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Happy New Year.

Okay, maybe the subject is a little dramatic, but hopefully I've got your attention for a few minutes 😀

After a recent upgrade of my CH system, which included a LLH and a new circulation pump, my trusty old Vaillant ecoTEC 637 Plus system boiler seems to be spending a lot of time in pump overrun mode, especially in the mornings, when it first comes on.

It now spends the first 50 minutes heating up the water (DHW priority) before it starts to even think about heating the radiators. I've noticed a lot of that 50 minutes is spent in pump overrun mode, which I think means the hot water is cycling round the boiler circuit and is not getting to the unvented hot water cylinder to heat the water. During pump overrun the new circulation pump is off.

Am I correct in my assessment of the situation? Is there anything that can be done to resolve this issue so that the hot water is heated faster allowing the radiators to be heated, just like it used to do?
 
No both pumps should be on maybe depending on how the hot water is piped up is it after the llh ? / any pics ?
 
No both pumps should be on maybe depending on how the hot water is piped up is it after the llh ? / any pics ?
Hi Shaun, thanks for replying so quickly. A little more info about my system.

The system has been expanded over the last 6 years and the recent changes included an additional UFH zone, making it 3 zones (radiators, kitchen UFH, bathroom UFH) and DHW.
To achieve this a VR 71 wiring centre, VRC 700 and 2 X VR 91 remote thermostats were added. The LLH and new pump off the LLH flow was added to improve the efficiency of the different heating zones.

20201125_124623.jpg


Boiler is top top right, Vaillant unvented cylinder to the left and the new Grundfos pump is to the left of the new LLH, fitted to the system flow. After the pump, each zone has its own 2 port valve.

At the time the heating engineer did the work, he had to order a new component, called an X40, which was a mini circuit board that plugged into the main boiler circuit board. Vaillant told him to connect the new pump upto this, rather than into the VR 71 wiring centre. Here's a photo of the boiler circuit board with the X40 on the right, perpendicular to the main board:

20201126_105737.jpg



The engineer still seemed a little unsure exactly how to connect up the new pump as there are 4 connectors on the X40 and after reading the information that came with it, he connected it upto the yellow "rel 1".

The pump definitely does not come on when the boiler is running in pump overrun mode and I suspect therefore that the pump in the boiler is causing water to pump around the short boiler circuit between the boiler and the LLH? I assume it does this to try to cool down the water? The old system only had the boiler pump and so I assume when in pump overrun, water was being pumped around the system with the relevant zone valve open (DHW in this case), which possibly means that even in pump overrun mode, the boiler was heating the water in the cylinder? I'm guessing at this and I'm sure you guys will know better, but this would seem to explain why the current system takes so long to heat up the hot water and the previous version of the system did not.
 
Certainly sounds like a wiring issue, in HW Priority I’d expect your cylinder to reheat in 20/25 mins. Have you set the correct kw output of the coil for HW generation on the board?

Personally I’d of wired the additional pump to either the pump relay on the main board, or the S/L of the motorised valves.
 
Certainly sounds like a wiring issue, in HW Priority I’d expect your cylinder to reheat in 20/25 mins. Have you set the correct kw output of the coil for HW generation on the board?

Personally I’d of wired the additional pump to either the pump relay on the main board, or the S/L of the motorised valves.

The boiler and cylinder were installed 10 years ago, so I assume the correct kw output would have been set and used over this time. I'm not sure if the engineer would have changed any of the boiler settings during the recent upgrade (mid November), but it sounds like this particular setting wouldn't need to be changed?

Obviously, in terms of installing the VRC 700 (to replace a VRC 430), there were a number of config settings that had to be programmed to identify zones, heat curves and other stuff, but I guess none of this would affect the setting you mention.

Originally, the engineer had assumed the new pump would be connected upto the new VR 71 wiring centre, but after talking to Vaillant he had to order the X40 so that the pump could be controlled by the boiler, rather than by the wiring centre. At the time I thought this made sense i.e. any time the boiler pump is on, the new circulating pump should also be on, but this doesn't appear to be the case during pump overrun, when only the boiler pump is running.

I think the "pump relay" you mention is actually the X40. I believe this now comes as standard with new Vaillant boilers, but didn't with mine, which is now 10 years old. To connect the pump to the S/L of the motorised valves, it would need to be wired upto 4 different zones in the VR 71 and I believe this was also my engineers first thought, until Vaillant told him to connect it upto the X40.
 
The boiler and cylinder were installed 10 years ago, so I assume the correct kw output would have been set and used over this time. I'm not sure if the engineer would have changed any of the boiler settings during the recent upgrade (mid November), but it sounds like this particular setting wouldn't need to be changed?

Obviously, in terms of installing the VRC 700 (to replace a VRC 430), there were a number of config settings that had to be programmed to identify zones, heat curves and other stuff, but I guess none of this would affect the setting you mention.

Originally, the engineer had assumed the new pump would be connected upto the new VR 71 wiring centre, but after talking to Vaillant he had to order the X40 so that the pump could be controlled by the boiler, rather than by the wiring centre. At the time I thought this made sense i.e. any time the boiler pump is on, the new circulating pump should also be on, but this doesn't appear to be the case during pump overrun, when only the boiler pump is running.

I think the "pump relay" you mention is actually the X40. I believe this now comes as standard with new Vaillant boilers, but didn't with mine, which is now 10 years old. To connect the pump to the S/L of the motorised valves, it would need to be wired upto 4 different zones in the VR 71 and I believe this was also my engineers first thought, until Vaillant told him to connect it upto the X40.
Did you previously have HW priority before? The settings for this are I think D.75, D77, D.78. D.77 should be set to the KW rating of your Vaillant Coil.

Regarding the X40, yes you are probably correct. The newer Vaillants over a certain Kw have the relay built in now.

Has the probe been for the cylinder been properly wired in?
 
The second pump after the llh should run with any demand eg hot water, heating or ufh
 
Did you previously have HW priority before? The settings for this are I think D.75, D77, D.78. D.77 should be set to the KW rating of your Vaillant Coil.

Regarding the X40, yes you are probably correct. The newer Vaillants over a certain Kw have the relay built in now.

Has the probe been for the cylinder been properly wired in?

Yes, the system has always been setup for DHW priority.

D.75 = 45 (factory setting)
D.77 = 37 (factory default and maximum output I believe)
D.78 = 80 (factory default)

So, it looks like these settings are all defaults.

Yes, the cylinder temperature probe has been connected to S9 on the VR 71 wiring centre as shown in the attached schema diagram. This diagram was supplied by Vaillant as the nearest to my setup as none of the diagrams in the VRC 700 System Diagrams booklet were any good! However, this diagram shows each zone has having its own pump (and mixing valve), which isn't the case in my system.
 

Attachments

  • VR 71 Wiring Centre System Configuration.pdf
    480.9 KB · Views: 12
Are you sure the second pump is actually running when theres a demand?
Yes, most definitely.

The Grundfos pump has LED's that show green when the pump is running. The VRC 700 is very good at showing what the system is doing and which zone valves are open. I spent a bit of time ensuring all the zone valves and the new pump were operating correctly for each zone when the upgrade was completed.
 
To be clear about pump overrun, from memory, the boiler display shows a circle with a black triangle indicating the internal pump is running. To the left of this is a tap (hot water supply active) and the i button shows S.7 - Pump over run. During this time the LED on the Grundfos pump is NOT on. I also see this from time to time during the day, but instead of the tap, it is showing the radiator symbol in the display. Again, the Grundfos pump is not on, so while in this mode, no hot water is being pumped around this system, even when the thermostats are calling for heat!
 
There can’t be a demand then or cylinder isn’t upto temp as my thought is a soon as it’s finished heating the cylinder it should switch to heating if heating is calling if it’s not then it should turn off or pump overrun depending on what vaillants do
 
There can’t be a demand then or cylinder isn’t upto temp as my thought is a soon as it’s finished heating the cylinder it should switch to heating if heating is calling if it’s not then it should turn off or pump overrun depending on what vaillants do
I believe the cylinder is taking too long to get upto temp, so the system is in DHW mode for 50 minutes and a lot of that time it is showing pump overrun. Every time it goes to pump overrun (S.7) the new Grundfos pump turns off, so all that hot water that is running around the boiler side isn't going anywhere. Eventually, I assume it cools down and the pump is then turned on again and the water starts to be pumped around the cylinder. Then the boiler circuit heats up and its back to S.7. This continuous overrun mode seems to cause it to take much too long to heat up the cylinder, but eventually, the DHW circuit closes (I guess when it finally does get to temperature) and the other CH circuits open and finally the radiators etc. start to warm up.

At least, that's my novice understanding of what might be happening, but I could be completely wrong.

In the morning I will check the state of the system and the individual zones (specifically DHW) to be sure there is a demand whenever the boiler is in pump overrun mode. Given that it's supposed to be heating up the hot water cylinder, I would hope there is a demand.
 
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Is the pump on full speed ?
 
Is the pump on full speed ?
No, it is on the slowest speed (setting 1 of 3). The engineer left it on full speed when it was installed and when I called him and asked if there was any reason for that he told me there wasn't, so I turned it down.

It seems to me that the slowest speed setting allows the heat in the water time to dissipate while circulating around the system, so there's more chance of it being at a lower temperature on its return to the boiler/LLH. I'm not actually convinced the boiler is condensing as much as it used to, but I believe that is one of the side affects of using LLH's. But that's potentially another issue for another day 😀
 
Could this not be a control fault ie PCB. The function of pump overrun is to dissipate the residual heat after a demand is satisfied to protect the boiler. It shouldn't be active at the same time as a demand, although obviously the internal pump will be.
 
I've just noticed the boiler is currently showing the radiator and pump symbol, but the new Grundfoss pump is not on. "i" shows S.0, which surprised me as according to the VRC 700, the radiator zone is requesting heat i.e. it is on and the valve is open. The system status is showing "heat.mode". So, right now, there is demand by one of the zones, but the external pump is switched off and the internal boiler pump is running. This is what I assumed was pump overrun mode, but the boiler is showing S.0, not S.7!

This could be a different scenario to what is happening in the mornings, so I still need to monitor it first thing.
 
Could this not be a control fault ie PCB. The function of pump overrun is to dissipate the residual heat after a demand is satisfied to protect the boiler. It shouldn't be active at the same time as a demand, although obviously the internal pump will be.
That's the thing I don't understand about pump overrun. Isn't the best way to loose heat to pump the hot water around the system, without heating more hot water i.e. have only the pump running? You're saying there shouldn't be demand at the same time as pump overrun meaning all the 2 port valves would be closed, so any pumping would result in the water just cycling round the boiler (via the diverter valve)? Isn't this just wasted energy and at the same time a zone is waiting to be heated?
 
Sounds like it’s not set up / wired right next thing to do is speak to vaillant and make sure everything is plugged into the right kit
 
Sounds like it’s not set up / wired right next thing to do is speak to vaillant and make sure everything is plugged into the right kit
Yes, I have contacted Vaillant already about the setup, given the installer had a few issues, I got Vaillant to confirm all the wiring connections, which was when they provided me with the schema diagram.

I will ask them again, specifically about the pump connection, and explain the behaviour I'm seeing in the mornings.
 
As shaun says it does sound like it's not wired/set up correctly.
Ideally yes you would use the residual heat with pump overrun but in most cases it just circulates through a bypass to slowly drop the temperature of the heat exchanger, of course whats happening in your case is you're circulating quite a small volume so its no surprise its taking time for that temperature to drop.
 
As shaun says it does sound like it's not wired/set up correctly.
Ideally yes you would use the residual heat with pump overrun but in most cases it just circulates through a bypass to slowly drop the temperature of the heat exchanger, of course whats happening in your case is you're circulating quite a small volume so its no surprise its taking time for that temperature to drop.
So, do you think with my old system (pre LLH and external pump) that in pump overrun mode that hot water would have continued to circulate around the radiators/cylinder? I guess this depends on the 2 port valves being open, but if they weren't how would the water have circulated?
 
Certainly sounds like a wiring issue.
Basically what I said first time round 😉😜

After having a fair bit of experience with Vaillants, my concern would probably be whether your new controller and wiring centre is compatible with your older PCB. They say they are, but i'm not always convinced.

You need to set D.77 on 16kw (From memory this is what the coils are in the Vaillant uniStors) Otherwise you're going to get a lot of anti cyciling which may not be helping issues.

After heating each circuit the boiler will overrun the pump with the zone valve open, so after a HW reheat it'll over run and after a CH demand it'll over run. I can't exactly remember if it over runs also once the zone valve closes.

If your secondary pump out of the LLH isn't operating though on any demand, then that would explain your HW issue in a morning. The boiler will try and heat the cylinder (Failing) and then at 45 mins decide it can't do it, over run the pump then switch to CH mode.
 
So, do you think with my old system (pre LLH and external pump) that in pump overrun mode that hot water would have continued to circulate around the radiators/cylinder? I guess this depends on the 2 port valves being open, but if they weren't how would the water have circulated?
There is a bypass in the boiler, but yeah the Vaillant controls usually hold the zone valve open for a period of time. Well they do on the newer system from what I recall.
 
It would go round the LLH. That's effectively your bypass, plus an integrated one.
Yes, in the new setup it can cycle to the LLH and back. If the external pump was on, it could cycle further (and cool down quicker, while at the same time continuing to provide heat - to the cylinder in the morning) and I guess this is the crux of the issue that this doesn't happen in pump overrun mode.
 
Out of interest have you checked to see if there's power to the pump? I've had a couple of them newer UPM3s fail quite early on tbh.
 
I meant when it isn't running and the boiler is saying it should be.
Ah, sorry, I see what you mean.

The only situations I'm aware of where the external pump doesn't start, when the internal pump does start is S.0 and S.7. It is possible there are other status codes that I haven't come across, but these are the two I regularly see.

However, when there's demand and neither of the above status codes are showing on the boiler, then both pumps are on at the same time.
 
Ah, sorry, I see what you mean.

The only situations I'm aware of where the external pump doesn't start, when the internal pump does start is S.0 and S.7. It is possible there are other status codes that I haven't come across, but these are the two I regularly see.

However, when there's demand and neither of the above status codes are showing on the boiler, then both pumps are on at the same time.
Ah right OK, as I said in an earlier post then i'd configure your HW configuration correctly on the PCB first and see if that makes a difference. You're probably getting a lot of anti cycling.
 

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