Hot water, then none

MarkA

I’m having some trouble w a combi 151 Bosch. We have used it for five years with many issues along the way I’ve managed to solve but not this one.

We get hot water for about 10 seconds and then none. I cycle on and off and sometimes that works and it goes back to normal but lately it has remained cold and won’t work. I reset the unit and that did the trick for one shower. Nothing since. The shower will output up to 9gpm so drawing enough water isn’t the problem.

unclejohn

Thats a lot of water for a shower head.

Jamie Hall

How's the flow switch or sensor?

MarkA

It’s the output for the shower overall. I mentioned that because I’ve read if it’s not enough draw the system will start and stop.

MarkA

Regarding the flow switch or sensor, I’m unable to find one for the unit. Trying to figure out what sensor on the unit kicks it on when water is drawn. Best guess is the diverted valve is not working properly.

EdTheHeaterMan

@MarkA,
Are there any error codes?



part number 37 in the illustration is the flow meter location. There is a small strainer just before the flow meter. Have you checked the strainer?

EdTheHeaterMan

@MarkA
I hope this helps If you have not already replaced it with a conventional tank-type water heater.

I have had good success with these, very few problems. Just need to install them right.



these illustrations are from:
https://s3.amazonaws.com/s3.supplyhouse.com/product_files/ZWB-42-3-install.pdf

BillyO

does the Bosch have a transfer valve?

ResqDogz

I am experiencing similar problems with intermittent domestic hot water heating interruption with my Bosch Greenstar Combi 151 ZWB 42-3 unit (installed in 2015).

Thought initially the secondary heat exchanger might need cleaning/descaling, but ruled that out as I'm occasionally able to get sustained hot water enough to shower, and once it's kicked in as long as the hot water continues to flow, it doesn't appear to fall off - at least, not when showering with the bathroom sink faucet also fully open.

Once it's shut off, I've great difficulty achieving sustained hot water flow again, for more than mere moments, and not at full temp.

The diverter value appears to function proper, as does the pump motor.

Suggestions would be most gratefully appreciated!

David Penz

I suggest that you remove the flow sensor and disassemble it. This may be a turbine meter, and it may be that unnumbered part between washers #29. I think I see three pins, which could be a Hall effect pickup, so there is a magnetic rotor. If the turbine rotor has a coating of black powder, that may be your root cause. This has caused many flow sensor failures in my neighborhood. There is a separate thread going on this, see "tankless water heater flow sensor fouling". Usually heater operation becomes intermittent and then quits altogether, with no error code. No water flow can be normal for the heater, depending on how it operates, so can't be alarmed.

You should be able to clean the rotor by brushing it or vacuuming it or blowing on it with compressed air. Try not to breathe the dust. This material is either MnO2 or FeO2, harmless in small quantities but could be harmful if ingested into your lungs.

David Penz

ResqDogz, I just noticed that the earlier thread was from 2020. In any event, check the flow sensor on your Greenstar. I would be most interested to confirm other cases of black powder fouling.

ResqDogz

David, do you believe it's the #39 DHW Filter Insert (apparently inserts into that unidentified device you're referring to, which I believe to be #35 - the flow sensor - in line with #34 the flow turbine adapter) that might be the problem?
Everything seems to work, intermittently, but there seems to be an interruption from the control board (circuit relay?) because I hear audible "clicks" and the pump motor shuts off..
Thank for your help!
Steve

David Penz

Per pp. 96-97 of the manual (table 51 group 7 and figure 106), part#35 is the "flow turbine complete", which is likely the spare part item that Bosch sells. It appears to be an assembly of several parts, including the unnumbered part that is in between the two orings #29. That unnumbered part is (likely) the actual flow turbine. Inside is a wheel with multiple vanes. When there is flow, the rotor spins. There should be a 3-pin electrical connector attached. Three pins are characteristic of a Hall Effect speed pickup, triggered by passage of the magnetic rotor vanes. It generates pulses that can be counted or timed by the controller board. More pulses per minute = higher flow rate. No pulses = no flow, therefore no heat.

This is a similar flow sensor to my Rheem heater. Your part looks to be pretty easy to remove from the heater. Pull it out and look into it with a strong light, and see if it is fouled with black powder. You may be able to remove the rotor from the housing, there may be a retaining ring of some sort. If it is coated with black powder, see earlier post for instructions. Clean it and reassemble/reinstall it. Take photos please, and post with your results.

Your symptoms sound exactly like what is happening in my neighborhood. The heater shows no error code but won't make hot water. Sometimes, turning on a shower and several other faucets can get the heater to fire. Eventually, nothing helps but to replace or clean this part. My theory is that the powder either stops the turbine from spinning or interferes with the magnetic field enough so the pickup does not see the wheel spinning.

There is as yet no way to prevent this from occurring, but I am investigating possible fixes.

mattmia2

If the vanes themselves are magnetic then it will collect ferrous particles in the water.

chuckB523

did you ever get it worked out. what was it?

chuckB523

I am having the same problem. Everyone is pissed off. please help

hot_rod

There are some You Tube videos showing how to remove that flow sensor. If this looks like the same boiler you have?

https://www.youtube.com/watch?v=zRodAou4wqc