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Marc, Plumber
Category: Plumbing
Satisfied Customers: 1126
Experience:  Plumbing heating & home maintenance contractor
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Just trying to figure out why my Bosch Tankless heater cuts

Customer Question

Just trying to figure out why my Bosch Tankless heater cuts out only when we are in the cooler months down here in Fl
works fine all year then when the air temp drops below 70 degrees the heater cycles on and off
Submitted: 1 year ago.
Category: Plumbing
Expert:  Phil replied 1 year ago.


Turn the shower on as hot as it goes, and one other hot tap, and tell me if it still cycles on and off when it is below 70F outside... this is step number one.

We can go from there.

Customer: replied 1 year ago.
it does as a matter of fact if you turn the temp control down you can get it to stop cycling but then it is warm water coming out . Plus if you open two outlets it will shut the heater off
Expert:  Phil replied 1 year ago.

Thanks, ***** ***** not at all a common failure mode.

Please attach a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs to be taken from straight on, in very good light.

You can use the paper clip- icon at the right end of the tool bar at the top of this dialog box to attach it here.

If you have an error code showing, tell me what the code read out is on the wiring diagram or a sticker next to the diagram please.

If there is no paper clip-icon, look for the '+attach files'link at left of your send button.

I can mark it up with trouble shooting instructions.

You may need a multi meter ($30 at a hardware store) and some experience working with appliances, let me know a lot about your skill sets in those regards.

We can go from there.

Customer: replied 1 year ago.
No wiring diagram & there is no place to get codes. I have a digital multi meter
Expert:  Phil replied 1 year ago.

Hello again, I am at a complete loss on this, I suspect no one else here will have seen such a problem either. None the less I will opt out in case one of our other experts wants to respond, meantime I will sleep on it. and pick your question back up in the morning if anything comes to mind.

Customer: replied 1 year ago.
Ok thanks let me know
Expert:  Phil replied 1 year ago.

Hello again, the only thing I can think of so far, is that for some reason the city water pressure might be dropping in the cooler months... that would affect the flow rate through water heater and its flow switch control.

Can you comment on that?

Customer: replied 1 year ago.
It averages about 48 psi give or take throughout the day. I have noticed the heater cut out is more prevalent in the lard evening & early mornings. When the ambient temperature is lower. Usually when it is lower then 70 degrees outside it cuts out
Customer: replied 1 year ago.
Expert:  Phil replied 1 year ago.

Hello again, the web is giving the world problems today and it is affecting both our servers and my ability to pull up manuals from the web... please accept my sincerest apologies.

Expert:  Phil replied 1 year ago.

48 psig is more than sufficient. Let me know if drops into the 30 to 39 psig range in those late evening and early morning hours.

We can go from there.

Customer: replied 1 year ago.
Between 45 & 48 psi everytime I checked. Which was about 6 times plus this morning at 47 psi
Expert:  Phil replied 1 year ago.


I have never seen this to be the case with your type of water heater, but it is possible that it is set up by accident or on purpose with an outside air temperature sensor that shuts it down when it is below 30F outside. That is a real long shot guess, but all I can think of right now.

Try adjusting the 'potentiometer temperature adjustment' shown on page 9... I do not expect that should resolve the problem but lets see what happens, be cautious of scalding water after you make the adjustment.

Customer: replied 1 year ago.
Did that quite often. As mentioned above... When you adjust the temperature down to a warmer setting is stops the heater from cycling off and on but then on leaves a warm to cold water at the tap
Expert:  Phil replied 1 year ago.

Thanks, ***** ***** not see an outdoor master temperature sensor on the units wiring diagram. and there should be no reason for one either... this leaves me at a complete loss for an answer.

It makes no sense at all that your water heater should cease to operate below 30F.

however if the water line outside is exposed or buried less than a foot deep or so,. and a long run, the water could cool to 32F, and electric heating elements could lack the capacity to warm the water up more than say 40F at high enough flow rates, that would leave you with 72F water on a cold day.

Tell me if that possibility makes any sense on your end or not.

we can go from there.

Customer: replied 1 year ago.
If we were farther north maybe but here in Orlando Florida it very rare that we get below 35-40 degrees & this happens when we are at or below 68 to 70 degrees for a six or more hour time. The line from the meter to the house is 3 ft deep min. I replaced it shortly after I bought the house 10 years ago. All the plumbing has been done & this issue started beginning of January last year and has happened 4 or 5 times so far this year. It hasn't got that cold yet... So far
Expert:  Phil replied 1 year ago.

Thanks, so the problem occurs in the 68 to 70F range as well? that points to more of a coincidence sort of thing and failure of the electronics, bad printed circuit card possibly... that shrinks a few thousands of an inch when it cools and opens a crack (flaw) in one of the printed circuits.

You could test that with a piece dry ice in the cabinet to cool that space to 35 or 40F then notice what happens.

This is a common situation with furnaces, but happens when the printed circuit cards warm up and cracks open or close causing the card to change its behavior.

Do that cool test and let me know how it goes. if you can access the compartment with the printed circuit card in it, a much smaller piece of dry ice would work. One of those 'blue blocks' of glycol water that you can freeze in your freezer would also work.

Customer: replied 1 year ago.
Actually the problem is temp related at or around 70 degrees as I stated in my very first post to you. Let me see what will happen.
Expert:  Phil replied 1 year ago.

Sorry, somehow I got 30F on my mind, I deal with so many different questions, sometimes the data gets crossed up.