Vaillant Combi Boiler - F28 fault
- Thread starter boiledegg
- Start date
Sponsored Links
right so last night just before going to bed I thought I might try a slightly different approach. I turned the boiler from constant onto timer ( the timer was in "off" position for heating so heating turned off) and at that point the pressure was 2.5 bar. So I waited for about 20 minutes whilst the rads slowly cooled down. After about 20 + minutes the pressure had dropped to 1.5 bar. So I waited another 10 minutes and there wasnt much more movement. The rads hadnt cooled down properly yet though.
At this point I thought I would add another .5 of pressure just to see if this would stop it going down to far and would help it fire back up in the morning on its own.
Anyway woke up this morning about half an hour ago. House is freezing, boiler is flashing F28 and pressure is at zero again. The boiler should have come on with timer over 2 hours ago. Anyway I refill pressure to one bar and hit the ignite button and boiler comes on straight away. I have just now checked the pressure and its shot up to 3 bar - its been on for half an hour (I expect it to drop shortly as the pressure release valve kicks in).
Can the cold force the pressure down that quickly? It means during a six hour spell overnight that the pressure has dropped from 2.5 to Zero because I have turned heating off. Yet it somehow finds that 2.5 and whatever other pressure I have added once I have re fired it up. For a while I thought maybe pipes in the house were freezing for the pressure to drop so much but I dont have any problems refilling the pressure and switching back on (i dont have to boil any kettles).
Does this definitely make it the expansion Vessel?
At this point I thought I would add another .5 of pressure just to see if this would stop it going down to far and would help it fire back up in the morning on its own.
Anyway woke up this morning about half an hour ago. House is freezing, boiler is flashing F28 and pressure is at zero again. The boiler should have come on with timer over 2 hours ago. Anyway I refill pressure to one bar and hit the ignite button and boiler comes on straight away. I have just now checked the pressure and its shot up to 3 bar - its been on for half an hour (I expect it to drop shortly as the pressure release valve kicks in).
Can the cold force the pressure down that quickly? It means during a six hour spell overnight that the pressure has dropped from 2.5 to Zero because I have turned heating off. Yet it somehow finds that 2.5 and whatever other pressure I have added once I have re fired it up. For a while I thought maybe pipes in the house were freezing for the pressure to drop so much but I dont have any problems refilling the pressure and switching back on (i dont have to boil any kettles).
Does this definitely make it the expansion Vessel?
uhu... and these electronic sensors then.. they are thermistors or something similar, the resistance of which changed when heated.. so they ARE heated up..
how else do you sense a flame?
or is it a gas spectrometer measuring the amount of gases in the flue?
Sponsored Links
Just thought I'd drop in my view - mickyg and agile are almost certainly right about the expansion vessel needing a recharge with air, these are precharged with air (or sometimes, nitrogen/argon) -see Vaillant faq's for correct pressure and procedure, because water (or any other fluid) does not compress to any noticeable extent. The EV is required on a closed heating system to allow for the expansion when the boiler heats up the fluid in CH system. Basically, the EV is open to the sealed CH system at the bottom and is fitted with a flexible diaphragm, air (or some other suitable gas) is pumped into the upper part of the EV, above the diaphragm (normally something like 0.1- 0.2 bar above usual system operating pressure-but check Vaillant instructions) This precharge must be done with the CH system depressured as per instructions. The air in the top of the EV allows the CH fluid to compress the diaphragm and absorb the extra pressure caused by thermal expansion. The O.P's problem sounds very much like there is little or no air in the EV - if pressure looks about correct when cool (about 1.0 bar) but rises rapidly towards 3.0 bar as boiler heats up CH system, it strongly suggests that the water has nothing to expand against and is operating the relief valve (dumping water out of the closed CH system) which is then causing abnormally low system pressure when the boiler cools down overnight and the water contracts. The relief valve sounds as if it is doing exactly what its designed to do and may well be OK. Get the EV precharge checked first. It is possible that the diaphragm, or the EV itself may have failed, - but check air pressure first, if you get water out of the air valve when you test the pressure, I'm afraid the diaphragm has probably failed. You can test the diaphragm by pumping to, say 1.5 bar with boiler off and cold (preferably with closed system depressured and open to atmosphere), and recheck air pressure after 30 minutes or an hour - drop in pressure almost certainly means the diaphragm has failed. Another clue would be getting air from the radiator bleed valves after putting air in the EV. NB. the EV usually has a Schrader (car tyre type valve) on it somewhere.
Sorry for the long post - Regards, Warrior193.
Sorry for the long post - Regards, Warrior193.
i am still in training, but would suggest an idea yo get you over the xmas period.
since it is probable that the expansion vessel has gone you need to get the warm water to expand somewhere to avoid the pressure relief valve from operating. pick a rad that you do not need perhaps the hallway or spare bedroom if you have one. shut both valves. drain down the radiator so that all the water has gone. close the air vent and open just one end up fully (preferably the thermostaitic end if one is fitted). re charge your boiler to 1bar when cold. turn down the temp on the Central heating a little and flash the boiler up if it will reset. the radiatior will have a certain amount of air in it which should help in keeping the pressure slightly down to prevent you lifting the PRV again. The PRV will probably be leaking now as they always do so that will need changing together weith the expansion vessel. This is only a Real short term solution and should not be left, but hopefully you will get heating!
since it is probable that the expansion vessel has gone you need to get the warm water to expand somewhere to avoid the pressure relief valve from operating. pick a rad that you do not need perhaps the hallway or spare bedroom if you have one. shut both valves. drain down the radiator so that all the water has gone. close the air vent and open just one end up fully (preferably the thermostaitic end if one is fitted). re charge your boiler to 1bar when cold. turn down the temp on the Central heating a little and flash the boiler up if it will reset. the radiatior will have a certain amount of air in it which should help in keeping the pressure slightly down to prevent you lifting the PRV again. The PRV will probably be leaking now as they always do so that will need changing together weith the expansion vessel. This is only a Real short term solution and should not be left, but hopefully you will get heating!
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
