Controlling the hot water flow into the coil on my HW cylinder

[TRK's dad]

The device which controls the flow of hot water from my boiler to the coil in the cylinder is a TRV, set to 6 and connected to rudimentary temperature probe held up against the side of the cylinder. This, due to the hard water where I live has acquired the habit of sticking shut so periodically we don't have hot water and I have to fiddle about with the TRV to try and free it. There must be a better solution than this, surely ? Can anyone suggest anything please ? I intend to replace the whole system in spring next year when extending the house so I don't want to spend a load of money now, I suppose that I may end up having a new TRV fitted.

Thanks

[Dave Liquorice]

Erm the water in the coil is the primary water and doesn't get changed so scale build up like in a kettle is not (shouldn't be) a problem.

Has the primary got any Fernox, X100 or what have you in it? I suspect not and it's just sludge, a full flush through of the system and refill adding Fernox etc will probably sort you out.

Though if you really are going to replace the system in the next 9 months thats a fair bit of work. I might be tempted just to drain enough water from the primary to empty to header tank plus a bit, pour in a goodly amount of inhibitor and refill. Inhibitor has some lubrication properties and may well tide you over.

[TRK's dad]

I take your point about the coil water not changing but upon dismantling the valve a bit in the past I have seen white crusty deposits around the mechanism which when cleared have made the thing work again.

There is a tag tied to the pipework in the airing cupboard which states that Fernox was added in July 2002 and I have a receipt from a heating company for draining the system and adding Fernox which matches this date (we moved in in OCtober 2003).

Anyway in trying to free the plunger (apologies if this is not the technical term) last night I managed to pull it right out and most of it is covered in a black sludgy stuff. This appears to have completely broken the TRV so I need something new to regulate the flow of water into the coil and I may as well get some new Fernox put in as well just as a way of hopefully tiding me over.

Anyone got any more ideas ?

[Christian McArdle]

If you get black sludge, the entire system needs some serious flushing. However, this doesn't mean you should pay 1000 quid to BG to do it for you.

Also, you should check that the system isn't being constantly refilled. This would usually happen if you have an expansion tank that frequently (or continuously) dribbles out of its overflow.

Christian.

[Dave Plowman (News)]

It's a TRV and they all fail - eventually. Somewhere between 5 and 15 years depending on quality.

A motorised valve would be a more elegant solution, and this might be used on your new system. However, a three port type is the more usual these days, but this would require substantial re-plumbing on the existing system.

[TRK's dad]

Apparently it's not a TRV it's a tapstat - see here :

[Dave Liquorice]

A "TRV" for HW cylinders. Principle is the same as a "TRV" for radiators.

Strikes me as a good idea on gravity HW systems, stops the HW rising to the full primary temperature when the CH is running. You still need a electrical tank stat to shut the boiler down when the only demand is for HW though.

[Dave Liquorice]

Or leaks...

Or pump over and an uncovered tank leading to lots of evaporation. Even without pump over you'll still lose measurable quantities from an uncovered tank.

[Dave Plowman (News)]

Not that much. Servowarm used to fit header tanks with no cold water supply - you simply topped them up with a bucket etc. Usually every other year.