I am a Gas Fitter/Heating Tech for a School Board in Western Canada (basically responsible for keeping the heat on). We have a wide variety of schools from a age of school standpoint, some with original heating plants. We have many old, heritage (early 1900's) schools, and some brand new schools and everything in between. We have heating systems with old low pressure steam boilers & steam emitters, to some old cast iron or steel hot water boilers with univent & fan coil heat emitters, to newer plants with condensing boilers and/or cast iron boilers that heat with univents, fan coils, or radiant heat sources like panel rads and in the case of a couple brand new schools, radiant floors.
So there is a wide variety of heating plants to see.
Now, one of the of the common themes in schools with any heating plant is intermittent use. The schools are only in operation for the most part around 8 hours a day (with the exception perhaps of if someone is using the school for some night event but this is an exception). So naturally most school operations departments want to take advantage of night time & weekend set backs to save money on natural gas. They will run the schools to 20 degrees celsius (68 F) during working hours and would to set back as much as possible (typically to 10 degrees Celsius which is around 50 F) to save money due to reduced heat loss, but still protect the school from freezing and provide a reasonable temperature to bring the school back up to temp from on Monday morning.
So naturally we have a few issues here. The main one is flue gas condensation on non-condensing steam & hot water boilers, the other one is design of old systems versus design of new where the new ones don't have as much pick up or oversizing as the old ones - the old original steam boiler plants, of which we have some still around (some old 1940's HRT boilers, some old 1950-1960's cleaver-brooks or similar fire tube variants) typically have very little issue bringing the temp back up. There is only a fixed amount of water in a steam boiler that the burner has to heat up so condensing isn't an issue really as once that water is up to temp and producing steam everything is good. The steam boilers typically have oversized emitters and can bring a school back up fast, and due to this fast recovery, can be set back relatively low. They also don't have much in the way of pumps other then vacuum pumps and feedwater pumps, so very little electricity use compared to a hot water system. Hot water boilers, due to the large volume of hot water in most systems in comparison, condense for a longer period of time (dependant on plant size versus building heat requirements) and take longer to bring a school back up to temp. There is also higher electricity use due to the many pumps required. The new schools have very little pick up as the boilers seem to be sized more tightly to the heat loss. This allows less of a set back. I've always been a firm believer in low temp hot water heating and I think if operated more correctly using DDC can probably work out, but a lot of the guys in the heating department I'm in who have been here a lot longer than myself are much bigger fans of the steam heating systems. They pick up faster, have less pumps to maintain/fix, don't condense as long, and seem to last and last without issue - some of our fire tubes are over 50 years old and still have original tubes or have had very few vessel (tube) repairs/replacements. Sure steam traps need some love every now and then and the feed water/vacuum system needs some attention, but I'm wondering what your experience/preferences are? What have you seen?
What, from your experience, is the best heating plant/emitter combination to maximize fuel efficiency for buildings with intermittent use/operatin to take advantage of setback to some degree. Are high mass radiant heat emitters out?
Being a fan of low temp hot water heating my solution (if I were to design a school or design a new heating plant/emitter retrofit) would be to use condensing boilers with say 150% redundancy (100% to pick up the plan on the coldest day of the year, the other 50% for redundancy, and for pick up on monday morning when a big dump of BTU's are required), and then use panel rads as they are a low mass heat emitter that can pick up faster than a high mass radiant floor and still provide pretty good comfort.
Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber