Baxi 105HE Combi Boiler Problem - Expansion Vessel?

Discussion in 'Plumbing and Central Heating' started by superchoward, 16 Oct 2012.

  1. superchoward

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    Hi,

    I have a problem with the central heating on my Baxi 105HE combi boiler which I have been putting off fixing over the summer (logical time to fix it I know!).

    The boiler is producing hot water fine with no problems.

    On the heating side, the pressure of the system is almost if not at 0. When the heating is turned on the boiler operates and the pressure rises to around 1.5BAR. I have tried to bleed the system and top the pressure up to 1.5BAR when it is cold but this drops over a few hours (can't remember if this is when the boiler is on or whether it does it regardless of whether it is off).

    The radiators are getting warm but not as hot as I think they should be.

    According to the lights on the boiler, the temperature rises rapidly to 80deg C in about 20-30 seconds and then the boiler flame goes off with the pump still running. This cycles on and off continuously whilst the boiler is in operation.

    Has anybody got any ideas of what the problem could be? I think it might be that the expansion vessel diaphragm is broken so when I top the system up and turn it on the pressure relief valve kicks in quickly when there is no expansion in the system.

    I am planning to check this tonight by refilling the system and then keeping an eye on the blow off outside to see whether this is occuring.

    The flue goes out behind the boiler though and the boiler has been recessed into the wall by about 40mm which means I can't get the plate off to check the pressure of the expansion vessel very easily.

    Thanks in advance
    Chris
     
  2. MANDATE

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    On the assumption you have no other leaks the pressure drops due to one or both of the following.
    The expansion vessel having lost some of it's air pressure pre charge means the vessel is probably half full of water before expansion takes place.
    With a reduced volume of air to compress the pressure goes quite high, well it would, but the PRV opens at 3bar. The water pressure drops quickly and the PRV closes. Problem here is that the PRV does not always seal properly, which means you're still left with a problem after you have rectified the expansion vessel problem. The difference between the two leaks is good indicator. A longish period would suggest the PRV.
    The expansion vessel needs a pre air charge of around 0.8bar/ 1bar,but just as important it must be empty of water so the water pressure must be zero with the system open to atmosphere.
    If you have the air pressure at say1bar With vessel empty of water and you pressurise the water to 1 bar the diaphragm basically has not moved so this gives the max volume of air to compress.
    So action is needed to check vessel and recharge.
    Check PRV, clean or replace.
    Sometimes the pipe connecting boiler to expansion vessel can become blocked or restricted which also stops the vessel from operating as it should.
    If the diaphragm is broken you would get water on the air side and when depressing the air valve you would have water loss.
    With water pressure at 1 bar you can expect it rise by about 1 bar when heating is on and it should drop back when heating is off
     
  3. superchoward

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    Thanks for your response Mandate. I have topped up the boiler and bled the system tonight to get the water pressure to 1.5bar in the CH system. The system has been on for the last 1hr 15mins and since I turned it on the boiler is cycling on and off with the pump running continuously. The pressure has reached 2.5bar and is going up to a maximum of 2.75bar. No water has left the system through the pressure relief valve to outside.

    The boiler ignites and within about 30 seconds the temperature lights illuminate up to 80deg C and then after another 30 seconds the flame goes off for about 3 minutes and then the boiler ignites again.

    Do you still think it is the expansion vessel? I just want to try and make sure this is the problem before I knock plaster off the walls and cut the boiler frame work so I can get at the schrader valve!!!

    Thanks
    Chris
     
  4. Xpertgas

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    Sounds like more than 1 problem the exp vessel is defo flat and will just be a matter of time before its coming out the prp, it's also cycling so that could be a water quality issue poss a chocked return filter and the heat isn't leaving the boiler fast enough, have you been adding a lot of water to the system recently
     
  5. superchoward

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    I have filled the heating system up tonight but other than that not since early summer. Ill have to check the expansion vessel and go from there! Thanks for your help, I'll let you know in due course.

    Chris
     
  6. MANDATE

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    You won't be able to maintain the system if you can't check the air pressure or charge up the vessel so access is needed regardless.
    When the water is heated and it expands the rise in pressure will be about 1 bar so you're going from 1.5 bar to 2.5 bar. Why not try setting water pressure to 1 bar then there's less chance of opening the PRV.
    If pressure is lost then there is a leak.
    In most cases it's related to the expansion vessel loses its charge and PRV opening as it should
    Or the PRV not sealing. Best to secure plastic bag over pipe and see what gets collected. Otherwise can be misleading.
    Of course you could have a leak elsewhere. You could isolate boiler to gain information. Will gauge still show pressure drop or not?
    Also the air valve on the vessel could be leaking/loose, this means the diaphragm gradually moves towards the air side over time leaving less and less air to compress and pressure gradually going higher and higher
     

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