Pump overrun is killing my heating!

[milleniumaire]

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]

No both pumps should be on maybe depending on how the hot water is piped up is it after the llh ? / any pics ?

 

[milleniumaire]

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.

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:

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]

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.

 

[milleniumaire]

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]

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]

The second pump after the llh should run with any demand eg hot water, heating or ufh

 

[SJB060685]

Are you sure the second pump is actually running when theres a demand?

 

[milleniumaire]

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.

 

[milleniumaire]

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.

 

[milleniumaire]

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]

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

 

[milleniumaire]

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.

 

[ShaunCorbs]

Is the pump on full speed ?

 

[milleniumaire]

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

 

[SJB060685]

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.

 

[milleniumaire]

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.

 

[milleniumaire]

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?

 

[ShaunCorbs]

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

 

[milleniumaire]

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.

 

[SJB060685]

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.

 

[milleniumaire]

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?

 

[EvilDrPorkChop]

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.

 

[SJB060685]

If there was no circuit you wouldn't of had flow. Did you have a bypass?

 

[EvilDrPorkChop]

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.

 

[EvilDrPorkChop]

If there was no circuit you wouldn't of had flow. Did you have a bypass?

It would go round the LLH. That's effectively your bypass, plus an integrated one.

 

[milleniumaire]

If there was no circuit you wouldn't of had flow. Did you have a bypass?

The only bypass is the one in the boiler.

 

[SJB060685]

It would go round the LLH. That's effectively your bypass, plus an integrated one.

He said prior to the llh lol

 

[SJB060685]

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.

I wish all boilers were this advanced lol

 

[milleniumaire]

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.

 

[EvilDrPorkChop]

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.

 

[milleniumaire]

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.

Yes, there's definitely power as the LEDs are on and I can feel the pump vibrating, which increases as I increase the speed.

 

[EvilDrPorkChop]

Yes, there's definitely power as the LEDs are on and I can feel the pump vibrating, which increases as I increase the speed.

I meant when it isn't running and the boiler is saying it should be.

 

[milleniumaire]

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.

 

[EvilDrPorkChop]

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.

 

[EvilDrPorkChop]

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.

Can I just ask, who did that drawing for you? It's great, I struggle to get an answer for basic things out of my rep (Although he isn't bad to be fair).

 

[milleniumaire]

Can I just ask, who did that drawing for you? It's great, I struggle to get an answer for basic things out of my rep (Although he isn't bad to be fair).

I don't think Vaillant created that drawing especially for me. If you look at the date, it is 30/07/2018.

The VRC 700 came with a printed System diagram book, unfortunately none of the diagrams seemed to come close to matching my setup and I pointed this out to Vaillant when asking them to check the installation for me (via email) as I wasn't 100% convinced that everything had been connected up correctly by the heating engineer. He's a great guy and does all my servicing and upgrading, but even he admits its difficult to keep up with technology! As an IT consultant, I'm used to getting into the detail and like to understand how things work, so had no problem liaising with Vaillant support and have made a couple of changes as a result.

As I explained earlier, although the diagram reflects the zones I have (DHW via cylinder, 2 x UFH and radiators) there are some differences, mainly in the area I'm having issues; the external pump. The diagram shows each zone having its own pump, so I suspect these would be wired up in the same way as the zone valves so that when the valve opens the pump switches on. It appears to be a little more complicated to use a single pump for all zones and even my engineer was surprised that the pump had to be connected to the boiler, rather than to the wiring centre!

One day, when I have time, I may create my own schema diagram using Microsoft Visio

 

[milleniumaire]

Just found the installation guide for the X40 expansion board, which plugs into the boiler PCB and provides for the external pump connection.

Here are the relevant pages:

I'm trying to understand how the above relates to the actual device and the way the pump has been wired upto it:

On the boiler d.27 = 5 and d.28 = 2

What's the difference between "Circulation pump", "External Pump" and "Storage charging pump"?

Based on the photo, showing the pump is connected to the yellow "rel 1" connectors, which 1, 2, 3, 4, 5 or 6 does this relate to?

Because it is wired to rel 1 (open) and d.28 is 2, I assume it relates to 2 in the table. Are 1 and 2 mutually exclusive. 1 shows a ZP & tap and 2 shows a radiator, so I'm thinking they relate to DHW and heating! I'm not sure what "ZP" means.

Note: Just realised the last paragraph is nonesense as d.28 relates to "rel 2", so I'm now wondering if either the connection is wrong or the setting of d.27 is wrong!

 

[ShaunCorbs]

Those connections need to be done better as there’s cores nearly touching each other

 

[SJB060685]

As shaun says those connections are poor and the chance of a short circuit are real. Looking at the amendment should external pump go to relay 2?

 

[EvilDrPorkChop]

Just found the installation guide for the X40 expansion board, which plugs into the boiler PCB and provides for the external pump connection.

Here are the relevant pages:

View attachment 46989

View attachment 46991
I'm trying to understand how the above relates to the actual device and the way the pump has been wired upto it:

View attachment 46990

On the boiler d.27 = 5 and d.28 = 2

What's the difference between "Circulation pump", "External Pump" and "Storage charging pump"?

Based on the photo, showing the pump is connected to the yellow "rel 1" connectors, which 1, 2, 3, 4, 5 or 6 does this relate to?

Because it is wired to rel 1 (open) and d.28 is 2, I assume it relates to 2 in the table. Are 1 and 2 mutually exclusive. 1 shows a ZP & tap and 2 shows a radiator, so I'm thinking they relate to DHW and heating! I'm not sure what "ZP" means.

Note: Just realised the last paragraph is nonesense as d.28 relates to "rel 2", so I'm now wondering if either the connection is wrong or the setting of d.27 is wrong!

Should be down as external pump I think.

 

[milleniumaire]

Should be down as external pump I think.

So, assuming external pump is correct, does the chart suggest it can be wired in either of two different ways:

a. Connect to rel 2 and set d.28 to 2
b. Connect to rel 1 and set d.27 to 2

I'm concerned about the distinction between "circulation pump" (1) and "external pump" (2). Does this imply the pump will only work during hot water demand (1) or heating demand (2), but not both?

 

[SJB060685]

Your observation is like mine. Possibly connect to relay 2 and set to D.28 and see what happens?

 

[John.g]

Following this very interesting thread.
Just a few basic observations.
Pump overrun should not occur unless CH and/or HW satisfied, if boiler output still too high on max turndown then burner off but boiler circ pump and secondary pump should continue to run but overrun should not be called for. When pump overrun is called for normally, would expect secondary pump to be off. Suggest to check if pump overrun comes on with burner off with call from CH or HW.
Slow rad heat up/output not mentioned but with a LLH the boiler circ pump and external pump should be matched with boiler circ pump flowrate slightly higher?. so possibly wrong settings with boiler circ pump or more likely secondary pump or faulty pump. Also assume LLH connections correct.

 

[John.g]

Also S.0 refers to "no heating demand" which I would think is indicative of the boiler sensing no heat demand even with zone valve(s) open, I would have a good look at that Grundfos Pump as it may be the main cause of your problem, hopefully.

 

[ShaunCorbs]

Also don’t forget about if there’s flow through the pump the leds will light up as the unit is acting like a generator eg there’s no power going to the pump plug

 

[milleniumaire]

Good morning. Just catching up on the new posts.

Also don’t forget about if there’s flow through the pump the leds will light up as the unit is acting like a generator eg there’s no power going to the pump plug

Shaun, are you saying the LED's on the pump would still work even if the pump didn't have power? When I press the speed button on the front of the pump, I can hear/feel the pump toggling through the different speeds. Also, if the pump wasn't working, surely the radiators/UFH and hot water would never warm up as there's no way the internal boiler pump could push water past the LLH and around the system!

Also S.0 refers to "no heating demand" which I would think is indicative of the boiler sensing no heat demand even with zone valve(s) open, I would have a good look at that Grundfos Pump as it may be the main cause of your problem, hopefully.

Given my comments above about the pump and the fact that I've done tests on each thermostat to force demand for each zone and can see the pump LED's turning on/off as demand is requested/removed, I'm feeling very confident the external pump is working. Whether it is connected and configured correctly is another matter!

Following this very interesting thread.
Just a few basic observations.
Pump overrun should not occur unless CH and/or HW satisfied, if boiler output still too high on max turndown then burner off but boiler circ pump and secondary pump should continue to run but overrun should not be called for. When pump overrun is called for normally, would expect secondary pump to be off. Suggest to check if pump overrun comes on with burner off with call from CH or HW.
Slow rad heat up/output not mentioned but with a LLH the boiler circ pump and external pump should be matched with boiler circ pump flowrate slightly higher?. so possibly wrong settings with boiler circ pump or more likely secondary pump or faulty pump. Also assume LLH connections correct.

So, I set the heating and hot water to start at 06:30am this morning, which is half an hour later than I usually set it, but I wanted to monitor what happened. At 07:00am the system was still heating the cylinder. The external pump was on, the system status (via the VRC 700) was showing DHW and the boiler status was S.24 (Burner ignited), so all looked to be okay, apart from that fact that the cylinder was still not to temperature (65 degrees C) after 30 minutes. There was absolutely no heat in the radiators or UFH.

10 minutes later it was showing S.20 (Warmstart demand). At 1 hour since turning on I noticed it had switched to heating demand and the radiators finally started to warm up.

I confess, I didn't see any sign of pump overrun this morning. Maybe I just didn't look at the right time (in the first 30 mins) or maybe I'm mistaken about this. However, should it really take an hour to heat up the water in a 260 litre Vaillant unistor cylinder?

Looking at the current config of the external pump. It is currently wired to rel 1 (open) and according to the chart, this suggests that d.27 determines what will happen. As this is currently set to 5, the chart says "External gas valve" for this value. I wonder if this is just incorrectly set and the engineer set d.28 to 2 when he meant to set d.27 to 2 as according to the chart, d.28 has no effect unless the pump is connected to rel 2 (open). As far as I'm aware, my system doesn't have an "External gas valve", so I'm not sure how this impacts on the pump starting/stopping.

From my observations yesterday (not this morning) it appeared that whenever the gas burner was on, the external pump was on. So, when in pump overrun mode, the gas burner wasn't on and therefore the external pump wasn't on. Again, this could be a complete red herring, but I'm trying to make sense of the current configuration and what is happening, given the fact that setting 5 for d.27 mentions "gas valve".

As the pump cable connections are inside the boiler, I don't want to take the front off to change them, so it may be better for me to simply change d.27 to value 2, rather than 5 and see what impact (if any) that has.

I appreciate the connections need to be re-done and I'll ask the engineer to look at them as he's due back soon to service the boiler.

So, although I didn't see any evidence of pump overruns this morning, I've definitely seen it happening on other days, so I'm not ruling this out completely just yet.

If it is expected to take an hour to heat the cylinder to 65 deg C then it would appear everything is working as normal and I'll just have to turn on the hot water earlier in the morning to give this time to happen, followed by time to heat up the house. However, this never used to be a problem prior to the recent upgrade, when the heating/hot water started up at 06:00 every morning and the house was toasty by the time I got out of bed at 07:00am.

 

[Murdoch]

What happens if you set the water to run more than an hour before the heating comes on? i.e. allow the water to run and then turn off before running the heating?

 

[milleniumaire]

Following this very interesting thread.
Just a few basic observations.
Pump overrun should not occur unless CH and/or HW satisfied, if boiler output still too high on max turndown then burner off but boiler circ pump and secondary pump should continue to run but overrun should not be called for. When pump overrun is called for normally, would expect secondary pump to be off. Suggest to check if pump overrun comes on with burner off with call from CH or HW.
Slow rad heat up/output not mentioned but with a LLH the boiler circ pump and external pump should be matched with boiler circ pump flowrate slightly higher?. so possibly wrong settings with boiler circ pump or more likely secondary pump or faulty pump. Also assume LLH connections correct.

Sorry, I meant to comment on your post.

Regarding matching the flowrates for the boiler and external pumps, the external Grundfos pump has a touch button that allows me to increase/decrease the flowrate and it is currently on minimum. I'm not sure exactly how the flow rate of the internal pump is controlled, possibly through a boiler config parameter, but I seem to recall it being on a low setting.

When you say assume LLH connections correct, I assume you are referring to flow and return connections to the system and boiler? With my basic knowledge they appear to be correct and I know my engineer has fitted this type of LLH before, so I would hope he got it right. If they weren't connected up correctly, I'm sure I would be seeing other, more obvious issues, such as heating zones not warming up etc.

I think I understand what you are saying about pump overrun. Basically, you wouldn't expect the external pump to be turned on when pump overrun is on. This is what I am seeing. Usually, when the boiler shows 2.7 (pump overrun) the burner symbol (hour glass) is NOT displayed.

 

[milleniumaire]

What happens if you set the water to run more than an hour before the heating comes on? i.e. allow the water to run and then turn off before running the heating?

Do you mean set the timer to stagger the start times for hot water and heating, so:

06:00 Water on
07:00 Water off
07:10 Heating on

I would expect the hot water demand to end before 07:00 i.e. boiler and pump turn off, due to the cylinder reaching the required temperature. This morning, the heating demand started around 55 mins after the hot water demand started, suggesting it took 55 minutes to heat up the 260 litre cylinder.

If I've understood you correctly, I'm not sure what this would prove.

I have wondered it it is possible to change the settings so that hot water does NOT have priority i.e. heat up both the cylinder and radiators/UFH at the same time?