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Rick, Construction Supervisor
Category: Plumbing
Satisfied Customers: 21765
Experience:  Licensed Master plumber with 40+ yrs. experience.
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My Bosch 250SX LP supplies only one bathroom with 2 sinks

Customer Question

My Bosch 250SX LP supplies only one bathroom with 2 sinks and one shower with single handle control and anti-scald feature. Only one water source is ever used at one time, i.e. one shower or one sink only. If my single handled shower control is on all
the way to hot it produces hot water too hot to shower. If I adjust it to a point where the water is comfortable the demand water heater cycles on and off continuously causing water too cold than too hot for comfort. If I turn a sink hot water faucet on than
I can adjust the shower for the right temperature but I must keep the sink faucet running. I had a whole house water softener installed after flushing out the demand water heater. It is very unlikely the supply pipes are clogged partially or otherwise. I am
not a plumber but assume that a temperature sensor is defective or calcium deposits on some internal control or sensor may be causing the problem. Do you have a solution? Keep in mind that I can purchase relatively inexpensively a small demand water heater
since only one faucet or shower will be operating any one time.
Submitted: 2 years ago.
Category: Plumbing
Expert:  Rick replied 2 years ago.
It sounds like you don't have enough flow to keep the unit firing when you mix the water at the shower. The first thing to try is to clean the intake screen on the cold water inlet on the heater.
Customer: replied 2 years ago.
When your first reply came to me it was partially covered by a request from Marc which blocked me from responding to your answer. Marc's page looked like I was to respond to him since I could not respond to you. So I did. After responding to him and entering my payment information again your reply popped up and I am now replying to that.
Information I'm giving you I gave to Marc already but I will repeat it. I put a new filter on the cold water supply. I also purged the water heater as directed by Bosch procedure. I also installed a whole house water softener. I am not a plumber abut it seems to me that there may be a sensor, probably a temperature sensor of some sort that is defective. Perhaps a mineral buildup around an internal sensor which was not cleaned enough by my cleaning procedure. Some minerals may require actural scrubbing for example. I have seen the same symptoms on some other repair sites but never responded to those sites because none of the sites seemed to be able to solve the problem or even know what the problem was. It was a series of try this; try this than; etc. etc.
Expert:  Rick replied 2 years ago.
It sounds like you're describing a water flow issue like I outlined in my original answer. The inlet filter is only one possibility. The only way this is temp sensor related is if the temp readout on the unit doesn't conform to the actual water temp. Like I said in my original answer lowering the water temp is the first thing to try for this issue. If the temp is already below 120 and you still have the same problem then the temp sensor is a possible culprit.
Customer: replied 2 years ago.
I replied to your last query incorrectly I believe. I don't think you received it so I will try to answer your last query.
I adjusted the water temp. from 122 degrees down to 106 degrees. No change. The water is either too hot or cycles on and off if I adjust it to a comfortable level. If I turn a hot water faucet on at the sink than I can adjust the shower at a comfortable temp. and take a full shower. Can you provide me with the location of any temp. sensor or similiar item on the water heater that I can replace and may solve the problem.
I have no problem replacing the whole unit since I only use one water supply at a time. I believe a unit in the range of 3.1 or so gallons per minute should suffice.
Units of this size are relatively inexpensive. A rinnae 3.1 gallons per minute is about $230.00 on line upt to about $500.00. I would appreciate your opinion on this. ThanksJack
Expert:  Rick replied 2 years ago.
I just checked this thread and it appears a large part of my original response has vanished. The fact that the unit functions properly when you increase the flow by turning on a sink tells me this is a flow problem not a temp problem. But too hot at 106 could be a temp sensor issue. You should be able to stand water at ~106 without mixing any cold. If you can get another heater for that kind of money and do the work yourself then it probably isn't worth fixing this unit if the fix is to replace the water valve (part item #14) which would be the likely culprit causing a flow problem. However replacing the sensor is a pretty easy and relatively inexpensive fix. Here is the install manual with parts on page 30 & 31