Vitodens Operation

Mike T., Swampeast MO

Here's a particularly interesting day...

REMEMBER: Outside temp is measured on the South side and has solar influence. "Supply Target" is based on outside temp with 141° @ 8° outdoor design and 0.8 ratio--VERY close to the curve in the Vitodens. Also, the supply temp is measured on the surface of 1" black iron about 3' from the boiler. (See photo for sensor location. Sensor is very well insulated and is in a dab of heat conductive paste.) Also VERY important: the "Boiler Temp" reported by the control display will be rock-steady and VERY close to the "Supply Target" during periods of modulation and space temperature maintenance.

Rainy/heavily clouded day so almost zero solar influence on the outside temp sensor. Heavy rain until the first rapid temp dip at about 2:30 a.m.; lighter rain until the big temp dip around 11:00 a.m.

Note how the "pulses" become stronger after the first dip in outside temp, but how their frequency remains unchanged.

The response to the sudden outside temp drop around 11:00 a.m. is where things get <I>very</I> interesting to me...

Imagine what is happening: As the outside temp falls rapidly, the house begins to loose heat and the mean radiant temperature tries to fall--all delayed by the insulation of course.

The TRVs on the standing iron begin to open wider and convection increases significantly as the average temp of the radiation climbs faster than it can be liberated via radiation.

The difference between flue temp and measured supply temp increases as the radiation attempts to stop the fall of space temperature via convection.

The measured supply temperature comes very close to the supply target once the convection has done its job. While I have no way of tracking the actual level of burner modulation, I'm willing to bet that once this happened, the flow of fuel decreased.

Once the house has resumed a reasonable state of mean radiant temperature stasis, the difference between the supply target and the measured supply temperature increases <I>as the convective output of the radiation drops and the radiative portion increases</I>. (At least I believe this a logical and possible explanation of the "phantom" delta-t.)