If you hire them watch their work
I hired them to do the venting ,I watched them and the cleaning brush only went into a portion of the vent. If you hire them measure the length of the vent and measure how far the brush goes in . They will see if your watching when they are doing the work . They only did half the job because I had traditional steel vent and they said there were screws in the piping catching the brush. ALL STEEL VENTS HAVE SCREWS TO HOLD THEM TOGETHER.They will show you a picture of a vent which is very dirty ,does not mean yours are . I will never have my vents done again ill just change filters more.There so no way to check their work unless you watch them do it.I thought it was a fancy machine with a really big vacuum , NOT. The employees were nice though.But they do not have the right equiptment.