Never ending drain down

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Hi,

On Saturday morning I started draining down my system ready to do a few jobs that I'd been storing up for a while. It's a big system with 8 x double panel rads, 1 x single, and 3 x towel rails plus the hot water. Within a few hours, the system had drained enough for me to be able to do the jobs I needed to do. However, by lunchtime yesterday (24+ hours later) there was still water draining out of the system at a reasonable trickle - quite clean-looking cold water.

I then opened up some more radiator bleed valves to try to speed things up and this resulted in water coming out of the return pipe on the boiler (I'd disconnected that temporarily so I guess water was being forced into the heat exchanger via the flow pipe and was then coming out of the return pipe onto the floor. :(

The speed of water escape from this return pipe slowed a bit when I shut the motorised valve for the hot water.

The system is a standard 1960/70s setup with a vented indirect DHW tank that'd fed from a cold tank in the loft, and two 2-port motorised valves (one for CH and one for DHW). About 12 years ago, it was converted to a sealed system by adding an expansion vessel and filling loop next to the boiler in the garage and capping off the vent pipe and the supply to/from the F&E tank in the loft.

To try to establish why the drain down looked like it was never going to end, I drained the hot water tank completely. Soon after that, the draining finally stopped.

I then refilled the hot tank and the draining started again. Also, even once the tank had had long enough to refill I could hear water flowing through the pipe that supplies the hot tank with cold water from the loft.

Once I'd finished all the jobs, I closed the drain valve and all the radiator bleed valves. But I did not refill the system. However, it has refilled itself to an extent and there is now 0.2 bar of pressure in the system.

It seems to me like I must have a hole/crack in the hot water coil.

However, I have never noticed any discolouration of the hot water nor any bits of nastiness in it.

Is it possible for there to be a leak in the hot water coil that would be bad enough to mean the system never finishes draining but not bad enough to mean that we get contamination of the hot water during normal operation?

Also, the water draining out was cold, even after I had had the immersion heater on for a few hours. Mind you, even at that point when I had lots of hot water available, the bottom of the tank and the coil in/out pipes felt cold so not sure that proves anything.

The only other thing perhaps worth mentioning is that over the past few months I've noticed that the pipes to all the radiators get hot even when only the HW is on and the CH is off. I'd put that down to poor system design and/or the CH motorised valve letting a bit of water by. But, if I do have a leaky coil, could it cause the radiators to get hot like that?

I'm happy to replace the tank but wanted to get your thoughts on whether there could be a different cause for all this other than a faulty HW coil.

Thanks in advance,
David.
 
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Drain the heating system till no water comes out, disconnect the connections to the coil on your cylinder the fill the cylinder and you will know for sure if you have a pin holed coil, this is of course assuming with the information you have provided that you have a traditional open vented HW cylinder and a sealed CH S plan system
 
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Surprising that the sealed system doesn't require frequent top ups (except that it has a auto top up PRV) with a differential head of > 1 bar between the coil and the HW cylinder, the coil disconnection should reveal all.
 
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Surprising that the sealed system doesn't require frequent top ups (except that it has a auto top up PRV) with a differential head of > 1 bar between the coil and the HW cylinder, the coil disconnection should reveal all.

Depends whether the boiler has a pressure cut off switch which it almost certainly doesn't if it's a conversion...
 
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Surprising that the sealed system doesn't require frequent top ups (except that it has a auto top up PRV) with a differential head of > 1 bar between the coil and the HW cylinder, the coil disconnection should reveal all.
Throughout the time I've had it as a sealed system (about 14 years) it's required top-ups every month or so. I've always put that down to things like a weeping bottle vent (fixed at the weekend!) and other small leaks that I've assumed would be inevitable in a system that dates from the last century.
 
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Drain the heating system till no water comes out, disconnect the connections to the coil on your cylinder the fill the cylinder and you will know for sure if you have a pin holed coil, this is of course assuming with the information you have provided that you have a traditional open vented HW cylinder and a sealed CH S plan system
Thanks. I'll try that at the weekend.

And yes it is an S-plan system. It is still the original open-vented HW cylinder, although I'm not sure where it vents to now that the pipes to/from the F&E tank are capped off. Presumably it vents to the expansion vessel in the garage and then to the outside if that causes the pressure to exceed a certain limit?

If it is just a pinhole, would that explain why in normal usage I've not noticed anything unusual about the water coming out the hot taps?
 
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Its a strange one because if a pin holed coil is causing the system to partially refill reasonably rapidly with just 0.2bar pressure then IMO there is no way that the system would last a month without a top up, each top up "only" requires ~ 2.5 to 3L.
 

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