Short cycling

Clank Clank

Typical recommendation to combat short cycling on this site is to ensure main vents are properly functioning. Once you get beyond the first cycle, where the vents and hot and therefore shut, how is it that having main vents prevents subsequent quick rises and falls of pressure?. It takes a long time for them to cool down and open back up. Too much time to help with anyone's short cycling problem.



Explanation would be greatly appreciated.

Jamie Hall

There's short cycling and short cycling

and we really should have two different terms.  At least...



What you want to avoid is a situation where the pressure starts to rise noticeably -- above a few ounces -- before all the radiation is filled with steam.  Good main venting will avoid that problem.



On the other hand, once the radiation is completely filled with steam, but the thermostat is still calling, you don't want the pressure to rise further  -- so the pressuretrol or vapourstat kicks in and shuts off the burner -- but you still need heat.  So, within reason, the shorter the off period (within the capabilities of the burner) the better.  Why?  Because that way the boiler never has a chance to cool down and therefore you don't lose any energy bringing the kettle up to a boil again.  It would be better, perhaps, to refer to this short cycling -- which is matching the boiler output to the capacity of the radiation -- as modulating, but that term has been used to refer to variable firing rate, rather than on/off firing.



So -- "short cycling" near the beginning of the cycle is not good.  "Short cycling" near the end of a cycle is not a problem and is expected unless you should be so lucky that your boiler exactly matches your radiation.

Clank Clank

Makes sense.

That finally boxes in the benefit of main vents. Thank you.



In regard to the modulating, back half short cycling, makes sense that the off periods are kept to a minimum given that explanation but it sounds like a balancing act of pros/cons with those who are putting in purposeful time-delays, actually extending those off periods, to give time for the heat in the radiators to do their work before needlessly restarting the boiler. In this case, you save some cycles but spent more energy reheating the boiler.



Is this a difference in opinion or is one approach clearly more efficient?

Jamie Hall

Probably at this point

more of a difference of opinion, since I don't think anyone has done a study on it.



I think I'm right (naturally!), and I think my point of view has a good deal of logic associated with it.  But I'm not going to make a Federal case out of it!

Fred

Extended Cycling?

Maybe the short cycling at the end of a cycle should be call "Extended Cycling" ?

Mark N

Reason for Cycling on Pressure

Clank why are you cycling on pressure before satisfying the thermostat? Are you recovering from set back? Is the boiler oversized?

Clank Clank

Right both times

Everyone likes it cold at night so there is a decent recovery in the morning (especially this winter). In addition, boiler is certainly oversized. That's the reason I'm interested in knowing which theory on the "rhythm of the boiler" gives the most efficient result. Both agruments make sense.