Single Feed Indirect Cylinder overheating.

[AllanCB]

Hello

I have a Single Feed Indirect Cylinder [SFIC] (the Primary System is filled from the Cylinder itself, so does not require a seperate header& expansion tank).

My cylinder is an 'Orto' supplied at some point before I moved to this house by Albion. The cylinder does not look like the SFICs I see in the shops and on the internet these days - with the 'extra bit' on top and the emmersion heater low down - it looks more like the one on:

http://www.mcdonald-engineers.com/products/cylinders_04.htm

My boiler is a Potterton Kingfisher II, and I believe is part of a gravity system.

One year ago I was awoken with a noisey 'boiling sound' coming from the cylinder. I got up switched of the heating and ran some hot water from the bathroom taps - which came out discoloured.

Heating Guy turned up a few days later he explained about the air pocket that exists in the SFIC cylinder, and sort of explained that the 'bubble had burst due to overheating and the my primary water had been contaminated with the secondary water - hence the discoloured hot tap water.

Does this mean the water that potentially goes through my radiators mad it out through my hot taps? And if over heating and expansion from the secondary water made is way to the overflow in to the cold water tank, that we could potentially have been drinking water that was once in a radiator? Would this explain the madness in my family - only kidding.

Anyway, Heating Guy drained the system (through the radiator low-point, as someone put a garage beam over the proper drain pipe), changed the thermostat on the boiler and filled the system up again. (Unfortunately I was not there when he did any of this).

Last week it all happened again.

This time Heating Guy tested the Thermostat, and said it was doing everything it should be doing, he then just switched the heating back on and left - no draining and refilling of the system. Things seem to be okay just now.

Sorry for the long story.

Question 1: What, other than a faulty thermostat, can be making the water in my SFIC boil/bubble?

Question 2: How bad can the contamination be when this happens?

Question 3: How dangerous could this be (high pressure explosion-wise) if it happened whilst we were out of the house for a couple of days?

Question 4: Would the SFIC system have recovered quite happily on its own since last weeks occurence - i.e. would the air pocket (and other bits) have fixed itself without a drain and refill?

Question 5: I'm about to change the sink and bath in my bathroom, and as
neither have stop valves, I need to drain the system - again through the radiators. I pretty much know what I'm doing regarding draining and filling up and bleeding etc, but worry about this air pocket in the SFIC - is there anything extra I need to be aware of during the drain and refill?

Many thanks if you've read this far.

Regards,

 

[PTH]

Firstly, yes, your heating water and hot water are effectively the same stuff, normally separated by an air bubble, the discolouration is just oxides and precipitated solids, nothing too nasty but it smells a bit. There should be no nasty chemicals as primatic systems should never be dosed with inhibitors etc.

The primatic has it's own overflow from the upper vessel so any expansion above the 'normal' will leave the system there and should not find it's way to any other storage cisterns. I doubt if any drinking water points are taken from a storage tank anyway.

Q1 Cylinder boiling or primary system boiling into the cylinder can only be caused by a stat not operating or a temporary lack of circulation. Most gravity systems on primatic installations have a check valve on the flow connection off the boiler - these sometimes stick or, if they are in a vertical pipe, lumps of sludge sitting on top of the flap will prevent operation. A whack with a hammer sometimes does the job!

Q2 not bad - see above

Q3 maximum pressure that can exist in a primatic is that created by the height difference between the top of the cylinder and lowest bit of pipe plus the pressure needed to push the air bubble into the upper tank. The system then becomes an open vented system - albeit one that's overflowing it's feed tank. It sounds worse than it is - but should be sorted out.

Q4 - system should be drained partially in order to reinstate the air bubble properly.

Q5 - you can minimise the drainage problems by isolating all the radiators before you start, turn off the valves at each end - make a note of the number of turns of the lockshield valve on each one as you close it - that way you can reinstate the heat balance roughly. Albion will send you an owners manual for the cylinder which should give proper directions for reinstating the bubble.

good luck.

 

[PTH]

Further tought - Is the immersion heater on or on timed operation? if so check that the integral thermostat in that is working. Sorry if this is insullting your intelligence but it's one that I'vee missed before!

 

[AllanCB]

Thanks PTH

The emmersion is never used.