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Has nothing to do with the expansion tank, the expansion tank whole enough air so that when the heater heats the water it does not build up pressure. You know from school that you can compress ait. you CAN NOT compress water. When you haet water it expands until it turns to steam. So lets rule that out.
First look at all the plumbing in the house where there is a cross conection. This would be like a shower valve that has a personal shower head on it. and you turn of the water using the personal shower handle and not the valve at the wall. This then would let cold water to go into the hot water side piping.
Most people do that with out knowing it at the laundry sink. they put a hose on to was the dog and forget to turn off the faucet and then BANG cold water in the hot side.
Go around the house and turn off all the faucets and valves by using the handles. no add on things.
Now last could be the shower valve itself. . Do this happen in just one shower or all of them???
If its just the one and you spend the day doing the walk around, even check the bidet if you have one
Tankless heater are a little frustrating sometimes but this is generally an issue of waterflow I believe. The fluctuations are do probably to the amount of water and rate of water going through the heater. If you have a shut off valve on the cold side of the heater, shut it back about half way. This will slow the water going through the heater and allow the heater to heat it up more efficiently. Then go to the shower, turn the hot on all the way (no cold) and allow it to heat up. Once it has heated up, test it by hand for a good temperature that you like. If it is too hot, turn the cold on just a bit until it comes to the temp you like. If it is too cold turn the hot down just a little (still no cold) and it will warm up. Ideally you should be able to turn the hot on full blast and then just turn on a little cold to get the right temp. Once you learn how these work then it makes it easier. You really cant run the water like you do with conventional hot water systems and expect to get the same results. Also, wanted to give you another little tip. Every 6 mos to a year, you should flush you heater with vinegar for about 30-45 minutes using a small cheap submerible pump like for a water feature in a garde. Just put in a bucket of vinegar and and run a hose to the "in" side of the heater and another from the "out' side of the heater and then back into the bucket. This will circulate the vinegar inside the heater and clean all the deposits that will DOA your heater. Funny how the manufacturer wont tell you this but these deposits will ruin the connectiosn on the heating elements. You can make your heater last for a long time if you do this.
There are several things you can do to fix the problem.
1.You can turn the temperature up on the water heater.
2. Turn the maximum temperature setting all the way up on your shower faucet.
3. Clean the Bosch tank-less unit by flushing it with vinegar.
4. Remove the flow restrictor from your shower-head.
I had the EXACT same problem (installed my tankless last winter and worked fine, now this summer it fluctuates, have to turn the shower completely off and then on again to get any hot water and then it only lasts for a couple of minutes!) Argh - it is frustrating!
Here's what I found: the demand for hot water is lower during the summer (hot) months because the "cold" water coming in is already quite warm. You don't need as much hot water to make the shower temp comfortable. SO, my tankless was shutting off due to low flow demand. I've removed the flow restrictor from my shower head and it helped alot but didn't completely solve the prob. Next step is what has been recommended to me by several people but is much more expensive - change the pressure valve on your shower to a temperature valve. Or just wait til winter and it should work ok again :)
Not sure but check the inlet water at the heater, the filter may be clog up a little that restrict the flow. Did u use the heater at all during the winter? If not, the heat exchanger may be clogged with built up in there. U need to descale it. Good luck. Chinhcu
if you have build up in the heat exchanger it will cause the unit to use more gas to heat your water, if the gas line isn't sized properly then your going to kick the error codes off. it sounds like you have two problems, first you have build up in your heat exchanger and it needs to be flushed and second you need to get a bigger gas line to it
WE THOUGHT WE NEEDED ANOTHER THERMOSTAR, BUT WHEN WE WERE LOOKING FOR THE PART THE EMPLOYEE ASKED WHAT WAS THE TROUBLE. WE EXPLAINED, HE SAID YOU DEFINATLY DO NOT HAVE A PROBLEM WITH YOUR TANK AND ASKED IF WE HAD A SINGLE HANDLED FAUCET WITHIN THE HOUSE. NO WE DONT. HE SAID IT SOUNDS LIKE WATER IS MIXING WITH THE COLD WATER THATS THE REASON FOR THE WARM--COLD--WARM WATER. COME TO FIND OUT SINCE WE ARE REMODLING THE BATHROOM, WE HAD BOTH THE HOT AND COLD WATER TURNED ON IN THE SHOWER--BUT WERE PLUGGED OFF AT THE SHOWER HEAD AND THE FAUCET UNTIL THE TILE IS INSTALLED. SOOO ALL MY HUSBAND HAD TO DO WAS TURN OFF THE WATER IN THE SHOWER. NOW WE HAVE PLENTY OF HOT WATER, NO WARM COLD. YEA!!!