Viessmann Vitodens 100-W trouble maintaining temperature

dr911

I have a 2017 Viessmann Vitodens 100-W condensing gas boiler connected to a radiant floor heating manifold. No external pump. 120m2 floor area. Recently had the annual service. Boiler has been on for about a week now heating the underfloor cement slab.

It is my impression that the boiler seems to have trouble maintaining the central heating temperature (at any rotary dial setting) on the digital display.
Burner starts and heats up water (central heating side), only 1 notch on the modulation indicator, heats to ~40-50C for 30 seconds then stops and water temp goes down always to 24-27C for some minutes then cycle starts again ... Same on all dial settings 1-2-3-4-5.
Outgoing water pipe is hot. The return of the central floor heating water is colder to the touch. Return pipes temp ~25-26C. Floor temp is around 22-24C. Temperature in the rooms 20.5-22C.
Room thermostat on MAX in order to keep the flow open (could this be an issue? - I wanted to see how hot the rooms would get). Rooms further away from the boiler seem colder by ~2C. Didn't notice this last year.
Outdoor temp here now is around +14C day +2C night.
No outdoor sensor installed. No error codes.
Previous years have had it set between 3-4 and that gave a constant flow temp of around 35-45C as I remember. Do not recall this odd start/stop pattern. Previous heating seasons rooms at 23-24C and floor ~25-27C.
Domestic hot water is functioning fine. Set between 4-5. Slightly slow to heat up but gets to 60-70C and holds there temp for showers.

Any thoughts or am I just being worried for nothing?

Jamie Hall

My first thought is... for some reason the flow rate is down. You seem to have a delta T in the floor of around 25C, if I'm reading you right, and that's high -- should be around 10C. So I'm wondering if either a the pump is tired, or if there is a filter it's partially clogged, or even the heat exchanger in need of cleaning, or... something restricting the flow.

dr911

The boiler brings the temp up to 40-50C only for a few seconds then the burner goes off and temp on the display gradually drops to 24-27C over a minute or so. After some minutes it fires up again and does the same cycle over. Thus flow temp in the pipes never really stays steady at 40-50C to get that delta T of 25C...

Jamie Hall

No... true. But the point is that the output temperature shouldn't get that high. I'm thinking it's tripping a high limit.

dr911

Jamie Hall said:

My first thought is... for some reason the flow rate is down. You seem to have a delta T in the floor of around 25C, if I'm reading you right, and that's high -- should be around 10C. So I'm wondering if either a the pump is tired, or if there is a filter it's partially clogged, or even the heat exchanger in need of cleaning, or... something restricting the flow.

After some late night forum research it seems the symptoms match 'boiler short cycling' ?
I'm going to have to look into the exchanger plate cleaning. I don't remember them taking this out and cleaning it during service.

GW

What do you mean, no external pump? Something must be moving the water.

dr911

GW said:

What do you mean, no external pump? Something must be moving the water.

The Viessmann Vitodens 100-W B1KA has an integrated circulation pump. No pump with the manifold.

GW

Ahh thanks for clarifying which Vitodens 100

still, no external pump sounds problematic. Tiny zone perhaps you’re good 

mattmia2

Look in the manual to see if there is a strainer built in to the boiler. It isnt uncommon for mod cons to have one. Could be one in the piping somewhere too.

CMadatMe

Possible Diverting Valve isn't fully opening or some type of obstruction/blockage. What are the loops lengths and size of tubing? Is the pump set to setting 3 from the factory or has it been changed? Have you overstepped the residual head availability of the boiler pump? Last thing, Possible kink in a loop and/or air bound?

dr911

CMadatMe said:

Possible Diverting Valve isn't fully opening or some type of obstruction/blockage. What are the loops lengths and size of tubing? Is the pump set to setting 3 from the factory or has it been changed? Have you overstepped the residual head availability of the boiler pump? Last thing, Possible kink in a loop and/or air bound?

Very good questions. Unfortunately I don't have the answers for those as the original installer is unavailable. I was hoping it was just a setting because it was recently cleaned/serviced ut it seems I need to get a Viessmann tech back or an underfloor heating service tech to come take a look and do some troubleshooting.

Some videos of the short cycle and 'odd noise':
https://sendvid.com/3wa0f0eq
https://sendvid.com/x7z1px7i

dr911

Update on the problem - here is what the Viessmann tech did so far:

- Replaced plate heat exchanger
- Flushed floor heating & added ADEY MC3+ to the floor heating system
- Replaced boiler internal circulation Grundfos pump
- Power flushed with chemicals/acid the main heat exchanger

Next step diagnosis from Viessmann tech: clogged heat exchanger - replace it. €650/$750.

I've already spent close to a grand on parts and labor. Is this standard way troubleshooting is done at Viessmann and on gas boilers, part by part replacement at the customer's expense whether or not that particular part is the problem? I'm very shocked that the heat exchanger would 'clog' after 5 years especially after the 2-3h of chemical power flushing did zero to improve the symptoms.

I asked the tech if we order the new heat exchanger and it turns out to not solve the issue if they would take it back so that's left to be seen what they answer. What do you guys suggest we do? Update the Christmas gift to myself to a new boiler & install and throw it in the garbage or take the chances continuing the whack-a-mole with the parts?

jinbtown

dr911 said:

Update on the problem - here is what the Viessmann tech did so far: - Replaced plate heat exchanger - Flushed floor heating & added ADEY MC3+ to the floor heating system - Replaced boiler internal circulation Grundfos pump - Power flushed with chemicals/acid the main heat exchanger Next step diagnosis from Viessmann tech: clogged heat exchanger - replace it. €650/$750. I've already spent close to a grand on parts and labor. Is this standard way troubleshooting is done at Viessmann and on gas boilers, part by part replacement at the customer's expense whether or not that particular part is the problem? I'm very shocked that the heat exchanger would 'clog' after 5 years especially after the 2-3h of chemical power flushing did zero to improve the symptoms. I asked the tech if we order the new heat exchanger and it turns out to not solve the issue if they would take it back so that's left to be seen what they answer. What do you guys suggest we do? Update the Christmas gift to myself to a new boiler & install and throw it in the garbage or take the chances continuing the whack-a-mole with the parts?

Seems like a couple pressure gauges on the supply and return would tell you a lot about the pressure drop and where it's occurring

dr911

Any ideas what I should do at this point?

* find yet another Viessmann service partner to come and look and pay the call out fee to get an additional opinion
* agree to replace the heat exchanger and hoses $$$$... and pray it's the actual problem as "no return policy on the HX" and if that doesn't work...
* buy a new boiler + install @ at least double the cost of the HX replacement, try to sell the old one for parts to make up some of the $ spent on the part replacement.

What worries me about the heat exchanger suggestion is the strange bubbling noise behind the pump, maybe due to the alleged blocked heat exchanger some water is flowing backward or bubbles somehow making this noise? I'm worried it could also be something with the AquaBloc which is all one big (expensive as usual) piece on the Viessmann which causes some obstruction or problem with circulation.

EBEBRATT-Ed

And once again it would seem that any savings by using a mod con are lost. And after only running 4 years or less??

This is a bad situation

dr911

EBEBRATT-Ed said:

And once again it would seem that any savings by using a mod con are lost. And after only running 4 years or less??

This is a bad situation

Agree.

Looking back I would have been better off buying a cheaper combi from some high street brand at half the cost and just throw it out after 5y should it break and replace with a new one. Same at the cost of a single shiny high quality Viessmann.

Alan (California Radiant) Forbes

At this point, do we really know it's a problem with the boiler? Personally, based on what I've read here so far, I think it's a system problem.

psb75

Has anyone contacted Viessmann tech support? Send them a text first.

dr911

psb75 said:

Has anyone contacted Viessmann tech support? Send them a text first.

yes, the viessmann service partner told me he contacted Viessmann tech support and they just told him 'likely calcified heat exchanger, replace'. He told me also they won't take the new part back in case he puts it in and that's not the issue. So I have to spend over $1000 to 'test' if this is really the problem. That's why I'm leaning toward buying a new boiler and selling this one for parts and never touching this brand again.

Alan (California Radiant) Forbes said:

At this point, do we really know it's a problem with the boiler? Personally, based on what I've read here so far, I think it's a system problem.

nobody is really sure what it could be... several different techs just replaced parts with new parts so far, none of which fixed it. underfloor heating flow was tested and it's not blocked. funny enough I've read quite a few horror stories on the Viessmann Community German forums as well about this part whack-a-mole. A few people with similar issues of short cycling from a few years ago but no solid solutions. I even asked there in German, no help at all.

GW

Combis—-no thanks 

hot_rod

The plate HX is just for DHW, it should not have anything to do with heating output unless the valve that toggles between the two is not functioning properly.
a calcified HX would show up as a lack of DHW?

dr911

hot_rod said:

The plate HX is just for DHW, it should not have anything to do with heating output unless the valve that toggles between the two is not functioning properly.
a calcified HX would show up as a lack of DHW?

The DHW was fluctuating luke-warm hot, and the plate HX was dirty with flakes, however not really calcified when they replaced it. There is a new one in there, DHW is 'fine' quite hot, but the boiler temp runs high during DHW operation 75-80C (165-175F) and at times cuts out. This seems to indicate a lack of circulation. Thus they replaced the pump. Pump noise behind pump continues and low flow/lack of circulation still there. Next is the theory of calcified HX. Ran acid via power pump for 2h from flow/return and back. Boiler behavior identical after. In central heating mode, boiler only operates on setting higher than 120-125F otherwise it short cycles. Flow rate is quite low 0.75-1L/min (0.25Gal/min) through the floor heating.

Alan (California Radiant) Forbes

So, you're using this boiler to heat 400 square feet of floor space? That's 2 loops. No primary-secondary, no closely spaced tees, no LLH, no buffer tank? No wonder it's short cycling.

dr911

Alan (California Radiant) Forbes said:

So, you're using this boiler to heat 400 square feet of floor space? That's 2 loops. No primary-secondary, no closely spaced tees, no LLH, no buffer tank? No wonder it's short cycling.

No, ~1300 sq ft. 9 loops.
It's 26kw, but modulating down to 6kw.
No primary-secondary, no closely spaced tees, no LLH, no buffer tank? -> yes, but I didn't install it - had no gas heating system knowledge at the time of install. Trusted the installed to do it correctly.
Worked fine at low temps with good flow for 5 years...

Alan (California Radiant) Forbes

Nine loops at ½ GPM per loop = 4½ GPM = 4,500 BTU when all loops are flowing.
Six Kw is your boiler's minimum firing rate = 10,236 BTU

So, your boiler is creating more than twice as much heat as the system can absorb and will short-cycle. The system worked fine for 5 years and then this happened. There is a restriction somewhere and you've done a few (expensive) things to try and fix it. I'd say the next thing is to pipe it properly.

Here is a Viessmann 100 installation manual. Page 39 has an "Installation Example", Layout 2 that shows Item H as a Low-Loss Header. (Ignore the indirect water heater.) I think this is what you need to make things right.

dr911

Alan (California Radiant) Forbes said:

Nine loops at ½ GPM per loop = 4½ GPM = 4,500 BTU when all loops are flowing.
Six Kw is your boiler's minimum firing rate = 10,236 BTU

So, your boiler is creating more than twice as much heat as the system can absorb and will short-cycle. The system worked fine for 5 years and then this happened. There is a restriction somewhere and you've done a few (expensive) things to try and fix it. I'd say the next thing is to pipe it properly.

Here is a Viessmann 100 installation manual. Page 39 has an "Installation Example", Layout 2 that shows Item H as a Low-Loss Header. (Ignore the indirect water heater.) I think this is what you need to make things right.

Thank you for that link. The US manual is much more detailed than the European one.

Alan (California Radiant) Forbes

BTW, where exactly are you in the world? Both my parents were German (Berlin and Worms) as were my grandparents (Sinzig and Rucheim).

CA_homeowner

Did you ever figure out what was wrong and the fix for your system? It's almost as if you were me, writing your post.

My Viessmann Vitodens 100w (B1KA) from ~2019 is having an issue where it doesn't seem to start circulating the floor heating water if the thermostat kicks on after the boiler has been on in a non-heating mode for more than a few hours. DHW works.

For instance, my thermostat heating set point gets flipped from 62 degrees to 68 degrees overnight at 2am, and if the temperature is 66 degrees, the thermostat sends the command to start heating, the boiler turns on the fan and heats up, but then there isn't water outflow to the floors (there is some heat dissipation along the outflow pipe, but the cold return pipe stays cold). If the thermostat is calling for heat and I turn off the boiler and turn it back on, then it starts pumping heated water through the floors, and it continues until the thermostat stops calling for heat. But the next time the thermostat calls for heat, it's likely the boiler fan will turn on and it will heat up, but won't circulate water again.

Monitoring the boiler power usage, in the heat requested but not circulating scenario (boiler fan on), it uses about 115-120 watts, while after power-cycling it and with water flow, it's using 140-150w steady.

Our system is one zone, about 6 loops?, no outdoor temp sensor, no low-loss header, ~`700-1800 square feet of heated flooring I think.

dr911

CA_homeowner said:

Did you ever figure out what was wrong and the fix for your system? It's almost as if you were me, writing your post.

My Viessmann Vitodens 100w (B1KA) from ~2019 is having an issue where it doesn't seem to start circulating the floor heating water if the thermostat kicks on after the boiler has been on in a non-heating mode for more than a few hours. DHW works.

For instance, my thermostat heating set point gets flipped from 62 degrees to 68 degrees overnight at 2am, and if the temperature is 66 degrees, the thermostat sends the command to start heating, the boiler turns on the fan and heats up, but then there isn't water outflow to the floors (there is some heat dissipation along the outflow pipe, but the cold return pipe stays cold). If the thermostat is calling for heat and I turn off the boiler and turn it back on, then it starts pumping heated water through the floors, and it continues until the thermostat stops calling for heat. But the next time the thermostat calls for heat, it's likely the boiler fan will turn on and it will heat up, but won't circulate water again.

Monitoring the boiler power usage, in the heat requested but not circulating scenario (boiler fan on), it uses about 115-120 watts, while after power-cycling it and with water flow, it's using 140-150w steady.

Our system is one zone, about 6 loops?, no outdoor temp sensor, no low-loss header, ~`700-1800 square feet of heated flooring I think.

It sounds extremely familiar to my situation with a few differences. Our floor heating thermostat is almost always calling for heat at some zone. It's rare that all zones are warm enough for the central heating mode to shut down. I believe I did try turning it off/on many times and it did not make a difference.

I get water flow through the zones are measured by the flow meters in the UFH manifold however it's quite slow 0.5-1L/min (0.1-0.25 gallon ?). The pump area makes a strange hissing noise as well.

I've had 3 Viessmann guys look at it and all scratched their heads. We replaced the heat exchanges and the pump and had the main heat exchanger power cleaned with some acid. Nothing helped one bit. Their diagnosis was blockage inside the main heat exchanger and solution to be exchanged at the price of 80% of a new boiler. I decided to change the boiler. I'm still waiting on it being done due to various issues so now it's sorta heating but at too high level for what I would want it to be at and cycling quite a bit.

Do you have any strange noise around the pump?
Does the pump work?
Do you have flow meters in your underfloor heating? You can have a service provider come by and check it and put a long screwdriver in the middle hole of the pump and see if you can push in and turn while boiler is off or lightly push in while it's on to see if it's turning.

I'm not sure how things work in the US with the Viessmann service providers but I was shocked that in my area, multiple service companies just wanted to replace bit by bit inside the boiler until they found out which worked. This process would cost twice the price of a new boiler!

CA_homeowner

Thanks for responding- I guess there are some differences in our situations.

Q: Do you have any strange noise around the pump?
A: No, the pump is pretty quiet when it's working- I generally don't hear it much above the noise of the burner fan when it's heating.

Q: Does the pump work?
A: Yes, the pump does seem to work if I have power cycled the boiler when the thermostat is already calling for heat (the boiler power requirement of ~140w when working/pumping is 20w more than the 120w when it's not pumping but the boiler fans are on. (20w actually seems like a somewhat low pump motor requirement, now that I think about it).

Q: Do you have flow meters in your underfloor heating?
A: Unfortunately, no flow meters in the underflow heating. It's old, original 1960 copper in concrete, with one zone. I currently verify that the underfloor heating is working with:
1) The temperature sensor I've attached to the floor return to the boiler rises.
2) The boiler power usage increases to the "pumping" range ~140w.
3) I can confirm the heated loops in the appropriate flooring areas with a thermal camera.

Since the Viessmann tech is 70 miles away, I'm not looking forward to attempts at service (since it's a difficult to reproduce problem- basically would have to leave the heat off for a day or so until the tech arrives, and demonstrate to them that the pump doesn't come on when the thermostat asks for heat, and then demonstrate that it does after power cycling the boiler. But then if they change anything and want to see if that fixed it- it would likely take at least 24-48hr to see if the heating still works after disabling the process I'm currently using to cycle the power to the boiler each morning after the thermostat requests heat. Unless they recognize the symptoms as a specific known problem with these units, it's likely going to be cost prohibitive to do a bunch of part replacements and service calls which is frustrating because this boiler was advertised as being reliable and easy to service.

Wondering if I may just have to write off this boiler as a bad decision and search for another manufacturer.

Alan (California Radiant) Forbes

@CA_homeowner I had the same issue with a 200-W. Went around and around with tech. support and finally replaced the control board which did the trick.

@dr911

We replaced the heat exchanges

Do you mean "exchangers"? There should only be one heat exchanger (HX) - in your boiler. Do you have a low loss header?

Both of you have different, unrelated problems.

GGross

One thing that will stop a Vitodens from firing, with no error code, is a no flow scenario. If there is air, if you ever add water to your boiler, if you ever drop pressure, or if you have an auto fill, it is possible that flow has stopped through the heat exchanger, or an air pocket is travelling through the heat exchanger intermittently stopping flow. both of these systems in this post are direct pipe, both are exhibiting symptoms of low/no flow. These boilers should be piped primary/secondary to help guarantee proper flow through the heat exchanger.

Additionally, if either of these boilers is a combi, or has an indirect water heater tank piped to their own tappings on the boiler, it is possible that you have a stuck diverter valve, though that is extremely rare, in fact I haven't seen one on a Viessmann with the exception of a direct piped install that had 6 system pumps (the valve couldn't move against the pumps). The diverter valve changes position depending on whether your boiler is making DHW or heating your house. There is no internal way the boiler knows the valve is in the correct position, it sends a signal to change the valves position and must trust the valve moved correctly, if you have an external pump flowing through it, or some other form of restriction in the piping, that valve may not move position, and the boiler will not fire

@CA_homeowner
It makes sense to me that you may have a stuck diverter valve. When you turn the boiler off and back on the diverter valve resets its position, and the pump is able to flow through the heat exchanger. I would start off by looking at what else in the system may affect flow, do you have any external pumps on the system etc.

dr911

@Alan (California Radiant) Forbes

Do you mean "exchangers"? There should only be one heat exchanger (HX) - in your boiler. Do you have a low loss header?

Viessmann tech replaced the plate heat exchanger.
Second Viessmann tech replaced the pump, which did not help. Third Viessmann tech ran some decalcifying acid through the entire unit for hours with a pressure pump from flow to return and then recommended to replace the main heat exchanger due to calcification.

No low loss header unfortunately but boiler worked well for around 4-5 heating seasons with the current install.

Alan (California Radiant) Forbes

@dr911 I've been looking at the parts diagram for your boiler and noticed the bypass cartridge, #4. Just after it is a check valve, #11 which could be the problem. Back in 2021, I spent days looking for an obstruction in the flow path of a Viessmann 222-F combi boiler and finally found a destroyed check valve that was the culprit. It should be a simple thing to drain the boiler and inspect both the bypass cartridge and check valve. Replace as necessary.





This all harkens back to what @mattmia2 and @CMadatMe suggested earlier.

learningcurve

did you ever fix or diagnosed the issue?

dr911

learningcurve said:

did you ever fix or diagnosed the issue?

Nope. I think Alan's suggestion of the destroyed check valve is good to take a look at but I did not get a chance to go through with it.

chrisl75

I wanted to add my experience with a Viessmann boiler that failed often and mostly it seems when its temp got low and it had idled fora while.

The heat exchanger was replaced under warranty. The cable to the flame rod was replaced with a new one. The electronic control for the igniter was replaced. My pressure gauge on the system now worked again. Stopped and didn't know why one day. Turned out it had sediment in it.

Despite all this, the issues still occurred if the system idled. The biggest pattern I noticed was it had issues starting if below 60C. Using the test mode, I was able to get it to fire a few times. Once above the 60C, it usually would catch and keep going on its own.

I spoke to a family friend who is an HVAC tech. I told him it made me wonder if it was a sensor. Describing the issue, he thought it was a condensation issue. I think he was right. I read up on it and and the condensate trap needs to drain into an open drain. Mine was going to a pipe that went to my sump pit. I don't think the location was the issue but wanted to mention my thought process.

I did wonder if the plastic drain pipe had any blocks. I moved it to a bucket I have for overflow. Some of the gunk that I think was in the pressure gauge came out. I don't know if that was impeding the condensation from coming out but the boiler has since been doing well even if idle. Now that the condensation drain is in a bucker, I actually notice water drips when it fires up. At least now I know excess moisture comes out on startup.

Hope this may help anyone searching this group.