Vitodens 200 - fixed boiler temp with ODR on mixed circuit?

Gordan

The WB2B comes with built-in reset for the boiler supply temp and one mixed zone. Is it possible to run the boiler supply on fixed temp, and the mixer circuit on reset?

Ted_5

Yes it can

if you use the Viessmann mixing motor that connects to the Vitodens then you can set a reset temp on the mixed system.  On the boiler you would give a contacted closer called "External Heat Demand" that makes it run at a fixed set point and the mixing valve does it's job.

CMadatMe

Why?

You can limit the modulation range of the boiler whether it be input or output. You can also limit the input for DHW production. Its all done in the control. Why would you not want the boiler to modulate for heating circuit 1 and the mix for heating circuit 2. You should be using a Vitotrol for that mix vlv zone.

Simply Rad

And then some

What is your fixed setpoint demand?



I have used external demand only for snowmelt. I think its designed for short term heat. Might want to check on what all happens when the contacts are closed? I have used the shift adjustment of the heating curve for heating circuit A to flatten the curve. Simply turn up the shift... maybe start with 5C+. Then check the operation status heating circuit A for the setpoint and adjust if needed.



Jeffrey

Gordan

Argh...

Typed a long response and accidentaly hit one of those stupid extra mouse buttons that go forward/back.



Long story short, my setpoint load is the reverse indirect, at 120 deg. F. This also happens to be the design day supply temp. The reverse indirect has extra tappings, allowing it to be used as a buffer for system water. I can't use ODR for its temperature - DHW temperature is a pretty fixed quantity.



Looking at the efficiency curves, it doesn't appear that I'd be losing much if I maintained the buffer/DHW source at the set (lowish) temperature.



Another thing I could do is avoid mixing alltogether, and then just use TRVs (and an Alpha) to modulate down flow through the emitters (radiant floors/ceilings.) This would result in very low return temperatures. It would also result in very low flow through emitters for most of the heating season (possible source of air problems?) and a pretty big difference in surface temperatures between one end of the emitter and another. (That could be somewhat mitigated by making sure that the warmer part is around the exterior walls of the room.)



Why do this? One less circulator, one less buffer tank. On the other hand, perhaps the cost of the circulator and the buffer is a small price to pay for the peace of mind from separating the DHW function from the space heating function and not running a "weird" configuration.

CMadatMe

DHW

Temp is not a fixed temp. You can also take it out of automatic mode and run it in individual time mode. There are alot of options for DHW in the control.

Gordan

Double post, ack!

Please ignore.

Gordan

Chris-

Thanks, I didn't mean to imply that there aren't controls that allow you to vary DHW temp, I meant that I wouldn't want to have that on outdoor reset. For most of the season, my system supply temps will be well under what would be considered usable DHW temperature.



I'm leaning toward a separate DHW circuit, with its own buffer. But even so, looking at the thousands of options available on the Vitotronic 200, I don't see any that are buffer-oriented. The thing seems to want to run the boiler circulator all the time (as long as there is a call for heat) and not only when the boiler is firing. Is this true? If so, this would mix up the contents of the buffer tank and prevent the coldest return water from making it to the boiler.



What I'd like to have would be for the boiler to only run its circulator when it figures out that the buffer needs recharging. This shouldn't be based on a fixed differential but on tracking the rate of temperature drop in the buffer. The boiler should always try to recharge the buffer at lowest modulation, and only modulate up when it detects that it's not keeping up. Pretty much the same thing that it does with system supply; in fact, the buffer tank could be a drop-in replacement for the LLH, but is there a way to set the boiler circulator to only run when the boiler's firing?