Viessmann Vitodens 200-W B2HA heating issue

[John.g]

Is this a low loss header?.

 

[ShaunCorbs]

Yes John

 

[ShaunCorbs]

I am not sure that it is done like in UK here in Lithuania. I checked papers, the only thing I managed to find was the information I provided in earlier posts. I might do some calculations, but I have to find information about insulation etc., have to go through some more papers. Last year heating season I burnt ~4000 m3 of gas, if it matters.

Might be best to see if you can work out the outputs of your rads eg find similar ones in the same sizes they should state outputs etc

 

[kgfx_5]

In the whole house there are 18 radiators, sum of their output at 75\65\20 is ~46.8 Kw. Plus UFH, 3 zones, ~65 m2 combined. On each floor there is a distribution manifold. On the first floor there is also a UFH pump unit. There are also 2 towel warmers, not sure about their output, and 3 small underfloor radiators near windows.

 

[gpbeck]

There will be no condensation at those high temperatures. System should be balanced to produce a 20 deg flow/return drop. Boiler can do water priority (higher flow temps for water heating) so can you not try it at lower flow temp, 55 or 60?

 

[kgfx_5]

There will be no condensation at those high temperatures. System should be balanced to produce a 20 deg flow/return drop. Boiler can do water priority (higher flow temps for water heating) so can you not try it at lower flow temp, 55 or 60?

Those are just numbers from radiator seller. Right now my boiler is working (when it is working) on 55 C, and it is fully enough for now. I would like to try it on 45, but it just short cycles, never actually starts.
Flow is around 53, return around 38.
——
Hot water is prioritized, there are no problems with DHW.
——
Question - best way to deal with this problem would be buffer tank, boiler change to 32 kw with better modulation, or 2 boilers 25 kw each (worst scenario for me)?

 

[John.g]

Those are just numbers from radiator seller. Right now my boiler is working (when it is working) on 55 C, and it is fully enough for now. I would like to try it on 45, but it just short cycles, never actually starts.
Flow is around 53, return around 38.
——
Hot water is prioritized, there are no problems with DHW.
——
Question - best way to deal with this problem would be buffer tank, boiler change to 32 kw with better modulation, or 2 boilers 25 kw each (worst scenario for me)?

Have you any idea what the primary flow through the LLH is?, assuming it is running above at ~ 12kw, then a dT of 15C means its circulating 11.47LPM , 0.69m3/hr, this primary flowrate is determined by the primary circulating pump setting and is presumably fairly constant through the boiler Hx, the burner will trip at 60C so the boiler output can only rise to 17.6kw before burner trip (assuming boiler return temp remained at 38C). One might think that the primary boiler flowrate might be based on the boiler output and a dT of say 20C, this means a primary flowrate of 35.1LPM, 2.11m3/hr. this would then, theoretically result in a dT of 5C at 12kw output.
But more importantly it means the boiler could fire at say ignition conditions of 30kw without exceeding 50C from a return temperature of 38C, maybe this is why you have to raise the SP to 55C?.
The boiler manual should show the max flowrate recommended.

 

[gpbeck]

If your boiler is pumping more than the system pump off your low loss header this would shut it down. Think I’d try range rating the boiler down to say 20kW and see what happens. Your radiators are probably oversized so are no guide to your actual heat loss. I think I’d do my own heat loss which would give an idea.
Perhaps the best thing would be a heating engineer to visit you and advise.

 

[John.g]

One (good) reason for a LLH is that it allows primary and secondary circuits to operate at completely different flowrates, for example you may have the primary circulating 35.1LPM as above and the secondary only requiring say 3.6LPM, the primary will recirculate 31.5LPM and supply 3.6LPM to the secondary with the secondary returning the 3.6LPM to the primary to give the 35.1LPM primary flow&return?.

 

[kgfx_5]

Have you any idea what the primary flow through the LLH is?, assuming it is running above at ~ 12kw, then a dT of 15C means its circulating 11.47LPM , 0.69m3/hr, this primary flowrate is determined by the primary circulating pump setting and is presumably fairly constant through the boiler Hx, the burner will trip at 60C so the boiler output can only rise to 17.6kw before burner trip (assuming boiler return temp remained at 38C). One might think that the primary boiler flowrate might be based on the boiler output and a dT of say 20C, this means a primary flowrate of 35.1LPM, 2.11m3/hr. this would then, theoretically result in a dT of 5C at 12kw output.
But more importantly it means the boiler could fire at say ignition conditions of 30kw without exceeding 50C from a return temperature of 38C, maybe this is why you have to raise the SP to 55C?.
The boiler manual should show the max flowrate recommended.

Those are some next level calculations for me, lol. I don’t know about flow levels, unfortunately. Right now, for example, I see the picture of boiler that turned off burner and for whatever reason temperature doesn’t drop fast, so this time it managed to heat something. But, difference between flow and return on LLH is 2 C. While there are still 5 radiators open, not including UHF and other stuff.

Manual suggests using 6th position on boiler’s pump that equals 2.8 m3/h when deltaT is 15C? I am not sure how installers set them up. Are those real numbers or am I looking the wrong way?

If your boiler is pumping more than the system pump off your low loss header this would shut it down. Think I’d try range rating the boiler down to say 20kW and see what happens. Your radiators are probably oversized so are no guide to your actual heat loss. I think I’d do my own heat loss which would give an idea.
Perhaps the best thing would be a heating engineer to visit you and advise.

Yeah, I will try finding someone who can help me locally. Regarding range rating, You mean like this?-

 

[kgfx_5]

One (good) reason for a LLH is that it allows primary and secondary circuits to operate at completely different flowrates, for example you may have the primary circulating 35.1LPM as above and the secondary only requiring say 3.6LPM, the primary will recirculate 31.5LPM and supply 3.6LPM to the secondary with the secondary returning the 3.6LPM to the primary to give the 35.1LPM primary flow&return?.

Yeah, but I have only one heating circuit.

 

[gpbeck]

Yes but range rating of 20% would be 10kW which is less than boiler minimum. 40% would be 20kW.

 

[John.g]

Yes, 2.8m3/hr at a dT of 15C = 49kw. Presume this pump is internal to the boiler and isn't one of the Wilos where its very easy to derive their circulation rate from the pump curves?.
Anyway, IMO, the main thing to try and confirm is that the primary pump is set as high as possible, don't think you can do much more than that.

 

[John.g]

Yes but range rating of 20% would be 10kW which is less than boiler minimum. 40% would be 20kW.

Maybe the boiler has been range rated to 20kw (40%) since installation??.

 

[kgfx_5]

Yes, 2.8m3/hr at a dT of 15C = 49kw. Presume this pump is internal to the boiler and isn't one of the Wilos where its very easy to derive their circulation rate from the pump curves?.
Anyway, IMO, the main thing to try and confirm is that the primary pump is set as high as possible, don't think you can do much more than that.

View attachment 79151

Yes, I was referring to internal Viessmann pump.

 

[kgfx_5]

Maybe the boiler has been range rated to 20kw (40%) since installation??.

No, I just did it to check as suggested. Unfortunately, even when I fully opened all my tado TRVs, boiler didn’t fire up at all while being on 40%. I just don’t get why… The demand is clearly there, and it fires up when I do this on 100% power. Maybe 20 kw is not enough for the boiler to fire up? Sounds a little bit stupid.
Forget it, heating curve changed when I changed rate to default 1.4, and 1.4 means 37 C flow when it is 10 C outside. This thing never showed such temperatures.
I guess maybe I should connect my tado thermostat back. At least it allowed me to heat my house😃.

 

[gpbeck]

Maybe the boiler has been range rated to 20kw (40%) since installation??.

Well with a minimum of 10kW that would be a sin!

 

[John.g]

I guess maybe I should connect my tado thermostat back. At least it allowed me to heat my house😃.

Does the thermostat control a number of zones or are there a number of them?.

Still a bit surprised the boiler doesn't seem to be able to get away, 10 minutes anticycle with the circ pump circulating through the LLH should knock the flow/return temps well down one would think, have you noted them towards the end of the anticycle time?

 

[kgfx_5]

Does the thermostat control a number of zones or are there a number of them?.

Still a bit surprised the boiler doesn't seem to be able to get away, 10 minutes anticycle with the circ pump circulating through the LLH should knock the flow/return temps well down one would think, have you noted them towards the end of the anticycle time?

In tado system, as far as I understand, every TRV has the ability to call for heat via so - called “extension kit” that connects directly to the boiler. So every TRV is essentially a thermostat. Yes, I have several rooms as “Zones” in tado.

Technically, my boiler is old enough it still supports KM-Bus, to in theory Tado should be able to modulate it. In practice however, tado struggled. I will speak with their support one more time, maybe they could suggest something.

Regarding flow-return temperatures, those are -
~25-28 C in the end, boiler’s pump is working whole time. Flow and return temperatures are the same, maybe 1 C difference. And it just keeps circulating that 28 C water through radiators. But it won’t fire up until I set a heating curve very - very high. I imagined WC should modulate a temperature set by heating curve and provide it constantly to the system, like if it is +10 outside and curve is set to 1.4, in a perfect world boiler should fire up and modulate itself so it can provide stable 37 C to the system until outside temp will change?

 

[John.g]

That's strange, normally once the boiler is enabled at SP-5C and the anticycle time has elapsed then the boiler will refire.