DHW Priority with Viessmann Vitodents 200 boiler

Tarpan

Hi all. I hope you all doing all right and staying safe.

I'm confused about my Viessmann Vitodents 200 B2HA-28 boiler. Any mod con boiler can do DHW priority and outdoor reset. Except Viessmann. WHY???

So, I have a 4 zone radiant piped over a LLH (low loss header) and a indirect DHW heater piped in parallel to the LLH (separate pump). The heat is controlled by thermostats with zone valves. The thermostats are wired to the external demand input of the boiler. If a thermostat calls for heat while the boiler runs hot for DHW, the boiler starts system and zone pumps and starts pumping 160+F water into the radiant floors. I see a way to disable DHW production while heating the floors, but that would be just stupid. I want the opposite - typical DHW priority.

Obviously the boiler has problems with "external demand" input. Is there any other way to set it up?
Any ideas?
Thanks you

Alan (California Radiant) Forbes

I'm familiar with Viessmann Vitodens boilers, but we normally wire our thermostats and zone valves to a separate zone valve controller and not to the "External Demand" contacts. This allows the boiler to work off the outdoor sensor and DHW priority may come into play.

If you don't get any answers here, text your boiler model, serial number and issue to Viessmann tech. support at (800)288-0667.

The correct number is (844)649-5886. They have gone to texting ever since shelter-in-place.

GW

Doesn’t make sense all 200 series you can chose priority, I thought.

psb75

Dhw priority over radiant space heat is a fairly basic control strategy. A Viessmann Vitodens 200 boiler with LLH is a very good set-up. Definitely contact tech support for help.
I can assure you...a Viessmann "mod con can do DHW priority."

Paul Pollets

The external demand is not used to provide power to zone valves. The zone valves should be powered by a zone relay control (taco or equal) and the power comes from the system pump bus plug to the Taco zone controller. Unfortunately, the Viessmann schematics do not show this application.

GW

He may be using DE terminals which could change the phraseology, maybe

PerryHolzman

I have a Vitodens 200 - and it does DHW priority just fine....

Something is wrong with how you have your system wired and/or the program settings for your specific boiler.

Please follow the suggestions above.

Perry

Tarpan

Thank you all for the answers.

Yes, the thermostats and the zone valves are wired to Taco zone controller (which is also provides power to the valves). However there is a wire running from Taco's "pump end switch" to the boiler's DE1 (external demand) input. When any thermostat calls for heat, Taco opens the valve and closes the external demand input, which starts the boiler.

Alan, as you mention you use Taco zone controllers as well - how do you wire Taco to the Viessmann?

Thanks!

Tarpan

Wait a sec...

Should DE1 be set as "heating program changeover" instead of "external demand" ?

PS. I found the whole manual extremely confusing. What "heating program changeover" even means?

GW

we all tend to think that English speaking German engineers write these manuals.

I think you leave it as external demand, “weather compensated”

The trick is to get your taco to revive the DE 1 signal. If your Taco ZVC has a ZC terminal then you’re golden, simply connect the hot wire from de1 to ZC and then drop the jumper on the taco

If you don’t have a ZC terminal then you kinda need to get a relay to allow the DE1 to power the Taco

I don’t work with Taco ZVC enough to know if you have ZC terminal

Tarpan

GW, thanks.

however, according to manual external demand cannot be weather compensated and it trumps DHW priority.

By now, I am almost sure "heating program changeover" is the way to go. I will test it tonight if I have time, it takes just 5 min to override a few registers in the boiler program.

GW

Cool, I’ve done about five of the newer 200s, 4 or 5 newer CU3A boilers with DE. Still have not mastered the phraseology, especially when I’m not standing in front of the boiler.

I thought about it more, if your Taco control does not have a ZC input, my previous idea will kill power to the thermostats. Not a big deal if you’ve got the old school mercury thermostats, but you probably have battery units.

You could just take the hot 28/20 leg and run that power through the “end switch” on the Taco, then off to the heat pump- much easier then what I previously stated

If you posted a picture of the wiring diagram of the taco, I could confirm.

Alan (California Radiant) Forbes

Tarpan said:

Alan, as you mention you use Taco zone controllers as well - how do you wire Taco to the Viessmann?

On our jobs, the zone valve control does not communicate with the Viessmann boiler. The boiler will sense that there are no end switches connected to any of the DE terminals and it starts heating when the outside temperature drops below the warm weather shutdown (WWSD) temperature whether or not there's a call for heat.

I think they gave the U.S. DE terminals because we are so used to using end switches and they didn't want to shove their technology down our throats.

How you lost DHW priority, I don't really know.

Tarpan

All, an update to this old post - "heating program changeover" was the answer. The Viessmann's manuals are very difficult to understand. They use lots of special terms, like "heating program changeover" or "external demand" without any explanation what they mean. Awful!

That said the boiler itself is quite nice. It's pouring like a kitten for most of the day, modulated way down. Nice and comfy in a house and my heating bill is about 1/2 of what it used to be with my oil-fired central air furnace.

sunlight33

Coding address A2