Viessmann Vitodens 100-w trouble maintaining central heating temp

[dr911t]

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?

 

[Gasmk1]

It is just going in circles the flow is not enough the viessmann does not like a large difference in flow and return temp so will cycle the most obvious way is to follow manufacturers instructions and fit a low loss header.

 

[Ric2013]

It is just going in circles the flow is not enough the viessmann does not like a large difference in flow and return temp so will cycle the most obvious way is to follow manufacturers instructions and fit a low loss header.

The manufacturer's instructions state to fit a LLH in certain conditions. We don't know that the OP's installation meets those conditions.

 

[king of pipes]

Ric read what it says bud it states to fit one if it exceeds maximum flow or minimum flow or you don't know what you have it can't be any clearer. Merry Christmas kop

 

[John.g]

Ric read what it says bud it states to fit one if it exceeds maximum flow or minimum flow or you don't know what you have it can't be any clearer. Merry Christmas kop

I would say that we do know to a reasonable degree what the required flow rate is, as the OP did see all the loops flowing 2LPM, 9 loopsX2 equals 18LPM or 1080LPH and viessmann states that the use of a LLH is only required where the flowrates are < 400LPH or > 1400LPH, what could be clearer?. My simple calcs suggest that this flowrate of 1080LPH is achievable with a pump head of ~ 3/3.5M.
Again, all I would suggest is that the OP, if happy/confident to do is, is to measure this flowrate as suggested above using the boiler gauge as a aid.
Viessmann (like most gas boilers) does not like a big flow/return dT but if the flowrate is as calculated then this will not be excessive.

 

[hometech]

The important rule of thumb with water in pipes...that is that it must go somewhere...lol.

If the flow had its heat extracted to such an extent that the returning water was so cold it would damage the hot exchanger and so the boiler closes down (as is being suggested) then how is this happening so quickly (as in the video). That is not the answer.
Boiler shut down on return temp (which is what is happening) can only be
a. the water is actually at the temp for cut-off
or b. the temperature probe has failed.
If you can establish the resistance (ohm range for the sensor) from the manufacturer then that would be an easy test to do.

 

[Ric2013]

Ric read what it says bud it states to fit one if it exceeds maximum flow or minimum flow or you don't know what you have it can't be any clearer. Merry Christmas kop

I see what you're saying, but we have inline flow gauges so we do know the flow (though you might have a point about the minimum flow). And since the system isn't working even if we reduce the number of circuits being fed, I think it's fair to say the issue at hand is more the insufficient working head than that the boiler maximum flow is being exceeded. But we can also check the required head and then we'll know if the boiler is doing its job.

This is a high efficiency boiler but it will never run in condensing mode if we couple it to a LLH. Which is why, for me, that would have to be a last resort.

I'm not sure we can take that 'if the flow is unknown' comment literally. It would apply if we thought the flow might fit within the boiler parameters most of the time but might be exceeded on occasion or we couldn't prove we weren't pushing the envelope and I think what Veissmann is saying is fit a LLH if there is doubt about the flow being excessive.

But 'doubt' surely doesn't apply to a case where we have flow metres fitted. It might apply to a boiler serving 4 heating zones or multiple TRVs where we could assume flow is unlikely to exceed x lph but where it might on occasion giving rise to excessive flow / return delta t or if we thought the UFH might require as much as 3lpm per circuit. Take the comment literally, however, and you'd fit a LLH when performing a boiler swapout on a 2 bedroom flat because you won't know the exact flow rates required on the existing radiators that could be 50 years old.

Surely if it were that clearcut, Veissmann would already have said so?

 

[hometech]

any circuit will need a differential in pressure/flow for the water to circulate. The differential pressure/flow required to create this circuit is always very small, so a minor differential can have major impact..which then impacts something else and so on. I would not have believed just how small a difference can have an impact until it happened to me.

Water takes the path of least resistance so if the path is easier back to the boiler than through a circuit, that's what will happen and that is what is happening here. (well, the boiler is saying that)
The massive drop in temp will not be possible over these short distances so rule that out as Its only the hot temperature that is telling the boiler to stop heating.

If the pressure/flow is too high, a water brake will help in the balance but the water will still have the same characteristics (easiest path).

You either have a short circuit or the thermistor is faulty because this is a return water temperature issue.
Multi meter rules out faulty thermistor, IR thermometer shows actual temp.
Prove those first.

 

[Gasmk1]

The return temperature is too big a difference because the flow is not enough.

 

[ChrisMMM]

Did this ever get resolved? I have the same issue ..

 

[Lou]

(Just bumping a few older threads, they will drop off what is new soon)

Check Out Plumbing News!