How to Size a Downstream Chemical Injector
An injector that does not match your pump's flow will either not draw at all or draw so weak the soap is useless. Sizing takes 30 seconds once you know your GPM.
The chart
| Your pump flow | Injector range to buy |
|---|---|
| 2 - 3 GPM | 2-3 GPM injector |
| 3 - 5 GPM | 3-5 GPM injector |
| 5 - 8 GPM | 5-8 GPM injector |
| 8+ GPM or dual-gun | 10 GPM / high-draw injector |
Buy for your actual pump output, not the number on the engine sticker. If you are between ranges, size down: a slightly small injector still draws; a slightly large one does not.
Why injectors stop drawing
- Wrong nozzle. Downstream injectors only draw with a low-pressure soap nozzle (black tip, 40 degree, 3-4x your normal orifice). A high-pressure tip closes the venturi.
- Too much hose. Every 50 ft of hose after the injector cuts draw. Past 200 ft, move to a high-draw injector or a 12V system.
- Worn check ball or spring. Bleach eats them. Rebuild kits are a few dollars and take five minutes.
- Air leak on the chem line. Check the barb and the pickup tube before you replace anything.
Adjustable vs fixed
Fixed injectors are simpler and fail less. Adjustable ones let you back off the draw for delicate surfaces. For house washing with SH we recommend fixed plus a metering tip set.
Acid and high-draw models
Standard brass injectors do not survive acid or heavy SH use. Use a stainless or poly-body injector for roof mixes and F9 acids.
Shop Chemical Injectors, Soap Nozzles and Stainless Fittings. Bring your rig by the shop in Gaithersburg and we will size it on the spot.