Bosch singular 5200 DHW issues

fenkel

I been trying to figure out why this boiler will not control the DHW temp. The DHW will climb to 150* and then cycle off. then ramp back up to 150*. the temp of DHW will not stay at a constant temperature. Any help? Any ideas as to why this happens?

dko

Are you sure you are reading the DHW temperature?

Hold the mode button until the faucet icon lights up and you should be able to choose the set point. Maximum is 120F until you hold the + for 5 seconds, then you can set it max 140F.

150F is not an option

Are you reading 150F when the unit is in DHW mode with the faucet icon lit up?

fenkel

will try it.. Could the 3-way valve or water sensor cause these issues?

dko

What is your space heating temperature set at?

Prime suspect would be the dhw thermistor 7738007295
If your dhw temperature is indeed rising above set point every time.

Normally the initial hot water coming out may be hotter than set point if the spacing heating was running. But if it is consistently higher and even without space heating running prior, I would suspect the thermistor.

fenkel

I replaced the dhw thermistor, no change in results. did the hold of mode button for 5 seconds and "unlo" came on panel.. what's does this mean? unit still does not control the temperature of the dhw. the panel says its 122 and its cold at the sink. then temp at sink faucet will go back to hot, boiler states that its 122 thru the cycle.. Any ideas?

dko

It was only a suspect, more due diligence needs to be done before you go parts swapping until it starts working again. But your current description of the problem does not match your original.

I am not sure what you are describing. Are you saying when you turn on the hot water from a faucet.. it comes out cold and then become hot after a while? The temperature on the unit is what the temperature is leaving the boiler, not the temperature at your faucet. There will be a cold water sandwich until the hot reaches the faucet. Or are you still experiencing the hot cold hot cold syndrome? Do you have the same problem using the shower or just the one faucet? Same problem when running shower AND faucet hot water?

Was this an issue since the beginning of installing this unit or only recently?

Photos of installation?

fenkel

I got called in on this problem. owner installed it himself. states this is the second one he has installed. The first one the heat exchanged (main)sprang a leak.
Problem was there with the first boiler as well.: the dhw temp at the sink will go to set temp of around 120's then back to about 90, then back up to 120's. I tested all the temp sensors, and they are all in spec per service manual.
noticed the boiler, at dhw start-up making a loud sketching sound. wounder if the gas pressure is off? Might be to high?
if I cycle thru all the modes, the boiler will run okay and I will achieve a somewhat stead temp, until the next call for DHW and then it starts all over again.
I did pull the DHW exchanger out and had sediment present. I rinsed the entire system out and a lot of dark sediment did come out during the rinse. (Boiler side of heat exchanger). I replaced the dhw exchanger. better temps, but still on and off cycling.
I plan on checking the pressure regulator to see if it working correctly, then test the incoming gas pressure. set the boiler to correct gas pressure and see what happens.
I did take notice that of the control panel flashing "a.66" code once in a while.
never saw that before.

GGross

How old is the install?
Sometimes these units DHW heat exchangers need to be de-scaled

Check for correct fuel type, if a conversion took place confirm it was done correctly

Check gas pressure, static pressure before ignition, pressure drop on ignition, and measure the lowest pressure recorded, turn off the unit and measure the lockup pressure after the gas valve is closed


Complete combustion analysis

Confirm the actual flow rate of water through the DHW heat exchanger.
generally combi units need anywhere from 0.5-0.7 GPM sometimes manufacturers just say its a half gallon per minute, when it may actually be just a bit higher. Remember the flow rate is primarily what controls the burner during these cycles.

Check all inlet screens, etc
kind of sounds like you already did this, junk in the DHW side is a strong indicator that you may have reduced, or intermittent low flow.

I like to have water filters on the inlet side of any tankless or combi unit. Since these are flow activated heavy sediment can cause them to not fire, or fire intermittently.


Does the boiler function correctly 100% of the time for space heating?

fenkel

boiler is only 6 months old.. not sure if it heated that well, the heat system is only used for in floor heat.. this system is grossly oversized for the heating zones. () maybe a thousand squarefeet in the basement on four zones. gas pressure static was 8.53 at stand still. went down to 2.5 to 2.83wc fast and slowly moved back to the 7.63wc, I tried to adjust to 9.00 wc , but it wouldn't go any higher... either the gas pressure in system is low or bad regulator or possible a bad gas valve...
would the poor gas pressure cause the boiler to overheat? will post a video later today .

GGross

I'd start with what I know is incorrect. A 6" wc pressure drop on ignition is not a good sign. I would start out by sizing the gas line as though it were a new install. It is possible that the gas line is undersized to begin with.

I wouldn't suspect the gas pressure to cause overheating DHW temps, however this needs to be addressed first in my opnion

fenkel

I'll take out the DHW exchanger and check for sediment on both the heat side (boiler) and DHW side.. wonder if the main heat exchanger is plugged as well. and yes the gas line is undersized, main meter is 250 BTUs. Will add up BTU required next visit.