It’s quite rare but the coil that holds the water from the boiler inside your hot water tank can split.
This is generally down to age, corrosion or just a manufacturing fault.
You may also find the water slightly discoloured or has a strange taste.
Two different hot water systems
The hot water tank has two separate water systems. The large volume of clean hot water in the tank itself and the dirty central heating water held in the coil. I’ll refer to these as clean and dirty water from now on.
The clean water can come from either a storage tank at a high point in the house (mostly in the attic/loft) or from the mains.
The dirty water comes from a different smaller storage tank at a high point in the house (mostly in the attic/loft) or can be a pressurised system from the boiler or central heating system.
In both cases there is a difference of pressure between the clean and dirty water. This difference in pressure is the cause of the problem.
Water system with two tanks in loft
This system has the lowest pressure difference as both clean and dirty water come from tanks at roughly the same height so have roughly the same pressure.
As the pressure difference is so small it will show itself as a gradual overfilling of one of the storage tanks.
The tank located at the lowest level will alway be the one that overflows even if the float valve is isolated or tied up.
Water system with a mains pressure tank
This type of hot water tank doesn’t have a storage tank at a high point in the house. They operate at much higher pressure than a traditional system.
This cylinder will have much higher pressure clean water than the other systems. This means that the clean water in the tank will force its way into the dirty central heating system.
This fault will show itself in two ways.
Sealed boiler system
If the boiler has a sealed system the pressure will always go up too high.
The hot water tank operates at a much higher pressure than the central heating system. If the coil is split the pressure difference between clean and dirty water is so big it will always overfill the central heating system.
Open boiler system
The clean water pressure in the hot water tank is far higher than that of the dirty water feed tank. A split coil in this system will show by the feed tank for the dirty central heating water constantly overfilling even with the float valve isolated or tied up.
Proving a split coil
Two tanks in loft (one small one large)
To prove the two feed tanks the hot water and heating need to be out of use for a few hours or even longer depending on how small the leak is.
- Isolate the water supply to the tanks either at the service valves near the tanks or tie up the float valves so they are closed.
- Open a hot tap until the water level in one tank is lower than the other.
Next you need to wait to see if the water level in the lower tank starts to rise. The two tanks should be separated by the coil so if there is a split they will start to find their own level which means the higher tank will drain into the lower tank.
As the split in the coil may be quite small it may take a while for the water level in the overflowing tank to go down.
The only solution to this problem is to replace the cylinder.
Mains hot water tank with a sealed system
- Remove the filling loop and cap the valves.
- Isolate the mains cold water supply to your hot water tanks.
- Open hot water taps to remove the pressure from the tank.
- Wait to see if the pressure in the boiler rises. If it still rises with the cold feed (not the hot outlet) isolated then the fault is elsewhere. If the pressure doesn’t rise after being left for a few hours turn the cold feed to the tank back on. This should cause the pressure to rise.
Mains hot water tank with small tank in loft
- Tie up the ball valve and remove some water from the small overflowing tank in the loft. You can use a drain off on a radiator or scoop some out into a bucket.
- Turn off your heating and hot water so the boiler doesn’t operate.
- Turn off the cold supply to the hot water tank for several hours. The water level in the (now emptied) overflowing tank should not move.
- Turn on the cold supply to the mains hot water tank and leave for several hours. The water level in the (now emptied) overflowing tank should slowly rise to the point where it overflows.
What to do next
Unfortunately once you’ve proved the coil is split there is very little you can do to repair it.
The only real solution is to replace the tank.
I would recommend getting a second opinion as it’s an expensive repair.