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Boiler Controls

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by rreihart, Nov 7, 2007.

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  1. rreihart

    rreihart New Member

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    I think I'm pretty much set on a piping and controls plan. After I posted my original piping diagram, I kept finding problems with it. I think as attached, it finally looks sound. Another big concern has been how do I add a wood boiler and heat storage without adding a bunch of expensive to my existing Tekmar tn4 control. I want to use the wood boiler when its firing and use the storage when its temperature is sufficient and only use the oil boiler as a last choice. I think a couple relays as shown in the other attachment will do what I need.

    For anyone patient enough to follow the diagrams and the logic description that follows, I'd be interested in any input you might have or any problems you see.

    When Wood Boiler fires, Wood Pump is on, which closes coil on R1.
    A heat demand will power Primary Loop Pump. This will power R1-6, which directs wood boiler water to the primary loop through a 3-way valve.
    No heat demand means no power to R1-6, which will direct boiler water to storage tank through same 3-way.

    When no Wood Boiler fire, Wood Pump is off, which opens coil on R1.
    A heat demand will power Primary Loop Pump. This will power R1-5, which will initially power Storage Tank Pump through R2-5. Also at this time the Tekmar 422 will be sensing the temperature of the storage tank through R2-4.
    If 422 sees low temp it will close boiler contacts to fire oil boiler. This will close the coil on R2, which will power the oil boiler pump, R2-6 and switch the 422 temp sensor to the primary loop, R2-3.

    Thanks, Rob.

    Attached Files:

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  2. Nofossil

    Nofossil Moderator Emeritus

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    I don't have any experience with the Tekmar controller - how does it know that there's a wood fire? How do you set temperature thresholds? I get concerned that the system might have ambiguous states or situations where it cycles back and forth between states based on small fluctuations in temperature.
  3. rreihart

    rreihart New Member

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    Thanks for looking Nofossil.

    I'm by no means an expert, but I'll try to give a brief description of the Tekmar 422 control as I understand it. Basically when there's a demand for heat (in my case it could be DHW, cast iron baseboard zones, or radiant zones) the 422 sets a target temperature based on outdoor reset. This demand turns on the corresponding zone pump and the primary loop pump. If the supply water sensor sees that the temperature is lower than the target temp it will fire the oil boiler. The control has a lot more to it, like controlling the radiant mix temp and set back times etc..., but I don't think anything else is relevant to firing the boiler.

    The 422 wouldn't actually know when there's a fire. However, when there is a fire the wood circulator pump would be powered. I would use this circulator voltage to switch relay R1. This would prevent the oil boiler from being able to fire since it would not have a 120V supply.

    When the fire dies down the circulator would turn off and R1 would lose power to the coil so it would switch back to the NC contacts. This would allow power to the oil boiler when necessary. The second relay would basically choose between the storage tank and the oil boiler. If the storage tank was warmer than the 422 target temp then the storage tank pump would be used. If the storage tank was too cool, the 422 would fire the oil boiler, which would close the coil of R2 and turn off the storage pump and turn on the oil boiler pump.

    I hear what you're saying about ambiguous states or situations. I'm trying to think of possible scenarios and how this set up would respond and right now I'm not seeing any problems. But I could certainly be overlooking something and that's why I'm posting this. To see what others see and think it through a more thoroughly.

    Thanks again and by all means, keep looking.
  4. termite

    termite New Member

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    rreihart,
    I have the same wood boiler. I noticed your schematic (assuming it is a front view of the boilers) shows the return on the left side of the Biomax. I was told it was the short pipe on the right. Do you have reason to believe it is the way you have it shown? I noticed there is what I think is a thermistor (or maybe it's the pressure transducer) near the short pipe on the right which would indicate it is propbably the output like you have it shown. I'm getting ready to plumb mine and want to be sure I'm doing the right thing.
  5. rreihart

    rreihart New Member

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    Termite,

    I'm not trying to accurately depict the supply and return locations. I'm just showing what lines connect and in what order.
  6. Nofossil

    Nofossil Moderator Emeritus

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    My apologies - I'm buried with real work today. Haven't had a chance to really study this. I'm assuming that the Tekmar has built-in hysteresis or 'deadband' - that is, if it changes state based on temperature, the temperature would have to change by more than a miniscule amount before it would change back again. First pass, looks like this would work. I'll study it more when I have a few minutes.
  7. rreihart

    rreihart New Member

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    No apoligies necessary, I understand the real work thing. I'm not sure about built-in hysteresis, but I don't think the control is messed up by minor temperature changes.

    I did contact tekmar and one of their reps was pretty helpful, but the solution he came up with involved a boiler stager and a couple setpoint controls. I'd probably have $1000 in them. The plan I put together almost seems to simple but I don't see a problem with it.
  8. Nofossil

    Nofossil Moderator Emeritus

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    Seems like no one wants to stick their necks out on this. I'm printing copies so that I can color in pressure and suction areas in each mode to make sure that water doesn't want to go in the wrong places. Does each of the pumps have an integral check valve?
  9. Nofossil

    Nofossil Moderator Emeritus

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    More questions - what is the function of the three-way valve labeled 'low temp'? Don't see a control signal for it, and I don't see when it would open horizontally.

    In the Tekmar, what is the effect of applying power to R1 or R2 (terminals 7 and 2)? Does that enable the other relays to operate?

    I have to wrap my head around the whole concept of switching the sensors, but dinner beckons.....
  10. rreihart

    rreihart New Member

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    The pumps will be Taco 007 with integral flow checks.

    Your right, I did not address the low temp 3-way valve. This would be to protect either boiler from low temp return water. Actually, I'm still not sure if I'll use this set up or a circulator. Either way I'm thinking I would place an aquastat in the return line just before the boilers. I think this would be independent of the other controls.

    Terminals 2 and 7 are for the relay coil. When a 120V circuit is completed across 2 and 7 the NO contacts are closed (8 connects to 6 and 1 connects to 3). This would also break connections 8-5 and 1-4. When 2 and 7 are not a 120V circuit, connections 8-5 and 1-4 are made (8-6 and 1-3 are broken). If you already knew that then I'm not sure I understand the question.

    Thanks again for taking the time to look.
  11. Nofossil

    Nofossil Moderator Emeritus

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    Got it - R1 and R2 are each a pair of SPDT relays with a common coil. I think my only question is this: If the 422 is using the sensor input to fire the oil burner, what happens to it when the sensor input is switched to the primary loop? Will the oil shut off as soon as the primary loop temp is above the setpoint?

    For low temp protection, I just used a zone valve that connects the outlet to the inlet just upstrem of the wood boiler pump. It has a small orifice (about 5/16") and that seems about right.

    Here's my block diagram:

    Attached Files:

  12. rreihart

    rreihart New Member

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    In the current setup (oil boiler only) the sensor is attatched to the primary loop close to the oil boiler. The control uses a target temp and a differential (maybe 10 degrees) to fire the boiler. When the loop temp is less than the target temp minus 1/2 the differential the boiler will fire. When the temp rises to the target plus 1/2 the differential the boiler shuts off. I can choose a fixed differential or use an auto differential where the control would try to minimize short cycling.

    When the wood fire would die, the initial state would have the 422 sensing the storage tank temp through terminal 4 on R2. When the tank cools below the target temp the 422 will fire the boiler. At this point I will no longer want to monitor the temp of the storage tank, as it will not be changing. I will want to return to the setup I currently have to run the oil boiler. So the oil would shut off at the setpoint plus 1/2 the differential.

    For the low temp do you think I could place a zone valve just to the left of where I'm showing the 3-way low temp? I guess with a smaller zone valve like a 3/4", the main piping is 1 1/4", much of the flow would continue to the loop but some would pass through the valve to raise the return temp. Would I be correct to place an aquastat in the return line just before the boilers to control this zone valve? I'm not very familar with zone valves and aquastats so help me out here. I think zone valves are generally 24V and some aquastats have built in 24V transformers, correct?

    Thanks, Rob.
  13. Nofossil

    Nofossil Moderator Emeritus

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    Very elegant - I understand. The differential is effectively what I had called hysteresis or deadband earlier.

    Makes sense.

    Attached Files:

  14. rreihart

    rreihart New Member

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    My thought on zone valve placement is that if I put it to the left of the 3-way low temp (I'll actually use a tee there instead of a 3-way), then I will be able to protect either the wood or the oil boiler.

    It sounds and looks like you do not use an aquastat. Are you sensing the return temp with a thermocouple and powering the zone valve with your controller?

    I think the limited participation is probably due to the fact that it takes some time and effort to think through and try to figure out what's going on. I'm not sure I'd want to if it wasn't mine.
  15. Eric Johnson

    Eric Johnson Mod Emeritus

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    I'm in the middle of getting my tank piped (or re-piped) for the new hx, and that's enough confusion for me at the moment. Nofossil seems a lot more comfortable with this stuff than I am, so I don't want to create any confusion by making observations and suggestions that might not make sense. In any event, it sounds to me like you're getting good advice, R.
  16. Nofossil

    Nofossil Moderator Emeritus

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    I don't use an aquastat. Since I have made the decision to have a full-blown controller anyway, it was easy and cheap for me to have the controller manage the valve. I use the recirc valve to keep the inlet temp above the condensation point as well as to keep the outlet temp high enough to be useful when the fire is dying down.

    I actually use thermistors rather than thermocouples because they're less expensive and require less interface electronics. I do use a thermocouple for measuring combustion temperature.

    As far as I know, oil boilers generally don't have inlet temperature protection, perhaps because the HX is usually cast iron and less sensitive to rust. Perhaps it's also because oil boilers have more consistent and higher combustion temperatures - I really don't know.
  17. Eric Johnson

    Eric Johnson Mod Emeritus

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    I think it's a consideration on any boiler, even cast iron, but more so with wood gasifiers for some reason.
  18. rreihart

    rreihart New Member

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    I have a 3 way valve with a 24V actuator that I would like to use to direct the wood boiler flow to the system or to the storage. It just struck me that the valve is meant to be proportional controlled with a 2-10V signal. I don't want to use it proportionally in a mixing application. I want it to be fully in one direction or fully in the 90 degree direction. Any ideas on how I can generate a 10V signal to actuate the valve? As described previously in the thread, I'll use a relay to send the signal to the actuator. But I don't know how to generate the 10V signal.
  19. Nofossil

    Nofossil Moderator Emeritus

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    I don't have the specs, but it's likely that a 12v signal wouldn't hurt it. If that's the case, lots of the little plug-in 'bricks' that power computer accessories, cell phones, etc. put out 12 volts. Any self-respecting geek (like me, for instance) has accumulated a box of them for just this sort of application. You'd have to know how much current the valve draws, but it's probably not much. If you have a make/model for the valve, I'd be glad to see if I can dig up a reference on-line and check it out.
  20. rreihart

    rreihart New Member

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  21. Nofossil

    Nofossil Moderator Emeritus

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    This is a very cool device - I wish I had me a couple. Looks like the actuation power is the 24V power, and the 0-10V is just a position signal. It has a 100kohm input impedance, so it's drawing no power at all from the 10V input. To be safe, I'd get me a 12V brick and a potentiometer in the 10k-50k range - Radio Shack still carries stuff like that. Connect the end terminals of the pot across the 12V supply, and adjust the pot until the center terminal reads 10V. Connect either 10V or ground to the actuator as so:

    [​IMG]
  22. rreihart

    rreihart New Member

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    By 12V brick I guess you mean the transformer block that plugs in to a 120 outlet and powers speakers, battery chargers, etc... I see the ones that power our cordless phone bases are 9V, so I could possibly find one that is 10V already? I'll have to check all the extra cords that I've kept and had no idea why. So all along I've been a self respecting geek and didn't even know it.

    Assuming I have a cord to use, I can clip off the plug on the end, strip back the wires and connect them to the valve to test it?

    One more simpleton power question. To power the actuator for a test, can I use an outdoor lighting type 24V transformer?

    Thanks.
  23. Nofossil

    Nofossil Moderator Emeritus

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    Yup.
    Yes, but they are notoriously innacurate., often producing 50% above their rated voltage. Measure a 9V unit and see what you get - it may be good enough.

    These projects tend to bring out the inner geek in many of us. Side benefit: you can now justify hanging onto assorted cr*p if such justification has previously been an issue ;-)

    I'd test with a voltmeter first, before even cutting the wires.

    Almost certainly yes - as long as it provides 200ma of current, you're good to go. I expect a lighting transformer provides an order of magnitude more power than you need. A doorbell transformer may be a better long-term solution.
  24. rreihart

    rreihart New Member

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    Thanks for the help. The valve works. I did have one set back with the 24V outdoor lighting transformer. I couldn't figure out why it would only output 12V until I look at the label and discovered all 3 units were 12V outputs. I thought I might be stumped for lack of a 24V transformer but the aquastat from the old boiler had one integrated. So it wasn't a pretty setup, but it did prove the point. The 9V brick actually read 11.8V but it still stopped a little short of 90 degrees of rotation. I don't think that will be a problem.
  25. Eric Johnson

    Eric Johnson Mod Emeritus

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    I'm glad to see some practical solutions to problems and questions. Thanks to everyone who contributes to this resource.
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