Vaillant ecoTEC not reaching desired flow temperature

[Yikes]

I have a Vaillant ecoTEC plus 418 on a vented system with twin 2-port valves, ABV, VRC 470f, VR65 and outdoor temperature sensor.

The CH Part Load is set to 5kW. HW is timed to come on at 5am and CH at 6am.

HW is working as I expected but CH cuts off when the flow temp reaches 50C, well below the target set by the controller:

Any suggestions why this is happening please?

Mike

 

[Mottie]

5Kw? Unless I’ve read this wrong, surely 5Kw is surely too low. Why do you have an 18Kw boiler? I have the same boiler in an sealed/unvented system. Lowest I set it at is 12kw and that’s in summer for the hot water only. I wouldn’t expect the heating to get warm enough at 5Kw.

 

[Yikes]

I have an 18kW boiler for the same reason as everyone else - the installers don't know any different. My heat load rarely exceeds 8kW, in the current weather it's a lot lower. If I've set it too low I would expect it to take longer to heat up, not cut off at 50C. The HW part load is left at 18kW as you can see from the modulation curve.

Mike

 

[Johntheo5]

I have a Vaillant ecoTEC plus 418 on a vented system with twin 2-port valves, ABV, VRC 470f, VR65 and outdoor temperature sensor.

The CH Part Load is set to 5kW. HW is timed to come on at 5am and CH at 6am.

HW is working as I expected but CH cuts off when the flow temp reaches 50C, well below the target set by the controller:

View attachment 300569

Any suggestions why this is happening please?

Mike

Can you actually see the target temperature set by the outside temperature, if, for example the OT has set the target temperature to 45C then the minimum boiler output may be greater than the CH demand, in which case the the burner will trip and the boiler recycle when the actual boiler temp is 5C > than the target temperature, in your case this would mean that the OT control has set the target temp to 45C, can you post the heating curves.
You can see the modulation going to a steady 30% at a boiler temp of ~ 35C at around 0610 but the boiler flow temp keeps rising yntil cut out at 50C??.

 

[Yikes]

Can you actually see the target temperature set by the outside temperature, if, for example the OT has set the target temperature to 45C then the minimum boiler output may be greater than the CH demand, in which case the the burner will trip and the boiler recycle when the actual boiler temp is 5C > than the target temperature, in your case this would mean that the OT control has set the target temp to 45C, can you post the heating curves.
You can see the modulation going to a steady 30% at a boiler temp of ~ 35C at around 0610 but the boiler flow temp keeps rising yntil cut out at 50C??.

The target temperature is shown in the graph above as Flow Temp Desired, which I presume is derived from the heat curve according to the OT.

Here are the Outside and Room temperatures for the same period. The target Room temperature is 18C:

The heat curve is currently 2.5:

Mike

 

[Johntheo5]

You were looking for ~ 58C boiler target temp but only achieving 50C, the 30% modulation is close to your range rating of 5kw (18x30%) so possiblt the boiler just can't achieve 58C, its possible that the boiler logic sees this as a problem and trips the burner??, so maybe increase the range rating (d.00) to 10kw or let it off on Auto even temporarily and see what happens.

 

[Yikes]

You were looking for ~ 58C boiler target temp but only achieving 50C, the 30% modulation is close to your range rating of 5kw (18x30%) so possiblt the boiler just can't achieve 58C, its possible that the boiler logic sees this as a problem and trips the burner??, so maybe increase the range rating (d.00) to 10kw or let it off on Auto even temporarily and see what happens.

Thanks, I will try that.

Mike

 

[Yikes]

I set d.00 to 12kW and on the next CH cycle at 1200 hrs it reached the (albeit lower) desired temperature. I shall have to wait until the morning to see if there is something else stopping the CH flow temperature from going over 50C. I have a long (60 minute) anticycle time to prevent short cycling which is not ideal but I can't get it to run continuously on a low burn. Also, it's set to Return regulation because otherwise it microfires continuously.

Mike

 

[Johntheo5]

Vaillant were (whatever about their modern ones) notorious for maintaining ignition conditions for up to 60 secs from firing at ~ 60% before allowing modulation which could cause the flow temp to exceed the target temp by 5C and recycle. You seem to have had a flow/return dT of 10C during that 10/15 min continuous firing period around 0615 hours. By calculation, a 5kw output with a dT of 10C gives a flowrate of 7.2LPM, if the boiler fires at 60% output (10.8kw) then the dT is 21.5C which means that the flow temp will not exceed 58C on firing as long as the return temp is 36.5C or below, this then gives 5C allowable temperature rise (to 63C) in that 60 sec period before the boiler modulates or burner trip.
Is there a ABV, (automatic bypass valve) installed?

Return temperature regulation is interesting but I've rarely, if ever, seen anyone using it, if no OT control then there must be some form of cascade control or link to the flow temperature otherwise the boiler would fire flat out after firing since the return temperature rise is much slower than the flow temperature rise, it possibly assumes a boiler dT and sets the target flow temperature accordingly.
The anti cycle time of 60 mins, ~ 16 minutes at 58C target flow, seems a bit more than adequate as it increases with reducing target temperature.

I don't know if microfiring is the same as shortcycling, I would associate microcycling with firing times of only seconds, say less than 10 secs, is this whats happening?.

I know you have allready done so but maybe flow temperature target control is the better option in view of the OT control?.

 

[Yikes]

Thank you so much for you detailed explanation. This boiler is a bit of a mystery to me even though it was installed in 2009!

The manual specifies a minimum flow rate of 12.9L/m. Is this proportional to output, i.e. 6.5L/m at 50% power?

The ABV was changed in December as my heating engineer concluded the old one was broken. He fitted and adjusted this one for me before I replaced the original Danfoss on/off controls with the OT control. As he's not a Vaillant expert he left me to play with the settings!

Here's what it did with Flow regulation, which is what I understand to be microfiring:

And this is Return regulation but with an anticycling time of 20 minutes (default):

Mike

 

[Johntheo5]

Thanks Mike,
What make, model, mode and setting is the circ pump set to and what is the ABV setting?

 

[Yikes]

The pump is a 3 speed Grundfoss UPS2 15-50/60-130 on speed 2. The ABV is just below 3.5.

Mike

 

[Johntheo5]

In answer to yout original question of a flowrate of 12.9 LPM, this gives a dT of 20C at the boiler output of 18kw but does not adjust automatically, the dT will be 10C at 9kw etc.
If, as I calculated, your flowrate is 7.2LPM then the pump head is 4.8M, the ABV setting of 3.5M may be a bit low, if its still hot with boiler firing normally maybe increase it to 4.0 to 4.25M.
How many rads are on the system?

 

[Yikes]

Thanks. It's a 1970s 4 bed detached house with 10 rads. As far as I can tell by touching the pipes the ABV is not open when the boiler is running.

Mike

 

[Johntheo5]

Surprising that the flowrate isn't greater than 7.2LPM with 10 rads but if so the pump head should be 4.8M and the ABV should be passing at a setting of 3.5M.
If the pump head is 3.5M then the circ rate is 21.7LPM which certainly won't cause any problems on boiler firing. Did Vaillant offer any advise?