Vailant ecotek plus 438 + dab evoplus 80/180m - proportional or constant curve

[ScottishGasMan]

These boilers need a high flow rate to correctly ignite and modulate properly, if you look in the manual it actually states equivalent of about 1600 l/hour flow rate minimum. The head loss across the boiler alone at that flow rate is just over 4m.

They don't like variable speed pumps as they are not in control of them, you tend to get better performance off these boilers having pump at a fixed output.

Part of the issue is people quite often "over specified" this boiler, as in used it where a much smaller unit would have worked, The burners obviously do modulate, but they need to flow enough water to ignite, test the sensors, then modulate to what they need, which takes about 30-60 seconds going between 60-100% of full power regardless of what its range rated to on D.0

Unless youre 13 radiators are all 2 meter long double panel double finned units, then theres a fair chance the boilers bigger than it needs to be. However it's not that much of an issue so long as you provide it with plenty of flow rate.

I'd be tempted to set you're output to winter output, fix the pump speed, and when its running at the fixed output use D.40/41 to gauge the difference between the flow and return and try get the pump speed to a point that gets you around 10-15 degree difference on the radiators, then run the DHW on its own and if it holds between 5-20 degree difference without cycling then you should be good.

 

[Johntheo5]

I think most boilers fire up with a high firing rate of up to 65% and then modulate, and some say the Vaillant maintains this for up to 60 secs (is this your reference above, as well?) and some then experienced problems with the flow temperature exceeding the target temperature +5C and tripping the burner and constantly cycling in trying to get away.

A lot of Vaillants can be run on Return Temperature Control, d.17, but I rarely, if ever see anyone posting re this as it seems a nice option to at least try anyway, here is a quote from me re someone who had briefly run in this mode.

"(Vaillant Ecotec 418) briefly c/o to Return Temp control, it had been set to Flow control with a SP of 70C which gave a return temp of 55C, the target temp was then set to 55C on c/o and the flow/return temps then fell to 47C/37C, with a observed d.05 of 36C which is the "Determined heating flow set target temperature", presumably, this was now the calculated return temperature required?. Unfortunately, as there were more pressing problems at the time, the test was not continued."

 

[Johntheo5]

Checked my own flow rates now, I get a flow rate of 0.82m3/hr on a CP setting of 5.5M on DHW only, almost twice that of your flow rate of 0.4/0.5m3/hr.
I get 1.15m3/hr at a pump head 5.5M with CH only on, 12 rads, ~ 20kw total output rating.
You might note your readings on CH only as that pump might be a little suspect, you were getting 1.1m3/hr originally on DHW only,post #5, almost identical to mine and which seems more normal to me.

 

[ant2]

These boilers need a high flow rate to correctly ignite and modulate properly, if you look in the manual it actually states equivalent of about 1600 l/hour flow rate minimum. The head loss across the boiler alone at that flow rate is just over 4m.

They don't like variable speed pumps as they are not in control of them, you tend to get better performance off these boilers having pump at a fixed output.

Part of the issue is people quite often "over specified" this boiler, as in used it where a much smaller unit would have worked, The burners obviously do modulate, but they need to flow enough water to ignite, test the sensors, then modulate to what they need, which takes about 30-60 seconds going between 60-100% of full power regardless of what its range rated to on D.0

Unless youre 13 radiators are all 2 meter long double panel double finned units, then theres a fair chance the boilers bigger than it needs to be. However it's not that much of an issue so long as you provide it with plenty of flow rate.

I'd be tempted to set you're output to winter output, fix the pump speed, and when its running at the fixed output use D.40/41 to gauge the difference between the flow and return and try get the pump speed to a point that gets you around 10-15 degree difference on the radiators, then run the DHW on its own and if it holds between 5-20 degree difference without cycling then you should be good.

Thanks for the input. Mine is always set to winter mode all year round.
As per above for hw I never found any benefit with the cc, the variable does boost up to 6m at times when I was watching it.

John-

Definitely only getting 0.4-0.5mh3 registered via the Q stat on the pump for HW as just checked- unless this reading is something else.

Never had any issues with heating on PP, heat up speed has always been amazing compared to our old grundfoss pump and compared to others.

Just tested CH on 38kw
, head is 5.0m at 0.5m3h With 27 degree diff on flow and return for first 5 mins

After 10mins, flow 73, return 50. 5.5m = 0.8m3h

And house starting to get too hot so stopped test.
In winter I'll be reducing flow back to 65 and will check d40 and d41 to see if it's worth hard setting but I never had issues with cycling for heating.

All rads are doubles, most are 1.2m+. Boiler probably is a bit oversized as house is modern.