What is the story on high pressure washers (water)?

I've got algae growing on my vinyl siding (only on certain areas) and its either bright green or dark green, but I need to clean it off. Conventional wisdom is to do it with high pressure water and I could pay a guy to do it, or buy my own 'unit' and do it myself.
Question: there is a range of units from cheap/low pressure (i.e. 1100 lbs specified) on up to expensive (and gas engine)/high pressure (i.e. 2000+ lbs as specified) and my question is: do you really need that higher pressure? Is there a kind of threshold where below the threshold, you get significantly less cleaning performance?
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I clean my vinyl siding all the time with my 1200 psi.Coleman pressure washer and find it more than adequate to remove whatever accumulates on the siding (short of spraypaint).
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Get a siding cleaner that kills mold first , you really wont get rid of it with a power washer . It will grow right back as you didn't change anything affecting growth, or its roots. you probably wont need a powerwasher with a few applications. Bleach works but i don't know what it will do to vynal . 1200 lb are good just slower.
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I live in Houston, a swamp of algae and mildew, so my 'new' house has some algae on the bricks. I wanted to try chemical removal first before pressure washing. I'm not certain if bleach will have any effect on algae (as it does on mildew). I browsed at Lowe's and found several spray-on cleaners for houses. Then I found this stuff at Ace Hardware: www.sprayandforget.com specifically for algae. It was expensive per gallon but it seems to work well at killing the stuff, plus it leaves a residue that keeps the stuff from growing back. It doesn't have the immediate effect that power washing would but then power washing can damage siding, etc.
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I've never faced this problem, but our local paper carries a column by a home inspector with a couple of decades' of experience. He points out that since you'll be power washing from ground level, you'll be forcing water BEHIND the siding. Proceed at your own peril.

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You seldom have a use for the high pressures you get from the big gasoline units but if you use a softer nozzle you get a much wider spread at a more reasonable pressure and things go a lot faster. I have a 1200 electric unit and access to a 6500 gasoline unit. If I have a big job I use the big one. The electric is small and easy for small jobs.
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On 06 Jul 2004 15:46:41 GMT, snipped-for-privacy@aol.com (Greg) wrote:

A follow up question, if I may.
I have a large pecan tree in my back yard that needs spraying with fungicide/pesticide. My hand sprayer won't reach high enough so I need to know if a high pressure washer will do the job.
Pj
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Like Doug said:
If you are going to use a power washer, do it from ABOVE the siding. Now I know that may sound difficult, and you may have to do it from a ladder, but if you do it from ground level and spray UPWARDS, you are going to get water BEHIND the vinyl and then you are in for big trouble. Whatever is behind it will stay soggy and rot. Then dirty siding will be the least of your problems.....
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It's amazing to me that this isn't completely obvious to anyone considering power washing. I have immense respect for the guy on our local radio show for keeping his language civilized when callers ask about the procedure. :-)
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straydog wrote:

I had a bunch of algae? Mildew? Green on the joists to a porch. Hit it with "jet" on the hose to get the gross dirt/dust of, then 1/2C bleach in a couple gallons of water with a bristle brush (small broom) and about 20 minutes (with a yellow rubber rain jacket on after the first minute :) )
Is it a LARGE area you need to clean? Out of reach of a broom?
3rd cheap option: Hit any rental place and rent one for a day.
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