Vaillant F20 fault code

Discussion in 'Plumbers' Talk' started by jtothep, Nov 9, 2017.

  1. jtothep

    jtothep New Member


    We've recently had 3 new radiators added to our heating system at home. System is Vaillant EcoTec Plus 428 with UV Gold cylinder.

    Since the radiators have been added, the boiler keeps on cutting out with F20 errors. When I reset it, the temperature on the boiler goes up really quickly and works for a little bit then cuts out again with the same code. It sometimes sounds like there's just water running through the pump but the little flame icon on the boiler screen isn't showing.

    If I bleed some air from the front of the pump while the boiler (I hear a difference in the boiler when I do this) is running it works for a little while longer, sometimes for a few hours before cutting out again.

    I also bled some air from all of the radiators.

    As I said, resetting the boiler and bleeding air from pump, fixes it temporarily.

    Boiler was serviced a couple of months ago. Heating and hot water were working fine before radiators.

    Can anyone help out on this?

    Many thanks

  2. KIAB

    KIAB Well-Known Member

    Safety temperature limiter by NTC activated, & the cause Flow-NTC not correctly thermal-connected or defective; appliance does not
    shut down.

    Installation manual with fault codes here:
  3. Dave does Gas

    Dave does Gas Well-Known Member

    Is the auto air vent above the pump open this will clear all aair from the boiler.
  4. jtothep

    jtothep New Member

    Thanks both for your replies.

    Dave does gas — what does an auto air vent look like? I've attached pics above pump and boiler.

    Last night when I got home, it had f.20 so I reset it, bled some air from pump and then again. It made some strange noises at first but after more bleeding it started heating up gradually and stayed on for couple of hours for heating and hot water. I opened up the little valve above the boiler (shown in pic) and a tiny bit of air came out but not much. I was able to turn the heating on and off successfully later in the evening. But this morning it hadn't come due to the f.20 thing again.

    Attached Files:

  5. Jto, does this boiler have a pressure gauge on it? (Ie - is it a 'system' boiler, unvented, with a pressure gauge, or is it a 'heat only' type (vented, with a small F&E tank in the loft)?

    Is having to bleed air from rads and the vents an on-going issue, or has that stopped now?

    The wee thingy on the very top of that pipe above the boiler looks like a manual air vent - do you have to use a radiator 'bleed' key on it? That short piece of pipe collects any air passing out the boiler, and then you can bleed it off whenever you need (but always with the boiler turned off - same as with rads).

    'Auto' air vents are much bigger than that, and will almost certainly be inside the boiler, especially if it's a sealed, unvented, 'system' boiler.
  6. jtothep

    jtothep New Member

    It's a system boiler. Pressure gauge is on a red expansion tank. Reads just under 2 bar. Before these issues bleeding the rads was never an issue but a few days ago there was quite a lot of air in one rad. It was expelling air for about 30 seconds or so.

    The thing above the boiler has what looks like a dust cap for a car tyre that twists off. This has a small hole in it.
  7. KIAB

    KIAB Well-Known Member

    If that valve is on the expansion tank, then it's for recharging vessel.
  8. Ah, these boilers are designed for both vented and unvented systems, I think. For 'unvented' (sealed, like yours) an exp vessel and pressure gauge needs fitting to the pipework.

    Can you tell me what that pressure gauge reads from when you start the boiler from cold, to when it heats up?

    I wonder if the exp vessel has failed, and the air side of it is where your system sir is coming from? In which case, you should see the 'cold' pressure go up significantly when it heats up, and will probably reach 3 bar.

    Can you also confirm - that BRASS item in your second pic in post #4, the thingy that sits at the top of that short 15mm(?) pipe has a car-type dust cover on it? Could you take a photo? Have you ever tried to release air from that thingy? (But DON'T try this unless you know how it works).
    KIAB likes this.
  9. KIAB

    KIAB Well-Known Member

    Valve is a schneider valve, so can use tyre pressure gauge or the gauge on footpump.
  10. dobbie

    dobbie Well-Known Member

    I have a Schrader valve on my expansion vessel.
    I do use a lot of Schneider products though, as they have bought out a lot Electrical product manufacturers.
  11. KIAB

    KIAB Well-Known Member


    Yes ok, I got my Schneider & Schrader mixed up, but Schneider are involved with Schrader valves.:rolleyes:
    jtothep likes this.
  12. Hopefully that wee brass thing at the top of the water pipe has neither a Schneider nor a Schrader valve...
    jtothep likes this.
  13. jtothep

    jtothep New Member

    I thought I'd give it a couple of days before posting it again to give a better update on what's going on.

    Only Friday evening it had f.20 on boiler in evening. I did the usual of resetting the boiler, bleeding air out the pump. Then went around all the radiators and removed air from a few. System was working great until Saturday evening. Then had to reset it again later in the night. Then again this morning.

    Pressure gauge on red expansion vessel reads 1.8 most of the time. I noticed it has decreased .1 or .2 from 2.

    Not much more to report other than that really. So pretty much the same things are happening as when I first started this thread
  14. jtothep

    jtothep New Member

    Early on that day I had waited for things to cool down and release a tiny bit of air from the little valve above the boiler. Later in the day I went up to the loft to check out the boiler and the water and heating were off at that time, no f.20 fault, but the pump was running with water going through and the boiler display said 76c. There wasn't any flame icon in the display. I pushed flame button and the temperature quickly shot up to 104c before reducing.

    Later on the f.20 fault came back. And today we have no heat
  15. jtothep

    jtothep New Member

    Here's the pic of the valve.

    Noticed the pressure had dropped a little this morning from 1.8 to 1.6

    Attached Files:

  16. dobbie

    dobbie Well-Known Member

    That is an air vent.
    jtothep likes this.
  17. Ok, assuming that your system is sealed (unvented) which I guess it must be if that exp vessel is for the 'system' and not the gold cylinder, then one possibility is that the exp vessel is faulty. There's a rubber diaphragm inside it which separates the pressurised water system side from the pressurised 'air' side. If that diaphragm is leaking, then the pressurised air can make its way in to the system water.

    However, I would expect your pressure reading to start going a bit crazy as a result - with little air left in that vessel, the pressure should be shooting up quite rapidly when the systems heats up, but you reckon it's pretty steady?

    I can't think where else the 'air' is coming from - unless you have hellish corrosion going on in your rads!
  18. The Teach

    The Teach Well-Known Member

    Did the radiator installer bleed the boiler/system according to the makers instructions ? If not...

    Ensure the auto air release inside the boiler (often adjacent to the pump) is open and any other additional auto air vents are open.

    iirc,that boiler has a test program and will run the boiler with no gas burning for a set time then introduce gas at low fire until the test programs is complete, then it run's at normal settings,They are accessed on the front controls have a read of the installation manual.P0 is the bleeding program.

    Run the test programme on hot water only,bleed any manual air vents.then on hot water & central heating.

    looking at your pictures it may take a few attempts to bleed enough air to run the boiler.

    also make sure the pump is running at a fast enough speed (3) with pump valves fully open !

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