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.
***Update before I even post***
Writing this out helped me realize that the Viessmann Service buletin likely does apply...its just that this boiler never gets to throw a code because the one 1929 non-TRV radiator is capable of maintaining return temps below the point where a code would be thrown. Given the bulletin and the boiler's behavior since install, I bet the sensor was binned wrong at the factory as opposed to being "bad." This gives me a little more confidence that the ordered parts will actually make a difference. Hopefully posting this will help the 1 or 2 other people with an older CU3a and an overhead gravity system...perhaps 1 or 2 others is an optimistic estimate?
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