Vitodens Operation

Mike T., Swampeast MO

Attached .pdf document shows the results of my 2nd "Eurocave Response Test" this year.

Fully TRVd, standing iron, gravity conversion, no low-loss header.

Reset curve is cut VERY close this year. In other words, the "Sun Dial" setting corresponds almost exactly with the <I>maximum possible space temperature</I>.

For 1st response test this year I raised all TRVs in setback spaces up to the level corresponding with the sun dial. Room temp response was torturously slow--I discontinued the test after about 24 hours with setpoint still not achieved.

This time, I changed both boiler setting (sun dial) and only two TRVs.

The "boiler target" on the graph can only handle one curve, so it is not accurate BEFORE the test began with "Sun Dial moved from 63°F to 72°F".

Boiler was near the end of a "burst" when I increased the sun dial setting--in other words the actual temperature was slightly above the target temperature. This is normal operation when the system load is less than minimum modulation.

When I raised the sun dial, there was no change in operation even though the Comfortrol™ correctly calculated and displayed the new target temp quite rapidly. Boiler temp was 104°F with 106°F the new target. Had a beer while watching...

Still no discernable change in boiler operation. Quiet as a mouse and operating a minimum output.

Then I raised the two TRVs as indicated. The one raised to 2¼ is in my office--the other another part of the master suite, but separated and isolated by closed doors. By the time I made it back to the basement, boiler output had increased significantly and circulator speed had also increased. Had another beer while watching. Boiler achieved target temp rapidly and it stayed nailed at the 106° target. Back upstairs--the two rads were fully heated with almost no discernable (by the hand) delta-t across them.

Back to basement for another beer. Boiler output had dropped slightly but was still above minimum. Circulator speed [seemed] unchanged. Boiler stayed nailed at target.

Went for brisk walk. Came back and space temp in both rooms with the increased TRV setting had increased by about 3°F. Was immediatley struck with "radiant coziness". Sat down to work at the computer a bit and for the first time at night this year pushed up my sweater sleeves. (Neither the beers nor the walk are uncommon at such a time...) This was Saturday night. Returned early Monday morning. Office was about 64°--it should have been warmer (like 66-67 for that time of day at that TRV setting). Checked datalogs and found that after the initial and quick increase that it had resumed following the solar cycle with again, a torturously slow increase in temp. Other space with the increased TRV setting had achieved setpoint.

Gave one 10 minute or so forced air electric "boost". Since then, my office has maintained setpoint (64°F-68°F at the red triangle TRV setting) following the solar/occupancy cycles as normal. Curiously, I would swear that the room "felt" like a room at about 68° but the air was not that warm. Just that one quick boost of warm air was all it took for the radiator to maintain conditions... Kind of interesting, huh?

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Observations of the graph:

Remember: I'm sensing the supply temp on the surface of black iron pipe about 3' from the boiler. The computed target temperature is VERY close to the actual boiler target temp (with the exception of the change in sun dial mentioned earlier).

Prior to this test, the sensed supply temp and target temp have been in near perfect agreement during true modulation and nicely averaged when loss is below minimum modulation.

First, note the sudden and significant plunge in return temp when just two TRV settings were slightly changed. Then note how the sensed temperature stays a number of degrees below the target <I>until the TRV settings were changed again</I>.

For all of last season I used a significantly higher reset curve with abundant "headroom". Sensed temp was ALWAYS significantly below target, very similar to this portion of this graph.

Then with the second TRV change something utterly new occurred. Sensed temperature was actually slightly ABOVE target during what appears to be modulation! I've NEVER seen that before and can only surmise that the boiler was running at minimum output but still slightly higher than necessary.

I sincerely believe there is an opportunity for enhanced control of the Vitodens. As with every other test I have ever made, increasing the setting of even a single TRV by a fairly slight amount (say 3°-4°) results in a nearly instant plunge in return temp.

This is a simple and reliable mechanism for feedback of the occupants' desires.

BUT, it takes a little bit of history to understand what to do after this trigger event. The boiler should be comparing it's internal temperature with surface supply temp some distance away from the HX.

Once any reset curve change is made, the brain should start comparing these two temperatures. If they are nearly identical than a simple assumption is made: the curve is almost perfectly suited to <I>maintain</I> the sun dial setting. After detecting a sudden drop in return temp, the boiler should fire to achieve setpoint at the REMOTE location--not the internal. This is sort of like changing both the sun dial and TRV(s), but with a difference. This too results in a rapid increase in output, BUT only until the boiler itself nears setpoint--instead this increase should continue until the entire system is satisifed, e.g. when internal and remotely sensed temps are identical.

With a little bit of none-too-fancy programming, the boiler could easily learn valuable information regarding the mass and response time of the system.

If, on the other hand, the temp in the boiler itself has been somewhat higher than the remotely-sensed temp, the boiler "knows" that it is <I>already</I> producing a temp higher than required for temperature <I>maintenance</I> at the current sun dial setting. In this case there is <I>already</I> "headroom" in the curve and response to TRV increase will be rapid.

Americans don't typically live in solid masonary, externally insulated, multi-family dwelling units. While I GREATLY appreciate the Viessmann control philosophy (including nighttime setback <I>of boiler temperature</I>) in such a structure, it doesn't work quite as well on this side of the pond. Give us German efficiency with consideration for American structures! Granted, it's always somewhat inefficient to raise space temp, but we can do it MUCH more rapidly (and efficiently) in stick-built homes with reasonable insulation than you can in a typical German dwelling in a relatively predictable climate.

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Does ANYONE out there have a Vitodens driving a TRVd panel rad system with modern piping and no low-loss header? If so, please contact me. You can help me enormously with simple tools and a bit of time!

Paul Pollets

Vito w WPR

Mike, I'm about to do a 6/24 with 9 panel rads using Pex-Al-Pex distribution. Rads were sized for 140 deg water. The designed flow rate is just under 6.5 gpm with a connected load of 61K Btu's. Will also use a VitoCell 300 53g tank. First time I don't have to use a LLH. This project will install in May. Maybe I can help, but I don't have a datalogger.

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Mike T., Swampeast MO

Nothing more than a simple remote reading digital thermometer nicely insulated against the supply piping just before your manifold.

Then an inquisitive person who wants to learn how to adjust their system and will sometimes spend an hour or so watching the boiler and feeling the emitters. Exercise for the brain and body...