This is really a Hydrostat 3150 question, not a generic question. If the answer isn't in
the manual , call their technical help line to find out if cold-start means truly cold start when enabled, or if some other progammable setting interferes with it.
On page 4 of the manual it states:
"
To set COLD START operation
Operates on call for heat only.
Low Temperature Limit
Make sure Low Temperature Limit is turned fully
counter-clockwise (OFF position)."
Like you, I read that to mean that it should not be firing when there is no call for heat from either the indirect or the heating zones, yet it sounds like the behavior is something else.
FWIW: Most homes don't need anywhere NEAR 170F water to meet the heat load on design, day, but depending on how much radiation you have relative to the boiler's output you might need it to be that high (or higher) to keep it from short cycling, especially if the system is cut up into zones.
I assume you have it set up to use the pre-purge function? (Recommended for efficiency, since it lowers the average operating temp for lower standby losses, and minimizes the total burner-on time.)