The consequence could be imploding your cylinder and flooding your house if you don't know how to safely isolate and drain the cylinder to change the expansion
If you don't fit the right thing or do it wrongly or don't know how to check its working properly then you may blow your cylinder up and potentially the top floor of your house.
If your budget is less than the cost of a qualified engineer to fix it and you value yours and your families life I would advise you resort to boiling the kettle and bathing with a bucket and a sponge.