Coalpail.com Forum

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.

I just noticed your 2006 posting recommending a LOGO timer.

Your idea predates my same idea by 6 years! Cool!

People would have saved a lot of coal and boiler wear and tear over the years if they listened to you then!

vermontday wrote:People would have saved a lot of coal and boiler wear and tear over the years if they listened to you then!

I am one of the stone-age guys using a pinwheel-type timer. Have you done a short study to compare the number of on/off cycles and daily coal consumption (or just run time) with a pinwheel timer vs. the LOGO unit?

e.g. One week with a fixed 30 minute cycle, and another week with the LOGO determining when to cycle.

It would be tough to quantify.

It would vary by heating season. The more natural calls for the stoker, the less the need for an out fire timed stoke, and the more you would save.

While a percentage of the heat generated during an outfire stoke does go into the boiler water, it probably does not do it as efficiently as during a longer natural stoker call.

Then there are the summer times when the boiler water is near the high temperature limit after a natural call, and the outfire timer drives it into the high temperature setting.

vermontday wrote:It would vary by heating season. The more natural calls for the stoker, the less the need for an out fire timed stoke, and the more you would save.

At first I thought the same thing, but then I thought that if there were more heat calls, the boiler would also sit idle less and be more likely to use the heat stored from the timer cycles. More timer cycles = hotter boiler = reduced run time on next heat call?

I experimented with no timer at all during the winter and didn't notice much difference in daily run time, but I also didn't closely monitor the outside weather during that time. Summer DHW is a more predicable load, for me at least.

Edit: It probably seems that I am negative towards this "smart" timer...that is not the case, just stating that my timer experiments in the past haven't worked out like I expected and I wondered if you or someone else had a different experience.

I can do the study you suggested. The LOGO already displays totalized stoker run time. I could set the LOGO for a outfire call time of 999 minutes, so it does not call and turn on my Will-burt timer for a week. Then have the LOGO do it the next week.

Give me a few weeks to do it.

The following are the results just heating domestic water with the EFM.

Willburt stoker minutes per day 130, 112, 128, 144, 130, 121, 131 Avg 128
Logo stoker minutes per day 106, 116, 121, 133, 101, 118, 113 Avg 115.4

Difference 9%

This difference is surprising considering how seldom the EFM has a natural stoker call in comparison to hold fire stoker calls in the none heating season.

The more natural stoker calls there are, the more you would expect to save by not doing unnecessary hold fire stoker calls, so you would expect an even larger savings in the heating season.

However, this test did not have a lot of sample points, and even just domestic water heating load varies day by day.

It is starting to get colder here in Vermont. My family is starting to ask why I have not turned up the thermostats, so I am afraid the domestic water only test will have to continue in the spring.

I will try to do a Willburt versus Logo test during the heating season by trying to normalize the stoker run time data with heating degree days.