Why 12″ Pop-Ups (1812) Crack More Often—Especially with Side Inlets
Imagine a tall thermos with a hole on the side near the bottom. After you pour, a cup of water still sits below that hole. In winter, that trapped “cup” expands about 9% when it freezes—pushing outward on the spray body until plastic gives. Side-inlet 12″ bodies are notorious for this in freezing climates.
Manufacturer guidance: side inlet in freezing regions
Rain Bird’s own 1800-series tech sheets state: “Side inlet installation not recommended in freezing climates.” That applies directly to 1812 bodies. It’s not about brand—it’s about physics and geometry. :contentReference[oaicite:19]{index=19}
The geometry: why water stays trapped
- Taller column, bigger force. A 12″ body holds more water above grade than a 4″ or 6″—more expansion energy when it freezes.
- Side inlet = internal bathtub. When the feed enters the side of the can, water below that inlet has no gravity path out. Even after a blowout, a pocket can sit under the inlet—exactly the “cup” that splits cans mid-winter. (Install sheets warn against side inlets in freezing climates.) :contentReference[oaicite:20]{index=20}
- Cold sinks. North-facing beds and shallow installs chill faster, so that trapped pocket freezes early and repeatedly.
Why low CFM makes 1812s crack more often
Bill-Nye version: Blow a weak puff across a puddle—the surface ripples, but the puddle stays. That’s a small plug-in compressor: low CFM means air rides over liquid; water in the base of tall, side-inlet cans never really sweeps out. With professional volume—your ~80 CFM Viper or a 185 CFM tow-behind—you can clear to fine mist at safe PSI, so the base pocket is minimized. (Manufacturers and extensions emphasize volume and modest pressure—generally ≤50–80 PSI depending on piping.) :contentReference[oaicite:21]{index=21}
Field fixes that actually work
Use a swing joint to feed from below whenever spacing allows; this gives gravity a way to help drain. (Matches manufacturer guidance.) :contentReference[oaicite:22]{index=22}
Target ~50–60 PSI with high CFM and short cycles to mist. Avoid long blasts on dry pipes. :contentReference[oaicite:23]{index=23}
On the Winterizing Report I note tall cans in cold exposures for spring re-plumb or body swap.
What I look for during blowouts
- Where mist stalls. If a 12″ head keeps spitting slugs late, I suspect a trapped base pocket.
- Side-inlet tell-tales. Older beds with tight edging often forced side feeds; I’ll flag those for spring.
- Post-freeze cracks. Most splits show up along the vertical seam 1–4″ above the inlet—classic trapped-water signature.
- Rain Bird 1800 Series Tech Spec (side inlet note) :contentReference[oaicite:24]{index=24}
- Hunter — Winterizing (safe PSI) :contentReference[oaicite:25]{index=25}
- CSU Extension — Air volume vs. pressure :contentReference[oaicite:26]{index=26}