Vaillant Turbomax plus 828E low water pressure F28 lockout

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Hi guys, Hope you can help. I've viewed all the relevant and some of the irrelevant posts on similar subjects but nothing fits my exact problem.

I have a five year old Vaillant Turbomax plus 828E. It has worked flawlessly for all of that time until the last 3 weeks. When the central heating turns off on the time clock the water pressure drops to zero and when there is demand for central heating or hot water the boiler locks out. F28 fault. Top up the boiler from the filling valve to 1.5bar and restart and everything remains fine until the next time it shuts down again.

If I run the central heating on 24hrs it continues to work although when the thermostat has signalled off the water pressure plummets to about 0.8bar. I've checked the whole circuit for leaks, nothing. I've checked the PRV out let, not wet. When the boiler is going full chat and the flow temperature is above 50degs the water pressure climbs.

I have no idea where to go now. Could the expansion tank be faulty? or is it something simple?
 
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Put a bottle or plastic bag over PRV vent.

Classic symptom of a failed expansion vessel. See FAQ for testing and repressurising.

One day I will write my own explanation of how to do that properly.

Tony
 
Okay Thank you.
I've put a bag around the PRV outlet and after topping up the system to 1.5bar and restarting I got about 3/4 of a litre in 2 hours. Pressure gauge reads about 0.8bar.

I've read the FAQs on repressurizing the expansion tank. Does the water collected in the bag prove anything conclusive or do I still have to find the expansion tank schrader valve? Not frightened of testing it just short on time and if It's definitely the expansion tank I can order one for next Saturday.

I'm expecting the 'short cuts cost money answer'

Thank you

Dadllfixit
 
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Okay Thank you.
I've put a bag around the PRV outlet and after topping up the system to 1.5bar and restarting I got about 3/4 of a litre in 2 hours. Pressure gauge reads about 0.8bar.

I've read the FAQs on repressurizing the expansion tank. Does the water collected in the bag prove anything conclusive or do I still have to find the expansion tank schrader valve? Not frightened of testing it just short on time and if It's definitely the expansion tank I can order one for next Saturday.

I'm expecting the 'short cuts cost money answer'

Thank you


Dadllfixit

Hi I`m assuming that the pressure goes up when the heating system is hot?The PRV should only open at 3 bar pressure allowing the water to be expelled to outside the building ie into your bag.Because you have then lost that volume of water(into the bag)then when your boiler goes off and cools the pressure drops.
If this is the case then your expansion vessel requires checked /pressurised as follows
1. turn boiler off(switch at front will suffice)
2.Locate pressure relief valve under boiler on RHS(if there is a bottom cover you will have to remove this)
3.Turn PRV anticlockwise to a position where you catch it at the top of the cam(to manually keep valve open).The water will drain from the boiler and the gauge will drop to zero.
4. On top of the boiler behind the flue outlet right at the back you will see the expansion vessel incorporating a schrader valve at the top of it.
5. remove dust cap and attach a foot pump and pressurise to 0.5 bar
(opening the prv is to ensure there is no pressure on the other side of the expansion vessel diaphragm)
6. remove footpump,close PRV(turn anticlockwise) refill boiler and test boiler operation.

NB Before starting this procedure you can try pushing in the shrader valve,if a significant amuont of water comes out then it is probable that the diaphragm is perforated requiring replacement of the vessel.
Also after pressurising the vessel check the valve for passing (soapy water or leak detection fluid)

Hope this helps Martin Gas Safe registered engineer
 

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