[michael]

cannot help to sometime cast advice when it is about housing rehabilitation. having read the previous posts, may or may not be able to shed light and/or advice on the issue. for one, the CORRECT solution of white vinegar mixed with water can be a very good disinfectant as well as a cleaner. in essence it is muriatic acid, and when using spot testing can be effective even on painted surfaces. i never have used a power washer on siding, but have used on masonry. white vinegar ( one part to five) i have used on vynil recently ( at my elderly moms house) and because of foliage/flowers at foundation perimeter mixed a mild solution in an attempt to save plantings. due to soot from close by wood burning devices, it did require sweeping/loosening the surface with a common straw broom 4 or 5 minutes after solution being applied; and it did come clean with a good rinsing. used a garden hose with one of those spray bottle attachments. 60 or so days, plants are flourishing.
when referencing vynil siding the above posts for the most part are acurate, with the acceptance of what type of application was used when the siding was applied. at minimum a breathable layer of TYVEK or a perforated breathable styrofoam underlayment ( sometimes called insulation) should be installed underneath the vynil. also backflashing at soffits,windows, and doors should be applied prior to vynil installations.
the weep holes mentioned are usually below the upper panels hanging flange, in an attempt to prevent water from infiltrating from below the siding system. if siding is installed properly it should be protected from lower water infiltration, if is not than the power washing will probably blow the sidings seams apart, if the presuure is not set properly.
as usual PLEASE READ INSTRUCTIONS ON THE BACK OF SIDING BOX, and do not clean siding during the colder seasons.
my 2 cents