Question about Vitodens Operation...

Larry_10

From what I've read here on The Wall, the Vitodens is most happy when it's setup for constant circulation.

How does the Vitodens operate if the system design doesn't allow circulation unless a particular zone calls for heat and starts the circulator on that zone? From my reading it seems like the Vitodens can be setup to only operate on a call for heat (via a dry contact), but if you do this, outdoor reset is disabled and you can only operate at a fixed temperature. Can this type of setup be done such that outdoor reset is still available?

Constantin

Paging Mike T to the O.R.!

In the meantime, let me take an unqualified stab at it...

IIRC, the Vitodens can be set up as part of a pri-sec circuit. Its pump can run constantly, along with that of the primary-circuit. Depending on the results from the OR sensor and the heating curve calcs, it'll adjust the primary loop water temp and try to maintain it.

Anytime a secondary heating loop starts to siphon off BTU's, the primary loop water temp will start to drop and the Vitodens will start going up in its firing rate to recover the difference. Problem solved, or am I missing something?

Enclosed is a Viessmann diagram showing one possible, proper implementation. Note how the boiler is its own pirmary circuit, while the secondary loops pull BTU's out of the low-loss header. Naturally, if you have only one type of heat emitter, the motorized three-way valve can get nixed.

Mike T., Swampeast MO

On My Way

to Post Office and Happy Hour computer location. Lacking details on operation from Viessmann can only give speculation based on observation...

Mike T., Swampeast MO

The short (and slightly idealized) answer is that the Vitodens is driven by the emitters. If the emission portion of the system is giving off zero heat, the boiler is producing zero heat. If the emission portion is giving off 500 BTUs per MINUTE the boiler is producing 500 BTUs per MINUTE.

Will try to keep Johnny Walker from making the rest too long...

Excepting very unusual circumstance, you should NEVER connect a digital wall thermostat to a Vitodens! Why? The boiler is designed to detect proportionate heat loss.

How? Really quite simple. If it cannot maintain the boiler temperature as modified by the outdoor temperature and selected on a true curve by the END USER, it produces heat.

While I'll still say that a fully proportional system with 100% constant circulation AND proportional delivery of heat to the system is the ideal of the Viessmann designers, I'll describe a more common (here in the U.S) situation first.

If you don't have it, download and print the Technical Data Manual available from Viessmann US.

Refer to the annotated illustration of the low-loss header. You will see an ALL-IMPORTANT comment. HEAT IN = HEAT OUT

If you're using wall thermostats, you're also using zone circulators AND the low-loss header--such is the "Vitodens way". Once any thermostat calls for heat FROM THE CONNECTED CIRCULATOR, the boiler is FORCED to produce heat to ensure that BTUs IN = BTUs OUT. Why? The "boiler temperature" will fall in proportion to the amount of heat being given off by the zone with that circulator.

In this instance the END USER should establish a reset curve that:

1) Is adequate in all circumstances.

2) Is BARELY adequate to give the desired setback recovery.

The faster and deeper the setback recovery desired, the higher the curve and the less constant circulation in that zone. With no setback and very well-balanced zones you can establish a curve such that the CIRCULATOR will call for heat nearly continually.

A Vitodens driving a system via a low-loss header is NOT (IMHO) a "constant circulation" system.

--------------------------------------------------

Without the low-loss header you can ONLY have a constant circulation system. If the system is VERY well balanced the only "thermostats" needed are the room temperature dials (normal and reduced) on the control unit. Note however that the control unit DOES NOT KNOW THE ROOM TEMPERATURE! Set up this way, either locate the boiler in living space where it's convenient to use the "thermostat" or remote mount the control unit!

In more complicated or less ideally balanced systems, USE TRVs or FHVs!!!!!!!!!!!!!!

From my observation, the "perfect" system for a Vitodens will find a well insulated (main AND branch) driving panels (including floors/walls) or standing iron with a design supply temp of 160 degrees or lower.

Piping should be JUST ADEQUATE IN SIZE to keep head losses within parameters of the built-in or suggested circulators. FORGET ABOUT DELTA-T! Concentrate ONLY on DELTA-P!

Larger than necessary piping creates a problem--VOLUME--and the Vitodens is forced to learn to turn that volume into a buffer. Such is great if you're working with an old gravity conversion system with enormous main volume, but it's only waste if you're working with new...

joel_19

well

The last guy just described the ideal. So what, we live in the real world. We have several Vitodens working as replacement boilers on mostly baseboard systems without a problem. they all work very well . I also have them working on Hydro air!We always use some type of header and dial in the curve to try and make zones run as possable. It's pretty easy. just turn up the stats to say 75 and keep turning down the curve till you've got as close to 68 as you can get. Simple but effective.

Mike T., Swampeast MO

Real World

Is EVERYTHING about the Vitodens. It tries to idealize ANY system to which it is connected by realizing that goal of BTUs IN = BTUs OUT.

Your description of setting the curve, "just turn up the stats to say 75 and keep turning down the curve till you've got as close to 68 as you can get" is exactly what I've suggested. BUT, once you hit that "magic" point where everything is 68° and the thermostats are calling constantly to the circulator(s) you have lost nearly all ability to raise space temperature.

Jump the T-T connections on a Munchkin piped Primary/Secondary and adjust the curve similarly and you'll find similar operation.

Even if you hit that "magic" point PERFECTLY at some set of outdoor conditions, it would FAIL at another set of outdoor conditions at that same outdoor temperature.

That's where TRVs/FHVs come in so handy. You need only supply the temperature required by the LEAST radiated space--usually the kitchen. Those spaces with comparatively more radition gain "headroom".

Mike T., Swampeast MO

EEEK!!

Even my "short" answer was long...

The zone circulator(s) provide the call for heat by lowering the boiler temperature.

Jeffrey Campbell

Indoor feed back

Mike T
I have been reading your posts on the Vitodens as I installed my first Viodens. I am quite pleased. I have Warmboard radiant system and slab. The design temp is a low 115 for an outdoor temp of -20. I just had another customer call and he is very intersted and educated about the Vitodens. He was also interested in indoor feedback. This is a gray are to me. I am unable to understand the control strategy. Do you need a logic control to process the indor information first and then tie into the vitodens. I do not think you can tie directly into vitodens with their logic themostats? I have used Tekmar 369 zone controls which process the information from the indoor feedback themostats 079(?) and then send the signal to the master heating control. Nay ways the Vitodens ROCKS! I love it.
Thanks Jeffrey Campbell

Mike T., Swampeast MO

I know it sounds strange, but the indoor "feedback" comes from the emission devices themselves. As long as they have reasonable mass, the Vitodens works without any indoor space temperature information.

Viessmann recommends the space temperature sensing remote control only for systems of extremly low mass like fin baseboard. It compares the sensed temp to the desired and makes adjustment to the boiler temperature.

If you've used wall thermostats operating circulating pumps, the flow of water and resulting drop in boiler temp is all the indoor feedback required.

A radiant system using FHVs (Floor Heat Valves by Danfoss, Oventrop and maybe others) should be absolutely awesome when connected to a Vitodens, but I've yet to hear from anyone who has tried it.

In that note, FHVs and TRVs provide a sort of indoor feedback to the boiler by altering flow which in turn forces the boiler to change output to maintain the boiler setpoint temperature.

What seems to make all of this possible is the truly modulating nature of the Vitodens burner. If the boiler temperature tries to fall below the current setpoint, but boiler draws more fuel with the combustion air automatically and non-mechanically adjusted for best combustion. If the boiler temp tries to rise below the current setpoint, the opposite occurs--if the temp continues to rise at the lowest possible fuel input, the burner turns off until the boiler temp falls below setpoint.

The Vitodens seems to know how to modulate perfectly "out of the box". The learning features of the boiler [seem] to be mainly? fully? concerned with operating the burner efficiently when heat loss is less than lowest output.

Larry_10

Mike,

What is the logic that determines the speed of the built-in variable speed circulator? If you have a low-loss header, I assume the temp sensor at the low-loss header provides info to the control and that manages the boiler so the temp at the header supply is equal to the calculated setpoint based on the reset curve. Is there some pre-programmed relationship between the modulation level of the burner vs. the speed of the internal circulator?

Mike T., Swampeast MO

About all I know is that the built-in circulator works in two different ways:

When used with the low-loss header it is in "Boiler Circuit Mode". (See illustration of the low-loss header.) The circulator changes speed to keep those equations regarding flow/heat true.

In "Boiler Circuit Mode" the coding addresses pertaining to circulator speed have no effect.

Without the low-loss header, the circulator functions in "System Circuit Mode". According to Vitodens literature pump speed is a simple function of outdoor temperature--the colder it gets outside, the faster it turns. No fancy differential pressure sensors to regulate the circulator speed. In "System Circuit Mode" you can adjust the minimum and maximum possible pummp speeds in the coding.

Image doesn't have the best scan quality. The third line that looks like "T2 - T4" is actually "T2 = T4".