T-Stat's number of cycles

NY_Hammer

How the rate of cycles setting in the T-stat affect one-pipe steam system?

Default is 1, but I read somewhere here that 2 is also fine for some systems.



Just trying to understand how this setting may affect the system





Thanks.

Rod

Thermostat Cycles

Hi-  The surface of a radiator is very hot (200 + degrees) when the

thermostat shuts off the burner.  The idea of the cycle function is to delay the thermostat coming back on so as to allow the hot radiators to "coast" a while as they are still giving off heat. Cutting down on the burner on time saves fuel. The normal cycle setting for steam is one per hour though if the house cools very quickly or you have thin wall radiators then you might be better off at two cycles per hour.

- Rod

Dave in QCA

Another example

If you have cast iron radiators, they have a lot of mass.  If you have ever moved one, you come to that understanding very quickly.  When all of that iron (or a portion of it on a mild day) comes up to the temperature of steam, it will continue to give off heat after the boiler has shut off, and the steam goes down.  Usually, about 45 mins to one 1 hour later, the radiator has cooled down considerably, and is only luke warm.  Next steam cycle starts....



The objective is two fold.  One, keep the room temperature as steady and even as possible, with no over shoot, and no cold spells.  Two, make sure that each boiler cycle is long enough to fully distribute steam to at least the first section of every radiator.  Generally speaking, one cycle per hour for cast iron radiators will do the best job of achieving the objectives.  Keep in mind, one cycle per hour does not mean an "off" time of one hour; it means that there is an hour between the beginning points of each cycle.  On a very cold day, if the boiler runs for 45 minutes, there will only be an off time of 15 minutes, and you would not even perceive that the heat had shut off.



However, ever if you do not have cast iron radiators, your system will probably not hold a large amount of residual heat.  If, for example, you have copper fine tube convectors, they will probably cool off completely in 5-10 minutes.  So, for this type of system, the recommended cycles per hour is 2, or sometimes 3.  This is what is necessary to provide a stable room temperature, additionally, in this type of a system, it heats up so quickly that it takes less time to establish the distribution of steam when the boiler starts.