FAR 23.1325   Static pressure system.

(b)(3) If a static pressure system is provided for any instrument, device, or system required by the operating rules of this chapter, each static pressure port must be designed or located in such a manner that the correlation between air pressure in the static pressure system and true ambient atmospheric static pressure is not altered when the airplane encounters icing conditions. An antiicing means or an alternate source of static pressure may be used in showing compliance with this requirement. If the reading of the altimeter, when on the alternate static pressure system differs from the reading of the altimeter when on the primary static system by more than 50 feet, a correction card must be provided for the alternate static system.

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The way I read this is that if you don't have a heated static port, and there's any chance that ice will block your static port, you need an alternate static, VFR or IFR.  I know this seems to apply to "normal, utility, etc., and not experimentals, but your op limits will say you need a static check per part 43 appendix E, which then refers back to part 23.

We had an a customer's plane go all goofy when water in the static line would freeze at altitude.  Utterly VFR but an alternate static source still would have come in handy.

McMaster-Carr makes an nice little valve (P/N 6791T21) that looks like a toggle switch that fits the bill nicely.  It's about $15.

Dave Saylor

831-750-0284 CL