Viessmann CU3A 57/199 Boiler temperature sensor issue

DKomoroske

Hi all,

I have a boiler supply temperature issue with a Viessmann Vitocrossal CU3a 57/199: the boiler supply temp sensor thinks its temps are 7ºF higher than the water actually coming out. This causes weird run-on issues while heating the house and the DHW tank. During heating, the boiler runs at 20% 16-18 hours a day regardless of outdoor temperature; on this system, that's 8-9 Therms on a 60ºF day or on a 45ºF day--Design day is 39ºF. The boiler is chasing the heating curve set point but the actual supply temp is always 7ºF too low to actually reach it...so the boiler just keeps running and the returns temp just keeps rising.* Same thing for reheating the DHW tank--however, the boiler does shut off eventually since even the 13ºF differential is enough to raise the tank to its set-point...it just would have done it faster at the programmed 20ºF differential.

The boiler dual-temp sensor tests fine across a range of known temperatures and the boiler isn't throwing any codes. I've ordered replacement sensor and sensor-well; I'm just not convinced it'll fix it. Has anyone seen this run-on issue with these boilers?

Viessmann does have a service bulletin regarding an error code caused by faulty boiler temp sensor but this boiler isn't throwing any codes. Perhaps this sensor is on its way to the bulletin's noted behavior without having fully failed?

*the return temps rise to within 3-4ºF of the actual supply temps (10-11ºF of what the boiler thinks is happening) because this is a large-mass overhead gravity system from 1929 where all but 1 of the radiators have TRVs.

GW

What’s you’re operating platform, you’re doing constant circulation with no indoor sensor? You’re guessing at slope and shift? You doing DE input? 

DKomoroske

Constant circ. w/outdoor temp, without an active indoor sensor--it does have a properly ignored Vitotrol 300A. No DE inputs. The gravity loop is NOT circulated via pump; the ECM heating circuit pump merely injects heat. The loop is not short-circuited by the injection pump, the gravity system heats up quickly and evenly. I introduced a [2] Supply Temperature Sensor on the gravity loop this season but it didn't change the outcome: the boiler starts out at higher temperatures but then lowers as the common flow begins to heat up--all the while the Vitotronic shows boiler supply temps 7ºF above what is actually coming out the back of the boiler. I'm certain I can control the problem away...I just shouldn't have to. The boiler is designed for this. If it'd output the temp it says it is, there wouldn't be a problem or need for further controls.

I'm hoping the boiler temp sensor is faulty after all. And if not the sensor then that the sensor well is fouled in a way that somehow makes the sensor read hotter than it actually is...I'm not holding my breath. If this fails, i'll just control the problem away. Hell, I can even program it away with a daily punctuated setback. I'm just hoping to find the cause.

GW

Ok I’m not fully understanding, 300a isn’t connected? 

Did you ohm out the sensors? 

DKomoroske

The 300a is connected/allocated to a Heating Circuit (HC1) w/o mixing valve but I have B0 set to 1. Does B0:1 use room hook-up for both Heating & Reduced? I'm under the impression that B0:1 only uses room hook-up for Reduced Mode..Since the kid was born, the system stays in Heating Mode and never goes into Reduced Mode, which is why I said "properly ignored," so I failed to give relevant details. Sorry about that.

For the sensor, I did ohm out both of #3's probes before I ordered the replacement. When the new #3 sensor arrived, I checked the old one again. At the time of the swap the Vitotronic read the boiler temp as 104.0ºF and my FLIR camera, analog boiler gauge, and digital OnSet data-logger read the boiler's water at 97ºF; that 7ºF delta I'm still trying to track down. The boiler had been off for hours so its temperature was relatively steady. In the service manual, 104ºF for #3 equates to 6 k Ohms, and the old sensors ohm'd out to 5.522 & 5.517 k Ohms. 5.5 k Ohms should be ~108.5ºF. I swapped in the new #3 sensor but, once up to temp, they read effectively the same (5.566 & 5.567 and the vitotronic computed boiler temp @ 102.2ºF).

For reference, Service Bulletin TSB 010 did not apply here since Error F6 was never thrown and the #3 sensors ohm out the same. Error F6, not in the earlier manuals, means the delta between two probes (in the #3 sensor) is too large, and #3 sensor has gone bad.

GW

This is what I set the boiler to when I use a 200a/300a

you want some indoor feedback, or not much sense in having this control on the wall. Sure, you can turn things up and turn things down, buy your boiler doesn’t know what’s going on  

night time temp is set at the boiler. Daytime is set at the 300a. Setting the time between day and night and the day of the week is set at the boiler of course.

SymonLie

Hi....the thing is it may not be the sensor but some other part playing up and you could end up paying loads for parts till you get the right one, normally you'd test the sensor using a meter to see that it is working (as temp rises the voltage increases on the signal wire).

pcb online ordering