snap disk to cut off fan ?

End of burn draft fan shutdown is better done with flue temperature. Here's a link to a snap disk and timer solution, just search the page for 'snap':

https://www.hearth.com/talk/threads/1000-gallon-tank-piping-pictures.73343/#973718

It is simple and should work very well, but I would be nervous about running 120 VAC over to a snap disc on the boiler flue. It would be straightforward to run 24 VAC through the snap disc and use that signal to control a relay for the draft fan.

I'm putting together a digital temperature controller using a K-type thermocouple which will be programmed for on-off operation, low limit, and a hold or suppression function which will override the low limit on boiler startup until the flue temp rises above the low limit, then the low limit will be activated to turn off the draft fan when the wood load has burned down and flue temp (or other sensing location) falls below the low limit. This will be an addition to a Wood Gun E500, which requires manual shut-down of the draft fan. A timer was considered, but since the Wood Gun is operated by different staff members, the concern is that an operator may forget to set the timer for a sufficient burn time, and the timer will shut down the boiler before the wood load has burned out.

Last year I did what jebatty is doing. My sensor, that shuts the fan off at end of fire, stopped working. The sensor was no longer available. The sensor and/or a stock manual timer powered the control circuit of a cube relay. The relay latched power to the fan.

I was able to put a k-type thermocouple in the gasification chamber. It is interesting to see the temperature throughout the burn. However, the thermocouple needs replacing a year later due to what looks like too much heat. It wasn't melted. It was split. It is still working. I suspect it would last longer in the boiler exit or chimney pipe.

The thermocouple is cheap (1-7 dollars) and easily replaced annually if necessary. The PID digital temperature controller wasn't expensive. You can get them on ebay or auberins.com.

You can then set up the shut down at the temperature it works best.

I bread-boarded the control components this afternoon, and it works. I will provide more info and a circuit diagram soon.

You can use a simple reverse action A-stat wired ahead of your boiler A-stat. If temp is too low power would be cut to boiler A-stat and no fans. Of course you'd need to wire in a bypass to start boiler from a cold start. You are doing the same thing with the snap switch arrangement you are talking about. But either way you will have to bypass this for a cold start.

But either way you will have to bypass this for a cold start.

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True -- and my object is to make the bypass nearly automatic. Strategies I considered were/are:
1) On a cold start have a manual on-off switch to turn on the draft fan until the fan "off" thermocouple is at a temperature above the off setpoint, then turn off the manual switch. Issues: forget to turn on the bypass switch and fire fails to start or starts and smolders; forget to turn off the bypass switch and fan continues to run after fire is out; need to have a thermocouple temperature read-out to indicate turn off temperature has been exceeded to provide notice that the bypass switch can be turned off; boiler goes into idle and thermocouple temperature falls below the off setpoint producing a false fire out condition, draft fan shuts down, and boiler does not restart.
2) Same as (1) but the manual switch is timer switch which does not shut off until a sufficient time has gone by to assure a successful firing. Issues: same as (1) except no need for a person to stay around to turn off the switch; fire fails to start and switch turns off; boiler idles and fails to restart.

In the situation I am dealing with the boiler is fired/loaded by staff with variable levels of experience and understanding. The goal is to minimize the opportunity for operator error by requiring the operator to stay around while the fire buildsand then to turn off the bypass at the appropriate time or by requiring the operator to turn on a manual bypass timer switch to an appropriate length of time.

There probably is a more elegant solution, but what I now have is:
3) On cold start operator pushes a reset button and the control does a bypass function which turns on the draft fan even though the temperature is below the thermocouple "off" setpoint. Similarly, if the boiler is turned off because heat not needed, when the boiler again is turned on the bypass function is automatic.
4) As the boiler fires and the temperature rises above the setpoint the bypass automatically is "off" and the control turns off the draft fan only when the temperature again falls below the setpoint.

Issues yet to consider: 1) if fire fails to start, the draft fan will remain "on" -- consider an automatic timer that shuts the draft off if the "off" setpoint temperature is not reached within a fixed period of time; 2) boiler idles, temperature falls below the setpoint and boiler fails to restart -- need to have the temperature sensing point at a location and setpoint which better assures the fire is actually out (this issue is the same as with a manual or timer switch) without running the draft for an unnecessary period of time.

An another goal is to keep the cost down as low as reasonably possible and still achieve the objectives. Additional ideas are welcome.

Here is a diagram and picture of what I made to provide automatic draft fan operation for a wood boiler (in this case a Wood Gun E500) that is not equipped with a draft fan shutoff at end of burn. Operation is pretty straight forward, but the keys to making this work are a PID with a low limit function, an alarm function, and the alarm function allowing suppression.

Assume the boiler is firing and flue temperature is high. The K-type thermocouple is located in the flue or other area where it can sense falling temperature as the boiler wood load burns out. The DPDT relay is normally not energized, current flows through one set of contacts to turn "on" the draft fan [in my case to a contactor which turns "on" a 240V draft fan].

The PID is programmed to set the alarm at the low limit temperature (200F or so). When the temperature falls to 200F, the alarm relay closes, current is sent to the DPDT relay coil, which then disconnects and cuts off current to the draft fan [for me, the contactor], and the draft fan is "off." Boiler flue temperature continues to fall.

Without suppression, the PID would remain below low limit and the draft fan would remain off. This is where the OFF-RESET / ON switch and suppression come into play. The switch is turned off to cut power to the PID and then turned back on to power up the PID. Suppression operates to suppress or ignore the prior low limit and opens the alarm low limit relay. Opening that relay cuts current to the DPDT relay, the relay snaps back to its normal position, current flows to the draft fan [for me, the contactor] and the draft fan is "on." And the cycle repeats.

The OFF-RESET / ON switch and suppression provide another benefit. Assume the boiler went into idling, flue temperature fell, and the low limit was activated to turn off the draft fan. The boiler operator notices this happened, adjusts the wood load, and now wants to restart the boiler. All that needs to be done is to switch the OFF-RESET / ON switch off and on again, which then immediately will open the alarm low limit relay and restart the draft fan.

The light shown in the diagram is an indicator light which merely shows that the system is active. The PID also will be lit up when active, so the indicator light could be skipped.

In most case a contactor probably is not needed. I had a used one and thought it would be a good idea to use it to control the 240V draft fan on the Wood Gun. A typical draft fan is not a high current device and the DPDT relay alone likely could be used to operate the draft fan.

It's often difficult to get all the PID specs from web postings, and it took me quite a while to find a PID with specs showing an alarm with a suppression feature. I hope this might be helpful to some boiler users. PID Alarm Suppression

jebatty said:

In most case a contactor probably is not needed. I had a used one and thought it would be a good idea to use it to control the 240V draft fan on the Wood Gun. A typical draft fan is not a high current device and the DPDT relay alone likely could be used to operate the draft fan.

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Not sure if leaving one leg hot on the 220 VAC motor is permissible, I've never seen a motor contactor application where off doesn't mean all hot legs off, lockout/tagout notwithstanding.

Why is second pole of DPDT relay being used to interrupt the neutral leg of the contactor coil circuit?

What happens if the entire system loses power for a second, a minute, or two hours?

It looks like you have an auber(ins) controller. I can't tell if it is ssr or relay model.

Why not hook up your dpdt relay to pins 7 and 8 so the main output of your PID controls what it was designed to do. The green numbers will show your off temperature and your red numbers will show real time. The alarm can be adjusted to do the same thing, but is a low amperage internal relay on a made in china product, at least mine is a made in china product.. You have to push a button to see where the alarm is set, then it disappears. The main output is always displayed.

My uneducated opinion only, I don't think the alarm relay on your PID is stout enough to do anymore than turn a low power light on and off. The output relay on pins 7 and 8 I suspect are a little more durable.

It looks like you should use a cross between figures 5.2 and 5.4 in the auberins manual to get contactor control on 240v. I have the redundancy of the dpdt relay, like what you pictured, on my boiler. I have a ssr relay controlling a second cube relay's control circuit on 120v. The manual timer also controls the control circuit of the cube relay. The cube relay controls the draft fan. I don't have a contactor.

Once you set the off temperature, you need to find how much to set the hysteresis. Mine is set at 100 degrees. My boiler off temperature is set to shut off at 450 degrees. Once the fan is turned off at end of fire, the residual heat from firebrick might heat up the sensor 50-75 degrees. It might stay above the off temperature for 15 minutes. If you have a 450 degree off temperature and 100 degree hysteresis, that means on start up the PID wont turn on until 550. For my boiler and location of the sensor, that is about 45 seconds after the match is struck.

If you do a flue pipe sensor location that hysteresis as well as the on/off temperature I think would be set lower. Also I have the P set to off, the I set to off and the D set to off. So the temperature controller is in on/off function only.

Maybe the discussion at this point, on use of a PID controller, deserves its own thread. Based on the comments I have corrected the drawing as to the 240VAC circuit.

Not sure if leaving one leg hot on the 220 VAC motor is permissible

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My mistake in doing the drawing, the contactor switches both legs on the 240VAC circuit.

Why is second pole of DPDT relay being used to interrupt the neutral leg of the contactor coil circuit?

Click to expand...

I'm not sure what the intent of this question is or its import. Is there a problem from doing this?

What happens if the entire system loses power for a second, a minute, or two hours?

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A system loss of power for any length of time (including a loss of power only to the controller) would be the same as switching OFF-RESET / ON to "off" and then back to "on." On power-up of the controller the suppression feature on the alarm circuit opens the alarm low limit relay and restarts the draft fan, probably what is desired on a brief power outage, maybe not on a long outage, but then the operator would be aware of a major problem and probably has lots of problems to deal with, including restarting a fire in the boiler.

It looks like you have an auber(ins) controller. I can't tell if it is ssr or relay model.

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It is an Auber controller, relay model.

Why not hook up your dpdt relay to pins 7 and 8 so the main output of your PID controls what it was designed to do. The green numbers will show your off temperature and your red numbers will show real time. The alarm can be adjusted to do the same thing, but is a low amperage internal relay on a made in china product, at least mine is a made in china product.. You have to push a button to see where the alarm is set, then it disappears. The main output is always displayed.

Click to expand...

I thought about doing something like, because essentially all I'm using the PID for is to use the alarm circuit with suppression, which then operates like a limit switch with automatic reset on power-up (and on power failure/power restore). But it was the search for an automatic limit switch that led me to this PID controller. My thought on the low amperage internal relay is that it is only switching the coil on the DPDT relay, very low current draw. As to using pins 7 and 8, I don't think this provides for suppression of the low limit on power-up. Wouldn't the fan have to run on a cold start to get above the low limit? And it was automatic running the fan on cold start that was my primary goal. Do you see another way to do this?

... to get contactor control on 240v.

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The PID will operate on 240VAC. The contactor I had has a 120VAC coil, and it as well as the DPDT relay, also 120VAC coil, were salvaged parts I had collected and I wanted to use them rather than purchase 240VAC parts. The circuit diagram could be simplified with use of 240VAC parts.

If you have a 450 degree off temperature and 100 degree hysteresis, that means on start up the PID wont turn on until 550. For my boiler and location of the sensor, that is about 45 seconds after the match is struck.

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With the Wood Gun E500, there is little or no natural draft after the match is lit, so it is not possible to light a match and get the temperature up to a temperature to turn on the PID at 550F, for example. Do you see another way around this? My use of the alarm relay deals with this situation.

If you do a flue pipe sensor location that hysteresis as well as the on/off temperature I think would be set lower. Also I have the P set to off, the I set to off and the D set to off. So the temperature controller is in on/off function only.

Click to expand...

While not relevant for use of the alarm relay, I also have these set to off and use the PID for on/off only. As to location of the sensor and the "off" limit setting, I think I will need to experiment a bit to make sure it goes off at a temperature about at the point where the boiler is not providing any heat to the system, probably a temperature of 10-20F above maximum boiler hot water output. The Wood Gun has a high limit of 185F, so probably a draft fan "off" limit of about 200F. On my Tarm the high limit is 190-195F and I have the draft fan shut down at 210F, and the Tarm sensor is located in the smoke box before the exhaust gases enter the flue.

ewdudley said:

Why is second pole of DPDT relay being used to interrupt the neutral leg of the contactor coil circuit?

Click to expand...

jebatty said:

I'm not sure what the intent of this question is or its import. Is there a problem from doing this?

Click to expand...

Just can't see any purpose for routing neutral though the spare relay contacts and was curious if I was missing some subtle design goal.

jebatty said:

A system loss of power for any length of time (including a loss of power only to the controller) would be the same as switching OFF-RESET / ON to "off" and then back to "on." On power-up of the controller the suppression feature on the alarm circuit opens the alarm low limit relay and restarts the draft fan, probably what is desired on a brief power outage, maybe not on a long outage, but then the operator would be aware of a major problem and probably has lots of problems to deal with, including restarting a fire in the boiler.

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We can see what the effect of cycling power to the fan shutdown controller subsystem would be, I was wondering what the effect of cycling power to the whole system would be, to include the boiler and any components that supply input signals to the boiler.

The Wood Gun has pretty simple electrical-mechanical controls. The PID controller only will cycle power to the draft fan and will be inserted in the draft fan circuit immediately before the motor. Whatever controls the Wood Gun has shouldn't even notice that this control has been added.

jebatty said:

As to location of the sensor and the "off" limit setting, I think I will need to experiment a bit to make sure it goes off at a temperature about at the point where the boiler is not providing any heat to the system, probably a temperature of 10-20F above maximum boiler hot water output. The Wood Gun has a high limit of 185F, so probably a draft fan "off" limit of about 200F.

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The best part about end of burn cycle draft fan shutdown control is the ability to shut the fan off while there is some charcoal left to jump-start the next fire. With a K-type thermocouple protruding into the flue neck I've found that anywhere in the 250-300 degF range works well. At that point all there is left is red-glowing charcoal that does not smoke when draft is removed, and it re-lights with just a few seconds of propane flame or newspaper kindling.

jebatty said:

The Wood Gun has pretty simple electrical-mechanical controls. The PID controller only will cycle power to the draft fan and will be inserted in the draft fan circuit immediately before the motor. Whatever controls the Wood Gun has shouldn't even notice that this control has been added.

Click to expand...

No, what I'm asking is what would the actual behavior of the system be.

The best part about end of burn cycle draft fan shutdown control is the ability to shut the fan off while there is some charcoal left to jump-start the next fire.

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Very true and very beneficial to an easy boiler firing.

No, what I'm asking is what would the actual behavior of the system be.

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I'm not seeing what you are asking. How about an example or two to align my neurons?

ewdudley said:

No, what I'm asking is what would the actual behavior of the system be.

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In the schematic the power to the fan is being fed from the mains. So if power is cycled to the boiler does the boiler controller resume the burn cycle or does it reset to off? If the boiler is off and the draft fan defaults to on in your scheme, does the boiler controller still run the circulator or does the system start blowing off steam? If power cycles in June will the draft fan run for the rest of the summer? What would the actual behavior of the system be if power is cycled to the entire system in the middle of a burn cycle, in the middle of summer, or at any other time?

[Edit: The question was based on the schematic, which contradicts the design commentary.]

I think I see what you are saying when you say the commentary does not match the schematic, and I will try to correct that, if I understand the PID alarm relay suppression feature.

Assume the boiler is firing and flue temperature is high. The K-type thermocouple is located in the flue or other area where it can sense falling temperature as the boiler wood load burns out. The DPDT relay is normally not energized, current from the supply flows through one set of contacts to turn "on" the draft fan [in my case to a contactor which turns "on" a 240V draft fan].

The PID is programmed to set the alarm at the low limit temperature (200F or so). When the temperature falls to 200F, the alarm relay closes, current is sent to the DPDT relay coil, which then disconnects and cuts off current to the draft fan [for me, the contactor], and the draft fan is "off." Boiler flue temperature continues to fall. Boiler burns out wood load.

Without suppression, the PID would remain below low limit and the draft fan would remain off. Upon a reloading over remaining coals/embers, or upon the next cold boiler firing, boiler remains below the limit setting and the draft fan will not come on.

This is where the OFF-RESET / ON switch and suppression come into play. The switch is turned off to cut power to the PID and then turned back on to power up the PID. Suppression operates to suppress or ignore the prior low limit and opens the alarm low limit relay. Opening that relay cuts current to the DPDT relay, the relay snaps back to its normal position, current flows to the draft fan [for me, the contactor] and the draft fan is "on." When the low limit temperature is exceeded, the PID re-enables the low limit function. And the cycle repeats.

Are the commentary and schematic now consistent? I will address your questions in a follow-up post.

The two following diagrams show the relay/limit switches in the boiler/draft fan "on" scenario and the low limit/draft fan "off" scenario.

jebatty said:

Are the commentary and schematic now consistent?

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The schematic shows the fan contactor as being fed by mains L1 and L2, but the the design commentary says that the fan contactor is fed by T1 and T2 of a contactor in the boiler control. If wired as drawn there could be some problems, but if wired as implied by the discussion then the draft fan current is gated by the boiler controller and the system should work at least as well as designed by the boiler manufacturer.

(And again, I believe there is no such thing as switching the neutral leg.)

The Wood Gun has an elementary control system, namely when the boiler is fired or turned "on" the boiler control opens flue gas dampers in the firebox and then turns on the draft fan to purge combustible gases. The boiler may then be loaded with wood. There is an operating limit aquastat (set at 180F) which shuts off the draft fan when the boiler water reaches that temperature. There is a high limit aquastat (set at 200F) which shuts off the draft fan and allows the spring actuated flue dampers to close the air supply to the boiler to shut it down. Circulator on/off control is independent of the boiler and is on/off based on the demand of the system. A master switch is present to disconnect power from the circulator.

In the system in which the WG is operated (Deep Portage), a total of 7200 gallons of water is available for storage. The WG is always operated at full output and never idles. It is operated with a full load and reloaded so long as the storage temperature is less than the target storage temperature. The target storage temperature varies but normally is not greater than 165F, and at that temperature about 3000 gallons of water is available at a temperature of not greater than 165F (500,000 btu minimum available storage capacity to 185F assuming no system draw). If storage is above the target temperature, the WG is allowed to burn out. The WG supplies a primary loop from which the system makes demands. Following burn out, the WG circulator as originally and remains installed continues to operate until manually turned off by the master switch.

My schematics show the 120VAC and 240VAC supplies but do not show where they come from. In both cases the supply is between the WG controller and the draft fan. My controller is wholly dependent on power being available from the WG controller.

So if power is cycled to the boiler does the boiler controller resume the burn cycle or does it reset to off?

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Good question. I've never seen this happen during boiler operation. With all the risks of assuming, I will assume on power off that the draft fan shuts down and the spring actuated dampers close. On restart, if operating limit and high limit have not been exceeded, I will assume that when power is restored, the dampers open and the draft fan restarts. And I need to verify the assumptions.

If the boiler is off and the draft fan defaults to on in your scheme, does the boiler controller still run the circulator or does the system start blowing off steam?

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As mentioned, the circulator operates independently of the boiler and is always "on" so long as power is available and the master switch is "on." The DP system always has capacity to absorb the btu content of a full wood load in the WG. If power is available, there should never be a steam event. And on power out, WG is designed to kill the fire by closing the flue dampers.

My draft fan control is between the boiler controller and the draft fan, so if the boiler controller would turn the draft fan "on," my controller which defaults to "on" also would turn the draft fan "on."

If power cycles in June will the draft fan run for the rest of the summer?

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This one is easier. My controller is between the WG controller and the draft fan. In June the WG would be shut down, thereby also cutting power to my controller completely. But if not, by turning the on/off switch on my controller to "off," power is cut to my controller and the draft fan also will be off.

What would the actual behavior of the system be if power is cycled to the entire system in the middle of a burn cycle ....

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An excellent question to answer and one which I will pursue along with the chief boiler operator at DP. I will suggest shutting down the system (power outage event) during a full burn to see what happens, and then resume power and see what happens. To the best of my knowledge this has never occurred since the WG (and Garn) were installed in 2009. Time to find out.

I don't know the controls on the wood gun.

For my situation I only put a PID in place of a stock sensor that shuts down the fan at the end of fire.
So the manual timer in my case, or if you got one, is turned on to start the fire. The timer will make the fan run, no matter what, if you turn the timer to any number. After the temperature gets above the set point (plus hysteresis) the fan is on auto with the PID, regardless of the timer.

It looks like you are wiring the middle relay to be normally closed?

Just looking at the manual for the auber relay PID, it looks like you could get rid of that middle relay. The PID would control your conactor on the output relay.

On edit, looking back at your posts, my boiler wont turn off the fan at a high temperature limit. Yours looks to cycle?

What boiler do you have? Are you sure you don't have a shutdown at high limit? If not, how is your boiler protected against excessive high temperature and steam blow-off through a vent or release of a temperature/pressure relief valve?

I'm not using the PID regular relay for low limit shutdown but the alarm relay which is NO. The goals of this project were to provide automatic shutdown at end of burn and no manual timer required at the beginning of the burn. The suppression feature of the alarm relay allows start-up without a manual timer.

The WG is designed to cycle, but manufacturer's assurances notwithstanding, cycling is not a pleasant experience with the E500. Plus, so far I have not been able to be convinced that cycling is a desirable or an efficient way to operate a wood boiler. The pleasant problem with the E500 is that it is a beast and easily puts out its full 500,000 btuh output continuously (100 lbs of wood per hour). After two burn seasons of experience the E500 cycling, Deep Portage installed a 4000 gallon storage tank just to serve the WG (plus the 3200 gallons of the Garn can be available as well), and with that the beast was tamed, and now the WG never cycles and always burns full-out as I believe it should.

I have a dumont. It is a close relative to hobbyheater's Jetstream.

I have a manual reset over-temperature aqua-stat for over heat protection. There also is the 30 psi pop-off valve, and a low water cut-off.

The electricity has to go through the over-temp aqua-stat and the low water cutoff before it available to either the manual timer or the PID. There is also an off switch. All of that triggers a relay to close and turn the fan on.

I was just thinking you are doing it different than me, not that I'm an expert. Mine is working so good. I see that some of these other boilers cycle. It gets up to, say 180 degrees, and idles. Mine wont idle and come back on by itself. Storage is required.

The edit above is hind site after posting. I'm offering advice based on my system and I don't know yours. But your trying to do the same thing I did. I have looked at the manual for your PID and assumed you would to do the regular output relay (pins 7 &8) for 240 volt and a contactor. I think your loose some of the functions of the pid on the alarm circuit, but we are both using it for on/off and not the full proportional-integral-derivative part.

I haven't figured out your alarm suppression feature.

I just got parts in to do a PID controller for the pump. Right now my system requires too much babysitting. The pump originally was set up to run when the fan ran. It would come to a simmer when the fan and pump shuts off and the hot firebrick will keep heating the water in the boiler. I have had the pump run until I turn it off manually. If I don't turn it off at the right time, the stratification is lost.

Have you thought of a differential of the flue temp - water temp. This is more accurate for storage systems , garn incorporates this in the new controller. A diff temp insures the inducer shuts off when the flue temp is x deg above water temp regardless, of wood load or demand. A dual input thermocouple panel meter with a math function and relay will work. Also type k in a low o2 environment will shorten it's life, a ceramic sheath will help keep out of fire contact