How smart is my vitodens 200?

Tom Hopkins

I got my radiant circuit with the mixing valve running tonight. I have a couple of questions about how it's going.

Even though I have the HCA timer set to go on only 5am to 8am, with the slope at 1.8 (this is for the unico hydronic heat), the setback temp on that circuit is still higher than HCB (the mixing valve controlled radiant circuit). I have the setback dial all the way to the left. Is there a way to shift the setback temp even further? I really don't want to incur the cost of HCA unless it's 5am to 8am and there or there is a call for heat.

HCA slope is 1.8

HCB slope is 0.9

On the controller, HCA shows the setback temp at 41, but the actual setback temp at 70. I don't understand this.

Also, should I expect the "Mixing Valve: " item in the operating status to show something?

Do I have to give it a few days to learn the ropes?

heatboy

Slope A........

.....will always ride above slope B when a mixing valve is involved regardless of setback settings.

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joel_19

Unico

1.8 curve on your Unico is VERY high I would suggest lowering that dramatically a 1.2 or 1.4 should be plenty if not you can always turn it up. For warmer weather say 50f a 1.2 or 1.4 could be too low but you can shift up the bottom of the curve a little.

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Paul Pollets

Huh??

For a heating curve of 1.2, for example, the system circuit water would rise 1.2 degrees for every 1 degree fall in outdoor temperature.

HW coils in air handlers usually need 140 degree temps (min) to develop air temps that will heat the home, depending on design sizing. Owners tend to feel strongly about lukewarm air coming from the registers.

We've always used a 2.0 to 2.4 curve for the air handler HW coil circuit. Never seen a design using a 1.2 curve that would blow hot air. Am I missing something?

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scott markle_2

air handlers

Paul, it is possible to oversize A.H coils such that lower supply temps can be used. Air flow also effects this. I did a shop space with oversized modine vertical discharge unit heaters powered be a gb124, the gb operates in reset mode for attached panel rads. and 125 deg set-piont control when unit heaters operate. I provided speed controls for the fans so user can establish a comfortable supply air temp. The system works very well and I get good condensing from low return temps.

Paul Pollets

Shops vs Residence

What you can get away with is a unit heater hanging in a shop may be different than required temperatures for supply air in a plenum system.

From the original post, I take it that the orig heating curve is too low for the HW coil to get hot.




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Tom Hopkins

oversized hydro

Paul I have a hydro system as low as .9.Unico does really well with low temps however I also have some flat curves because of that exact problem

Mike T., Swampeast MO

I must ask. Why do you only want the hydro-air active from 5 a.m. - 8 a.m.? Are you setting back the radiant during those hours and wanting a forced air "boost" in the morning? If so, such is FAR from the "Vitodens way" of low, slow and constant as possible. Yes, you can certainly use setback with a Vitodens, but such is setback of the supply curve. If your boiler is also providing DHW, I cannot imagine a worse situation for your Vitodens than attempting to simultaneously boost the air temp via hydro-air, bring the radiant floors up to temp and provide heat for morning showers etc.

Mike T., Swampeast MO

The Vitodens 200 seem rather "smart", but it tends to "think" in the European way (e.g. constant circulation, TRVs, and about any form of emitter except hydro-air), not the North American way with wall thermostats, zone valves, zone circulators, hydro-air, etc...

Given your desired operation, here's a suggestion:

While often frowned upon, this would seem to be a good place to use the "external call for heat" function of the Vitodens 200.

Here are the basic steps to enable an "external call for heat":

1) Set address 011:001 This enables external switching OR an external call for heat. One or the other may be used--not both at the same time.

2) Set address 027:001 This sets the function to external call for heat.

3) Then connect your hydro air to connections X 4.1 and X 4.2 inside the boiler. Note that these are DRY (NOT POWER) connections. DO NOT SUPPLY POWER TO THESE CONNECTIONS. Unless the air handler provides relay contacts for such a purpose you will probably have to add a relay. Again--all you have to do is complete the circuit between these two connections--DO NOT provide power to either connection!

4) Set address OC5 to the appropriate value. There are many choices for this address. Likely you will use OC5:005 but it will depend on how (and if) your secondary circulators are connected to the boiler's power supply. The OC5:005 setting will not affect any circulator (except ensuring that any connected and dedicated DHW circulator is off) and your mixing valve will operate normally and maintain the target contained in HCB.

5) Set address 0A2 to the desired fixed target temperature during the external call for heat. Note that the value you add is in degrees Centigrade--even if you've set the boiler to display in Fahrenheit!

6) Set the curve of HCA exactly the same as HCB. (No, this will not cause a problem--I've verified such previously with Viessmann.)

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The main reason I suggest this is that as long as HCA is programmed with a higher curve than HCB, the boiler will fire to meet the needs of HCA even if your air handler's thermostat is not calling for heat. As you've discovered, the target temperature when a heating circuit is operating in reduced mode (to the moon dial setting) isn't always what you think it should be. Why? I have zero idea. I use the "external changeover" function with my 6-24 to approximate warm-weather shutdown. Once the outside temp goes above 55F I close the circuit and usually (but not always) the boiler will stop producing heat and the circulator will stop running.

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Connected and adjusted this way, you will only incur the cost of increased target temperature when the hydro-air is calling and the mixing valve will continue to modulate properly to serve the radiant. You'll also be free to use the sun/moon dials and setback timer for reasonble setback of the radiant if desired. You won't be wasting anything as you still need the mixing valve to properly supply HCB during an external call.

scott markle_2

Is mix is redundant in this configuration?

Mike, this sounds like good advice. However I was thinking that if the AH is sized such that it can deliver warm air at at 125 (radiant safe) then why have the mix at all?

If ,as you have assumed,the intention of HCA is to speed recovery of night time setback then the radiant would benefit from a boost during this change of program as well. Piped direct return (is that the right term) the AH return could temper the radiant supply by probably 10 deg or so too, Room to play. I'm sure you will agree the 200 is very capable of modulating its supply temperature for the radiant with no mixing at all. If HCA is all about a punch of heat what's the harm in dropping ODR curve for HCB during these periods.