Vaillant combi boiler - DHW pump overrun error

[DIYer18]

Good morning all, I hope someone could help me out here. My boiler is a Vaillant ecoMAX 828/2E. Five years ago we changed the internal pump when we power flushed the whole central heating system but apart from that it has always worked fine both in winter and in summer.

Since last week though I noticed that the pump keeps running with no hot water demand. Like...every minute! The "tap" symbol appears on the boiler system display for 1 second, the pump activates (no normal flame image though), then it stops for a little bit until when the tap symbol appears again and here we are again...I wonder if this is due to the extremely high temperatures of these days or if it got jammed or if there is a leak in the hot water pipes or...I don't really know...

To give you more background, the only way to stop it has been so far to switch the boiler off. When I turn it on again the system code is S.31 ("No heat demand boiler in summer operating mode") with the display showing 30 degrees. But within seconds the tap symbol appears again for 1 split second and the pumps starts running, with the S.17 code on. It's not a fault code but the manual says "DHW mode – pump overrun error". No hot water is used in the kitchen, basin or bath though.

Clearly central heating is switched off (e.g. thermostatic valves in the radiators set to ZERO, central Drayton thermostat set to 5 DEGREES, WiFi time controller set to OFF, etc.) hence the only demand can come from DHW.

I have tried to contact the 6 Vaillant-approved technicians closest to my post code and one of them serviced the boiler. The only comment that he made is that the boiler is approximately 10 years old hence it is meant to make noises...but the operation is fine according to him and it's not worth spending more money on it.

I don't think this is fine at all. The noise of the pump running constantly does bother me at night, but most importantly this has never occurred to date and I believe it may affect the performance and the life spam of the various components, if not the safety of the operations, hence I would like to understand what is causing the pump to run continuously and fix it if possible.

Have you ever come across something similar? Thanks!

 

[Gasguru]

Isolate the cold water to the boiler and see if that stops the tap symbol coming on.
It could be either a leak or trapped air somewhere in the pipework acting as a "spring".
On shared mains even a neighbour using water could cause water flow triggering the boiler as the trapped air expands and compresses.

 

[DIYer18]

Thank you Gasguru, this could actually be very good advice. I occupy a flat in a multi-storey block and a few weeks ago we received a letter from the company that undertakes the building's maintenance explaining that they were working on the water mains pipework to empty the common tanks located at roof level. We were asked to not use any water for a few hours and to not leave the plugs in the basins to avoid potential air blockage. Although this issue started only a few days ago, I now realise that the two things could be connected. My wife reported that a few days ago she opened the kitchen taps and there was a lot of air coming through both cold and hot taps, but didn't think this could affect the boiler. I don't think there is a leak, otherwise my neighbours below me would have complained, I presume. Do you share my concerns regarding sorting this thing out as soon as possible or do you agree with the Vaillant technician's comment?

 

[DIYer18]

Isolate the cold water to the boiler and see if that stops the tap symbol coming on.
It could be either a leak or trapped air somewhere in the pipework acting as a "spring".
On shared mains even a neighbour using water could cause water flow triggering the boiler as the trapped air expands and compresses.

I spoke with the Vaillant technical service team and apparently in their opinion the issue could be due to the low DHW set temperature (42 degrees). They recommend to use 55 as a minimum, better 60. However my boiler is less than 5 metres away from the toilet, with insultated pipes to the basin (which has separate taps, by the way) and the shower, hence supplying hot water at 55 or 60 degrees to wash my hands or to have my shower in the morning would be a waste of energy (and water), as I would need to mix it with cold water to avoid getting burnt! What's your take on that?

 

[Gasguru]

I wonder if Vaillant are assuming you have comfort mode (hot water pre heating) turned on and the boiler is kicking in. 60 degrees is the recommended temperature to ensure any Legionella bacteria are killed off in the pipework.
You can often tell if there trapped air within the pipework by opening the hot tap and observing the water flow where it hits the basin surface. If the initial flow tends to reduce after a second to a steady flow it's a sure sign of trapped air.

 

[DIYer18]

I wonder if Vaillant are assuming you have comfort mode (hot water pre heating) turned on and the boiler is kicking in. 60 degrees is the recommended temperature to ensure any Legionella bacteria are killed off in the pipework.
You can often tell if there trapped air within the pipework by opening the hot tap and observing the water flow where it hits the basin surface. If the initial flow tends to reduce after a second to a steady flow it's a sure sign of trapped air.

Thanks again Gasguru. The "warmstart" option is off. As you say, they have asked me to triple check if it was on, but this is not the case (it's Summer, I don't really need it, and the boiler is literally 5 metres off the taps anyway, it takes nothing to get hot water in the toilet). I don't think there is any trapped air. Some plumbers are proposing another couple of options: either the PLC being faulty (and have proposed to "tap" it vigorously to unblock it!) or some thermostat issue informing (wrongly) the pump that hot water is needed...I don't really know what to do.

 

[Gasguru]

Have you isolated the cold feed to the boiler?