No Hot Water but Central Heating OK

Discussion in 'Plumbing and Central Heating' started by dougieunited, 26 Feb 2010.

  1. dougieunited

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    We have been in our house for 6 months now and have just had the house rewired. During the process of rewiring the electrician managed to cut through a water pipe. At the time the electrician wasn’t aware what pipe it was and switched off the main stop-cock and turned on all the taps including the hot water taps. He then worked out that it was actually one of the central heating pipes, so we closed of the supply to the header tank and drained the central heating system.

    The pipe was repaired, the supply to the header tank was re-opened and all radiators were successfully bled and are now working ok. Unfortunately the problem we have now is that we can’t get any hot water unless we use the immersion heater.

    Now I’ve read a few posts on here by people that have had similar problems but I’ve not been able to action the solutions due to differences on my system. I’m therefore asking for some advice relevant to my particular system.

    This is what I’ve been able to deduce about the system in our house. I believe it is an open vented gravity domestic hot water and pumped central heating system.

    We have a Potterton Kingfisher MF CF 60 boiler, with a Randall 3060 timer and a Myson Compact CP53 pump. The “flow” pipe comes out of the boiler and passes through a tee. Horizontally out of the tee the pipe runs to the pump and vertically up through the house to the hot water cylinder. The boiler also has connections for return pumped and return gravity.

    The hot water cylinder has an input and an output for the flow and return and a cold water feed and a hot water output. I can find no bleed valves on the hot water circuit. The hot water output does have a pipe that goes off into the chimney breast that I believe is probably some sort of vent? There is also no thermostat or timer connected to the cylinder.

    In the loft we have two tanks, the big cold water storage tank and the header tank.

    So in summary, we have central heating and flow of water through the hot taps and can get hot water if we use the immersion heater but no hot water when using the boiler to heat hot water.

    I think that’s about it, I’m no plumber and I wasn’t in the house when the system was installed so I hope I’ve explained the problem in enough detail. Any advice would be appreciated.



    Thank you
     
  2. tininho

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    hi,

    could be a few things.... given the burst pipe issue it seems most likely to be a bugger of an air lock, or perhaps the cold feed may be choked.... What is the water content of the system like??? clear? black and sludgy???

    If it is choked, it is quite possible that when the system was re-filling it has pulled sludge in from the header tank, and prevented the calorifier at the cylinder from filling.... I assume the cylinder is either in the attic or upstairs bedroom.... the first thing I would do is operate H/W only and check a few things.... 1: has the boiler fired? 2: check the flow and return to the cylinder, ie are you getting any heat at all.... if not it suggests either it's air locked, calorifier is blocked, or cold feed is choked.....

    Hope this helps

    Martin
     
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  3. dougieunited

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    Hi Martin, thanks for the reply. This is what we've done so far, perhaps we are narrowing down the possibilities?

    The electrician supplied a plumber and his diagnosis was an airlock. In an attempt to fix this he removed the temperature sensor from the boiler and used the boiling water to clear the airlock. This sent hot water through the entire circuit.

    Anyway, we get up this morning and we still have no hot water. On the plumber's advice I tried the boiling trick again and let it run until water came out of the vent into the header tank. However as soon as you put the temp sensor back in the boiler will only run for 30 seconds or so. We've had the H/W selected for about five hours now and in that time the boiler hasn't fired and so hasn't produced any hot water.

    How many times or how long should you use the boiling method to clear an airlock? Or do the symptons point to a blockage rather than air lock

    In answer to your question, the cylinder is in an upstairs bedroom.

    I'll be back on the phone to the electricians tomorrow but any advice in the meantime will be appreciated.

    Ta
     
  4. Agile

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    The focus should be on the connections to the heat coil on the cylinder.

    Normally the flow carries on past the cylinder to be the vent pipe. Thats self venting.

    Sometimes when its wrongly piped its necessary to loosen a compression screw at the cylinder flow connection to let air out.

    I dont understand from what you have said how your system is connected.
    Tony
     
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  5. tininho

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    hi,

    allowing the system to "boil up" is a very handy trick.... particularly with back boiler units, however if it doesn't work on the first attempt I would tend to assume this isn't correcting the fault.... from what you've said it is quite definately a circulation fault and is almost certain to be as a result of an air lock.... as agile says, depending on whether the system is piped correctly or not there should be an open vent pipe at the flow, just before it enters the cylinder. You should be able to identify this by it's size, 22 mm or 3/4" pipe and will extend to the header tank and end in a "swan neck" shape.... If opening the nut on the flow pile as agil has suggested does not help, I have found "back filling" particularly helpfull..... you would need your plumber/heating engineer to do this, but basically works as follows.....

    Open main drain for heating(if there is one)
    Connect a hose between an outside tap (again if you have one) and drain point and open the tap....

    The mains pressure has in my experience proved effective in filling the system and keeping air locks to a minimum..... This is of course dependant on having both a drain and outside tap.....

    I would also suggest to your heating engineer, introducing water into the system via the open vent can help.... However, the plumber/engineer will know how to do this anyway.....

    If I can help further, just let me know

    Martin
     
  6. jackecole

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    What's the boil up trick? I have heard of this. I had a big airlock in a slow filling CH system a while back and I tried everything and finally got it to shift by putting HW and then CH on at the programmer for a minute or so to get the pump going and to shift it and allow the system to refill.

    Of course the boiler locked out during this but it did the job.
     
  7. tininho

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    To do this on a back boiler for example, the stat phile is removed from it's pocket and heating and hot water left on..... In effect the system begins to boil because essentially the stat senses no temp increace, thus pushing any trapped air out of the open vent..... Only recomend this for RGI's....

    Martin
     
  8. dougieunited

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    Just wanted to say thanks for the responses to my query and to say that the "boil-up" trick finally cured the problem or at least appeared to anyway. CH and HW now running all ok....touch wood!

    Cheers
     

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