I need to get my 2400 sq.ft., 2-story house pressure washed, and had a few services visit the house to provide quotes. The quotes are widely different so I need some advice to decide. Service A said he will use a 3000psi jet, put up a tall scaffold to stand on while washing, and charge $600 total. Service B said he will do a soft wash using a low-pressure jet (< 200psi pretty much same as the house water supply pressure), use a pole to reach the second story, and charge $300 total. Is Service A's method overkill and/or potentially damaging to the house? Does Service B's method sound sufficient given the amount of mold/mildew on the house is moderate? Both services use the same chemical (bleach).
Water – Help with hiring pressure washing service
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Assuming metal pipe, I'd set it to about 60, because that's a good compromise between too-high pressure and disappointingly little pressure. That's what most well pumps I've seen are set for, and significantly less than most municipal water systems (I've seen one city water system that delivered 125psi to the house). Much lower than 60psi and you'll be frustrated by the slow flow.
With resilient (not PVC/CPVC) plastic pipe, I'd go a little lower than that because unlike metal pipe, which has very strong, secure connections, plastic pipe has only clamp-over-plastic-over-barb connections, which can fail more easily than metal pipe. With PVC/CPVC, I'd go even lower because it gets brittle with age.
There are a few reasons this can happen.
Other uses of water + small lines
Depending on the size of pipes and the way it's plumbed in, it could happen if something/something else in your house was using water (dishwasher, washing machine, shower). This happens a lot in subdivision homes (which are built using the bare minimum amount of money required to meet code), where all the branch lines are 1/2".
Hose clogged
You can also check the hose when this happens: disconnect it from the hose bib (aka spigot), and see if that has a reduced flow as well.
If it doesn't, there may be something clogging your nozzle or hose.
Pressure regulator
You may also have a pressure regulator on your house (see pic below). These can have trouble if the supply pressure is below what they're set at -- I'm not 100% sure what happens but I'd bet it would reduce the pressure like what you've said.
They can also get clogged with debris (though unlikely with city supply) and sometimes fail for no apparent reason (though again, unlikely).
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Best Answer
Ideally you would use the lowest pressure that will be effective, to minimize the chance of damage. A soft wash generally focuses on letting the detergent dwell and do the work, while a harder wash can make do with fewer (or no) chemicals. Some jobs clearly require one or the other, but siding is somewhere in the middle.
Assuming that both operators are skilled the high pressure guy has the potential advantage of using fewer chemicals, which is nice for plants and animals. The greater height from the scaffolding should also make it easier to aim the spray down, which tends to work better with siding than up. On the other hand, the high pressure stream is more likely to force water into places you'd rather not have it. Got any poorly flashed windows?
The low pressure guy is potentially less dangerous to your house, potentially more so to your garden if he's loading up on the chemicals. Using a pole puts him further from the work. That being said, you really care how the house looks from the ground. It doesn't sound like there are any features that couldn't be cleaned with the pole. Are there gargoyles or corbels that need the close up touch?
In the end you'll have to decide who is more trustworthy, whether that's by asking questions or seeing their previous work. Neither sounds too outlandish, though neither sounds extremely compelling. Two hundred psi sounds very low to remove mildew that's grown into the siding while three thousand sounds fairly high for a house.