Discuss Viessmann Vitodens 100-w trouble maintaining central heating temp in the USA area at PlumbersForums.net
Lack of inhibitor may cause Hx blockage due to corrosion.
Still can't find the dP across the main (clean) Hx.
Did you note the change in boiler pressure on circ pump start up.?
You have a return temperature back to the boiler that is not a true reflection of the system temp.
Hot water flow is returning back to the boiler too early without dissipating heat which means the flow is mixing with the return and raising the temperature of the return higher than it would otherwise be.
This will always be a function of the pipework route. If the route has not been changed then a valve is causing the issue.
Buy a digital thermometer and measure the temperatures along the pipe, at valves, joints etc. Where the temp rises or drops within a few centimeters is your problem point. it only takes a very small rise in return to create this issue.
As you are using low temperature water for circulation, your problem will be unimaginable to 99% of plumbers who only know about basic combi boilers and room thermostats set at 22 degrees.
If you want a more scientific approach rather than the random guesses, have a look at heat geek's site. Go back to basics, its so refreshing
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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.
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.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.
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.
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 weekI 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.
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).
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
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).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...
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.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?
Inclined to agree, at this point. To some extent anyway.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
The manufacturer's instructions state to fit a LLH in certain conditions. We don't know that the OP's installation meets those conditions.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.
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.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
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