A coal stoker used for year-round hot water generation requires a way to turn on for short periods to prevent the fire from going out during periods of low demand (that is, roughly from mid-spring to mid-fall in NEPA latitudes).
Traditionally this is done with a one hour “pinwheel” motor based timer. Pins are added to one or more positions around the wheel perimeter, and are used to close a Microswitch. The minimum on time is the time it takes for the pin to move off of the Microswitch, and, depending on the manufacturer, is between 30 seconds and one minute.
The entire pinwheel turns once per hour. To turn on the burner twice an hour two pins are added, for instance, at the 12 o'clock, and 6 o'clock position of the wheel.
This circuit is a better replacement for a pinwheel timer in at least two respects:
1). It monitors when the burner is turned on due to demand, and restarts timing after the demand call has finished. This cuts down on the number of 'useless' burner start-ups (i.e. - those occurring right after a demand call).
2). On time and dwell time setpoints are both variable making it possible to “fine-tune” the system. In my case the Honeywell pinwheel timer originally installed had a minimum on time of 40 seconds, and set up for running every 30 minutes. I found through experimentation it can be reduced to 23 seconds in winter, and 28 seconds in summer. Hacking off 24 seconds every hour (40 seconds - 28 seconds, times two) doesn't sound like much, but does add up over the course of a year, and, in my case, enough to pay for itself in about 3 years.
A Siemens Logo! controller (usually used in industrial, and HVAC equipment, but also suited for general home automation tasks) is programmed with the attached logic.
Timer B003 is the dwell time between burn cycles, and timer B004 is the run time. If input I1 (connected to the 'hot' side of a 120V burner motor) senses no voltage then the NOT block B001 is turned on. This feeds the B003 dwell timer, and B002 AND block.
If the timer counts up to 30 minutes then AND block B002 turns on, and sets the B005 flip-flop, which, in turn, powers output relay Q1, and turns on the motor. Once the flip-flop is set the only thing that will turn it off is when timer B004 finishes.
If input I1 sees the burner motor turned on for a demand call NOT block B001 turns off, and clears the B003 dwell timer.
This may sound like over-kill, but a suitable Logo! can be picked up on eBay for anywhere from $50 to $100, and that at least puts it in the running cost-wise as a 1 hour timer replacement.
Even the original model 6ED1 052-1FB00-0BA0
(120V AC supply, (6) 120V inputs, and (4) 10A relay outputs) will work with the attached logic, although the newest version (6ED1 052-1FB00-0BA5
) offers quite a number of additional functions and capabilities. This circuit and logic uses only one input, and one output, and thus leaves free the possibility for doing other automation as may be useful.
For instance, this model has a RTC (real time clock), and can be programmed to turn devices on and off based on time of day.
A Logo! controller can be programmed from the front panel, and, although this is somewhat clumsy, especially for larger programs, it is easy enough to do, and eliminates the cost for the software and special programming cable.
Go to Siemens Logo! Home Page
to get more information.