The DC14 is better made of a more robust and flexible plastic. It is also a more complex design, incorporating more cyclone chambers and removable sections for accessing blockages.
It does however exhibit a few problems from time to time, but they are easily solved. It's ironic that devices designed to clean things (vacuum cleaners, dish washers, washing machines) are often vulnerable to dirt or debris.
"Screaming" This is when the cleaner makes a shrieking sound . I have traced this usually to dirt deposits on or around the various seals which couple the sections together. Either on the rectangular rubber seal on the main dust collection drum, or the various removable sections. Just wipe any dirt deposits away with a damp cloth and reseat. Small residual accumulation of dirt or debris here causes a small air gap, through which air is sucked. This acts like the reed in a wood wind instrument and creates the sound.
Cleaner does not pick up when used upright. This is when there is suction through the wand but it does not pick up from floor or carpet when used as an upright cleaner. This may be caused by a blockage, but in my case is caused because the cleaning head unit is not correctly parallel with the floor. The floating section which contains the rotating brushes should always be parallel with the surface being cleaned, in order to maintain sufficient suction. If you take a floor level look at the left side of 
the cleaner, you will see that the head is tensioned down by the same white pipe section which provides the suction. This short length of tube , approximately 10 cm in length, is made of pipe with a bracing metal wire coil. This seems to act also as a spring in tensioning the head of the cleaner , so it's always pushed "heel down". If this does not happen try twisting the pipe until it retains it's springiness and pushes down the heel of the cleaning headThe Cleaner smells bad. I have only made this mistake once ! If you wash the top foam and plastic filter section under the tap, it's important that it's completely dry before you put it back in. If it's replaced while slightly damp, bacteria will develop in it and it will start to smell. Worse the very action of the cleaner air flow will exhaust this bad smell as you clean. Just wash thoroughly the filter and leave to dry in the Sun or near a radiator for a day.
Cleaner motor cuts out. This happened on Friday. If the motor over heats it has a thermal cut out. This shuts down the motor until it has cooled. The cause is usually an obstruction, either in any of the complex piping between cyclone chamber and the tools or header unit OR an obstruction on the output of air. In the DC14 the motor expels air upward directly beneath the cyclone chamber. On the allergy model a small round chamber holds a HEPA 'lifetime' filter so that expelled air is also filtered. The chamber holding the filter can carefully be opened by gently raising 3 plastic catches. Inside you can remove the purple filter. The underside of my HEPA filter , from a 4 year old DC14 is black, probably from microscopic pieces of carbon from the motor brushes. In the UK it is possible to buy another HEPA for around �25 and I may do that, as suction seems to be declining.
Loss of suction through wand and when used upright. Generally I have been noticing that there is less suction of late. I emptied the dust container, removed all the inserts and checked for blockages. All OK. The top yellow filter had been rinsed through quite recently, but I eventually tried the cleaner with the top filter removed. The cleaner was back to great form. This suggests to me that the yellow "lifetime" filters have a finite life ? The blue foam looks clean and it appears to be just sponge. However I'm not sure if the filter integral with the yellow piece lasts for ever ? Have ordered a replacement.
This seems to have given the cleaner a new lease of life. Perhaps I didn't need to replace the lower HEPA filter after all. However the top filter looks like it did need a replacement. The cleaner now sucks up fine. I can only assume that the yellow plastic top filter does not last forever as claimed ?
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