6L toilet cistern to bigger sized

Question

I have a standard traditional handle flush toilet that has a 6L cistern attached. I need to change this to a bigger 9L or higher version as not enough water to get rid of my waste so drain keeps getting blocked as it's just letting the waste sit in the pipes

Steve · Accepted Answer

Hi yes you can get cisterns that hold 8 litres, older high level cisterns held more water up to 13 litres.

Ben · Answer

You won’t be able to buy a brand new cistern that is more than 6 litres because of the rules in: 
The Water Supply (Water Fittings) Regulations 1999.

If 6 litres isn’t enough to flush away the waste from your toilet, it could be that the flush isn’t powerful enough, efficient enough, or the flush mechanism is faulty or broken.

Todd · Answer

Hi, as of 2001 the water regs state any new installs have to be 6l or less, 9l is only to be used if existing toilet fitted is a 9l cistern, we are advised to change to 6l for water consumption

Lee · Answer

If your toilet flushed fine before and the issue has developed over time, the problem is likely a partial blockage, a siphon fault, or limescale buildup, rather than the cistern size. Increasing the cistern capacity isn’t necessarily the best solution, as a properly functioning 6L flush should be enough.

Possible Causes & Solutions

1. Partial Blockage in the Pipes
	•	If waste isn’t clearing properly and sits in the pipes, there may be a buildup of debris restricting flow.
	•	Solution: Try a toilet plunger or a drain unblocker. If the issue persists, a plumber may need to inspect the waste pipe.

2. Siphon Fault (Flush Mechanism Issue)
	•	If the flush isn’t delivering a strong enough surge of water, the siphon diaphragm inside the cistern may be worn out or faulty.
	•	Solution: Replace the siphon mechanism—this is a simple fix and much cheaper than replacing the cistern.

3. Limescale Buildup in the Pan or Jets
	•	Over time, limescale can clog flush holes around the rim, reducing the flow of water into the pan.
	•	Solution: Use a limescale remover or white vinegar to clear any buildup inside the pan and flush jets.

Best Next Step

Before considering a cistern upgrade, check for blockages, flush strength, and limescale buildup. If the flush is weak, replacing the siphon is a more effective and water-efficient fix than increasing the cistern size.

If using chemical drain unblockers or limescale removers, always check that they are suitable for your toilet and plumbing system. Some strong chemicals can damage seals, pipes, or septic systems. Always follow the manufacturer’s instructions and consider gentler alternatives like white vinegar or specialist toilet descalers if unsure.

Chris · Answer

You may have a problem obtaining a new cistern that holds more than 6lts. The syphon fitted may be a poor quality version.

Steven · Answer

Firstly before you go and spend money on a new cistern I would suggest first replace the siphon as I'm pretty sure this is fault if the problem your having 
As due to bs u shouldn't have bigger than 6L

Daniel · Answer

Would it be possible to increase the inch and a quarter pipe to inch and a half with the correct adapter - this would increase flow. Also, do the waste pipes have the correct gradients, is it possible to remove unnecessary elbow (90's) to also improve flow?

Jerome · Answer

That would not be an issue but are u sure it’s the cistern and not the pipes themselves that’s causing the blockage? If it’s the latter u could end up changing the cistern for it to just keep blocking and with more water u could potentially flood the toilet

Jamie · Answer

You can now get adjustable ball valves that can fill from 3-12L.

6L toilet cistern to bigger sized

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