Both the Hoover PowerDash and SmartWash are carpet cleaners, but there are big difference between them. Below we will list some major ones that may affect your buying decision:

images from Hoover infomercials

• Size
  The PowerDash is a compact size carpet extractor, it is smaller than the SmartWash which is a full size cleaner. For small homes and spaces, you may want to consider the PowerDash, otherwise, go for the SmartWash.
  

  Extractor	Dimensions(inches)	Weight	Cleaning Path Width	Cord Length	Clean Tank Capacity	Amps
  PowerDash	43.5x10.125x15.25	less than 13 lbs	10-inch	20-foot	0.5 gallons	7 amps
  SmartWash	43.5x13x18.9	about 20 lbs	12-inch	22-foot	1 gallon	10 amps

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• How They Work
  Both of them are easy to use. However, the operation is a bit different.
  When using the PowerDash, you fill the clean water tank with warm water to the
  Water fill line, then add solution to the Solution fill line mark on the clean water tank. And then press the Water/Solution trigger for a slow forward and backward wet stroke, release the trigger for a slow
  forward and backward dry stroke.
  When using the SmartWash, the process is different. First, the SmartWash has a separate solution tank for holding formula. So you fill clean water tank with warm water, and fill solution bottle with solution. No need to mix them manually, the machine will dispense them automatically when cleaning the carpet. Second, the SmartWash has No trigger to press. You turn on the SmartWash, move it forward slowly, it will automatically dispense the optimal amount of water and cleaning solution. Then pull backward slowly, it will extract the water to dry your carpet. There’s also a Dry Only mode that just extracts residue without dispensing water or solution to dry the carpets faster.
• Attachment
  The PowerDash has no accessories while the SmartWash comes with tool for cleaning carpeted stairs and upholstery. So, if you have stairs, sofa stains, or other similar spots to clean, the SmartWash is a better option.
  
  

  images from Hoover infomercials, tools may vary

• Brushroll
  The PowerDash has one brushroll whereas the SmartWash has two rollers.
• Maintenance
  This is another important factor to consider.
  * Brush
  These carpet/rug washing machines have removable brush rollers.
  The PowerDash requires much work to clean the brushroll. You have to flip the machine on its side, use screwdriver to remove a few screws, then remove the belt cover, finally, pull out the brushroll.
  As for the SmartWash, you don’t have to flip the cleaner, there is also no screws to remove. You just lift up on the Nozzle release then pull the Nozzle forward to remove the nozzle, then you can reach the brush rollers and pull them out to clean.
  * Dirty Water Tank
  The dirty water tank on the PowerDash is hard to clean up completely, it has only one small side hole, really takes some work to get all of the dirty water out. There might be gunk sitting in the bottom which is difficult to clean out.
  As for the SmartWash, the collection tank has a removable top which makes it easier to empty.
  To sum up, the SmartWash takes much less work for maintenance.
• Warranty
  The PowerDash carries one year limited warranty versus five year limited warranty on the SmartWash. For what is covered by the warranty, you can visit Hoover.com to download the manuals to learn more. Or just go to the end of this text to get the download links of Download Product Manual the user guide pdf files directly.
• Price
  The PowerDash is much cheaper than the SmartWash.

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More information:

• Both of them have clean water tank and dirty water tank to keep clean and dirty water separately.
• Featuring Heat Force technology, applies heat generated by the motor, to blow hot air to quicken the drying.
• Have no heater to maintain or hot up the water. It’s recommended to use warm water.
• To keep the warranty, use only Hoover cleaning solutions, they do not allow you to use your own formula or solution from other brands. But if you do not care about the warranty, just go ahead.
• They are not intended for dry vacuuming, always using your traditional vacuum to do the dry vacuuming before using these carpet shampooers.
• To rinse only, just do not fill formula to the tank.
• They have no brush off option.
• Can you use these machines on wool rugs/carpets? It is not recommended to use them to clean wool rugs/carpets. If you want to use them to clean anything that you are not sure if it is safe to clean, or anything that is luxury or you care about, you should always ask the rug/carpet manufacturers first. Some rugs/carpets cannot be cleaned by a carpet shampooer, not only cannot be washed by these machines but also other carpet extractors.
• These extractors are designed for cleaning carpet/rugs, they cannot be used to clean hard floors like hardwood and ceramic floors. Using them on hard floors may damage the floor.
• Do not store the machines with water or formula in tanks. If you do not use up all the solution, it is recommended to empty the tanks before storing. If you do not want to throw out the solution, you should detach the tanks and put them in a place where leakage is not an issue.
• The SmartWash has two versions, the Pet Complete version has a detachable wand for pre-treating dirty spots, you can read the comparison of the SmartWash models here.

Overall, the Hoover SmartWash is a bigger size carpet washing machine with automatic cleaning technology, it comes with attachment for above floor cleaning, and takes less time and effort to clean up after use. The PowerDash is smaller, designed for small space and high traffic area cleaning jobs, and it is much cheaper than the SmartWash. So, just buy the one that fits your cleaning needs.

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Manual PDF download links:
PowerDash manual:
https://tti.a.bigcontent.io/v1/static/FH50700_User_Manual
SmartWash manual:
https://tti.a.bigcontent.io/v1/static/FH52000_SmartWash_GL_Manual
SmartWash Pet Complete manual:
https://tti.a.bigcontent.io/v1/static/FH53000-manual