Measuring Temps

jerry scharf_2

Mike,

The easiest first thing to do is to put on a longer, higher R insulation blanket around the sensor. If you can get R in the 8-10 range (2 layers of 3/4" split foam, splits on opposite sides) and insulate all the way from the boiler to the sensor and at least 12" beyond, it should rule out most of the water vs. pipe jacket issues. Also run the sensor wires along the pipe and exit downstream at the end of the insulation. If nothing changes, then you can safely ignore this as a problem.

Next, I would so a completely different sensor system. I wouldn't cost you too much to get a Maxim 1-Wire temp sensor in TO can and hang it off the serial or USB of a laptop. It would give you an alternate reading system at the same location with no common parts. Switching sensors is good, but not really enough.

Third, where is the internal temperature sensor located in detail. Is it in the water flow, and if so where? Maybe Viessmann could send you a replacement sensor and see if that changes anything?

Other tests are more complex and would require temporary replumbing. These would be best left for summer. These involve running water past both sensors without the boiler firing, which would be a major change from what you have plumbed right now.

jerry

Mike T., Swampeast MO

This difference between "boiler temp" as reported by Vitodens and my measured supply temp is really freaking me out.

Small, round (no flat area) temp sensors strapped tightly to 1" black iron with a dab of conductive paste. About 3' away from boiler. About 10" (length) of insulation with sensor probe in center. Vitodens connection to system only about 4' in length and uninsulated except for that around the sensors. Rest of piping insulated.

Have flip-flopped sensors and checked with another for reference. Measurements within a degree or so. Measuring system seems VERY responsive as even one minute of reduced temperature mode produces a significant dip in supply temp.

Outside temp has been fairly steady for about 24 hours--around 16°-18°. Boiler temp as reported by Vitodens is dead on with my calculation of the curve set in the boiler--about 133°. Temp reported by boiler stays NAILED at target.

BUT, the supply temp I measure is even more closely nailed at 119°! Return temp nailed around 96°.

My reset curve was designed to produce a nearly constant delta-t of 37°. 133° - 96° = 37°!!!!!!

I can understand a couple of "missing" degrees in what I can measure, but what on earth is happening???? My ideas are getting bizarre--even for me. Any ideas?

Mark Eatherton1

The sensor of the Vitodens

is actually in the boilers header piping, fully immpersed in the working water. It could be seeing the mass of the heat exchanger and other things that you're not taking into consideration.

The strap on is looking at the surface tmperature. THere could be "hysterisis" betwixt the two of them.

ME

Mike T., Swampeast MO

This may sound strange, but I wanted to capture that differential (kind of hard to call it hysterisis when everything is modulating) but am a bit surprised with how much difference in such a short distance.

I somehow knew that the Vitodens would turn the system piping into a buffer. Is this the proof?

Presuming that the Vitodens is producing water at the temperature it reports, what's my delta-t?

Is it based on the supply I can measure only a few feet away or is it based on the supply only measured in the boiler?

Mike T., Swampeast MO

Sensor wire is running out in the direction of flow. Will add some extra layers of insulation near the sensor and extend the insulation a bit further.

Do understand that the black iron pipe wall can't fully reflect the temp of the water flowing through. My choice of sensor location was carefully chosen--isn't it similar to where you would place a Tekmar sensor?