A cold-start boiler is a boiler that is not damaged by letting it cool way down. Many boilers suffer excessive corrosion on the heat exchanger plates if it's allowed to drop below some minimum temperature between burns, due to the acidity of exhaust condensate (which is much more acid with oil fuels than with propane or natural gas.)
With oil boilers that temp is often 130-135F, and the usual manufacturer's recommendation for a 140F low-limit to give it some margin. This is/was particularly true for higher-mass boilers that would have a longer heat-up ramp time, with correspondingly higher condensate quantities. Many newer boilers are both lower mass and use materials a bit more condensate-tolerant on the heat exchanger plates.
Boilers with embedded coils for domestic hot water typically needed 160F or higher idling temperatures to produce reasonable hot water performance. But any oil boiler used for space heating would be fine with a 140F minimum temp, and many newer boilers are cold-start tolerant.
Even those not designed for cold-starting will not have a problem if you only cold-start it once per year, even if chronic daily cold-starting would ruin them quickly.
On p.16 of
the manual Slant Fin recommends flushing the boiler while it's still hot to get the sludge out of the bottom, then adding sodium chromate solution and bringing it fully up to temp for an hour when shutting it down for extended periods. That's a bit excessive if you're turning it off in May and firing it up again in September. It's worth doing the sludge-purge and heat-up at the end of the season, but I'm less convinced that the chromate is really necessary if it's only idle for a few months.
If you didn't purge it when you turned it off it'll still be fine, but early in the season fire it up (without pumping the water through the radiation- keep the wall thermostats turned way down or off), and purge any sludge that ended up in the bottom of the boiler. Your circulation pumps will thank you for it later.
Boilers that were also used for domestic hot water are usually ridiculously oversized for the space heating load of a typical house in CT, and will have poor as-used AFUE efficiency as a result. There are a couple of ways of improving the as-used efficiency:
A: Adding a heat-purging economizer control such as the Intellicon 3250 or Beckett Heat Manager,
....and...
B: down-firing the burner by installing a smaller jet more appropriately for your actual heat load, now that you don't need the higher-fire to keep the hot water flowing at reasonable rates.
Both are worth thinking about here.
The heat purging economizer is very low risk, and is DIY-able for those with electrical skills. They are usually good for a double-digit percentage in fuel-use savings when the boiler is 2-3x oversized for the load, and they reduce the total numbers of burns, taking some wear & tear off the boiler.
To down-size the burner jet requires a bit more analysis, (but it's analysis worth doing anyway), as well as combustion analyzer equipment. If you have the boiler serviced on a semi-regular basis they probably indicate on the service tag what size jet is installed.
If you have a fuel use history of exact fill-up dates & quantities &/or a mid-to-late winter fill-up bill with a "K-factor" and your ZIP code (for outside design temperature and weather data) we can get a very good handle on the total heat load. Odds are the smallest jet that can operate in that boiler is still more than 2x your actual heat load, but that's a lot better than 4-5x oversizing, from an operating-efficiency point of view.
The total length of baseboard on the system (per zone, if it's broken up into zones) would be also important, since too much radiation for the size of the burner jet could result chronic boiler operation at temps that are lower than prudent, and might need some near-boiler plumbing tweaks to work.
BTW: "Furnace" generally refers to a hot-air furnace that uses ducts for distributing the heat. A heating appliance using steam or pumped hot water and baseboards or radiators for delivering the heat is generally referred to as a "boiler".
