just thought i would confirm what tenet said,you could have a bomb in your house if the unvented hot water is not installed correctly.who ever installs has to carry ID to prove that they are fully trained,if you have it done by Joe Bloggs the plumber and it goes wrong you may have problems with your home insurance.The cylinder has temperature and pressure relief valves on it which could release water at 90 degrees C so the discharge for the T&P valve is a big part of the installation.If done correctly you will be ok ,so choose carefully.