Question Regarding Closed Combi Boiler System

Discussion in 'Plumbing and Central Heating' started by Gremlee, 24 Feb 2016.

  1. Gremlee

    Gremlee

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    Hi,

    Before I start, I have searched through the posts on the internet (and this forum) and can't find a direct answer to my question. If there is one, i apologise in advance.

    I have a closed combi system and want to replace my upstairs bathroom radiator with a towel warmer. The previous occupants have painted over the nuts connected to the radiator and when I removed the paint, the nuts are completely rusted solid and round. So, my question is, if I reduce the pressure in the system to 0 (at the boiler), turn off the valves at the radiator, can I remove the radiator valves (whilst attached to the radiator) from the copper pipes without losing water? From my physics lessons at school, the system should be in a vacuum and therefore no water should come out as no air can get in. Even though water is in radiators in the other adjacent rooms, this shouldn't come out through gravity as there is no air getting into the system. I expect to lose about a egg cup full which is in between the valve and pipe.

    I will then remove the radiator, complete with water inside thanks to the closed valves, and fit the new rad valves to the copper pipes. Attach the radiator, open the valves and bleed screw on the new towel warmer, slowly bring the system pressure back to pressure, whilst bleeding the rad, and then turn the electric back on to the boiler. Job done!

    Does this make sense and am I correct?

    Many thanks in advance.
     
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  3. Dan Robinson

    Dan Robinson

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    Life is never that simple. Especially when it concerns bathrooms and plumbing.
     
  4. John506

    John506

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    Any part of your central heating system that is holding water and is at a higher level than the two pipes in your bathroom will drain out.
     
  5. Gremlee

    Gremlee

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    OK, thanks John506.

    I thought it wouldn't come out (even though it was higher) because there was no way air could get into the system. If the water drains out and air can't get in, what fills the void where the water was? I just want to understand this logically.
     
  6. Your reasoning is actually quite sound Gremlee, but I suspect it won't work - but give it a go for the sheer hell of it. If you take the radiator off, and then unscrew the bleed screw on another radiatior, then air can get in, and the water flow out. If you don't open any of the other rads on that level, then they may well be okay, but as soon as water can get out of the rad valves that you're going to need to take off, then air will get in, and all the water in the pipes will start to flow out, and then the rads will (albeit very slowly) will start to empty as well. You need to remember that in taking the pressure down to 0, you've got to open the system up somewhere, and start the draining system off anyway.

    As the nuts have rusted, you're going to need to get them off, and put new ones on. You're going to need to use a junior hacksaw at 45 degrees on the olives, so that you can remove them, before the new nuts will go on. Remember to put inhibitor in the system before you refill the system.
     
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  7. Gremlee

    Gremlee

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    Thanks. I'll give it a go this weekend, and let you know how I get on. I will probably close the valves in the other radiators before I start just in case anyway.

    Thanks all.
     
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  9. flameport

    flameport

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    If you remove one valve, in theory no water will be lost. In reality, some will be lost. A wet vac should be able to deal with the amount of water.

    If you then remove the other valve, a lot of water will be lost as you now have 2 open ends, so it would be better to remove one valve, fit the new and close it before removing the second one. Or temporarily plug the open end with something.
     
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  10. Gremlee

    Gremlee

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    Excellent. Thanks I'll try that. I have a wet vac so will get it ready.
     
  11. Gremlee

    Gremlee

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    Hi, I said I'd report back on how it went and here it is.

    Straight off the bat, let me say, it worked. I switched off the boiler and depressurized the system at the boiler and watched the pressure go to 0. I then closed off the valves to both sides of the radiator in the bathroom which is the rad I was removing. I didn't close off any valves on other radiators in the house, I didn't think I needed to. I then unscrewed the rad valve from the copper pipe and did the same at the other end. I then removed the radiator complete with the water still inside thanks to the closed valves. I got a couple of drips from the radiator valve but nothing came out of the copper pipes sticking out of the floor. Even though there was water in the system at a higher level, it did not come out. It didn't come out as the system was in a vacuum and as no air could get into the system, no water came out. The water could be seen about 2 mm down in the copper pipes just sat there. I removed the olives from the pipes by cutting with a hacksaw at a 45 degree angle, and then prized them open with a screwdriver. I cleaned the pipes with wire wool, and put the new valves on using PTFE on the thread. I made sure the new valves were switched off. I slowly pressurized the system, whilst checking the valves were not leaking, and brought it back up to the correct pressure. I switched on the boiler and all radiators are nice and warm. To further test I tried to bleed one of the rads which seems to be the worst for air, and no air came out. I also very slightly turned one of the new valves to ensure it was working and it slowly started to show water, i switched it off.

    At this point, I have two valves with no radiator. The people who lived here before didn't tile behind the rad so I have a few days work to patch that up first before installing the new towel rail. When I'm ready, I will connect it to the valves and fit to the wall. I will then pour inhibitor into the rad, fit the bleed valves to the top and close one of them off. I will then turn the valves slowly on and bleed the rad. I'll probably have to keep filling the system whilst i'm doing this at the pressure will drop whilst the new rad fills. Once the rad is full, I will close off the bleed valve, ensure the system is at the correct pressure and stand back and admire a good job and enjoy warm towels!

    I'm writing this for anyone in the future and hope that it all goes as well as mine did. Remember, as long as no air can get into the system, no water will come out - this only goes for closed systems. You'd expect gravity to push the water to its lowest point but it can't come out as nothing with fill the void it would leave in the rad at the top. If I had opened any of the bleed valves in any of the other rads in the house, it would have poured out of the open pipes. I did expect water to come to come out of one of the pipes as I had two open at the same time, but that didn't happen. I even had a little finger of a pair of washing up gloves ready to cover one of them if that happened!

    Thanks for all your help and advice, and regards to all.
     
  12. AGAS

    AGAS

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  13. As to the PTFE, next time, just put a couple of turns between the olive and the valve as this is where the seal is, not the threads, but apart from that, well done, you should be proud of yourself.
     
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