What kind of cylinder is this and how to change the blending valve

[chapson]

Hi all, I am new here, help would be appreciated.
This is my cylinder, it is 35+years old and has worked flawlessly until now with very powerful high pressure water, 2 showers on no prob. Anyone know the model? Btw it can run the whole system on electric only if boiler is broken.
But we have a weird problem now, took me a while to diag - the blending valve - white cap thing on top has failed and it does not let the how water come out, so even though its full of hot water you only get cold water come out. I have located and ordered the part Overtrop XC007.
But how do I change it? Water does not seem to drain if I close the cold inlet gate valve and open hot water taps. There seems to be no cock anywhere to let air in the system from top. No drain holes apart from the one bottom left - which is infact for the primary water. The pipe on top of boiler seems to be just the breather pipe of the primary water circuit - as I found by knocking - which goes through the cylinder for some reason (probably used for internal pressure safety venting).
So I wonder if I stop the cold, open hot taps and and try to remove the blending valve - will I get a 50l splash of hot water from the tank head above it ?
Also I'd really like to know the exact path of waters in this tank, say why the radiator pump pipes on the left go in and out of the tank, when the right pump is the one heating the water.

 

[Madrab]

You have what's called a thermal store, it is an open vented design ergo the vent coming out of the top of the cylinder. All the water in the cylinder is kept hot by the central heating system. There is a coil inside that the cold mains water flows into, just above the blue expansion vessel, it is then warmed up and exits at the top where is connects into the blending valve, then out to provide mains pressure hot water. The hot water in the cylinder also flows out for the CH, when the rads are calling for heat.

Once the cold feed stop tap is turned off, open up a hot tap, that will release any pressure/water from the HW circuit, then it's just a case of disconnecting the blending valve, replace and refit the pipework. You do need to be careful and use the correct thread sealing methods to the cylinder tapping etc as that will be under mains pressure and make sure the blending valve temp is set correctly.

Is there a makers label etc on it, doesn't look like the usual Gledhills TS'.

 

[chapson]

Wow thanks, this answers a lot of the questions! I thought that the can radiators work on electric by reverse heat exchange into the primary water ... So interesting that there can be a fast enough heat transfer by the coil to shower in real time! There is no label anywhere, yes the valve is Gledhil/Oventron/Elton? depending on where you look, so is the strainer. What do the strainer and exp tank do ? It feels the hot water pressure is a bit less than the mains.
I think I will call a plumber to do that anyway in case water starts splashing around plus I have mislaid my second big spanner otherwise I would have attempted it already

 

[Madrab]

The strainer just ensures that no debris that may come in with the mains water won't get into the coil/mixing valve and the mini expansion vessel (EV) just absorbs any water pressure jumps (hammer) when any valves open and close

The thermal store is kept quite very warm and as the coil is highly efficient at heat transfer it can get the HW up to =>50 degC relatively easily. Some Mains water pressure will be lost through the coil and through the mixing valve to ensure it can meet it's performance figures.

 

[chapson]

Spot on, thanks. So maybe I can install roof solar water panels and plug them into the unused ports, does it make sense in this country? Or use solar battery storage to run the immersion heater at night, then use the heat daytime. Every time I do the calculations of such solutions it seems a futile cause..

 

[Madrab]

You could certainly use solar to augment the heat source be it either PV panels running the immersion heater. Not sure if it would be solar heat ready (vacuum tube)/compatible at that age.

 

[chapson]

Hm yes, on my close about 20 houses were built with same boilers in late 80ies I think. Of these only 2 neighbours tell me they replaced the cylinder 'cause it was too furred up'. Is there anything I can do to mitigate this - more inhibitor maybe? Had I known all tank was primary water I would have put more in. Now mine seems very powerful still, the original Potterton Prima boiler was replaced 4 years ago with Baxi - only because its plastic case crumbled like biscuit. Only tank problem I had is once after I had changed radiator valves air got in and I could hear sloshing inside and it did not heat the water. Then with a hose I forced water in the loft tank outlet until it came back out from the top vent pipe - this fixed it immediately. Was this a good way to fix it ? What about the strainer - does it need cleaning etc ?

 

[Madrab]

Thermal stores are unfortunately also know as sludge buckets. It is a side effect of the water in the store also being used to heat the CH system The rads are made of mild steel and if there was/is active corrosion in the system then it will tend to work back to the store and settle out at the bottom.

If the system was well inhibited and maintained then it may not be too bad. I have only ever flushed out one thermal store and that was a full disconnect, removed the immersion, taken outside and hosed out. Worked very well though it is a lot of work.

Yes you can always use a hose to force air out of an open vented system using the mains but make sure it is well inhibited after. The other option would have been to connect the hose to the rad via an adapter in the vent and backfill the system that way.

Strainers do need periodical servicing