Have a Vaillant gas fired boiler (140K Btu/117K Btu DOE/102K Btu Net) with perimeter baseboard radiators upstairs (3 zones - 2300 sq ft)& a heated slab (PEX tubing in concrete) in 60% exposed basement (4th zone - 650 sq ft). Even when outside temp is in teens, the boiler comes on at 185 deg, fires for about 90 seconds to reach 190 deg, it takes 3-5 minutes for the water temp to drop back down to 185 then the cycle starts all over again. The circulator runs as long as heat demand lasts - does not go on and off with the burner. The aquastat is a Honeywell L4080B 112. Upper limit is set at 190. I don't see any adjustment for the differential, so am assuming it is fixed at 5 deg.

Questions #1: Gut instinct tells me that the boiler is cycling way shorter than it should...any input on what a reasonable or desirable cycle time should be from an efficiency standpoint?

Question #2: Shouldn't the aquastat differential be more like 30 deg? Water temp drop time and reasonable AM recovery time from 8 deg night setback suggests I have adequate radiator surface area & margin for 160 - 190 deg water to do the job (Note: slab is not setback at night).