The story starts from when the energy prices started to rocket. I heard that some big savings could be made by installing Weather Compensation onto compatible boilers to smooth out fluctuations and modulate the boiler.
My boiler is an ecoTEC Plus 418 open vented condensing boiler. My original layout was a Y-Plan system with a 3-Port, Mid-Position valve, controlled by a 240V room stat and cylinder stat and a programmable timer, all wired into a standard wiring centre.
Looking at my ecoTEC boiler manual there seemed to be a better means of controlling the system. . The manual mentions that the Vaillant controls that could be used with this system are:
Now, before I get completely slated, the Vaillant brochure says the VRC700 is “Compatible with Vaillant combination, system and open vent boilers” and “Can control a Y plan system when used with our VR 66 wiring centre”. The Controls brochure on Page 33, titled “Wiring with Intelligence” says
Now, admittedly, the brochure also gives some typical examples and makes some suggested configurations. Also, the table on Page 37 does state to get a single zone, weather compensated system using a wired VRC700 requires a VR70 or VR71 wiring centre. However, given the other conflicting information in the brochure, I sought Vaillant advice on whether a VR66 wiring centre would meet my requirements and was assured that I could use the VR66. It is correct that I can use the VR66 wiring centre as part of the control system – it does work.
In the “I would Like to” pages of the document, on page 36 it shows that the Wireless VRC700f System Control Pack will control a system or open vent boiler with one heating zone and hot water, and control everything from a single weather compensating programmable thermostat. And on the Control Compatibility page of the document at page 37 it shows the same information. Now I grant you that I didn’t get the VRC700f wireless version I got the VRC700 wired version because I have a perfectly suitable wiring layout that was 240V but could easily be adapted to employ the eBus and I have a perfectly suitable north facing wall on which to mount the weather sensor and it can be easily wired directly into the boiler – oh, and it was available at a great price on eBay!
So far I am very pleased with the result. When the system is in heating mode it appears to be doing what it is designed to do in that the boiler runs at a flow temperature around about the target flow temperature based on a 1,2 heating curve and the current OAT. The heating is modulating beautifully. It is providing a very steady and constant heat output based on the target flow rate and, whilst it appears to be running for longer periods, there is much less boiler cycling and because it is running at a much lower temperature, there is a good condensation flow – all good.
However, I had a problem with my system when I first installed the VR700 Weather Compensated System Control and a VR66/2 Control Centre that I did not foresee. My new system configuration is now a Vaillant 418 boiler with a DHW Cylinder and a 3-Port Mid-Position Valve in Y-plan. The VR66/2 is set to Monozone mode (1). Because I wanted to be able to run the system for both CH and DHW at the same time and have the valve in the mid-position, I changed d.70 on the boiler from 0 (Hot water priority) to 1 (Enable mid position) as stated in the table at 3.2.2 of the VR66/2 Installation instructions.
The problem comes when, following a period of heating only but with heating being still demanded, DHW is being demanded. When this happens, the valve moves to the mid-position as expected, but the system goes into standby mode and the boiler stops firing. When I check the boiler status it says it is in S.28 Anti Cycling Mode. It appears to want to stay in this mode indefinitely (certainly beyond the 20 minute d.2 setting) until I reset the system which I was doing by either setting the DHW to OFF or turning the boiler off. But then my DHW demand has not been satisfied and never will if the boiler remains in Anti Cycling Mode. Doing a reset using the boiler reset button just means it goes into the start sequence for the boiler in HW mode and ends up back at S.28. Whilst in S.28 the boiler won’t fire so I had a look at d.67 (remaining burner anti-cycling time) and it was reading 0 - which implied that the boiler should no longer be anti-cycling - but when I return to view the status it still says it is in S.28 and the boiler is still not firing. The only way to come out of it seems to be to do a boiler reset.
The way I understand the anti-cycling it is a feature that is designed to prevent the boiler from attempting to re-ignite too soon when the system is calling for heat but the water temperature is too high. So I am guessing that it uses return flow temperature value to determine if there has been sufficient heat transfer into the radiators and the rooms and if it sees that the return temperature is too high it assumes that the radiators are shutting off via their TRVs and that flow around the system is reducing resulting in less heat transfer. In this case, if there is too high a flow temperature (and therefore return temperature) or insufficient flow through the system it could easily trigger anti-cycling mode. So on that basis, I assume that what is happening is that when the system is in both modes, the system sees a much higher flow temperature due to the boiler no longer modulating because it is trying to heat the HW and sees a return temperature that is too high for the current heat curve settings based on OAT and Room Temperature and therefore puts the boiler into an anti-cycling mode. But it is not the CH anti-cycling mode (S.8) it is a HW anti-cycling mode (S.28) that is coming up.
For now, I have reset the d.70 back to 0 (Hot water priority) and this seemed to sort the problem in that the system goes into DHW mode and the boiler immediately fires up. It happily runs in DHW mode until the DHW is satisfied and then the boiler stops, the valve moves to fully open and the CH starts up again nicely modulating according to the heat curve. But with these settings it means I can only run the system in the CH or the HW mode and not both – not ideal but probably liveable with.
Now I realise that I can’t have an open vent condensing boiler in a F&E (or sealed) system in a Y-plan configuration and not have the boiler flow temperature hotter than the CH might require on a modulating system if I am going to get my DHW up to 60+ degrees but I was kind of expecting that the clever controls might recognise what the system is asking for and modulate everything accordingly and if necessary close the CH port temporarily rather than the boiler going into lockdown due to some HW anti-cycling mode.
The main questions I now have are:
Is there a fundamental difference between the VRC700 wired and the VRC700f wireless controllers that I am not aware of? If I had bought the VRC700f wireless version, would I not have had this issue?
Does anyone have any idea why the boiler goes into S.28 Anti Cycle Mode when DHW is selected along with CH? Or indeed why it would have an anti-cycle mode for the DHW side in my configuration? According to the manual the S.28 Anti Cycle Mode is only applicable to either Combi Boilers or System Boilers.
Does anyone have any idea why, when entering S.28 Anti Cycle Mode, the boiler would appear to lock in this mode despite the d.67 timer saying that there are 0 minutes remaining and the only way I seem to be able to get out of it is to do a complete boiler reset?
Is there anything in the way of settings that I can try to adjust that might prevent this from happening so that I can revert back to having the 3-port valve capable of selecting mid-position without the system entering S.28 Anti Cycle Mode? (although I’m inclined not to bother and just live with HW priority mode).
Can anyone tell me what parameters are used within the algorithm that tells the boiler to enter anti-cycling mode? E.g. is it a comparison between flow temperature and return temperature or is it a comparison between target flow temperature and the actual flow temperature.
Why do Vaillant not have a simple software logic that says when the system has requested DHW, ignore the heat curve and run the boiler at the required temperature to heat the water then revert back to the heat curve with some simple logic that allows that to happen?
In the absence of any other ideas, I will leave the system with a hot water priority setting and see how things pan out over the winter months when I am more likely to see and experience how far the temperature might drop in the 20 - 40 minutes it might take to heat a full tank of water. Any thoughts?
My boiler is an ecoTEC Plus 418 open vented condensing boiler. My original layout was a Y-Plan system with a 3-Port, Mid-Position valve, controlled by a 240V room stat and cylinder stat and a programmable timer, all wired into a standard wiring centre.
Looking at my ecoTEC boiler manual there seemed to be a better means of controlling the system. . The manual mentions that the Vaillant controls that could be used with this system are:
- VRC400 1-circuit controller
- VRT360 room temperature controller
- Time Switch 140 Timer
- And a VR65 Control Centre which is the system solution for UK cylinder connection.
Now, before I get completely slated, the Vaillant brochure says the VRC700 is “Compatible with Vaillant combination, system and open vent boilers” and “Can control a Y plan system when used with our VR 66 wiring centre”. The Controls brochure on Page 33, titled “Wiring with Intelligence” says
- Works with our complete range of system and open vent boilers.
- Compatible with traditional S plan and Y plan heating systems.
- Replaces a 10-way wiring centre
Now, admittedly, the brochure also gives some typical examples and makes some suggested configurations. Also, the table on Page 37 does state to get a single zone, weather compensated system using a wired VRC700 requires a VR70 or VR71 wiring centre. However, given the other conflicting information in the brochure, I sought Vaillant advice on whether a VR66 wiring centre would meet my requirements and was assured that I could use the VR66. It is correct that I can use the VR66 wiring centre as part of the control system – it does work.
In the “I would Like to” pages of the document, on page 36 it shows that the Wireless VRC700f System Control Pack will control a system or open vent boiler with one heating zone and hot water, and control everything from a single weather compensating programmable thermostat. And on the Control Compatibility page of the document at page 37 it shows the same information. Now I grant you that I didn’t get the VRC700f wireless version I got the VRC700 wired version because I have a perfectly suitable wiring layout that was 240V but could easily be adapted to employ the eBus and I have a perfectly suitable north facing wall on which to mount the weather sensor and it can be easily wired directly into the boiler – oh, and it was available at a great price on eBay!
So far I am very pleased with the result. When the system is in heating mode it appears to be doing what it is designed to do in that the boiler runs at a flow temperature around about the target flow temperature based on a 1,2 heating curve and the current OAT. The heating is modulating beautifully. It is providing a very steady and constant heat output based on the target flow rate and, whilst it appears to be running for longer periods, there is much less boiler cycling and because it is running at a much lower temperature, there is a good condensation flow – all good.
However, I had a problem with my system when I first installed the VR700 Weather Compensated System Control and a VR66/2 Control Centre that I did not foresee. My new system configuration is now a Vaillant 418 boiler with a DHW Cylinder and a 3-Port Mid-Position Valve in Y-plan. The VR66/2 is set to Monozone mode (1). Because I wanted to be able to run the system for both CH and DHW at the same time and have the valve in the mid-position, I changed d.70 on the boiler from 0 (Hot water priority) to 1 (Enable mid position) as stated in the table at 3.2.2 of the VR66/2 Installation instructions.
The problem comes when, following a period of heating only but with heating being still demanded, DHW is being demanded. When this happens, the valve moves to the mid-position as expected, but the system goes into standby mode and the boiler stops firing. When I check the boiler status it says it is in S.28 Anti Cycling Mode. It appears to want to stay in this mode indefinitely (certainly beyond the 20 minute d.2 setting) until I reset the system which I was doing by either setting the DHW to OFF or turning the boiler off. But then my DHW demand has not been satisfied and never will if the boiler remains in Anti Cycling Mode. Doing a reset using the boiler reset button just means it goes into the start sequence for the boiler in HW mode and ends up back at S.28. Whilst in S.28 the boiler won’t fire so I had a look at d.67 (remaining burner anti-cycling time) and it was reading 0 - which implied that the boiler should no longer be anti-cycling - but when I return to view the status it still says it is in S.28 and the boiler is still not firing. The only way to come out of it seems to be to do a boiler reset.
The way I understand the anti-cycling it is a feature that is designed to prevent the boiler from attempting to re-ignite too soon when the system is calling for heat but the water temperature is too high. So I am guessing that it uses return flow temperature value to determine if there has been sufficient heat transfer into the radiators and the rooms and if it sees that the return temperature is too high it assumes that the radiators are shutting off via their TRVs and that flow around the system is reducing resulting in less heat transfer. In this case, if there is too high a flow temperature (and therefore return temperature) or insufficient flow through the system it could easily trigger anti-cycling mode. So on that basis, I assume that what is happening is that when the system is in both modes, the system sees a much higher flow temperature due to the boiler no longer modulating because it is trying to heat the HW and sees a return temperature that is too high for the current heat curve settings based on OAT and Room Temperature and therefore puts the boiler into an anti-cycling mode. But it is not the CH anti-cycling mode (S.8) it is a HW anti-cycling mode (S.28) that is coming up.
For now, I have reset the d.70 back to 0 (Hot water priority) and this seemed to sort the problem in that the system goes into DHW mode and the boiler immediately fires up. It happily runs in DHW mode until the DHW is satisfied and then the boiler stops, the valve moves to fully open and the CH starts up again nicely modulating according to the heat curve. But with these settings it means I can only run the system in the CH or the HW mode and not both – not ideal but probably liveable with.
Now I realise that I can’t have an open vent condensing boiler in a F&E (or sealed) system in a Y-plan configuration and not have the boiler flow temperature hotter than the CH might require on a modulating system if I am going to get my DHW up to 60+ degrees but I was kind of expecting that the clever controls might recognise what the system is asking for and modulate everything accordingly and if necessary close the CH port temporarily rather than the boiler going into lockdown due to some HW anti-cycling mode.
The main questions I now have are:
Is there a fundamental difference between the VRC700 wired and the VRC700f wireless controllers that I am not aware of? If I had bought the VRC700f wireless version, would I not have had this issue?
Does anyone have any idea why the boiler goes into S.28 Anti Cycle Mode when DHW is selected along with CH? Or indeed why it would have an anti-cycle mode for the DHW side in my configuration? According to the manual the S.28 Anti Cycle Mode is only applicable to either Combi Boilers or System Boilers.
Does anyone have any idea why, when entering S.28 Anti Cycle Mode, the boiler would appear to lock in this mode despite the d.67 timer saying that there are 0 minutes remaining and the only way I seem to be able to get out of it is to do a complete boiler reset?
Is there anything in the way of settings that I can try to adjust that might prevent this from happening so that I can revert back to having the 3-port valve capable of selecting mid-position without the system entering S.28 Anti Cycle Mode? (although I’m inclined not to bother and just live with HW priority mode).
Can anyone tell me what parameters are used within the algorithm that tells the boiler to enter anti-cycling mode? E.g. is it a comparison between flow temperature and return temperature or is it a comparison between target flow temperature and the actual flow temperature.
Why do Vaillant not have a simple software logic that says when the system has requested DHW, ignore the heat curve and run the boiler at the required temperature to heat the water then revert back to the heat curve with some simple logic that allows that to happen?
In the absence of any other ideas, I will leave the system with a hot water priority setting and see how things pan out over the winter months when I am more likely to see and experience how far the temperature might drop in the 20 - 40 minutes it might take to heat a full tank of water. Any thoughts?
