Replacing CH expansion tank - fitting question

TheGreenFrog

I drained and recharged my central heating expansion tank last year but it has filled up with water again so I am replacing it (it is 20 years old).  I have the replacement tank but I am wondering about the most efficient way of doing the replacement.  It looks as though the 4 way connector attached to the existing tank (picture attached) has to be rotated onto the tank - which means disconnecting it from the plumbing, which I would rather avoid.  Maybe I am misinterpreting the connection method but I want to be sure before I start on the replacement rather than find out only when I have disconected!
!

ThisIsWeird

Is that a top-up hose, with a valve on it?
Obviously you'll need to fully depressurise the system first, and surprisingly little water will come out once you do this. 
The existing connection has a fair bit of PTFE tape on it, so you'll need similar, or perhaps HawkWhite or other good sealer.
Is the tank fully, symmetrically, round, or does it have a mounting bracket welded to it? If the latter, then obviously you need to stop screwing just before the end of the last turn.
Add fresh inhibitor when you repressurise, and consider fitting a magnetic filter too, since you've depressurised.

Sorry - I missed your main Q. Can't you keep the fitting and pipes steady, and screw on the tank?
Or, the copper pipe is the only one that's rigid, so disconnect that at the 4-way if needed. It shouldn't cause any issues, but worth having Hawk/Boss for this.

TheGreenFrog

ThisIsWeird said:

Can't you keep the fitting and pipes steady, and screw on the tank?
Or, the copper pipe is the only one that's rigid, so disconnect that at the 4-way if needed. It shouldn't cause any issues, but worth having Hawk/Boss for this.

I don't think I can screw on the tank - there does not seem to be any intermediate coupling between the fitting and the tank so I will have to screw the fitting onto the tank (having first uinscrewed it from existing tank of course).  I would therefore need to disconnect the top up hose (easy) and, as you say, the copper pipe which I assume is compression fit.  To depressurise last time I ended up draining the tank through the pressure release valve and into a bucket - as its below ground level it's not straightforward to drain without a very very long hose.  It's 130 litres so took me a while!

nofoollikeold

1. Don't drain down via the pressure relief valve - they have a habit of not sealing properly once used.  
2. The following assumes you have a second valve at the other end of the filling loop.  Then:
2.1 Close both filling loop valves.  
2.2 Disconnect the filling loop.  Water will drain out of the expansion vessel so be prepared to catch it in a bucket or similar.  
2.3 Disconnect the fixed copper pipe (coming in from the left in your picture).
2.4 If you can unwind the four way fitting with the pressure gauge still attached (you should be able to) do so.  Make sure you unscrew it with a spanner on the hexagon part near the EV.  
2.5 Clean all the old PTFE of the male thread which goes into the EV, 
2.6 Clean up the compression fitting and olive which connects the fixed copper pipe to the EV.
2.7 Wrap Loctite 55 around and across the male thread of the four way fitting and screw it into the new EV (follow the instructions on the pack).  It should be a really tight fit into the EV, and of course should end up with the pressure gauge roughly where it is now.  You can back off the thread up to 1/3 of a turn and still have a seal, but if that doesn't work, remove and re-wrap with more Loctite 55.
2.8 Wrap PTFE tape (4 to 6 wraps) around the olive of the compression fitting on the left hand copper pipe, and screw firmly into the left hand side of the fitting.  Firm hand tight plus 1/3 to 1/2 turn with a spanner.  
2.9 reconnect the filling loop and turn on.  

FreeBear

nofoollikeold said: 2.8 Wrap PTFE tape (4 to 6 wraps) around the olive of the compression fitting on the left hand copper pipe, and screw firmly into the left hand side of the fitting.

PTFE tape should never be used on a compression fitting. Ideally, the olive should be replaced, and just the thinnest smear of a sealant used (e.g. Plumbers Mait or Hawkwhite) if required.

nofoollikeold

For FreeBear:

1. It is perfectly permissible to use PTFE tape on a compression fitting, but it should only be used on the olive, NOT on the threaded parts.  I agree it is better to replace the olive, but not always easy for a DIYer to remove the old one.  The other option is to use a sealant paste such as JetLube V2 or LSX. For a new olive in a new fitting, no sealant of any sort is absolutely necessary, but given the quality of many modern components I prefer to use a smear of JetLube and avoid a call back. 

2. In my view Plumber's Mait should be reserved for bedding wastes into basins / sinks, which are very low pressure joints.  

3. Hawkwhite would be OK in this situation.  I don't use it as it isn't suitable for potable water, so just keep a pot of JetLube which does potable and non-potable.  

ThisIsWeird

Surely it isn't a compression fitting, but most likely a tapered thread - or not even tapered. But PTFE would be a good call? (Although I prefer Hawk)

nofoollikeold

The copper pipe to the boiler is normally a sort of "inverted" compression fitting.  It has a hollow nut with a long externally threaded stem and a 15mm inside bore, the end of the bore being machined to a 45 degree chamfer.  The nut is slid over the pipe, an olive follows it, then the pipe and fitting are tightened into the female threaded bore on the manifold.  Some radiator valves have similar fittings, though I don't think they are used much these days.  

TheGreenFrog

ThisIsWeird said:

Surely it isn't a compression fitting, but most likely a tapered thread - or not even tapered. But PTFE would be a good call? (Although I prefer Hawk)

nofoollikeold said:

The copper pipe to the boiler is normally a sort of "inverted" compression fitting.  It has a hollow nut with a long externally threaded stem and a 15mm inside bore, the end of the bore being machined to a 45 degree chamfer.  The nut is slid over the pipe, an olive follows it, then the pipe and fitting are tightened into the female threaded bore on the manifold.  Some radiator valves have similar fittings, though I don't think they are used much these days.  

If the pipe from boiler is threaded then I am not sure how I will get it off (Step 2.3 in @nofoollikeold 's post)  as the connection to the vessel also seems to be threaded (vessel has male G 3/4 BSP fitting).  Unless I cut it of course....

FYI for compression I have new olive and Jetlube...

ThisIsWeird

Any chance of a wide shot showing the whole vessel in its location?

TheGreenFrog

ThisIsWeird said:

Any chance of a wide shot showing the whole vessel in its location?          

        Photo attached