Sizing a surface cleaner to your machine

The pairing

The trade rule of thumb is roughly 1 GPM for every 4 to 5 inches of deck. Manufacturers rate it a little more generously than that: Whisper Wash rate their 19 inch decks from 4 GPM, their 24s from 5, and the 28 and 31 inch decks from 6.

So 6 GPM will legitimately run a 28 inch deck — but rated minimum and working comfortably are different things. Below about 8 GPM on a big deck you are inside spec and still slower than you would be on a 24. If you are between sizes, go smaller.

Check the temperature ceiling, not just the width

This is the spec people miss entirely. A deck’s maximum water temperature is set by its cover material, and it varies more than the flow rating does.

Aluminium covers take 212°F. Xenoy and ABS covers stop at 180 to 185°F. Put a plastic-covered deck behind a hot water machine running at temperature and you will destroy it — and that is not a warranty claim, it is an operator error.

What going too big looks like

The deck starves. Each nozzle in the bar gets less water than it needs, so you slow your walking pace to compensate, and you still leave stripes where the passes overlap unevenly.

You end up with a heavy, expensive tool that is slower than the smaller one would have been. We see this constantly and it is always the same story: someone bought the biggest deck they could afford.

What going too small looks like

Nothing dramatic — you just cover less ground per pass. It is the safer error by a wide margin.

Castors are not optional if you do this daily

A deck without castors drags. On a residential driveway that is fine. On a commercial lot it will wear you out by mid-afternoon and it wears the deck skirt as well.