Pump overrun is killing my heating!

[milleniumaire]

Your showing S.53 because of the output still being 37kw, have you rated this down to 22kw on D. Setting on the PCB?

Okay, thanks, that actually makes me feel a little better - knowing its likely to be a config issue rather than a fault somewhere. As it's a boiler setting, I guess this parameter must have always had that value and it isn't something that would change with the upgrade of controller from VRC 430 to VRC 700.

I haven't changed this setting as this is one of the many questions I've put to Vaillant technical support (showing them what the current settings are and asking them to advise on the best values). Unfortunately, I haven't yet received a response.

 

[EvilDrPorkChop]

Okay, thanks, that actually makes me feel a little better - knowing its likely to be a config issue rather than a fault somewhere. As it's a boiler setting, I guess this parameter must have always had that value and it isn't something that would change with the upgrade of controller from VRC 430 to VRC 700.

I haven't changed this setting as this is one of the many questions I've put to Vaillant technical support (showing them what the current settings are and asking them to advise on the best values). Unfortunately, I haven't yet received a response.

Personally, if I was configing your system. I'd set -
HW Target Temp on 55oc
D.20 - 60oc
D.72 - 120
D.75 - 45 mins
D.77 to 22kw
D.78 to 80oc

D.19 - 3 (Always Speed 2)

 

[John.g]

Just had a response from Vaillant technical support regarding one of my emails and their response is that I need to wire the external pump upto the VR 71 wiring centre! The exact opposite of what they told my heating engineer. I'm taking this with a pinch of salt as at the time I emailed them I wasn't aware that the value of d.27 was 5 and they have assumed it is 2 (apparently this is the default). Also, they are telling me that a value of 2 means the external pump would NOT work during DHW mode and so if using the VR 40 to connect up the pump, I would need to connect a SECOND external pump to relay 2 and set d.28 to 3. The fact that I'm seeing the external pump work in both DHW AND heating mode doesn't match what they are telling me, possibly due to the d.27 value of 5 (I've now asked them about this).

So, currently, they are telling me I either have to wire the pump upto the VR 71 or get a second pump, which I believe is overkill for my system. I'm still waiting for answers to a number of questions I asked yesterday.

Based on feedback so far I think I need to investigate the DHW 2 port value to make sure it is functioning correctly. If this isn't fully opening then even if the pump is running I guess the flow wouldn't go anywhere, but then I would assume the cylinder water would never get hot, which it clearly is (eventually).

Reducing the cylinder temperature setting may help the situation, so I've now reduced it to 60 deg C and I will monitor the impact in the morning and also try to determine what the start temperature of the cylinder is before it starts being heated.

I'm still on the fence about the pump wiring/configuration, but wouldn't be surprised if there was potentially an issue, even though the pump always seems to come on when the gas burner is on i.e. when there is demand from one of the zones/DHW.

As far as pump overrun goes, I'm not really seeing that as much as I expected to, so although that is the subject line for my post, it may be a red herring! I still can't help thinking it would be better if the external pump was on during pump overrun, providing at least one of the zone valves was open. Maybe this doesn't happen as pump overrun tends to happen when the heating demand has been met and it is cooling down the water temperature in the boiler to protect the heat exchanger, so to continue to pump hot water around the system might increase room/cylinder temperatures beyond what was desired? Just a thought.

Back to basics for me.
Is there a clue in the label re UPS3 connection to VR 71 as they suggest, above.
TITLE 12-37kW, Low Loss Header, DHW (Secondary Circulation), 1 UFH Zone (3rd Party), 2 Radiator Zones (Wired or Wireless VRC 700, VR91)

Why not connect a indicating light between L/N in R4 where it says DHW Secondary Circulation and observe it, if it always lights up and goes out with boiler circ pump operation in both CH & HW mode , then maybe that's where the UPS3 should be connected?.
Also if boiler circ pump on and R4 "on" with UPS3 off then maybe another clue that it should be connected in to R4.

 

[John.g]

Don't think R4 is correct, that seems allocated to a cylinder secondary recirc pump.

 

[milleniumaire]

Back to basics for me.
Is there a clue in the label re UPS3 connection to VR 71 as they suggest, above.
TITLE 12-37kW, Low Loss Header, DHW (Secondary Circulation), 1 UFH Zone (3rd Party), 2 Radiator Zones (Wired or Wireless VRC 700, VR91)

Why not connect a indicating light between L/N in R4 where it says DHW Secondary Circulation and observe it, if it always lights up and goes out with boiler circ pump operation in both CH & HW mode , then maybe that's where the UPS3 should be connected?.
Also if boiler circ pump on and R4 "on" with UPS3 off then maybe another clue that it should be connected in to R4.

Hi John, I understand what you are suggesting, but as things stand I/the engineer have received different advice from different Vaillant support people, so I need to understand why they are making these different suggestions, hopefully once they respond to my email.

What they actually told me in the last email was:

"I would advise to remove the VR40 and wire the single system pump as shown in the example diagram below with a permanent 230V supply to the grey wires and use the oranges from all zone valves wired to the pump live."

So, they are essentially wanting to turn the external pump on and off whenever any of the zone valves open or close, which I believe is what the engineer was originally considering doing, but then got confused looking at the schema diagram they had supplied (see attached) and called Vaillant, who told him he needed to use the VR 40, which he then ordered and installed the following day!

It's an interesting thought about using R4 for the external pump, however in the schema diagram it does appear to be associating R4 with the cylinder, so my gut feeling is it is likely to only operate in DHW mode, but as you say it may be worthwhile testing it, but I'll wait until I have more clarity from Vaillant. Maybe they'll suggest yet another way of wiring up the pump

EDIT: Just seen your post from 11:36 last night, so it appears we agree about R4.

 

[John.g]

"I would advise to remove the VR40 and wire the single system pump as shown in the example diagram below with a permanent 230V supply to the grey wires and use the oranges from all zone valves wired to the pump live."

That is the normal S plan configuration, there is a permanent live to one side of the zone valve end switches and all the wires (switched live) from the other end(s) are marshalled together to provide a a run signal to the boiler which now normally controls the boiler circ pump, so if any one, or all, zone valves are open then the boiler has a run signal and when all zone vales are shut then no run signal, I suppose with all their fancy control systems now, they don't use the end switches now but employ some virtual means of determining this hence all these status alerts or whatever.
It's a bit surprising that there is no contact on the boiler from the circ pump run signal, there must be some means of picking up this and wiring it via a external relay (if required) to run the external pump, I think this was suggested previously, above. Also, this implies that the (a) zone valve is kept open for the duration of any pump overrun, oh no, the pump won't run unless a zone valve is open!.

 

[milleniumaire]

Got up early this morning, prior to the heating/hot water turning on at 6:30am, so I could monitor temperatures etc.

Took readings before the boiler started up. This showed the current cylinder temperature was 56 deg C (the desired setting is 60). So, 4 deg C heat loss over 7.5 hours, which seems much better than the other night

When the boiler started up it went straight into heating mode, I guess because 56 is within 5 of 60, so no need to heat up the cylinder.

Had a shower then re-checked at 06:55 and the boiler was now heating up the cylinder, which it did within about 10 mins as by 7:10 it was back to heating mode.

So, everything worked perfectly, even the shower controls! Had a chat with my son and he admitted he had been having late night showers every 2 or 3 days!!!!!! If only I had known that.

It would therefore appear that a combination of the cylinder temp being set to 65 and my son's late night, secret showers using up hot water are possibly the reasons for the cylinder taking so long to heat up in the morning.

Obviously, there are still settings to change and possibly wiring to change, or at least an explanation of the wiring settings, so I await feedback from Vaillant, but it would appear the "pump overrun" situation I thought was occurring is a red herring! I'll update when I hear from Vaillant.

 

[John.g]

It would b interesting during the day to run off some hot water until the system changes over to HW, then stop running off the hot water and monitor the exact time taken to reach cut off at 60C, also a full set of readings during its run including cylinder top and target temperatures, LLH temperatures etc, can then tell pretty exactly the performance of the cylinder "coil" and might then be able to understand why it should take so long to reach 65C, if required.

 

[milleniumaire]

It would b interesting during the day to run off some hot water until the system changes over to HW, then stop running off the hot water and monitor the exact time taken to reach cut off at 60C, also a full set of readings during its run including cylinder top and target temperatures, LLH temperatures etc, can then tell pretty exactly the performance of the cylinder "coil" and might then be able to understand why it should take so long to reach 65C, if required.

Rather than wasting hot water (I am Yorkshire after all), I'll wait until my son has his next shower and take some readings. He has now agreed to always shower before 22:00pm in future!

I don't understand how the system gets the "Cyl temp top" reading. As far as I'm aware, there's only one temperature sensor in the cylinder, wired into the VR 71 and this shows the "Current cyl. temp". As you can see below from the VRC 700 settings, the "Cyl temp top" is shown within the Buffer cylinder menu. The "Cyl temp bottom" has NEVER shown a value. So how does the VRC 700 get the value for "Cyl temp top"?

 

[John.g]

Don't know but in post #77 it was 73c and now showing 68C so must be sensing it somewhere?. a bottom temperature if available would reflect the mains temperature in general and the current cylinder temperature where the sensor is would normally be located maybe 100/150mm above the cylinder coil return as you require the reheat process to begin when a small vol of hot water is drawn off . you will, in any case have a almost full cylinder of hot water available at the target temperature.

 

[milleniumaire]

Don't know but in post #77 it was 73c and now showing 68C so must be sensing it somewhere?. a bottom temperature if available would reflect the mains temperature in general and the current cylinder temperature where the sensor is would normally be located maybe 100/150mm above the cylinder coil return as you require the reheat process to begin when a small vol of hot water is drawn off . you will, in any case have a almost full cylinder of hot water available at the target temperature.

Yes, the top cylinder temperature figure does seem to vary.

There are no other cables from the cylinder to either the wiring centre or the boiler, apart from the temperature sensor that is wired into the VR 71.

Another question for Vaillant technical support I think as it would be useful to understand the origin of this VRC 700 menu option. Possibly also, why is there a top temperature value and not a bottom temp setting.

 

[John.g]

With no cylinder sensor the closest temperature to it is the boiler flow temperature, if so, on change over from HW to CH, it should drop very rapidly.

 

[Ben-gee]

I imagine that the bottom cylinder temperature is available from some cylinders - but not the one you have.

 

[milleniumaire]

Hi John, I understand what you are suggesting, but as things stand I/the engineer have received different advice from different Vaillant support people, so I need to understand why they are making these different suggestions, hopefully once they respond to my email.

What they actually told me in the last email was:

"I would advise to remove the VR40 and wire the single system pump as shown in the example diagram below with a permanent 230V supply to the grey wires and use the oranges from all zone valves wired to the pump live."

View attachment 47032
So, they are essentially wanting to turn the external pump on and off whenever any of the zone valves open or close, which I believe is what the engineer was originally considering doing, but then got confused looking at the schema diagram they had supplied (see attached) and called Vaillant, who told him he needed to use the VR 40, which he then ordered and installed the following day!

It's an interesting thought about using R4 for the external pump, however in the schema diagram it does appear to be associating R4 with the cylinder, so my gut feeling is it is likely to only operate in DHW mode, but as you say it may be worthwhile testing it, but I'll wait until I have more clarity from Vaillant. Maybe they'll suggest yet another way of wiring up the pump

EDIT: Just seen your post from 11:36 last night, so it appears we agree about R4.

Just received the following confirmation from Vaillant regarding the wiring of the external pump:

"With D.27 set to 5 the external pump will only run when the burner is on for DHW or CH as this is for an external solenoid valve so when the burner switches off so does the external pump which does not allow for any pump over run.

The number 5 setting in D.27 external solenoid valve I have only used for a commercial application with an external gas solenoid valve or it has been used for a commercial application when a building management system requires a run and fault indicator. I asked my manager if he new of anyone in or outside technical that would use number 5 for an external pump, which he did not.

I think the best and only option is to disconnect the external pump from the VR40 and use the orange wires from all the zone valves to activate the single pump, this will ensure the pump continues to run all the time each zone valve is open.

I apologise for the conflicting information however the diagram shown below is the standard Vaillant diagram to control a single external system pump the system side of a LLH."

They have included the same diagram as before.

Very frustrating! Not only did I have to purchase a VR 40, but I had to pay the engineer to come back the following day and fit it and now I'll have to pay him to come out, remove it and rewire the pump - in the way he was originally going to wire it until told differently by Vaillant. They really should be accountable when they give bad advice!

One thing that confuses me about the diagram. I assume the SYS.FLOW is the external pump and this clearly shows the live wire from each of the heating zones (and the DHW, which isn't applicable in my case) being used to switch the live on the pump, but where are the neutral and earth wires for the pump connected? I assume my heating engineer would know this, or would he need to call Vaillant again, for yet more bad advice?

 

[John.g]

It should be easy enough to pick up a N&E somewhere, I wonder what logic the boiler uses to determine if any of the zone valves are open if it doesn't get a input back from them?. Why do you say ("and the DHW, which isn't applicable in my case)", don't you need a feed back from the orange wire on that Mot.valve as well.?

 

[milleniumaire]

It should be easy enough to pick up a N&E somewhere, I wonder what logic the boiler uses to determine if any of the zone valves are open if it doesn't get a input back from them?. Why do you say ("and the DHW, which isn't applicable in my case)", don't you need a feed back from the orange wire on that Mot.valve as well.?

Of course, you are correct. For some reason I was thinking the DHW symbol was for a circulation pump, but it is obviously for the cylinder zone valve, so yes, the live from R6 would also need to be included.

Couldn't resist taking the front off the VR 71 wiring centre to see what the engineer had done with the orange wires. He' has simply cut them off as only the brown, blue and green/yellow wires are connected for the Honeywell zone valves. Also, the two UFH thermal valves only have brown and blue wires, plus the cable from a more remote zone valve is only 3 core, even though the zone valve itself has more cores than this (they have also been cut off).

So, it would appear the instructions to connect up the orange wires won't work! What is the difference between the orange wire and brown live wire? Is it that the orange is switched live (whenever the brown is live)? In which case it looks like it can't be done for my setup due to the UFH valves not having a switched live wire!

 

[ShaunCorbs]

Yes when port valves are open the oranges become live on 2 ports

 

[milleniumaire]

Yes when port valves are open the oranges become live on 2 ports

So somehow, he would need to connect the brown cables to the live on the pump, but this doesn't seem to be possible without each zone valve causing all the others to open i.e. if all the live connections are connected together to the pump live!

Sounds like maybe an electrician and some sort of device that mimics the "switch live" functionality for the two UFH thermal valves would be required to achieve this, rather than my heating engineer?

Or, as I think you were alluding to in your earlier message, is there a relay or something on the wiring centre that could be used for the pump? If so, I would have thought Vaillant would have mentioned it.

 

[ShaunCorbs]

Pump normally is connected to the oranges I think basically your turning your system into an s plan

 

[John.g]

No, not the brown, the orange wires, are you sure those actuators had orange wires as I think you could, in some cases buy them with no end switches/orange wires.
Why don't you get back to them and ask them can they give you any contact on the boiler from its own circ pump, all you need then is a external relay to switch your UPS pump, there has to be a switched wire to the internal pump in the boiler, surely it should be easy enough to wire into this??, even though the UPS will also run then if/with pump overrun it doesn't matter as it wont deadhead and will just circulate water in its own half of the LLH.

 

[ShaunCorbs]

Might be worth emailing Kirby

https://mobile.twitter.com/2starkirbs?lang=en

 

[milleniumaire]

No, not the brown, the orange wires, are you sure those actuators had orange wires as I think you could, in some cases buy them with no end switches/orange wires.
Why don't you get back to them and ask them can they give you any contact on the boiler from its own circ pump, all you need then is a external relay to switch your UPS pump, there has to be a switched wire to the internal pump in the boiler, surely it should be easy enough to wire into this??, even though the UPS will also run then if/with pump overrun it doesn't matter as it wont deadhead and will just circulate water in its own half of the LLH.

The two Honeywell zone valves have orange wires, but both are cut back and only brown, blue and green/yellow are used.
The two thermal valves only have brown and blue, not sure how these could be connected up to the pump!

I have sent an email to Vaillant asking them the question, so will have to wait a few days for the reply, but I'm guessing some sort of relay switch will be required to "simulate" the switched live for the thermal valves.

I'm sure if there was a connection on the boiler or the wiring centre that could be used to control the pump, whenever any of the zones were switched on, then Vaillant would have mentioned this, or at least you would have hoped they would!

 

[milleniumaire]

....even though the UPS will also run then if/with pump overrun it doesn't matter as it wont deadhead and will just circulate water in its own half of the LLH.

This would be my preference anyway, for the external system pump to be on during overrun i.e. at ANY time the internal pump is running. This way, you would expect the water in the boiler circuit would cool down faster, plus the hot water is being put to use either heating the cylinder or the heating.

In the early days of my contact with Vaillant they issued the following statement:

"The pump on the secondary circuit only needs to operate When any of the valves are open, and does not require a pump overrun after the demand. Only the primary side requires the overrun (the Boiler side) this is to dissipate the excess heat in the heat exchanger inside the boiler after the burn process is completes and demand is no longer required."

I'm a little confused about this statement as I'm sure when the boiler was in overrun mode, the zone valves were open, which suggests that the external pump would be on if it was wired up to them. I'll need to try to catch the boiler in overrun mode to confirm the valves are definitely open.

As I don't have a switched live wire on the two thermal valves used for the UFH, I obviously need some mechanism to switch live for these. I did wonder about the other connections on the VR 71; R7/8, R9/1, R11/12 and R13. These aren't currently being used and in the schema diagram they show motorised mixer valves (for each zone) being connected to them. Because the schema diagram also shows the use of zone pumps, wouldn't you therefore expect the mixer valves and the pumps to be switched on or off at the same time? If so, doesn't this suggest that R7/R8 will be live when R1 is live etc, so couldn't these therefore be used to wire up the pump?

 

[EvilDrPorkChop]

Personally I think it needs to be connected the the boiler, this is how I've done it in all the systems I've done with additional pumps on Vaillants.

I think their first answer was correct.

 

[milleniumaire]

Personally I think it needs to be connected the the boiler, this is how I've done it in all the systems I've done with additional pumps on Vaillants.

I think their first answer was correct.

Can you please confirm how you usually connect it up? Is it the same as my setup:
- Pump is wired into Rel 1 on the VR 40 and d.27 is set to 5

Due to the value of 5, the external pump only works when the burner is on, so it doesn't work in pump overrun, or any other mode where the internal boiler pump is working but the burner is off.

Based on their suggestion to wire it up to the zone valves, I would assume this would cause the pump to work whenever any of the zones are open, which I believe would include pump overrun etc. If not, I wonder what the differences would be.

 

[John.g]

If you use the "orange wire" method as suggested then you must fit relays to the UFH systems to pick up if they are running and also to any zone valve with chopped off orange wires, if too short, one relay might do the wole lotl.
Re your own suggestion above, to prove/disprove it, connect a test light across one of those relays and ensure it stays lit during both normal boiler burner on and off. For pump overrun, observe it while you switch off the last heating circuit, the boiler circ pump should/might overrun and the relay test light should stay on until pump overrun complete.

 

[EvilDrPorkChop]

Can you please confirm how you usually connect it up? Is it the same as my setup:
- Pump is wired into Rel 1 on the VR 40 and d.27 is set to 5

Due to the value of 5, the external pump only works when the burner is on, so it doesn't work in pump overrun, or any other mode where the internal boiler pump is working but the burner is off.

Based on their suggestion to wire it up to the zone valves, I would assume this would cause the pump to work whenever any of the zones are open, which I believe would include pump overrun etc. If not, I wonder what the differences would be.

According to my Vaillant instructions;

d.27 Switching accessory relay 1 in the accessory module 1 = Circulation pump (default) 2 = Ext. pump 3 = Storage charging pump 4 = Flue gas flap/extractor hood 5 = External gas valve 6 = External error message

Why is it set on 5?

In my configurations I would always config to External Pump, so 2.

 

[milleniumaire]

According to my Vaillant instructions;

d.27 Switching accessory relay 1 in the accessory module 1 = Circulation pump (default) 2 = Ext. pump 3 = Storage charging pump 4 = Flue gas flap/extractor hood 5 = External gas valve 6 = External error message

Why is it set on 5?

In my configurations I would always config to External Pump, so 2.

It is set to 5 because Vaillant told my heating engineer to set it to 5! Apparently a value of 5 will cause the pump to run whenever the burner is on. Vaillant seem to be backtracking on the use of the VR 40 and I get the impression they are performing some in-house investigation to try to determine who gave the advice! I assume they record all their telephone calls.

Vaillant told me that if it is set to 2, then the pump will NOT run for hot water demand, which is no good for my system. The pump needs to run for ANY demand, heating or hot water, with DHW taking priority, as all flows (cylinder and heating zones) are from the pump.

I'm sure there are situations that would warrant two external pumps; one for the heating and one for the hot water cylinder, but it would be complete overkill for mine, especially as the cylinder is 1m from the boiler and it is DHW priority, so even if there were 2 pumps fitted, they would only ever work one at a time!

 

[EvilDrPorkChop]

It is set to 5 because Vaillant told my heating engineer to set it to 5! Apparently a value of 5 will cause the pump to run whenever the burner is on. Vaillant seem to be backtracking on the use of the VR 40 and I get the impression they are performing some in-house investigation to try to determine who gave the advice! I assume they record all their telephone calls.

Vaillant told me that if it is set to 2, then the pump will NOT run for hot water demand, which is no good for my system. The pump needs to run for ANY demand, heating or hot water, with DHW taking priority, as all flows (cylinder and heating zones) are from the pump.

I'm sure there are situations that would warrant two external pumps; one for the heating and one for the hot water cylinder, but it would be complete overkill for mine, especially as the cylinder is 1m from the boiler and it is DHW priority, so even if there were 2 pumps fitted, they would only ever work one at a time!

Have you tried setting it to 2 and actually testing it?

In my experience I'm sure it runs whenever there is a Demand.

 

[milleniumaire]

Have you tried setting it to 2 and actually testing it?

In my experience I'm sure it runs whenever there is a Demand.

I haven't, mainly because I assumed Vaillant technical knew what they were talking about when they said:

"Using the VR40 to run the secondary circuit pump run as a heating charge or a DHW cylinder charge Pump not both

Switching of relay 2 on the VR 40 "2 in 7" multi-functional module Options

1 = Circulation pump - DHW secondary Circulation pump

2 = External pump – Heating Charge pump

3 = Cylinder charging pump – DHW Charge Pump

4 = Extractor hood

5 = External solenoid valve

6 = External fault message"

I may just give it a try though if others, like yourself, are setting d.27 to 2 and it works for both heating and DHW.

 

[EvilDrPorkChop]

I haven't, mainly because I assumed Vaillant technical knew what they were talking about when they said:

"Using the VR40 to run the secondary circuit pump run as a heating charge or a DHW cylinder charge Pump not both

Switching of relay 2 on the VR 40 "2 in 7" multi-functional module Options

1 = Circulation pump - DHW secondary Circulation pump

2 = External pump – Heating Charge pump

3 = Cylinder charging pump – DHW Charge Pump

4 = Extractor hood

5 = External solenoid valve

6 = External fault message"

I may just give it a try though if others, like yourself, are setting d.27 to 2 and it works for both heating and DHW.

According to the instructions of the actual boiler, it states 1 as Circulation Pump, and 2 as External Pump. In my experience in fitting them, the boiler states this too when configing them up. Don't trust Vaillant technical, they hardly know out about their own boilers never mind wiring them

Try 2, there's no harm done. You can just change it back

 

[milleniumaire]

According to the instructions of the actual boiler, it states 1 as Circulation Pump, and 2 as External Pump. In my experience in fitting them, the boiler states this too when configing them up. Don't trust Vaillant technical, they hardly know out about their own boilers never mind wiring them

Try 2, there's no harm done. You can just change it back

So, I set d.27 to 2 this morning.

The system was in heat mode and the pump was on. This continued. I then turned up the temp of the water from 55 to 65 and it immediate switched to DHW mode. The external pump continued to run. It took around 15-20 mins to heat up to 65, then switched back to heat mode. I didn't see any pump overrunning so while this setting is obviously working for hot water as well as heating (contrary to what Vaillant suggested), I'm currently not sure if there are any situations when the external pump will run now, when it didn't before i.e. is there any difference between a setting of 2 or 5 in terms of when the pump runs.

I'll keep an eye on it throughout the day.

I'm assuming just changing the setting is enough? The boiler doesn't need to be reset to accept the new setting by power cycling it or anything?

 

[John.g]

D.27 5 is IMO only designed as a safety device, ie shuts a outside gas isolation valve when the burner shuts down or something similar, that's why the ext pump was only running with the burner on. D.27 2 works exactly as one would (now) expect, it runs a additional circ pump, period.
If you wish to check for pump overrun just decrease the wall stat setting (if on CH) until the boiler shuts down, it should then do a pump overrun, also check if it keeps the zone valve opened otherwise you may have to consider installing a ABV or a normally open mot valve in the secondary circuit.

 

[milleniumaire]

D.27 5 is IMO only designed as a safety device, ie shuts a outside gas isolation valve when the burner shuts down or something similar, that's why the ext pump was only running with the burner on. D.27 2 works exactly as one would (now) expect, it runs a additional circ pump, period.
If you wish to check for pump overrun just decrease the wall stat setting (if on CH) until the boiler shuts down, it should then do a pump overrun.

Thanks for the info on how to force pump overrun, you were spot on. As soon as I turned the temperature down for each of the heating zones, the boiler showed S.7 - pump overrun and the external system pump continued to run. Result!!!!

I think that just about solves all my issues and the external pump seems to be working at the same time as the internal boiler pump.

My heating engineer is visiting on the 25th to service the boiler so I'll have a chat with him to confirm who decided on setting d.27 to 5. I've always assumed it was as a result of advice from Vaillant as I know he was on the phone to them 2 or 3 times during the upgrade and specifically for info on wiring up the new pump. So maybe the advice to use the VR 40 was correct and the current advice to instead wire up the pump to the VR 41 is tosh! I'll play it out with Vaillant technical support and see where it goes as it appears they still think using the VR 40 was the wrong thing to do!

 

[John.g]

Might also be worth also checking if it keeps the zone valve opened for the overrun period (otherwise despite what I thought previously) the ext pump will have no pathway for the water, (with/if all zone valves closed), it may not do the ext pump any harm if the overrun is only for a minute or so.