Sorry, primary/secondary,where the distribution system of the hydronic heating system drive the terminals (radiantion) and the heat transfer from fire to boiler is drive by its own "secondary" pump. Old-fashioned cast iron boilers wasted enough heat up flue, out jacket and collected enough via heavy mass to go without an extra circulator (talking residential).
With low-mass boilers using more noble, lighter weight materials such as steel, copper, aluminum and stainless steel the output to mass ratio makes minimum flow critical as under-pumping will lead to cavitation (makes noise when under fire). This is not an excuse to over-pump however (waste electricity, lift pressure relief valves (PRV) and cause flow noise and erosion.
When designing hydronic systems with low-mass boilers the first order of business is to make sure that now zone in the system--able to call the boiler itself--is capable of unloading the boiler at minimum fire and providing the minimum flow required at that out put.
It is easier to use P/S for everything, like wearing you pulling a trailer behind your truck. But it doesn't make it right. When converting an old gravity boiler connected to cast iron radiators with 3" black pipe, one zone, essentially no pressure drop in the distribution piping. One condensing boiler, one pump sized to the boiler load, program outdoor reset, save the customer 50% on the fuel bill, and $500.00 on the pump he didn't need.
Don't try this with Buderus, Weil McLain or SlantFin as the have taken the P/S pill CYA with the customer's money.
More progressive manufacturers, Burnham, Bosch, ICB, NTI and Viessmann for example, will let a thinking man do the math and build the system that best fits the job.
Most of the ModCons we service are too big and could have been improved with better near-piping, some with P/S and some most certainly without.
Yes it is a P/S system.
The Alpine manual, althought I do not have it handy as am in Fla for a few days visiting friends, does show a P/S in all the diagrams I believe.
Correct me if I am wrong but the return temp is reading actually the return water temp from the zone(s) plus the boiler pump water which is much higher, so I am not reading the actual return water temp from the zones(s). I could put some plumbers black tape on each return line and with an infared thermometer gun i suspect. Am I wrong here or would that be the most accurate way to determine the delta T?