Direct HW Cylinder with strange internal fittings

[daGigsta]

Please excuse my text formatting as I have just copied and pasted below from some notes to myself and it has formatted like this which I can't seem to change or indeed make any formatting adjustments or even start new para's. I have just removed a leaking direct vented cylinder heated from a Johnson & Starley Janus 3 (a dedicated 62.5l/m side boiler fitted to a J&S hot air CH system). I assumed this had to be a direct cylinder as there are no F&E connections to the boiler and it obviously takes its water direct from the cylinder. Having removed the cylinder it wasn’tat all what I expected to see through the boiler side connection points.
Something was in there butdefinitely not a coil, it looked more like another cylinder wall.
The only way to check was to cutthe cylinder open.
What I found was a strange set upof three close piped cylinders, each different, which I have never seen before. I have nowworked out what it does, but am not convinced of its necessity!
The taller front (140mmOD) cylinder is open at each end with a double skin forming a 10mm wide sealed outerjacket which the boiler connections enter, and an expansion pipe flowing directlyfrom this back into the main cylinder.Water feeds into the jacket througha bottom piped connection from the closed (150mm OD) second cylinder which is fed from a top entry connection to the (150mm OD) thirdcylinder at its side, the bottom of which is open to allow water flowfrom the main cylinder.
This weird (to me anyway) andwonderful arrangement clearly allows cold water to syphon through the first twocylinders into the water jacket of the larger cylinder and then through theboiler returning to the jacket as hot water, which then indirectly heats the main cylinder water.
Effectively this is a direct cylindersystem with these three internal cylinders creating a sort of self-primingindirect boiler heating circuit that uses the main cylinder as its F&Etank!!!
I assume this is intended toreduce corrosion/calcification due to fresh oxygenated water continually flowing through theboiler heating circuit by making it a relatively closed circuit after itsinitial priming.
Although this has worked well for20+ years I doubt that it is a standard fitting, so must now decide to go witheither:
a) A direct cylinder, and risk corrosion/calcification of the heating circuit
or
b) An indirect cylinder, and install a small F&E tank justabove the boiler
I would welcome any/all comments/suggestions. My thanks in anticipation, John (blame my Username on the grandchildren)

 

[lame plumber]

sounds like u may have a primatic cyl, any replacement cyl should be indirect but may need to be unvented or continue have cw storage cisterns in the attic if you go for open vent. the primatic cyl uses an air gap to separate the primary heating water from the hot water going to your taps for info.

 

[tamz]

Its a primatic. You can still get replacements if you are keeping the warm air unit. They (the JS) were usually fitted to direct cylinders as the hw heat exchanger is copper and it heats quicker that way.

Page 15 of this
http://www.range-cylinders.co.uk/pdfs/sales/copper.pdf

 

[lame plumber]

never replaced one but i would think they arent that cheap compared to a standard ov set up, tamz will know.

 

[snowhead]

The Janus 3 is most likely a non ferous boiler, probably copper.

If there isn't a circulating pump between the boiler and cylinder then the cylinder layout is designed to promote circulation between the boiler and cylinder.

You'll need and identical replacement cylinder or if there is a height difference between the boiler and cylinder just a normal direct as per this;
http://www.johnsonandstarley.co.uk/downloads/ZZ0166.pdf

 

[tamz]

About £100 more than an indirect last one i bought which is a few years ago now.

 

[daGigsta]

Thanks guys, although different in design to those schematically shown on the internet I can see that the second small cylinder could create an air gap isolating the main cylinder HW from the boiler HW, but as this is connected to an unpumped dedicated HW boiler which does not feed a radiator or indeed any other circuit this seems irrelevant and I see no point in installing another Primatic type cylinder. This is on the bottom floor of a 4 storey block of flats with about 10m (1 bar) head from main CW F&E tank and no separate boiler F&E tank, so if I were to fit an indirect cylinder replacement the only practical alternative would be to install a small F&E tank in flat at most 4-5ft above burner. As far as I can see the obvious and easiest way to go from a practicality and cost perspective is to install a vented direct cylinder, unless anyone advises against. Again many thanks, John PS. I can work within JS guide for horizontal/vertical pipework ratio (less than 2:1).

 

[lame plumber]

make sure your replacement cyl is capable of withstanding a 4 story head of water, a standard one wont be upto it

 

[tamz]

Being a flat with a shared cwsc may be why a primatic was specified in the first instance.

Different manus had slightly differing designs but they all work on the same principle.

 

[daGigsta]

Thanks again guys, I understood that standard Gr3 cylinders were good for 1 bar and tested to about 1.5, but will discuss with manufacturer and if necessary increase to Gr2. As there seems no basic reason to stay with Primatic type I shall also discuss changing to a vented direct cylinder with him and if he sees no problems decide accordingly. Best Regards, John