Help with central heating expansion tank

Discussion in 'Plumbers' Talk' started by NormanW, Dec 11, 2017.

  1. NormanW

    NormanW New Member

    This is the tale of a southern European plumber, and me trying to fix the faults he has left me with!

    I am not a plumber by trade, but consider myself an advanced DiY work. I know how to solder and have a pretty good idea of how stuff works.

    I had a local plumber install central heating. It is an open system, heated by a 16kW wood stove boiler. In the unheated, uninsulated loft he installed a small 10 liter expansion tank. I am attaching photos.

    Unfortunately he did not insulate either the tank or the pipes and last winter during some extreme cold, I had a number of bursts, including one on a 22mm elbow from the cold outlet of the tank. (sorry, can't get the picture to rotate to portrait)
    cold burst.JPG
    I am now fixing it as he has refused, unless I pay him again, but what I have found leads me to question the settings and would ask for forum members help please.

    There are three pipes, a 15mm overflow pipe, a 22mm feed to the top of the tank which is from a Tee piece on the hot side of the system, and a 22mm cold outlet from the bottom of the tank, also from a Tee piece on the cold side of the system.

    There is a lever ball valve close to where the cold pipe exits the loft to the outside - all the pipework runs outside the building and has now been properly insulated. When I went to replace the 90 degree elbow, I found the valve was set in the off position. It is not visible for the loft hatch.

    Question: What position should the vale be set to? Open, or closed?

    My understanding of an open system is that as water heats, it expands. This expansion is taken up by water flowing into the expansion tank. But I thought that then, as the system water cools and contracts, the valve should be open so that the expansion tank in essence becomes a header tank and refills the system?

    In running the system last year I found that the overflow was often running and I was also having to top the system up every week as well. I wonder if this is the reason.

    Thanks for reading - any advice would be appreciated.

    NW

    cold out.JPG cold valve .JPG hot expansion.JPG
     
  2. Stan Lee Blade

    Stan Lee Blade Active Member

    I am no expert on this but I do know that an open system normally has a tank in the loft filled from the mains with a 15mm pipe and maintained at the correct level with a ball valve.

    There should be an overflow pipe of a larger diameter than the inlet to enable it to be able to drain off the water if the ball valve was to go wrong and stay open this would normally be 22mm

    There would be a pipe from the bottom of this tank connected to the return side of the boiler system . This maintains water in the system topped up from the tank that is kept filled by the ball valve .
    There would then be an expansion pipe coming from the flow side of the system that goes into the top of the tank in the loft
    The level of the water in the tank should be low enough to enable any expansion to fill into the tank without going out the overflow and then return into the system as it cools down
    In theory very little water is used, with the ball valve just topping up the tank as water is lost by evaporation .if you put a lid on the tank this evaporation is minimal.
    I have never seen a system like yours so cannot comment apart from how I understand it should work
    You say you have to top it up manually .how do you do this? as I understood with an open system it’s all automatic as I explained .
    I have a combi boiler now and I do have to manually fill it if the pressure drops below 1.5 bar
     
  3. NormanW

    NormanW New Member

    Thanks for your reply Stan. To answer your questions...

    The tank is just a tank, no ball valve. Unlike the open tanks used in the UK, it is a welded steel circular tank with no means of accessing the inside.

    There is a screw connection and valve at the lowest point in the whole system, below the pump, on a short 22mm spur of the main 22mm cold/return side. I connect a short length of pipe between an outside tap and the screw connection when the pump is off, open the valve and wait until I see a dribble of water from the tank overflow. Then I know the system is full.

    The overflow pipe is smaller diameter (15mm) than both the flow and return (22mm). The pipe from the bottom of the tank IS connected to the return side of the boiler system, but this is where the lever ball valve is, which is in the closed position - I do not believe this setting is correct.

    The only means of keeping the tank topped up is manually to fill it, which prior to the bursts, I had to do every week.

    The flow side does connect to the top of the tank, via a 22mm pipe.

    Although the two plumbers involved were "experienced", or so they said, and were college trained, my suspicion is that on the sump of the tank, there is a drain port, and a pipe should have been connected to this, with a lever ball valve, so the tank could be drained. They just put a brass plug into it to seal it. I think the pipe leading from the tank to the return side does not need and should not have a valve - but I am not a qualified plumber!
     
  4. Stan Lee Blade

    Stan Lee Blade Active Member

    Norman
    You say that the pipe from the bottom of the tank and connected to the return has a valve that is closed.
    Given a scenario where water is coming into the tank from the expansion pipe, then it has no way, as far as I can see of getting back into you system. It will just remain in the tank, that will eventually fill up and exit the overflow, resulting in you having to keep manually topping it up
    I would think that this valve should be open to enable the water to return to the system when it cools down
    Water does come up the expansion pipe as it expands that’s normal but should be able to return to the system when it cools down .
    Water should not be exiting the overflow pipe during the normal courses of events. It’s their as a safeguard only, as with all overflows.
    How do you know when the system need topping up ? With an open tank it’s automatic with the ball valve and with my own combi boiler I have a pressure valve to look at and if it drops below 1bar the boiler shuts down.

    Bear in mind I am no expert and just giving you my thoughts. Do not do anything on my advice that could cause damage
     
    Last edited: Dec 18, 2017
  5. NormanW

    NormanW New Member

    Hi Stan

    Thanks for your thoughts. There is no manual or anything with the tank and I can't find the makers online. Having fired the plumbers over the debacle last winter, I'm not going to ask them now! Pride etc....

    I don't understand why you would put a valve on the return anyway. Put a valve into the sump drain, yes, but the return?

    By trial and error I found that once every week or 10 days, I needed to top up the system, but it was guess work. There is no sight glass or any type of visible gauge.

    Having though this through and looked at all kinds of drawings online, I am going to open the valve and see how it works. I think there is more change of danger with it closed - the water level could get low and I'd not notice, than having the valve open. In writing this I've just realised something. The top of the floor to ceiling towel rail/heater in the bathroom, only ever was warm after the system had been topped up. I suspect this was caused because the valve was shut.

    Just waiting for a parcel of HEP20 from Screwfix this week, and I'll have everything working. I'll post a followup once I've tried it. Thanks again, your responses have clarified what I thought.

    NW
     
  6. Stan Lee Blade

    Stan Lee Blade Active Member

    Norman
    The valve on the return could be there for no other reason, than when draining for maimtances etc ..turning this off just means you dont have to drain the tank as well
    My old open system had a valve the same as yours ..turning this off meant I could drain the system down without tuning off the mains water or tying up the ball valve .
    Can’t see any other reason for it being there in an off position
    Keep me pasted i will be interested as to the outcome.
     

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