I have a Vaillant Ecotec 415 open-vent boiler with VRC430 installed as a roomstat. I have had a lot of difficulties with getting it to work at all and the situation is still not satisfactory. I believe that I have a very good understanding of the problem. I have been contributing to a number of threads about the VRC430 and, if you are interested, you can find my stuff by doing a "search by author" which is tucked away as a link at the bottom of the search page.
The problem I have found seems not to be well understood other than by some rules of thumb - "Vaillant boilers need very good flow", "In this country you need to set a high heating curve" , "There is a cycling problem". It seems unclear if there is a real issue that might bite people who have taken reasonable design decisions and got a competent installation or the merely the inevitable results of bad planning, installation and tuning.
I had a response from a happy user recently under the thread "Vaillant VRC430" which clearly shows that sometimes it can all work very well:
Rather than describe the problem again in the depths of a thread which was about a specific aspect of the VRC430's behaviour, I thought I would package up my reply in a sort of survey. I'm hoping some other users or installers will write in with the details of their systems and how well they work.
My system details:
Small 3 bed semi with cavity wall insulation and double glazing. Main pipes 22 mm copper, radiators on 8mm copper but large double in living room on 10mm copper. Main rad is 3kw at 70C and easily large enough - it has fully open valves and no TRVs. Aim was to heat only main room but I now also have kitchen and bathroom rads on medium TRV settings because it makes the system work better and I can afford and appreciate the extra warmth. I heat the main room to a frugal 18C, more if I have guests.
There is a Honeywell automatic bypass valve and this is finely tuned to get the best balance between a good flow through the boiler and through the rads. From temperature rise across boiler and other complex calculations I think I have a total of 11 litres/min through the boiler but only about 4 is sure to go through the main radiator. I use a Grundfos 16/60 pump for better flow.
The problems in technical detail:
The house cannot usually sink the minimum 5kw from the boiler which therefore has to cycle. The boiler modulates 5-15kw but starts at a steady 12kw and cannot modulate for approx 1 minute. This heats water by 16C so boiler cannot restart until return water is this temp below target. With normal settings it will not have cooled enough so retries fail. The proportion of flow through the bypass means that the heat input during failed retries remains in the "bypass circuit" and further delays success in restarting. May never restart properly if d2 is set low. With d2 set to maximum 60 minutes (resulting in actual delay in range 10 - 40 minutes at temps I use) then retries will normally work for temperatures of 45C and above so that is set as minimum in the VRC430.
The water going through the bypass arrives back at the boiler cooled only by mixing with a lower proportion from the radiators. This used to happen before the boiler was ready to modulate and would also cause retries to fail at a late stage after putting enough heat in to seriously compromise further attempts. I have fixed this by installing in the bypass circuit a radiator which acts as a sort of buffer. It delays the return of the heated water and mixes it so that the temperature rise is smoother and easier for the boiler modulation to deal with. The radiator could not be put anywhere where heat was required so it is fully insulated and is just a tank. This has been very effective.
With these modifications and a heating curve of 1.5 the system will maintain my target room temperature quite accurately. The temperature will only very occasionally fall to 17.5 and a little more often rise to 18.5. Response to changing requirements is not so good though. In particular, getting up to temperature in the morning is difficult. As the room temperature rises and often the outside temperature too, the VRC430 reduces the target temperature and this brings back the problems of boiler restarts described above. Increasing the heating curve doesn't really restore responsiveness, is less efficient and causes room temperatures to often overshoot the target which is less comfortable. I deal with the morning problem by setting a target room temperature of 21C for an initial period of 40 minutes followed by an off period of 1 hour. The initial overhot fill of the radiator reaches my target temperature reasonably accurately and maintains temperature until the water has cooled off enough to go into steady state mode for the rest of the day.
If I have honoured guests and wish to raise the temperature to a sociable level, I override the VRC430 by setting the minimum flow temperature to some figure which will give a high enough rate of heating and disable the heating curve by setting it very low.
I have lately noticed during one of these manual heatups that the VRC430 has started controlling the anti-cycling itself by stopping calling for heat when the boiler stops even with room target not reached. Actually it is rather more conservative than the boiler with d2=60 and this is a bit irritating because it waits longer than necessary. I'm rather worried that Vaillant may have set it up to be too stupid to get it right but just clever enough to stop me outwitting it. Just today I felt inspired to see if this VRC430 smartness might work in the no-fly zone below 45C. But no, it just keeps calling and leaves it to the boiler which is too optimistic so 45C remains the limit.
Problem impact
Had to modify the system at some expense
Ridiculous effort to measure, analyze and tune the system with no help from manufacturer
System is reasonably efficient but not responsive and needs a lot of hands-on coaxing to get particular results.
The good news
The overall efficiency is now very good and heating bills very low.
The problem I have found seems not to be well understood other than by some rules of thumb - "Vaillant boilers need very good flow", "In this country you need to set a high heating curve" , "There is a cycling problem". It seems unclear if there is a real issue that might bite people who have taken reasonable design decisions and got a competent installation or the merely the inevitable results of bad planning, installation and tuning.
I had a response from a happy user recently under the thread "Vaillant VRC430" which clearly shows that sometimes it can all work very well:
Rather than describe the problem again in the depths of a thread which was about a specific aspect of the VRC430's behaviour, I thought I would package up my reply in a sort of survey. I'm hoping some other users or installers will write in with the details of their systems and how well they work.
My system details:
Small 3 bed semi with cavity wall insulation and double glazing. Main pipes 22 mm copper, radiators on 8mm copper but large double in living room on 10mm copper. Main rad is 3kw at 70C and easily large enough - it has fully open valves and no TRVs. Aim was to heat only main room but I now also have kitchen and bathroom rads on medium TRV settings because it makes the system work better and I can afford and appreciate the extra warmth. I heat the main room to a frugal 18C, more if I have guests.
There is a Honeywell automatic bypass valve and this is finely tuned to get the best balance between a good flow through the boiler and through the rads. From temperature rise across boiler and other complex calculations I think I have a total of 11 litres/min through the boiler but only about 4 is sure to go through the main radiator. I use a Grundfos 16/60 pump for better flow.
The problems in technical detail:
The house cannot usually sink the minimum 5kw from the boiler which therefore has to cycle. The boiler modulates 5-15kw but starts at a steady 12kw and cannot modulate for approx 1 minute. This heats water by 16C so boiler cannot restart until return water is this temp below target. With normal settings it will not have cooled enough so retries fail. The proportion of flow through the bypass means that the heat input during failed retries remains in the "bypass circuit" and further delays success in restarting. May never restart properly if d2 is set low. With d2 set to maximum 60 minutes (resulting in actual delay in range 10 - 40 minutes at temps I use) then retries will normally work for temperatures of 45C and above so that is set as minimum in the VRC430.
The water going through the bypass arrives back at the boiler cooled only by mixing with a lower proportion from the radiators. This used to happen before the boiler was ready to modulate and would also cause retries to fail at a late stage after putting enough heat in to seriously compromise further attempts. I have fixed this by installing in the bypass circuit a radiator which acts as a sort of buffer. It delays the return of the heated water and mixes it so that the temperature rise is smoother and easier for the boiler modulation to deal with. The radiator could not be put anywhere where heat was required so it is fully insulated and is just a tank. This has been very effective.
With these modifications and a heating curve of 1.5 the system will maintain my target room temperature quite accurately. The temperature will only very occasionally fall to 17.5 and a little more often rise to 18.5. Response to changing requirements is not so good though. In particular, getting up to temperature in the morning is difficult. As the room temperature rises and often the outside temperature too, the VRC430 reduces the target temperature and this brings back the problems of boiler restarts described above. Increasing the heating curve doesn't really restore responsiveness, is less efficient and causes room temperatures to often overshoot the target which is less comfortable. I deal with the morning problem by setting a target room temperature of 21C for an initial period of 40 minutes followed by an off period of 1 hour. The initial overhot fill of the radiator reaches my target temperature reasonably accurately and maintains temperature until the water has cooled off enough to go into steady state mode for the rest of the day.
If I have honoured guests and wish to raise the temperature to a sociable level, I override the VRC430 by setting the minimum flow temperature to some figure which will give a high enough rate of heating and disable the heating curve by setting it very low.
I have lately noticed during one of these manual heatups that the VRC430 has started controlling the anti-cycling itself by stopping calling for heat when the boiler stops even with room target not reached. Actually it is rather more conservative than the boiler with d2=60 and this is a bit irritating because it waits longer than necessary. I'm rather worried that Vaillant may have set it up to be too stupid to get it right but just clever enough to stop me outwitting it. Just today I felt inspired to see if this VRC430 smartness might work in the no-fly zone below 45C. But no, it just keeps calling and leaves it to the boiler which is too optimistic so 45C remains the limit.
Problem impact
Had to modify the system at some expense
Ridiculous effort to measure, analyze and tune the system with no help from manufacturer
System is reasonably efficient but not responsive and needs a lot of hands-on coaxing to get particular results.
The good news
The overall efficiency is now very good and heating bills very low.
