How do I read runtime for the NSB170FT RED at my load?

From the discharge tables linked above rather than from the 170 Ah headline. The 10-hour figure is a rating at one load and one end voltage; at a heavier load the block delivers fewer amp-hours, and the tables give figures across a range of discharge times and end voltages. Pick the rows for your end voltage, find the load per block, and read off the time. If the plant load and the reserve target are all you have, send them with your quote request and a specialist will do the sizing.

How do RED, HT RED and BLUE+ differ?

By duty and by cabinet temperature. RED is the long-float-life standby build: 15+ year design life, -40°F to 149°F operating range, and a three-year EnerSys warranty. HT RED shares that range and adds sustained operation at 104°F as a published characteristic, and EnerSys warrants it for four years. BLUE+ is the cycling build, rated 2050 cycles at 50% depth of discharge with a 12+ year design life, and its warranty term is confirmed in writing with your quote. Match the build to how the site treats the battery and to how hot the cabinet gets, then size the capacity.

How often should an NSB170FT RED string be checked?

IEEE 1188, the recommended practice for maintaining VRLA batteries, sets out a schedule of monthly, quarterly and yearly checks: string float voltage and room temperature at the short interval, block voltages and an internal ohmic reading at the middle one, and a fuller inspection of connections once a year. Log every reading against the block's position in the string, because a pure lead block ages slowly and the value of the log is in spotting the one block that drifts away from the others.

Can I add one new block to an aged NSB170FT RED string?

Rarely a good idea. In a series string the weakest block sets the runtime, so a new block put into an aged string ages down to the string rather than lifting it, and the replacement delivers less than its own sheet says. A young string with one early failure is worth a call before anything is bought. If the string is several years old, the honest answer is usually to plan the whole string and keep the good survivors as spares. Send the install date and the last set of float readings and a specialist will tell you which case you are in before anything is ordered.