This is most frequently due to air building up in the radiator. It can usually be solved by bleeding the radiator using a key. However, if you are having trouble with this, or the problem continues, you can call Millers for advice and assistance. Please note that is to top heats up but the bottom remains cold, this can be more difficult. This is often caused by a build up of sludge in the radiator, and the best solution is to have a professional flushing and cleaning.