Vaillant boiler frost problem

[whistler]

I asked this question last winter, but no one seemed to have the answer, so as winter is here again I thought I would give it another try.

Boiler is a Vailant Turboxmax 824E about 8 years old and generally working ok. The boiler is located in an attached garage on a single brick external wall and so is subject to cold temperatures in winter.

When the outside temp drops to around freezing, the boiler starts up - presumably by the built in frost sensor. However after about five seconds it shuts down with quite a loud bang. When I say 'bang', its not an explosive bang but more of a metallic bang as though someone had hit the pipework with a hammer!. It only happens once usually in the early hours when the temperature is lowest. If I get out of bed and switch the boiler on via the room thermostat it fires up normally.

I discussed the problem with the service engineer and to be honest he had no idea what was causing this bang and suggested it would be difficult to identify the problem unless he sat in front of the boiler all night! I also emailed Vaillant tech support but all they could suggest was to contact a local engineer.

So the other day on the first really cold morning of this winter, it happened again at around 8am. As I was awake I heard the boiler fire up and sure enough after a few seconds 'bang' and the boiler shut down. There are no fault indications on the display and status is normal.

I wondered if it is possible for an engineer to simulate a frost condition to see what happens.

Any advice would be very welcome.

Any ideas ?

 

[Beerlover]

I am guessing you have had the new style diverter valve fitted. Is the pcb the original? The later ones have reconfigured software that slows the action of the diverter valve to stop clunking between DHW and CH operation.

To mimic run the CH, with the engineer there, and turn the hot water on and off to see if this is the cause of the problem. If you have insurance cover, get him to change the pcb anyway.

 

[whistler]

The diverter valve is the new type with oval shaped ball. The original had the sticking ball problem. I don't know if the pcb is original as the boiler was installed before we bought the house. I think i know the clunking sound you refer to which happens when the valve changes, but this is not the banging noise I hear which happened before and after the valve was replaced.

I have actually made sure the valve is in the CH position before going to bed just in case the valve was somehow causing the problem, but the problem still occurs.

I was hoping that it might be possible to simulate the frost situation by shorting the frost stat. I have the full service manual but although it refers to frost protection it doesn't show the thermostat in the schematic or wiring diagrams.

 

[Bunnyman]

Hey beer lover ur good mate u must b ex company man ? I agree with u it's annoying but u can cure by changing to later pcb or drill relief hole in bypass valve

 

[bobw1956]

Hey beer lover ur good mate u must b ex company man ? I agree with u it's annoying but u can cure by changing to later pcb or drill relief hole in bypass valve

Can you elaberate on the relief hole in the bypass, not heard that one before`lot cheaper than a new board

 

[Bunnyman]

3mm hole thanku 831 bunny for refreshing my memory!!

 

[The831Bunny]

3mm hole in the centre of the plastic bypass insert (drain boiler and remove bypass) will usually sort most DV banging probs. Do you live in a bungalow? I've found long runs of horizontal heating pipework causes the best bangs also FYI the early pcbs didn't stop the pump at all during DV changeover, hence pushing/pulling the ball against the seating with a lump.

Saves using the newer 'galloping horse' board eh Bunnyman

 

[whistler]

Yes I do live in a bungalow. All of the pipework is under floor and horizontal. I have wondered if it was a pipework bang, but why would this only happen when the frost protection activated?

Where is frost stat? Is it built into the pcb?

 

[Agile]

There is no frost stat!

The boiler measures the temperature of the water inside with electronic sensors.

Tony

 

[The831Bunny]

The frost protection is activated via the boiler NTC temperature sensors and turns the pump on first when the water temperature drops to 8C, it then looks for a rise in temperature (warmer water pulled back from house) and then if required raises the flow temperature to 30C. It's all part if the pcb software control and as such can't be turned on or off.

I'm guessing that when you hear the clunk any trv's on the system are wide open (rooms cool) and there is low system resistance and large water volume movement at that moment, which makes the noise more likely to happen. If you have warmstart enabled as well (green light on) there will be times during the night the DV moves for sure. It also exercises once in 24 hours too, so it's virtually impossible to fix it in heating before you go to bed. The other work-around as well as the bypass fix is to alter the pump speed to 1. This will impact on DHW temperature and heating performance though.

A quick check to see if there's any other problem is to turn one of the isolation valves off before you go to bed. If you still get the lump it's something else.

 

[whistler]

Thanks for all the replies.

We have warmstart turned off (it annoys us during the night) and don't have any TRV's so can count those out of the problem. I also considered the 24 hour startup and tried turning the boiler off electrically during the day thinking that the 24hour time would then always be the same time every day.

Turning the pump down sounds like an easy option. I'll give that a try.