Failure? of Viessmann Vitodens 200 control head 7450 182

6bev · November 25

I have in the US a 17 year old Weismann Vitodens 200 with a Comforttrol Control head 7450 182 GB/FR.

I had a routine service call in September. Last week I tried to heat the house... The DHW (faucet icon) works fine, but the radiant heating never starts up when rotating the top knob on the control head from "faucet" to "faucet+radiator". The thermostat, loop valves and 24VAC transformer all work fine, and are presenting a signal "run the heating loops now" to the main boiler. However, we can get the radiant heat loop to run by simply switching to "Party" mode on the middle switch on the control head, so this means radiant pumps work fine too.

The service tech hypothesizes that the top rotary switch in the control head is bad, and proposes the $1900 experiment to replace the control head with a new one. This seems like an expensive overkill.

Do these rotary switches in these control heads fail? Mine has no indication of corrosion, dust, etc, and the lithium battery is at 3.6V as expected.

How hard is it for a competent electronics tech to open the control head, remove the PCB assemblies as necessary, and clean the wipes on the rotary switch(es)? Or is the switch a shaft encoder that needs to be reset by the firmware?

I see on Austrian ebay a number of control heads 7450 180 DE/FR for about $200. Is the difference between the -180 and the -182 model merely the language (internationalization locale)? Or is it (also) 120V vs 240V?

Why won't Austrian ebay seller(s) ship to the US? (The lithium battery perhaps?)

GGross · November 25

This particular part number is a bit different than I have seen but there should be programmable times for each of the 7 days of the week for the heating and DHW system, party mode would bypass those settings but I do not think it would change the operating mode. is it possible someone messed with the time on the boiler, or adjusted the programmed day settings? have you tried moving both the daytime and nighttime setback rotary knobs up to 5 or higher and see if the boiler fires without party mode enabled? party mode should only extend the daytime setting, not override the top rotary dial. If you can take a pic of your control it might help, also I do see the only part that is available for the control is the 3 knobs, so it is possible they had some failures.

Is this control separate from the boiler like a thermostat or installed on the boiler?

one more question, when you run party mode does the boiler actually fire?

6bev · November 26

Thank you for your help. Photos below.

Below are front and back photos of the control head. It normally is mounted in the living area, but can also be mounted directly in the boiler, which is the configuration I use for testing and maintenance.

I reset the unit by pushing. the small recessed push button behind the door "factory default settings", confirmed the reset, and then confirmed the date and time.

I see no change in behavior after resetting to "factory default settings".

If I set the "heating program selector switch" (the top most 3-position rotary switch) to DHW (faucet icon), the front panel LCD says: Program A/ DHW on/ Space heating Off/ Boiler temp 97F, which is as expected.

If I set the top knob "heating program selector switch" to radiant+DHW (radiator + faucet), nothing on the front panel LCD changes.

Leaving the top knob at "radiant+DHW", I rotate the middle knob, which is "selector knob for normal room temp", from 5 to 8. The LCD changes to an error message: "First select Program faucet+radiator!" but that's where the top knob already is. After 5ish. seconds the error message goes away. The boiler does not start.

With top knob at "radiator+faucet", middle at 5 (mid range normal), bottom at 3 (mid range night), and press party mode, the LCD switches to party mode display to 68F (my setting), and the boiler starts right up.

It is about 45F throughout the house and 50F in the boiler room. All thermostats say they want radiant heat, and hot water is flowing through the corresponding pex loops.

GGross · November 26

You are changing the switch positions while the unit is attached to the boiler correct?

It certainly seems like that switch is not working as you suspected, that doesn't explain why the party mode is working though. Can you turn the night temp knob up to the same level as day time to see if any changes occur? How long will part mode let you keep it active for? Party mode is really just there to keep your day time settings active later than normal, in case you have a party and are up late 😁 Viessmann got rid of that button on new models which is unfortunate because it's fun! I don't have great access to the programming for these, the manuals all have 2 different controls in them as they changed these to a more complicated version (which is what most of this vintage have near me)

I am wondering if the heating needs to be setup, i would think out of the box everything is set to fire the boiler though

Viessmann does sell a replacement for this control (according to their US people I talked to) which is the following part number, it's kind of pricey, but should be a relatively easy DIY think as you have already done it at least once

7133 383

http://bostonheatingsupply.com/7133383.aspx

Viessmann also does offer the 3 knobs still as a set, but I doubt the physical knob is the issue, as you noted it has to be something about where the knob interacts with the board. If the knob pulls off of there easily you could try pulling it off and see whats back there. This is one of the controls I don't have sitting around here so I'm sorry I'm not more help.

GGross · November 26

I would also consider changing the batteries before doing anything drastic just in case

6bev · November 26

I'm doing all of my testing with the main control head plugged directly into the boiler, and with a "null" control head plugged in upstairs.

I replaced the 2 AA (AAA?) batteries a few weeks ago. The old ones were at 0.1V and had a tiny leakage, which I do not believe dribbled anywhere in the control head.

It does not seem to matter if the null control head is plugged in or not. The null control head has an LCD display which I presume is to display temp, but there's nothing displayed. The null control head contains a 4-bit NEC microcontroller, presumablly to decode the wire format data coming from to the boiler, and also to drive. the LCD.)

More. tomorrow. I have to leave the job site now.

HomerJSmith · November 26

If you are able to pull the knob off and have access to the switch, with the power disconnected, spray electronic cleaner that you can buy at any auto parts store and move the switch with a pair of needle nose pliers.

109A_5 · November 27

Hello 6bev,

At 17 years old the switch contacts may have oxidized, how often was the switch exercised ?

Also and more likely IMO it may not be the switch contacts, the solder connection of one of the switch leads to the circuit board may have failed.

GGross · November 27

https://forum.heatinghelp.com/discussion/comment/1822780#Comment_1822780

If that is a Viessmann branded room controller those may operate a bit differently than what you might expect. My experience with the newer ones is that they take complete control of the boiler less of a thermostat and more a remote boiler control. I would try cleaning the contacts as recommended above, and double checking that both controls are on, if settings can be changed on the room controller try to make sure both controls match as far as the operating mode. I'm wondering if this may be why switching to factory settings didn't change anything, it's worth a shot anyway before spending several hundreds of dollars

6bev · November 29

The "heating program selector switch" is exercised 2 times/year. It seems to work, though; see below.

I took apart the control head so I could work on the printed circuit board. (PCB), as necessary. Roughly:

Power off to boiler
carefully uniformly prying and pulling as necessary, remove control head from boiler or remote control site
Press in 4 tabs holding the back of the control head, and seperate the front from the back.
The back contains 2 AAA batteries; check that the batteries are new and any electrolyte leaked from prior batteries have not corroded contacts. Clean up and fix as necessary
Remove 5 small screws on the periphery of PCB. Don't loose them. Match screw driver tip shape carefully to screw. Screws at 3 corners, and 2 middle left, in the picture below. (Screws are out in the photo.)
Remove the three knobs on the right side of the front of the control panel. Carefully pry from underneath the pointer, gently exerting pressure. Set these aside. They appear to be identical, although the top knob had a different shaft color.
Carefully pry/pull the PCB free from the front of the control head. All that is holding it in place are the ~20 pins of the long vertical white connector on the lower left. Grab onto to the white connector and pull straight off; some rocking may be needed.
This will reveal the backside of the LCD PCB nestled in the front of the control panel. Do not turn this sub-assembly! If you do, you may loose the soft pliable plastic backsides of the push button switches (small black circles), as well as 4 spacers on the LCD PCB. See this photo:
The 3 position switch for the "Heating program selector switch" is self contained, and has 4 mechanical alignment pins (and ground), and 4 pins for signals. Nothing clever.
1. On my unit, 8 solder joints looked good, but I reflowed them and soldered to the PCB as necessary.
2. The wafer in the switch glistened slightly, presumably from some grease from the manufacture. I used electronics de-duster to blow away any crud, but did not have any electronics cleaner to use (yet).
3. Using the continuity tester of my multi-meter, the 4 signal pins appear to have a single common (pin #2 in my nomenclature), and 3 one-hot pins, one for each of the 3 positions of the switch. Nothing clever. Nothing changed before and after my reworking the solder joints.
Examine both sides of the PCB for bad or missing solder joints, burned parts, etc. Mine was in pristine condition. As far as I could tell, there is nothing charred, no nicotine stains, no smoke damage, no dog hair, no dust, lint, cracking etc.
Reassemble in the reverse order of take-apart.
Redeploy to boiler.

After all this checking, there was no change of behavior ☹️.

The "Heating program selector switch" still is not registering the fact that both radiant+faucet are selected.

I can't do much more diagnoses without a PCB schematic and parts placement diagram, which I doubt I can pry free from Weissmann, although I'll try on Monday. I really doubt they will give me the source code for the micro controller(s) on this board.

GW · November 29

Viessmann is a little stingy with their electrics schematics. Hope it works out.

109A_5 · November 29

WoW, you have gone way farther than I would have expected and nice detailed write up. With that detailed write up I was hoping for a more positive outcome.

My thoughts;

A sharp picture of the main board would help me and also the side that the potentiometers and switch was on would have been nice too.

On the display is there an Icon that normally acknowledges that mode selection ?

Is there anything else in the system that would lock out that mode ?

Back inside the control head it looks like there are surface mount chip resistors connected to the switch leads, I would have measured their resistance and inspected their solder joints too.

You could also verify the defective mode's circuit path back to the microcontroller or it may just be buffered by a transistor and simply goes out the 14 pin connector. Reverse engineering enough to gain a repair can be time consuming.

Also if you are brave and can manage it with the unit mostly assembled you could measure the voltage at the mode switch (pins on top side of the switch) to see if that reveals anything. I would use the negative side of the blue battery for the meters negative lead.

With Powered up troubleshooting you have to be extremely careful you can make things much worse with just a slip of the probe. However the advantage is it can tell you a much more dynamic story.

Failure? of Viessmann Vitodens 200 control head 7450 182

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