condensing boilers

JackR

Primary/Secondary has nothing to do with the condensing process, it is a way to De-couple the boiler from the system and ensure consistent flow through the boiler which is extremely important in a Mod/Con. Any boiler will condense with return water temps lower than roughly 135 degrees ( depending on fuel ), Mod/Cons are made of a material, either Aluminum or Stainless, that is made to withstand the acidic condensate produced. In my opinion it never hurts to pipe any boiler P/S. Hope this answers your questions

laurence salvatore_2

primary/secondary piping on condensingboilers

I have put in a couple of vitodens and primary/secondary was recomended by the rep from viesmann. Is this required on all condensing biolers inorder for them to condense. Should I plan on primary/secondary loop piping on all makes of condesing boilers? Thanks for you help in advance.

Mike T., Swampeast MO

Primary/secondary with condensing boilers is ALL about ensuring a proper range of flow through the heat exchanger at all times. It has nothing to do with allowing them to condense. In fact, in systems where secondary (emitter) flow significantly exceeds primary (boiler) flow--quite common--it can be easily demonstrated that condensation (and thus efficiency) are reduced.

If flow through a mod-con goes too low, it's VERY easy for these rapidly heating boilers to exceed their safeties before the control system can react. If on the other hand flow is too high, velocity through the HX as well as its head loss will increase dramatically causing excessive wear, excessive electric consumption (to circulate water) and even [potentially] a reduction in heat transfer efficiency.

Primary/secondary is not an absolute requirement with mod-cons, but you can't just plunk one into any old system without it. The general requirements for use without primary secondary are: generously sized radiation that can fully absorb the full boiler output at around 160F supply; non-electric (proportional, like TRV) flow control on all or nearly all emitters and; extremely low head loss in the emitters and associated piping.