seemed to overcome the residual heat.

This is all a theory, though. I have no idea where the sensors are actually located. For reasons unproven, turning up the temp did the trick.

I live in a new high rise. Not sure how the water is tanked / pumped, etc. I know the tank isn’t on the roof as the roof is a party area / garden.

I tried turning up the temp - it didn’t help.

I was looking online and Rinnai said that turning the temp down may work as the heater may be getting too hot and shutting down. I just turned it down so we will se in the morning if that works.

Yes , I had a similar heater and issue. The Engineer said that turning the water temp, down actually gave hotter water !!. Turned out to be a faulty Thermostatic Valve … fixed in an hour for about 2000 ntd total by Rinnai Techs.

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Damn, it was the battery. I replaced the D battery and all is fine. There is a battery low light but it didn’t come on.


What does it power? The heater already seems to have an electric plug.

Most units here have a battery for the spark. Just like my gas stovetop. I didn’t know the stove had one until a friend of mine showed me where to change the battery.

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So I replaced the battery. Then I realized the unit only runs on battery - no AC cord attached.

Water got a bit better but half way through shower flow slows down and gets cold. I have to shut off water for a few second and then wait for new hot water to work its way through the pipes.

Sometimes it does take an age for the Hot water to arrive. Then it is scalding hot. Then it goes cold as you turn down the tap mm by mm.,the freezing again…repeat process. Seems normal. Quite a few problems seem to stem from blocked water pipes or taps .If you try getting a few Plumbers to check…you may get lucky and find one who has a clue.