Viessmann Vitodens 100-w trouble maintaining central heating temp

[dr911t]

As already discussed it definitely appears to be a circulation issues, I don't deal with Viessmann but are there any gauze filters on the return side of the boiler. Many manufacturers have them in the return valve or close by. If missed during a flush it can stay blocked for a while and force flow through the plate to plate rather than the desired heating circuit. Just a thought, may not be relevant on the Viessann though.

Don't think there is a gauze filter, at least it's not visible and nothing in the manuals.

I could close all loops on the UFH, drain the boiler from the return side via the return manifold to and see if a lot of junk comes up against the usual direction of flow. Maybe some flakes or something are stuck in the hydroblock. Then again, the Viessmann tech did a power wash from the flow to the return for a few hours in both directions and everything was the same after.... Running out of ideas.

 

[John.g]

That boiler gauge test only indicates that the pump head is at least 2M but of course is not conclusive.
There are really only a few constructive tests left to do IMO.

First, I don't think using two pressure gauges will tell you anything as I stated previously because of the need to install 0 to 2bar gauges and very difficult to see a accurate differential between the two.

I would attach a 0 to 1bar PG on a T piece on the mains hose and do the tests as suggested on the manifold.
I don't know if you can back flush the boiler Hx from the connections to the manifold as the boiler diverter valve I think defaults to the DHW side, you can try it but don't exceed 1 bar, if you do establish a flow then set the mains pressure at exactly 4M and measure the flow rate into a bucket for exactly 1 minute, this may give some indication of the state of fouling (if any) of the main Hx, it may of course depending on the diverter valve position mean that you are measuring the pressure loss through both the main Hx and the DHW PHEW but worth a test anyhow.

KOP kindly supplied a graph of the main Hx pressure loss which I only spotted after going back through the posts and it shows a pressure loss of 2M at 1080LPH (=2LPM/loop) and ~ 3.8M loss at 1350LPH (=2.5LPM/loop). my calculated manifold losses are 1.16M@2LPM and [email protected], so with a clean main Hx you theoretically require a pump head of 3.16M@2LPM and a pump head of [email protected].
A flow rate of 2.5LPM/loop is not achievable even with the pump at 100% but 2LPM is achievable at a pump speed of 86% or higher which bears out what you were getting in former times.
KOP also supplied the recommended limits for the use of a LLH which are < 400LPH or > 1400LPH so a flowrate of 2LPM(1080LPH) does not require the LLH.

 

[IND_Nick]

Don't think there is a gauze filter, at least it's not visible and nothing in the manuals.

I could close all loops on the UFH, drain the boiler from the return side via the return manifold to and see if a lot of junk comes up against the usual direction of flow. Maybe some flakes or something are stuck in the hydroblock. Then again, the Viessmann tech did a power wash from the flow to the return for a few hours in both directions and everything was the same after.... Running out of ideas.

Fair point, what about in the UFH Manifold, inlet valves, etc. If the boiler works on DHW, surely that indicates something on the heating circuit. I appreciate you have flushed each loop but may be worth dismantling flow and return to check. If it worked previously there must be something different now. Obviously easy to say, but harder to find.

 

[John.g]

He is experiencing some problems with DHW after new circ pump installed, post #42 (and others).

 

[dr911t]

I would attach a 0 to 1bar PG on a T piece on the mains hose and do the tests as suggested on the manifold.
I don't know if you can back flush the boiler Hx from the connections to the manifold as the boiler diverter valve I think defaults to the DHW side, you can try it but don't exceed 1 bar, if you do establish a flow then set the mains pressure at exactly 4M and measure the flow rate into a bucket for exactly 1 minute, this may give some indication of the state of fouling (if any) of the main Hx, it may of course depending on the diverter valve position mean that you are measuring the pressure loss through both the main Hx and the DHW PHEW but worth a test anyhow.

I believe when boiler is off the diverter valve is on the CH side but I'd have to double check this.

Just to be clear, the suggestion is to turn off the boiler, close the manifold loops ( flow meters and actuators ), drain the boiler water and run mains pressure through the manifold back into the boiler without isolating the manifold?
Which direction do you suggest the mains into the return side of the manifold so water flows into the return pipe of the boiler, up through the heat exchanger and back down through the diverter valve into the flow side of the manifold?

 

[John.g]

No.
Regarding the boiler, First isolate the manifold in whatever fashion you know best, remove the boiler flow and return where they connect to the ufh manifold then connect hose to return and back flush main Hx and let water flow to drain from the now disconnected flow pipe. When complete connect hose to the (disconnected) boiler flow, let the return go to drain, set the mains pressure to 4M and then measure the flow rate by putting the return into a bucket or whatever for a exact known time, measure this and convert to LPM. Carry out the manifold flow test as described above, both tests done completely disconnected from each other.

 

[John.g]

Got a bit mixed up above.
Re boiler flushing, to back flush (if required) hose to flow side, return to drain.
Normal flush and Flow test, hose to return side, flow side to drain.

 

[John.g]

Alternative Test?.
If you don't want to be messing about purchasing/installing PG(s) perhaps connect hose to boiler return, flush complete system then measure flow rate using the (now disconnected) manifold return at the boiler and using the boiler pressure gauge for monitoring test(s).

 

[hometech]

I think the problem is the opposite. Too much temperature drop due to not enough flow.

Heating Rotary Knob Setting: Between 4 & 5
Boiler temp: 49-50C
Heating flow: 47C
Heating return: 27.5C
Manifold flow meters: ~1L/min flow meters open fully - all thermostats on, 9 loops.

I have it set like this (for now to have heating until the issue is fixed) because it only operates on these temps and higher, any setting below just causes it to short cycle.

where are you getting this accurate data readings from? You have analogue dials yet mention very accurate readings. 20 degree drop is way too high BTW (heat geek videos worth watching). I had a similar problem with low temperature heating that was resolved only this week

 

[dr911t]

Alternative Test?.
If you don't want to be messing about purchasing/installing PG(s) perhaps connect hose to boiler return, flush complete system then measure flow rate using the (now disconnected) manifold return at the boiler and using the boiler pressure gauge for monitoring test(s).

I'll probably be able to do this test although removing the pipes isn't something I would venture into.

I would hope it would be sufficient to just close the loop's actuators and flow meters and feed water at mains pressure through the return side of the manifold - up through the boiler/HX - and back down through the flow and out the manifold flow drain...

 

[dr911t]

where are you getting this accurate data readings from? You have analogue dials yet mention very accurate readings. 20 degree drop is way too high BTW (heat geek videos worth watching). I had a similar problem with low temperature heating that was resolved only this week

the 100-W B1KA's setting dials are analog but the boiler temperature display is digital
the other temperature readings are from a digital IR thermometer
drop is way to high, agreed, likely due to low circulation, been trying point out the circulation fault without spending 1000+ on a new HX...

how was your issue solved?

 

[king of pipes]

You seem to be going round in circles and getting nowhere it's pretty obvious that this problem is a circulation issue, the boiler is getting confused and modulating down the pump speed is also decreased so you have reduced flow you don't want that on ufh, what does the boiler manufacturer say ?? It's in the manual you have no way of knowing what your flow rate requirements are ? So what do you do in this situation read the manufacturers instructions what does it say ? Fit a low loss header Eureka We're getting some where, this will need a constant fixed rate pump with time and temperature control wired to work in conjunction with the boiler, DIY is not a option here get a competent engineer to make the necessary changes needed , it may have worked previously but it's not now is it and why is that simple it's not installed correctly and to the manufacturers instructions ,I very much doubt there's s restriction your hot water works ok? And the water ways in s Viessmann are huge compared to other boiler manufacturers you can fiddle all you like unless you make changes you will not improve the situation. Sorry if this seems a bit harsh but your getting nowhere at the moment . Regards kop

 

[John.g]

I'll probably be able to do this test although removing the pipes isn't something I would venture into.

I would hope it would be sufficient to just close the loop's actuators and flow meters and feed water at mains pressure through the return side of the manifold - up through the boiler/HX - and back down through the flow and out the manifold flow drain...

Do it whatever way you think most convenient and also try and estimate/measure the flow rate at a boiler pressure of 5M (0.5bar).

 

[hometech]

the 100-W B1KA's setting dials are analog but the boiler temperature display is digital
the other temperature readings are from a digital IR thermometer
drop is way to high, agreed, likely due to low circulation, been trying point out the circulation fault without spending 1000+ on a new HX...

how was your issue solved?

so low temperature heating had worked fine on automatic save mode setting (boiler decides based on air intake temp and return temp). In the summer, I changed the pipework (went to 15mm) leaving the exact same arrangement (replaced like for like) and it caused circulation issues (radiators would have flow then they would not have flow and this would then alternate. One radiator had no flow at all when other radiators on the same manifold worked fine. I had to keep raising the circulation temperature and increased the pressure but it seemed to make things worse and still it was not sufficient to keep the property warm. I then noticed that at a setting of 65 degrees (almost normal room thermost heating temp) some of the radiators were still luke warm and the property was cold and yet the boiler was not firing (it sensed that the return temp was sufficient so switched off). Checking the boiler return, it was 2 degrees higher that the flow temp. After pressure checking and temp checking, I discovered that the pipework that had been a former heating zone was creating a short circuit back to the boiler. I capped the pipework so I broke the continuous loop (it became 2 spurs for flow and return). Opened the isolation values and instantly the entire circuit came back to life, all radiators working and back to being able to have the boiler decide what to do. Property nice and warm.

 

[Ric2013]

You seem to be going round in circles and getting nowhere it's pretty obvious that this problem is a circulation issue, the boiler is getting confused and modulating down the pump speed is also decreased so you have reduced flow you don't want that on ufh, what does the boiler manufacturer say ?? It's in the manual you have no way of knowing what your flow rate requirements are ? So what do you do in this situation read the manufacturers instructions what does it say ? Fit a low loss header Eureka We're getting some where, this will need a constant fixed rate pump with time and temperature control wired to work in conjunction with the boiler, DIY is not a option here get a competent engineer to make the necessary changes needed , it may have worked previously but it's not now is it and why is that simple it's not installed correctly and to the manufacturers instructions ,I very much doubt there's s restriction your hot water works ok? And the water ways in s Viessmann are huge compared to other boiler manufacturers you can fiddle all you like unless you make changes you will not improve the situation. Sorry if this seems a bit harsh but your getting nowhere at the moment . Regards kop

Inclined to agree, at this point. To some extent anyway.

While the page you have provided suggests a LLH must be fitted when the maximum flow of the system exceeds the maximum flow of the boiler (which is 23 lpm) and this is probably not going to be exceeded (hence I'm still against the idea of a LLH as a fix, for now), but we don't know what flow was anticipated as a total for the UFH as we didn't design it.

That said, maximum flow or not, the boiler should give adequate flow for at least a couple of circuits, and if I remember this long rambling thread correctly, it isn't doing that. So I don't know what to make of it.

I've suggested some basic tests, but the OP either hasn't done these, or hasn't told us he's done them, hometech is talking about his own system which seems an unlikely comparison (though idea of a short-circuit - perhaps an internal bypass - is something we'd need to be there to rule out), and JohnG and the OP are talking about flushing the system and testing the flow rates in the same paragraph and confusing the life out of me (it's taken me a week to catch up). If they understand each other, fine and well and indeed probably better than my suggestion, and if the OP wants to test the pressure/flow characteristics of the UFH at the manifold, this may be useful information. But until that is done, any further comment is a waste of time.

If the OP can come back to us with a statement that x pressure at the manifold gives 2lpm flow on each circuit (I would be surprised if any circuit required greater flow), then we might be getting somewhere. If it needs 0.5bar pressure, we need a LLH, whereas if we need a 0.1bar pressure, a LLH might not be needed as maximum flow would be 18 lpm and therefore the system pump might be sufficient. But all that would then prove is you'd need a decent heating engineer on site. But I think I've already said this and we are, therefore, indeed, going around in circles.

 

[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)

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