gravity fed back boiler pushing water up flow and return

Discussion in 'Plumbing and Central Heating' started by Lotarogg, 12 Sep 2011.

  1. Lotarogg

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    Hi,

    This looked easy when I started but it has turned into a bit of a nightmare.

    Having gutted my house I have replumbed it all and everything is working great except the back boiler on my solid fuel stove.

    Having let water into the system I light the fire. The boiler on the back hisses (like an electric kettle) then once it's built up a head of steam blows straight out the vent into the header tank for a about a minute and will keep doing this every 20mins or so. The hissing tells me there must be a fair bit of air in the boiler which I guess is being blasted out the vent when it reaches pressure. I ran it for about 2 hours over the weekend thinking that once it has blown all the air out it will start to work as normal. But it just keeps boiling.

    I have the vent placed before the intake into the cylinder coil, is this right? Also the feed from the header tank enters the heating system on the return to the fire.

    The hot water from the fire also trys to go up the return pipe telling me either gravity isn't working or I've done something wrong.

    Any help very very gratefully received

    Cheers
     
  2. Nige F

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    Both :eek: Have you used 28mm pipe for the gravity primaries to the cylinder
     
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  3. spraggo

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    You either have an airlock in the flow or return from boiler to cylinder (have you checked that these are rising to the cylinder with no dips), or the feed pipe between the header tank and the boiler is blocked. Assuming that you have water in the header tank.
     
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  4. Space cat

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    This might be a stupid question but have you connected the boiler the right way round? :?: :?: :?:

    The vent must attach to the highest point in the circuit. You should have a clear path running upwards from the TOP of the boiler all the way to the vent. This pipe will also branch off to the top of the cylinder coil but it must run DOWN from the branch point to the cylinder. Look at it a different way; there must be a clear path running upwards from the TOP of the cylinder to the vent.
     
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  5. Lotarogg

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    Thanks very much all. Looks like I need to make some changes. For 3 metres from the boiler I have 28mm copper then reduce to 22mm pex at the same point for both flow and return. The flow from the boiler has a couple of 90 corners but is always going up so no dips. The flow meets the vent pipe then a 90 corner up to (400mm) the intake for the coil – so I guess that’s the first change to place the vent going vertically up from the coil intake.

    The boiler connects: Return: From bottom of cylinder to bottom of boiler. Flow: from top of boiler to top of cylinder.

    Is it vital that 28mm be maintained throughout boiler < > cylinder cycle? I have 22mm intakes on the cylinder.

    Cheers
     
  6. Space cat

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    That's the solution. If I read your post correctly, you currently have a giant airlock in the cylinder coil. :( :( :(

    You obviously have to step down at some point. What lengths of 22mm do you have in there?
     
  7. Nige F

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    sounds like you have a cylinder for pumped circulation - with those 22mm intakes - you might get it to work after altering the vent ;)
     
  8. Lotarogg

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    @spacecat - have got about a meter of 22mm pex going to and from the cylinder coil.

    Will make the changes and report back.

    Thanks again
     
  9. peaps

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    I have a multifuel system that I have just fitted. It is set up as a gravity/pumped y system with a three port diverter and had the same problem. The solution was to add a bleed valve on the 28mm flow at the highest point in the pipe work going into the tank. Bled out all the air and bobs your uncle. In older AGA systems the vent was connected to the return but I was advised to connect it to the flow pipe just below the diverter. You may find that if you don't pump the system it will vent when the tank get too hot so making sure you have a heat sink is important.

    I just have a problem with the wiring and turm turn the diverter over manual for now.

    Is it a stright gravity fed system?
     
  10. Lotarogg

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    sorry for delay in posting as have been on hols then other jobs on the house took priority.

    Have done the suggestion of making the vent to the header tanker on the return pipe the last outlet in the system. I took a hose pipe and ran mains pressure through the flow and out the return to take out any air locks. fired up the stove and gravity is at last doing its thing :)

    @peaps the only heat sink I use is run the hot want to draw cold down into the system, is there a better way?

    Finally have to connect the rads up. I'm thinking flow to be taken about a foot higher from where the flow from the stove enters the cyclinder - ie the last outlet on the vent pipe before it vents into the header tank? I will put a pump there triggered by a temp sensor on the flow pipe from the stove. The returns will come in through one way valves into the return to the stove.

    Thanks for all help and suggestions
     

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