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Discuss Pump overrun is killing my heating! in the Central Heating Forum area at PlumbersForums.net

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140
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?
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
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26,076
No both pumps should be on maybe depending on how the hot water is piped up is it after the llh ? / any pics ?
 
Messages
140
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.
 

EvilDrPorkChop

Gas Engineer
Messages
647
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.
 
Messages
140
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.
 

EvilDrPorkChop

Gas Engineer
Messages
647
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?
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Messages
26,076
The second pump after the llh should run with any demand eg hot water, heating or ufh
 
Messages
140
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
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Messages
140
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.
 
Messages
140
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!
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Messages
26,076
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
 
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