pressuretrol settings

I'm confused

in one post, you say that it does the cycle bit towards the end of the cycle and the radiators are hot.  In another you say that the radiators are just barely getting hot.  You also note that the thermostat is calling for 68, but that the temperature is only 64.



First off, is this one pipe steam or two?  It makes a difference.



Venting is important.  In either one pipe or two pipe steam, the venting of the mains must be sufficient to allow the air to escape reasonably quickly -- ideally, as fast as the boiler can make steam.  jpf has given you some sources for helping to determine that.  In one pipe steam, and in some (but not all) two pipe steam systems, the mains themselves need to be vented (some two pipe steam systems of the vapour persuasion are vented at the boiler, through the dry returns).  In one pipe steam, the radiators also need vents; in two pipe, they do not have vents.  Either way, a lack of venting can cause pressure to build and cause cycling before the radiators have a chance to warm.



Insulation is also much more important than many seem to realise.  All the mains must be insulated, and as much of the risers as is reasonable.  Lack of insulation may cause an otherwise adequate boiler to be unable to heat radiators towards the ends of the mains; in any case it will use up a lot of fuel to no good purpose.  However, lack of insulation will not cause cycling on pressure -- to the contrary, lack of insulation may make it nearly impossible to build any pressure at all.



Your mention of the temperature being 64, but the thermostat calling for 68, taken by itself, suggests that the whole heating system is undersized -- not just the boiler.  The question I would ask relative to that is do you mean that the system cannot raise the temperature past 64?  Or do you mean that it takes forever to get there?  It is not at all unusual for a heating system to take an hour or two to raise the space temperature 4 degrees.



You also seem to have some question as to what is causing the cycling in the first place.  The 0-30 psi gauge, while required by code, isn't much help in determining what your system pressures really are.  A good 0 - 3 psi gauge added to the system would be needed for that.  However, you should identify whether it's the pressuretrol or something in the burner that is causing the burner to turn off and on.  My guess would be the pressuretrol, performing as advertised -- which goes back to the question of why the pressure builds early, if it really does.



Ideally, a steam system, whether one pipe or two, should run for quite some time -- as I noted in my first post, perhaps 20 minutes or more -- without building any significant pressure.  During that time, the vents are mostly open, air is leaving the mains and the radiators, and steam is reaching the radiators.  At about that point, the radiators should be hot, although they will not necessarily be hot all the way across and down, and all or most of the vents closed.  When that happens, the demand for steam production drops to where the only demand is from condensation in the radiators, and the pressure should begin to build.  And the pressuretrol should turn the burner off.  As condensation continues, the pressure will drop, and the pressuretrol should turn the burner back on.  This cycling is perfectly normal and expected, and should continue until the thermostat is satisfied.  If you are just maintaining temperature, rather than raising it from a setback, it is likely that the thermostat will be satisfied first, and the system won't get to the everything full point and no pressure build up will be seen -- and no cycling on the pressuretrol -- before the thermostat is satisfied.