Homebrew controller design...

I have been rolling an idea around since seeing Rob’s timer thread. I have an old verti stoker controller from a parts stoker I picked up that is inoperable at this point. I have not been able to reverse engineer the board parts to troubleshoot it and a schematic is not available per AHS Legacy tech support. The replacement controller is $400+ and being a tinker-er & cheapskate I started thinking how I could make my own. I thought I might need the functionality of a small plc but I think using a couple relays and timers I can get the same functionality for a spare homebrew controller for the two stokers I run.

It is a pretty basic control for the older mag stokers like you are all use to seeing. A call for heat from the ‘stat turns on the stoker motor, combustion fan and distribution blower. When the ‘stat is satisfied the stoker and comb fan turn off and the distribution blower runs for a user set time from that point (8 min in my case). If the ‘stat does not call for heat then a timer circuit kicks in to hold the fire and the stoker motor, combustion fan and distribution blower turn on for a user set time (2 min in my case), then the stoker motor and combustion fan turn off for a user set time (12 min in my case), while the distribution blower stays on for a user set time (the same timer as used in the ‘stat scenario).

After a few iterations of recognizing issues and correcting them I came up with the sketch below. I think this covers it but wanted input on what issues I am missing. I believe parts would total about $80. The only downside I see at this point is the timer circuit would run immediately after the ‘stat is satisfied rather than waiting for the off time before turning on the stoker and combustion fan. If I understood Rob’s thread correctly maybe that could be addressed using the ‘off’ cycle initially for the stoker/comb fan timer. Any other issues I might be missing or am I way off track?

Wouldn't it be simpler to just use an ordinary "snap switch" for controlling the distribution blower?
It is supplied hot 120v for 24/7, switches when hot enough and stays on until cool enough all by itself.

That would lighten up the overall requirements to be only the feeder and comb. fan...and they both run in tandem, so only one real switching load...

It definitely would be simpler to use a snap switch for the distribution blower, i like that idea. One less relay and one less timer.

thinking - what if the system thinks that distribution blower motor is running -> but the blower is not actually moving air...and the stoker is feeding and the thermostat screaming for heat...

Suggest considering case of overheating as a result of some sort of electro-mechanical failure of air flow...motor has dropped out, pulley loose, belt broken...or a VFD failure if one is being used to control fan speed / hard starting.

simply using the 'snap' switch to shut off the stoker may be OK..however I prefer the idea that if there has been an overheat, the cause of the problem is most likely not going to go away on its own -> so lock out the thermostat with a latching relay and sound some sort of alarm to get your attention.

otherwise the snap switch will just go in / out / in / out until you realize there is a problem....

It would be nice to alarm on a blower failure. I wouldn't be worried about the stoker 'overheating' the stove is beefy enough to handle the heat without the blower running. But it would be nice to be warned about the lack of heat circulation.

Likewise it would be nice to know if the stoker motor is running but not making heat since that would mean an outfire for some reason and the hopper is being emptied into the ash pan.

wow - your stove can handle the stoker running full tilt without taking the heat away by air circulation?

I would be afraid of warpage or damage of something related to my Keystoker (since it happened to my Reading - warped the furnace case and the doors no longer sealed...and I got CO leaks

Lack of circulation can be warned by one of these 'spring loaded flow flags'... or a duct pressure switch as commonly seen in gas furnaces...combustion blower comes on, draught is sensed THEN the igniter activates and then the gas valve opens...3 failures and the ignition system locks out until power cycled.

https://www.dwyer-inst.com/Product/AirQuality/Air ... s/Model530

but you have to put them on a time delay since there is a time between when 'blower on' and 'flow sensed' do not overlap and trigger the alarm unnecessarily.

no-heat (fire out) is easy to measure with a thermocouple or a fireye type of sensor (probably over-kill)

all of this runs through my PLC in one fell swoop...if the combustion chamber gets below 100 degrees or if there is no flow sensed in the ducting...everything is knocked off.

crazysteamer wrote: ↑

Sun. Jan. 05, 2020 1:50 pm

wow - your stove can handle the stoker running full tilt without taking the heat away by air circulation?

Yes, it's not a Keystoker or a Reading, it's 1/4" plate steel.

titleist1 wrote: ↑

Thu. Jan. 02, 2020 12:32 pm

I have an old verti stoker controller from a parts stoker I picked up that is inoperable at this point.

That looks a lot more complicated than it needs to be. Back when I was running a Mag Stoker I came up with a simple plan if the complicated Harman controller failed.

Distribution blower; In a normal hot air furnace this blower is operates independently through a fan switch. The fan switch simply closes a set of contacts when it senses heat. A 120 volt snap switch would work too.

Combustion blower and stoker motor; My plan was to just run them together. One relay to control them both, activated on the 24 volt side by a timer and the thermostat.

If you connect it this way, a call for heat will activate the stoker/combustion fan. Then after the stove gains heat, the distribution blower will turn on. At the end of the heat call, the stoker/combustion fan turns off and once the stove cools down, the distribution blower turns off.

If you use that $30 timer that Rob is using, you can configure it so the timer resets at the end of a heat call.
Cheap Solid State Timer

-Don

StokerDon:

perhaps you are missing the point - of the difference between 'power to the fan' and 'air is circulating as a result of power to the fan'. Will the Keystoker handle such a situation? I don't know and honestly would prefer to not find out...hence the simple air-flow switch as a backup.

My background is industrial controls and designing in / around the consequences of potential points of failure of anything in the ladder.

Since I have my circulation fan running from a VFD, I am conscious of the added potential points of failure...the system can be saying 'air on' but somewhere in the chain...there is the possibility of failure.

is my control system more complex than it needs to be?

Probably..but then I have always been a bit nervous about the lack of backups with coal systems...and adding the redundancy is only a couple of extra lines of code in the PLC connected to industry standard components.

I honestly think that the commercial coal furnace control systems offered are a bit simpler than they should be with little or no redundancy where there should be....always covered by the 'be sure the system is installed by a competent professional installer.'

Yes they work and for-the-most-part they are safe enough for a system where there is someone handy to look at them periodically...but my experience with the Reading unit overheating and causing a CO leak is exactly a point to consider.

much of my concern is driven by the fact that I can be away from home for weeks at a time and my wife is left with dealing contingencies that others may not have to worry about. (like when is the ash pan full?')...

if nothing else..I like to present the concept of 'thinking outside of the conventional box'....

crazysteamer wrote: ↑

Sun. Jan. 05, 2020 2:55 pm

Will the Keystoker handle such a situation?

Again, this is not a Keystoker. Please try to stay on topic. The Harman is designed as a radiant heating stove. The occasional 85,000 BTU that the stoker puts out won't hurt it one bit.

crazysteamer wrote: ↑

Sun. Jan. 05, 2020 2:55 pm

if nothing else..I like to present the concept of 'thinking outside of the conventional box'....

I very much like the PLC set up that you have come up with to control your stove. It might not be practical for everyone though. The plan that I laid out above is simple, effective and inexpensive. I think that Titleist1 is trying to come up with a simple alternative to the Harman control box.

-Don

ah...sorry...completely missed the point that titleist1 is working on a STOVE and I am envisioning a Furnace.

that makes it a horse of a completely different color and many of my concerns do not apply.

StokerDon has it covered...the Harman is built to withstand the heat with no distribution fan running, it is not a concern at all. The sketch i posted was trying to replicate the timer control functions of the stock Harman controller but as SD and Macgeiver indicated there is an easier way by using a temp switch for the distribution fan. I'd like to find one that is variable with a setpoint in the 300 degree ballpark.

there are lots of industrial choices in that range...question is $$$

$50...?? + thermocouple

https://www.thermaldevices.com/product/novus-n103 ... gKqGPD_BwE