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setback thermostats and Harman Pellet stoves

Post in 'The Pellet Mill - Pellet and Multifuel Stoves' started by HarryBack, May 11, 2007.

  1. cantman

    cantman Member

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    I installed my Harman XXV with a Honeywell 7400 thermostat and everything works great.
    My only wish was that I had purchased the Honeywell 7500 because it has seven day programming.

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  2. Jim H.

    Jim H. Feeling the Heat

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    I too just set mine up as well.....took a few tries, seems as though the wires going to the t stat matter which way you put them. the other thing that slowed me down was the Rh and Rc connections......I have no Rh so assumed it was the R.....works like a charm now! time will tell as it gets cold how well it works.

    jim
  3. slink

    slink New Member

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    I usually recommend 3.5 to 4 for the feed adjuster rate. Lower settings take longer to get up to temo and higher settings can over feed the firepot as the stove tries to ramp up to temp.
  4. mkmh

    mkmh New Member

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    Southern, Maine
    Nice job Jim! It took me a while to get mine working properly too. If I recall, I fumbled over some of the same stuff that you did. Frustrating when your going through it, but very satisfying when it all comes together.
  5. Three3

    Three3 New Member

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    Hi. This is my first post here. I'm quite a regular (and a member of the moderator crew) at a similar forum for Subaru Forester owners, so I've got a good idea of the amount of helpful assistance and great knowledge that can be found at places like this. I'm also an engineer (retired, but once an engineer, always an engineer!), so I'm not afraid to dig into things, even things that are still under warranty! (And I've already searched around to see what people might have already posted on the subject of this thread!)

    OK, on with my question. I just got a Harman Accentra pellet stove insert. So far, so good, but I'm already starting to think of modifications, and the first one that comes to mind is nighttime setback. I'm not keen on thermostat approaches that actually turn the stove on or off, for many of the reasons already cited here. My thoughts are more in the direction of fooling the temperature sensor or altering the temperature setting. Three possibilities come to mind:

    - If indeed the temperature sensor is a thermistor as described in the parts list, then it should be possible to fool it by putting a resistor in series (to make it read low) or in parallel (to make it read high). The resistor could then be taken in or out of the circuit with a timer-controlled switch or relay.

    - Similar addition of a resistor at the stove's temperature control could alter the setting under control of a timer.

    - A non-intrusive technique would be this one offered by the X-10 remote control people , which employs a small heating unit to artificially raise the temperature in the vicinity of the sensor, fooling the unit into thinking that the room is already hot enough. My initial impression of this approach is that it might be harder to predict and control its actual impact on the sensor.

    Has anybody looked into any of these or other similar approaches? If so, what did you find in terms of resistance values, results, and the like?
  6. 603BOB

    603BOB Member

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    The thermistor requires about 3K ohms in parallel to drop back 6 or 7 degrees.

    I used a 5K Bourns trimpot and a time clock - set the pot for the set-back temp desired, with the stove set for normal temp desired.

    Works slick, and no blinking lights.
  7. Three3

    Three3 New Member

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    Great! Thanks. I've already got all the parts in my junkbox.
  8. 603BOB

    603BOB Member

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    The stove will work normally - if it is on Auto, it will go off and on.

    If it is on Manual - it will go to maintenace burn.

    I set the trimpot by watching what the temp dropped to on setback - then left it alone - that was three years ago !

    My Advance is 7 years old - still on the original ignitor.

    P.S. I have a Forester - just REPAIRED the knock sensor.
  9. Three3

    Three3 New Member

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    Come visit us at www.subaruforester.org!
  10. GVA

    GVA Minister of Fire

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    Sounds like a great plan, I am only concerned about the klinker build up as they seem to build up more easily when in maintenance burn rather than full on burn.
  11. jimx24

    jimx24 New Member

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    Southern NE
    I have an Accentra Pellet Stove Insert and have been following with great interest the topic of adding some kind of setback thermostat. I like the concept of fooling the temperature sensor or altering the temperature setting as described by three3 and bob603. For those of us who are not heavy in circuit design/construction could you guys give a little more detail on what components you used (ie trimpot, clock/timer, PCB board etc), and how they were incorporated into the sensor probe circuit. A schematic of the circuit would be very helpful. Thanks in advance for any additional information you can provide.
  12. 603BOB

    603BOB Member

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    I used a clock thermostat from Home depot - one that runs on 2 "AA" batteries.
    One terminal of the thermostat goes to one wire of the sensor connected to the back of the stove. The sensor remains connected as it was. (I used some flat blade "Y" connectors from a auto parts store to allow 2 connections to the same point)
    The other terminal of the thermostat goes through a 50K ohm trim pot to the other sensor wire at the back of the stove. The trim pot is connected as a rheostat - the center arm and one end only.(I mounted the trim pot in the back of the thermostat )
    This results in the trim pot being connected in parallel with the sensor when the contacts close.

    The themostat is then programmed BACKWARDS - i.e. - program it for 90 degrees when you want the LOWER setting - 50 degrees when you want the HIGHER setting. DO NOT program it for the night setback desired !! It is only acting as an ON-OFF switch.

    Set the desired setback by adjusting the trim pot with the clock contacts CLOSED. This wil be a trial and error setting - start at about 30000 ohms.
    Wire size is not critical - contact types and ratings are not critical - any device that will open and close a contact at the desired time will work fine. It is only a switch that places another resistance in PARALLEL with the thermistor sensor. DO NOT use a time clock designed to turn a lamp off and on - they provide 120 volt output. Use only "dry" contacts - like a thermostat.

    Theory: The thermistor sensor has a negative temperature coefficient - when the temperature goes UP - the resistance goes DOWN.
    When you parallel resistors the result is a lower resistance.
    The stove sees the lower resistance as high room temperature, and reduces the fire to lower the temperature.
    The stove will maintain the lower temperature until the clock contacts open, which removes the shunt resistor, restoring operation to normal. (The stove setting will now determine room
    temperature.)
    Note: The setback temperature will remain a constant difference when the stove setting is changed. If you adjust it to drop back 8 degrees - for example - at 70 degrees normal setting - it will drop 8
    degrees if you raise the stove setting to 77 degrees.

    The clock can be mounted in any convenient location - it is not sensing temperature. It can even be on the floor. I hung mine from a hole on the rear of the stove with a small "s" hook.

    CAUTION; These connections go directly to static sensitve components on the control board. Static electricity can cause damage, and is a big problem in the winter. The stove should only be operated from a properly wired and grounded outlet. Touch the stove to discharge any static build-up before connecting and or adjusting anything involving the control circuit !!!


    Set it and forget it - change the batteries once a year is the only thing i have to do - it is on the fourth year now with zero problems.
  13. Three3

    Three3 New Member

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    Dang, you're fast! And you've changed your recommendations of resistance values from 3k to 30k. I haven't done the mod yet, but the measurements I've been making on my thermistor do tend to suggest a value higher than 3k.

    I just finished drafting my response offline. In the face of yours, I'm going to go back and tweak it a bit. I shall return!
  14. Three3

    Three3 New Member

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    603BOB has already covered the theory of what's going on here. My implementation, which isn't yet up and running, is a bit different from his, but they both do the same thing.

    Circuitry is simple. Start with a trimpot as suggested by 603BOB (a small variable resistor, about $3 on eBay including S&H) and set it somewhere in the middle of its range.

    Then you need to open up the sensor cable. If you don't mind leaving evidence, you can just cut it at a convenient location. You'll find two wires inside. Alternatively, you can pull the insert out of the fireplace, locate the twisted pair of blue wires that connects to the sensor cable, and connect a new two-wire cable in place of the old one, running it out of the stove at the same place where the original cable was located. This could be a physical challenge, but it lets you return everything to stock in the event of warranty repair. Use appropriate crimp connectors and insulate well. Be sure that your wiring doesn't touch anything hot.

    Now all you have to do is connect the cut wires back together the way they used to be (or marry the new cable up with the old one if you did it that way), and connect the trimpot between the two wires through a switch or relay controlled by a timer. So when the switch or relay is open, it's just like the trimpot isn't there, and things will behave normally. But when it closes, the trimpot is connected in parallel with the sensor, this lowers the resistance, this fools the stove into thinking that it's gotten too hot, and the stove cools down till it gets to the point where the resistance is what it's looking for.

    My initial plan was to start with a cheap timer (Harbor Freight 40148, $5.99 but often on sale cheaper in their stores), open it up, separate the switch from the electricity that powers the timer, and then use the switch to connect the trimpot into the circuit. Unfortunately the connection between the switch and the AC was buried deep inside the mechanism and I just couldn't get to it from the outside. Disassembling it farther (in the interest of science) resulted in a mass of gears on the floor (and then in the wastebasket).

    This would still be a great approach if you could find a timer whose switch is easy to get at and to disconnect from the AC. But I decided to move to Plan B, which isn't quite completed yet. I've ordered a small relay on eBay for about $3.50 (including S&H) from the seller named loisn14rxh. She deals in sewing goods and electrical stuff! She carries lots of relays, most at that same price. Just be sure that you get one with a 120 volt AC coil. Current rating for the contacts doesn't matter in this application.

    Then it will be a simple matter of mounting the relay in a regular electrical box, cutting off an extension cord and wiring it to the relay coil, and connecting the trimpot's center contact and one of its end contacts across the two sensor wires via the relay's Normally Open ('NO') contacts. Use crimp connectors if you need to preserve the stock wiring, or just solder everything directly. Be sure that everything is well insulated. Use shrink tubing on all connections. You are mixing 120 volts AC and low voltage, sensitive control circuitry in the same box, so be careful!

    Then plug the cord into a timer like the above mentioned one from Harbor Freight, set the timer, and you're in business. Play with the trimpot for final adjustment of the setback.

    [​IMG]
  15. 603BOB

    603BOB Member

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    The 5K and 3K were typo's - SORRY.
    I get confused when I type with BOTH fingers !!
  16. Amaralluis

    Amaralluis Member

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    177
    The only concern that I have is the wear and tear on the ignitors? I am using a programmable thermostat right now that basically will turn the stove on twice a day. Its not cold enough to have the stove on manual and burn one pound an hour.
  17. jimx24

    jimx24 New Member

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    Southern NE
    Hey three3 and 603bob thanks for the detailed explanations of your trimpot circuits.
  18. Three3

    Three3 New Member

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    I've got my nighttime setback device up and running and am happy with it. After I sketched out the basic design (shown in an earlier post above), I realized that I could add a bit of versatility to it by including a switch (SPDT Center Off) that would allow me to:

    Operate it in the timed setback mode, or

    Take it out of the circuit completely so that the stove operates normally, or

    Run it fulltime at the setback temperature (like when I'm away all day).


    Here are the resulting diagrams of the device and of the entire setup.

    [​IMG]

    [​IMG]

    And here are a couple of pictures of the finished product. If you're really sharp-eyed you can see that the actual wiring deviated just a tiny bit from that in the diagram, though they are electrically equivalent. I drew the diagram for ease of comprehension, and wired it for ease of construction.


    [​IMG]

    [​IMG]


    Quickly reviewing the design, in normal usage the stove is controlled by a thermistor, which is a resistor whose value changes with temperature, and in the opposite direction from the temperature change. What this device does is place a resistor (I used a trimpot, which is simply a small, variable resistor) in parallel with the thermistor, under control of either the relay or the switch. This lowers the resistance seen by the stove and fools it into believing that the room has suddenly gotten warmer. So it cranks its heat back through its normal processes. The resistance of the thermistor increases as the room temperature drops, until the combined parallel resistance reaches the value the stove is set to expect as normal. Then the stove does its regular thing to keep the temperature at about that level. Taking the resistor out of the circuit returns the thermistor to sole command, and the stove heats the room back up. The stove never actually shuts itself off or exhibits any error indication; it just goes through its regular adjustment processes, though the adjustments it makes are greater than those it would usually encounter in normal operation.

    Before connecting this to the stove I used my multimeter to set the trimpot to 30 kOhms as recommended by 603BOB. I haven't run this long enough to get a reliable estimate of the actual setback that this accomplishes, but it seems to be a bit under ten degrees. Theory says it should be in that ballpark when room temperature is around the normal comfort range. I might tweak it a little bit, but I saw no reason to make the adjustment accessible from outside the box. It will probably stay untouched forever after I've finished tweaking. I think you could easily substitute a fixed resistor of about 30 kOhms and get quite satisfactory results. And save a dollar or two. A resistor of less than 30 kOhms will produce a greater setback; a greater resistance will produce a smaller one. Wattage of the trimpot (or fixed resistor) is not critical. This is a very low current situation, so most any wattage will work fine.

    If you do build one of these and want to use a trimpot but don't have a meter to set it with, be sure to adjust the trimpot to somewhere around the middle of its range before plugging it in. (Or buy a meter at Harbor Freight for $3.99. Everybody should have one!) You wouldn't want to take a chance on the setting being so low that it damages the control circuitry in the stove. And adjust it in small steps, never letting it get down too close to zero. My trimpot requires many turns to go through its full range, but I've seen others that have a very coarse adjustment and could easily get set too low.

    The trimpot and the relay are secured in place with mounting foam. Nothing moves around inside.

    There are lots of ways this could be connected into the sensor cable. I chose to use 3.5mm audio jacks and plugs since I already had them in stock and they presented a neat appearance. I made a short adapter cable to connect the box to the existing crimp terminals on the sensor cable, since I wanted to leave it unmodified in the event of warranty repairs. And I made up my own cable to connect the box back into the stove: crimp connectors on the stove end and audio plug on the other. Polarity doesn't matter; either way is fine. I would have done something like that anyway, because I wanted to mount the sensor farther away from the stove than its own cable would permit. If ease of disconnecting isn't important to you, you could simply run one pair of wires out of the box and connect them to the existing cable. It's not really necessary to run the sensor circuit through the box as I did.

    Do be careful to ground yourself to discharge any static electricity (especially common during these winter months) before touching any part of the device that connects back into the stove. You don't want to take a chance on zapping anything in there. And keep the AC and the sensor circuitry well separated from each other. Note the shrink tubing on the relay terminals, where the two are closest to each other.

    I'm thinking about putting in a pilot light that will tell me when the box is getting AC from the timer, if I can find one small enough to fit and that's capable of still providing the necessary safe separation of the AC from the sensor circuitry.

    I'll confess to one embarrassment. The timer I used is one I had already set up for a day/night program, so I just left it as is. That is, until I realized that the settings were backwards, and the stove was giving out regular heat when it should have dropped down to low. That was an easy fix.

    As always, measure, test, be careful, keep the AC well insulated and well separated from the control circuitry, know what you're doing, and proceed at your own risk. Neither 603BOB nor I can guarantee that your stove is configured the same way that ours are or that something along the way won't go horribly wrong.
  19. from France

    from France New Member

    Joined:
    Feb 26, 2009
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    Loc:
    France
    Hi,

    Here are my drawings, with on top : Stop the stove, and at the bottom : Go to "Eco temp" setpoint
    http://bayimg.com/oAiMnAaCp

    Picture of my "Eco temp" device :
    http://bayimg.com/CajBhaACc

    Potentiometer has been tuned for a 4°C decrease (20°C and 16°C temperature setpoints) :
    keeps an acceptable temperature during working hours and nights,
    allows a temperature rise of approx 2 hours,
    program from 16°C to 20°C setpoint at 5:00, for a 7:00 breakfast,
    program from 20°C to 16°C setpoint at 8:00, when leaving to work, etc.
    one program per day of the week...

    Tune of the potentiometer (50% of 100kOhm at the beginning) is easy using Control Led and the Setpoint Pot of the stove...

    I can put Manual-ON or Manual-OFF on the clock to stay on one of the setpoints,
    or simply unplug the relay to go to Normal temp (Harman) setpoint : easier for wife and kids !

    !!! Be very CAREFUL using these information !!!
  20. Three3

    Three3 New Member

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    Oui, à vos risques et perils!
  21. craigsta1

    craigsta1 New Member

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    western Ma
    I bought a Harman Accentra Insert last fall and hooked it up to a programable thermostat as described. It worked flawlessly for the entire heating season. I just fired it up for the first time this year and something strange is happening ... with the stove in room temp mode and on auto it should shut down when the thermostat open the circuit (stops calling for heat). I get the 4 blink status, so I know the thermostat is working, but the stove goes into maintainance mode and will not shut down. I even tried turning the stove control to the off position AND it still continued to feed pellets and burn at the maintainance mode. I finally turned the thermostat back up which eliminated the 4 blink status and the stove finally shut off ( the stove control knob was still in the off position).

    Has anyone else had any issues like this? Any ideas how to fix it or is something wrong with the control board?
  22. KGR1951

    KGR1951 New Member

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    I have a somewhat different need. My stove is in a vacation home and I want to control it remotely. During the winter, I keep the primary heat on at about 40-45 degrees, and use the Harmon Advance to bring the heat up to normal when I'm going there. It's a small house, so the Harmon can get it up to 68 degrees pretty fast. Basically I want to be able to turn it on before I get there so the house will have warmed up.

    I currently have the stove plugged into a X10 module and use the X10 telephone interface to turn it on. Crude but it works. The problem is that the X10 module can go to the "ON" condition if there has been a power outage when the power comes back on. Not a good situation, it's a flaw in X10 that's been around forever. Shutting it off would mean doing so without the benefit of a controlled shut down.

    My thought is to put an X10 controller on the temmperature sensor line. X10 makes a dry contact module that is nothing more than a controllable relay. So based on what I've read here, I could put a variable resistor in series with the sensor to raise the resistance to a level where the stove would not turn on. Then I would place the relay in parallel with the resistor to create a short circuit around the resistor when I want the stove to go on. This would also allow me to shut it down cleanly via the remote, and no confusing the controller by opening up the circuit.
    Does that sound reasonable?
  23. kcellwood

    kcellwood Member

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    Central Maine
    BOB603, thanks for the info on the thermostat setback. I just installed a P68 this year and was looking for a way to turn back the stove automatically. I ended up setting up a system like you described and it works great. I bought a Honeywell 7 day programmable to use as the clock. I could not get a trimmer locally so ended up ordering one. In the mean time I installed a 33k resistor in the thermostat. Not sure yet how much the setback is with the 33k. On the thermostat there is a HEAT/OFF/COOL switch. If I switch it to OFF it takes the resistor out of the circuit and runs off the thermistor. Great little feature. Again thanks for the info.
  24. Three3

    Three3 New Member

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    Sorry for late response. I haven't been here for a while.

    The resistor goes in parallel with the sensor to lower the resistance to a level where the stove would not turn on. The resistance of thermistors goes in the opposite direction of temperature, and you want the stove to think that it's gotten much too hot.

    But wouldn't you experience the same X-10 spurious turn on problem, one way or another?

    There was a hack going around many years ago about disabling the specified performance of the X-10 device turning itself on when its load was turned on by its own switch. It involved cutting a pin (7?) on the IC. I wonder whether this would also inhibit its tendency to come on when power is restored.
  25. mdoggm1

    mdoggm1 New Member

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    Oregon
    Can anyone provide a copy of that Harman tech bulletin that describes how to install this setback thermostat?
    The one provided earlier at oatstoves.com is not valid any more and I can't find the blasted thing anywhere.

    I've wired in the thermostat with the probe, guessing as to what "in series" means and which wire goes on what terminal.

    The stove will turn on when the thermostat switches on. When the thermostat clicks 'off', I get the 4-blink status light indicating an open circuit. If the stove is off at the time, this is good and the stove won't fire up. But if the stove is running when the thermostat clicks 'off', the stove appears to keep on running.
    I know it takes some time for the stove to shut off. But if my thermostat is 'off' why is the stove auger still cranking and feeding pellets to the fire?

    Matt

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