Optimising Vaillant Ecotec plus 637 always cycling
- Thread starter Fireitup
- Start date
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Page 44 onwards in the MIs.
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My IR gun doesn’t give correct temp when measuring the pipes but seems to be fine for everything else! I’ll pick another one up soon.
Yes, ABV installed
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Re your post #1
"The boiler runs steadily for 5 minutes around 49°C (flow temp D40 at 49°C, return temp D41 at 44°C). The manifold feed temperature reaches a little higher than 45°C, while the manifold return is just over 30°C then all of a the boiler significantly increases flame, driving flow temperature to 70°C and triggering cycling behavior."
Assuming UFh only on then the boiler flow was, based on your 11LPM UFh flow, 8.68LPM, return at 30C, if so then it would require a bypass of 24.3LPM at 49C mixed with the return flow of 8.68LPM at 30C to give a d.41 of 44C, mathematicall correct but seems logically "daft"? (maybe too much (me) Jameson's), maybe just shut that by pass to max 0.5/0.6? even temporarily.
I think overall, to try and pin down the problem, is to run (sometime) on rads only that you know or think have a output of > say 7 or 8kw, then take some readings.
"The boiler runs steadily for 5 minutes around 49°C (flow temp D40 at 49°C, return temp D41 at 44°C). The manifold feed temperature reaches a little higher than 45°C, while the manifold return is just over 30°C then all of a the boiler significantly increases flame, driving flow temperature to 70°C and triggering cycling behavior."
Assuming UFh only on then the boiler flow was, based on your 11LPM UFh flow, 8.68LPM, return at 30C, if so then it would require a bypass of 24.3LPM at 49C mixed with the return flow of 8.68LPM at 30C to give a d.41 of 44C, mathematicall correct but seems logically "daft"? (maybe too much (me) Jameson's), maybe just shut that by pass to max 0.5/0.6? even temporarily.
I think overall, to try and pin down the problem, is to run (sometime) on rads only that you know or think have a output of > say 7 or 8kw, then take some readings.
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No confusion at all. A high output underfloor heating system will give you about 80 watts heat output per square metre × 85m² = 6.8kW. 8 radiators adds another 8kW (ish) so around 15kW. In reality your UFH is probably running at around 50w/m² unless you have a particularly cold and draughty house, knocking it back to 4kW total, add those radiators on and you're at about 12kW.
So yes, boiler likely to be massively over-sized for the system it's attached to.
Have you been provided with a copy of the calculations?
sensoCOMFORT is excellent, I highly recommend it.
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Agree, but strange that it sits at 49C for 5 minutes or so with a target temp of 65C.
Refiring after cycling was allways(and probably still is) a challenge for Vaillants as not only do they fire up at ~ 65% of rated output (range rating has no effect on this) which is fair enough as quite a lot of gas boilers fire up similiarly but it then holds this setting for 60 secs before allowing modulation, a good long anticycle time is required + a decent flowrate, the 637, at a ignition setting of 24kw to achieve, ideally a dT of say 20C requires a flowrate of 17.2LPM. If the return temp can realistically be reduced to say 35C then the flowtemp will be 55C and allow another 10C before reaching 65C (target temp here) and 15C before trip at 70C, problem is that when the controls are released to modulate after this (if) 60secs then the boiler will fire flat out and can overshoot and trip at 70C, this is where the range rating can help, the trick is to get the recycle time just long enough so that the flowtemp is say within 5C of the target temp when modulation allowed.
Some boilers seem to start modulating down after fire up before they reach target temp, a relation of mine has a 8/9 year old 20kw Vokera VisionS and the controls are released to modulate 5/10 secs after fire up and I can hear the fan reducing speed before the target temp is reached resulting in a overshoot of ~ 2C and very occasionally 3C.
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Same issue on a 15 year old vaillant. Apparently it is normal, and if your new boiler is doing this then I suppose it is.
Have you considered calling vaillant out under warranty?
Put black PVC electrical tape around the pipe, as the target.
Understood, I assumed it was based on amount laid rather than coverage, i'll adjust and continue to monitor through out today.
Running a lot better today. only 1 cycle but happened when HW reached temp and actuator switched off.
Reduced flowrate to 1.5l/min
d.00 set to 16kWh
d.02 set to 25 mins
Boiler is running at 65°C with flame at minimum:
d.40 = 65°C
d.41= 57°C
I've checked for any s codes but only code I see saying burner on.
Using infrared temp gun
- Feed from under boiler is 65.9°C
- Feed just before entering manifold 65.7°C
- Return to boiler just after exiting mixer 34.0
- Return just before entering boiler 33.8°C
I could tell that by touching the return there is no way it's at 57°C, i've looked for fault codes and returns no faults on the display so any reason why d.41 report such a big difference?
Reduced flowrate to 1.5l/min
d.00 set to 16kWh
d.02 set to 25 mins
Boiler is running at 65°C with flame at minimum:
d.40 = 65°C
d.41= 57°C
I've checked for any s codes but only code I see saying burner on.
Using infrared temp gun
- Feed from under boiler is 65.9°C
- Feed just before entering manifold 65.7°C
- Return to boiler just after exiting mixer 34.0
- Return just before entering boiler 33.8°C
I could tell that by touching the return there is no way it's at 57°C, i've looked for fault codes and returns no faults on the display so any reason why d.41 report such a big difference?
Is this with UFH only on?
What was the total UFH flowrate and manifold (mixed)flow/return temperatures, if only from memory, (the return manifold is 34C?.)
The reason for the (57C) d.41 and the (33.8C) return temp difference just before the boiler is because the internal bypass is mixing boiler flowwater at 65C with the UFH manifold water at 34C, I can calculate that once you give me a few more numbers, the bypassing gets worse with reduced flowrate, I have seen plenty observations like yours, from others, the internal bypass opening pressure can be increased which will help.
What was the total UFH flowrate and manifold (mixed)flow/return temperatures, if only from memory, (the return manifold is 34C?.)
The reason for the (57C) d.41 and the (33.8C) return temp difference just before the boiler is because the internal bypass is mixing boiler flowwater at 65C with the UFH manifold water at 34C, I can calculate that once you give me a few more numbers, the bypassing gets worse with reduced flowrate, I have seen plenty observations like yours, from others, the internal bypass opening pressure can be increased which will help.
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