Confused about CH system.. seems to be linked to hot water?

15 Sep 2013
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United Kingdom
I "Thought" i had a standard open vented system. with gravity fed DHW and pumped CH.

Ive got a FE tank in the loft along with hot and cold tanks.
There are four pipes coming from the boiler.

I decided to drain the system to treat with X400 as some of the rads are quite sludgy. I drained and removed a small rad in the kitchen so i could get a pipe to the outside to drain.

I started draining the system, and noticed the water was crystal clear.
I went upstairs to start opening bleed valves and heard water flowing into my cold water tank, which shouldnt happen.
I closed the valve feeding the hot water tank and now the cold water tank is no longer being topped up.

It seems my radiator pipework is being fed somehow from my hot water supply... im assuming this isnt right.
Is there anything i can do to check whats going on?

TLDR: think my radiators and DHW are using the same water.
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Have you isolated the fe tank (usually the small one) as said you may have a primatic cylinder, if you have DONT use chemicals, if the air bubble goes it could mix with the hot water. Usually a primatic has only one tank feeding hw and heating. Check on the cylinder and let us know make/model.
The boiler is a Halstead Buckingham3 - which is only for a indirect cylinder.

When had a closer look at the FE tank, the ballcock had already been tied up - the tank was dry and looks like has been for a while (potentially over a year) judging by the fluff and stuff in the bottom.

Ive been in this house about 6 months now and this was the first time id checked the FE tank closer than just putting my head in the loftspace.

The Cyl is a copperform one - 900x400 - cant find what type it is, as the label is on the back.. It has a immersion heater element in the top. however this has been disconnected since i had the house rewired
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It sounds like as already said you have a primatic cylinder (which is a type of indirect cylinder), "Primatic" is usually imprinted on the cylinder wall somewhere.

It could be that when installed there was no central heating and just the cylinder heated by fire or boiler using 28mm (1" in those days) gravity circulation to the internal primatic chamber.

Then someone adds central heating and pops in a feed and expansion tank not knowing about Primatic cylinders. They would get the same shock as you when they attempt connection!

Clues to Primatic are the primary flow and return will be 28mm copper. It should be gravity circulation not pumped.

Because it is only an air bubble separating the hot water from central heating water, chemicals should not be used. If it is Primatic you really would be best to swap it for a modern indirect cylinder. That way you can use agents in your central heating to keep it in good condition.
Thanks for the replies.
Maybe it is a primatic cylinder then.. although i cant find any other markings other than a sticker with the size, "medium duty" and the fact the word "standard" had been crossed out and "auto" written on

Ill probably just bite the bullet and get the lot ripped and replaced while the house is a mess anyway - whoever lived here before put huge oversized non-convector radiators in (think a 2m double panel for a 3x2.6m room)

Is there any special precautions i need to use for re-filling the system?.
With a primatic fitted the following would happen:

If the fitted feed and expansion tank is lower than your cold water tank, not only will the ball valve be tied up but the feed and expansion pipes would need to be sealed off as the tank would backfill as it tries to fill to the same level as the cold water tank.

Of course if it were higher the reverse would happen and the feed and expansion tank would be draining back to the cold water tank.
Keep the hw taps open when filling, after filling u may need to let taps run for a while as brown water usually comes out!
Thanks for the help everyone.
Ive (slowly) refilled the tank while keeping the hot-taps open and then bled all the radiators.. seems i may have been lucky and not ended up with the CH water mixing with DHW.

Ill have a think about what to do next, as im borderline getting the whole system plus radiators replaced anyway.. flushing the system was going to be my attempt at a quick win.

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