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Harman Pellet Stoves "Room Temp" Sensor combined with a Setback Thermostat?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by moog5, Aug 24, 2006.

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  1. moog5

    moog5 Member

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    It wasn't until a few weeks ago that I saw the thread about putting a set back thermostat in the loop with the room temp sensor. Sounds logical.

    In my case I leave the room temp at about 72. At night, if I remember I turn it down to about 65, then turn it backup to 72 when I get up. If I were to put a set back thermostat in the loop (in series) with the room temp sensor and I would probably turn the set back to about 63 because it would likely take too long to heat the house up from 55 to 72 (55 being a common setback used with central heat to really conserve).

    I can undestand the logic for making sure the thermostat is set to be considerably higher than 72 (when I want it to hold the room temp at 72). When the set back kicks in to 63F (the series circuit is "open"), it will be just like disconnecting the room sensor (until the thermostat cools to 63F). Per the previous thread (I haven't verified this), disconnecting the room temp will essentially shut the stove down. I am a little concerned that connecting and disconnecting this sensor (i.e what the thermostat essentially does) may somehow harm the electronics in the stove. If this isn't a problem for the electronics, why hasn't Harman included this as an option? There has to be a reason, anybody have any ideas?

    All I can figure is;
    1) opening and closing the room temp sensor circuit may cause havoc with the electronics in the stove.
    2) they may be worried about issues with the thermostat and room sensor not being in the same location. If they design the setback thermostat and the room temp sensor to be in the same spot, it would obviously make it more difficult to market the ability to hide the room sensor.
    3) the average Joe (me) may have problems running their stove because the setting the set back thermostat further complicates operation of the system. I can see my self getting up in the middle of the night wanting some heat during a midnight movie and turning "room temp" up so the stove will kick on (even though it was already set at 72), then being puzzled because I forgot about the hitting the "setback override".

    I would like to try this setback thermostat, but I don't want to chance screwing my stove up. I would likely mount the set back thermostat behind the stove (out of view) and leave the room temp sensor in it's present position (just across the room). Do any Harman techies or sales people know if this "repetitive opening and closing" of the "room temp sensor" circuit will hurt the stove?

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  2. pelletheat

    pelletheat New Member

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    Is there anyone on here who disagrees with the theory of temperature setback to save fuel? I would rather keep the room & house at a constant temperature, reason being, if the room cools down to 62 overnight / it will take much longer to recover the heat lost with a stove (space heater). The stove will work much harder to raise the room temperature and everything in the room (furniture etc.) up 10 degrees and run with a higher feed rate / lbs per hour of pellets to raise the room temperature then if the stove was running / idling (lower feed rate) to maintain the room temperature of 72 degrees all night. Do you really conserve fuel?

    Any thoughts :question:
  3. Shane

    Shane Minister of Fire

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    I always run my stove on high. You can set the thermostat back too far but I don't think 10 degrees is too much and probably does save you fuel.
  4. HarryBack

    HarryBack New Member

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    I as well keep mine at a constant temp. I figure that it'll take more heat to reheat the far reaches of the house after I get home rather than keep them toasty with constant heat.
  5. Retired_Ted

    Retired_Ted New Member

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    It's my understanding that the Harman room sensor is actually a thermister whose resistance varies with the temperature. It isn't an actual thermostat which would be an on-off device. Therefore, I doubt if a thermostat in series will work since the Harman circuit is continually looking for the thermister's resistance. As for using a set back thermostat - as long as one isn't using a heat pump, it should save some $$.
  6. moog5

    moog5 Member

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    I think the question of "if a setback thermostat saves energy" has already been answered, at least with homes who use it with a central heating system. With a central heating system, people who set back the temperature also have to reheat the same items as a space heater.

    I think it is a good question with respect to a pellet stove (space heater). When a pellet stove cranks up (hi pellet feed rate), it seems to me that the efficiency is going to go down (more BTU's per lb of pellets goes up the chimney). The slower you burn the fuel, the longer the available time you have to extract the heat from the burn. The distribution blower blows the same cfm on high pellet feed rate as it does on low feed rate. The more air you can blow against the heat exchanger, the more heat you will extract.

    The question to be posed may be what the breakeven setback degrees setting would be to burn the same amount of pellets with or without a setback thermostat (all dependent on size of house, inslulation, setback hours, etc.).
  7. moog5

    moog5 Member

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    Some people apparently are using the themostat wired in series with the room sensor (the thermister). The only appreciable change in resistance with the thermostat wired in series would be to make the circuit a "closed circuit" (no change in resistance), or make the circuit an "open circuit" infinite resistance (the same as if you were to completely disconnect the room temp (thermister) from the stove.

    for example:-------------thermostat (open or closed)---------thermister


    room senor ___________/ ________________________////____________
    connections _____________________________________________________)

    With the switch open, aka setback mode (as shown above), "in the Room Temp" mode, the stove shuts down (so they say, I haven't tried it yet, it was 93F here today).
  8. pelletheat

    pelletheat New Member

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    Good response Pete,

    I do agree with limited setback on central heat. You are also correct in regards to more BTU out the pipe when a pellet stove is running on a high, the Harman combustion blower is a variable speed blower. As the temperature demands increase the need for more fuel (larger fire), the speed of the combustion blower will also increase, causing air to move more rapidly across the heat exchangers.

    The room sensor (thermeister) is a benefit that other manufactures ( I think) do not currently offer. It meters fuel every 60 seconds based on the "room temperature setpoint" on the control panel to constantly maintain your room temperature, only feeding pellets to maintain. A thermostat will create greater peaks & valleys //// as far as overall comfort.
  9. Shane

    Shane Minister of Fire

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    I don't think the efficiency goes down. The faster it's burning the more complete the combustion. All heat that can be extracted is extracted when you have close to 99% combustion efficiency. The key is heat transfer and with a properly designed exchanger system, which I'm sure the majority of pellet stoves have, a very high percentage of heat is transfered.
    Harmans control system is unique, but one thing that makes me question it's superiority is that they're the only ones that use it.
  10. Hammerjoe

    Hammerjoe Member

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    Two excelent points that I am having dificulty coming up with a counter argument to any of them.
    There might be 99% combustion efficiency with higher pellet feeding, but because it also means it burns faster, how efficient is the ehat echanger recovering the heat from that fast buring fire?

    I think both are in a sense correct. The only factor that determines efficieny imo on these stoves, is how efficient is the fan blower extracting heat from the heat exchanger!?

    To me it makes more sense that the longer the fire lasts then the more time there is to extract the heat without necessarily having to burn more fuel.
    Which means that the optimal setting is to have the blower fan at max speed to extract max heat from the heat exchanger and the feed rate as minimal as possible, to allow the pellets to burn more completely and to give time to extract the heat from the fire.

    I all confused. :)

    As for the set back, I think there are advantages to it, the question is how far back is the optimal setting? 5 degrees? 10 degrees?
  11. Shane

    Shane Minister of Fire

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    That's why there are paths for the smoke to go through. Most stoves have flue gas residence time down to a fine art. The exchanger systems incorporated with the smoke pathway ensure adequate residence time to extract the heat. So to me the hotter it burns the more complete the burn so long as the stove is of good design and ensures proper residence time for effective heat transfer it seems to me that burning on high would be the way to go.
  12. pelletheat

    pelletheat New Member

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    Combustion efficiency & Heating efficiency are totally different.

    % of fuel burned / all pellet stoves should have a high (99%) combustion efficiency if they are a quality stove.

    exhaust temperatures increase the higher the BTU's / most pellet stoves are rated in the high 70's to low 80's as far as heating efficiency. To attain those numbers the stoves are on low burn with minimal feed of fuel.
  13. Shane

    Shane Minister of Fire

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    In my stove on low and many many other stoves that I work on the combustion efficiency is lower when the stove is set to the lower settings. This is evidenced by an increased build up of soot on the window & exchanger tubes as well as in the burn pot. That in effect would lower overall efficiency right? How much I don't know I've never scientifically measured. Do you have test results/procedures showing lower efficiency on high? If so I will retract my statement and chalk it up to faulty perception, but from my personal experience I feel like I get better efficiency while burning on high. Maybe it levals out between the two?
  14. Hammerjoe

    Hammerjoe Member

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    Is that with the fan blower on low or the feed rate on low or both?
  15. Hammerjoe

    Hammerjoe Member

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    Now I understand that the best setting for the fan blower is to keep it on max and let the stove decide how to operate it.

    I am unsure about the feed rate thou.

    On my Harman Accentra I have it set at 4.
    I understand what it means is that every 60seconda the stove feeds pellets to the burn pot for whatever # seconds. And that is what the fee rate knob does, change the time that the feeder is on every 60 seconds.

    Now besides at shut down the stove override that setting or it is pre set?

    Either way why not lower the setting to a 3 or even a 2?
    Wouldnt that achieve total combustion of the pellets in the burn pot before fresh new ones arrive?
  16. smirnov3

    smirnov3 Feeling the Heat

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    As I understand it, on the Harmans, the Feedrate knob sets _maximum_ feedrate (ie when stove is going full blast). It is user set-able to adjust for diffent grades of pellets.

    And AFAIK, you can set it down to 3 or 2. Just make sure that the stove doesn't run dry (not certain the best way to check that)
  17. smirnov3

    smirnov3 Feeling the Heat

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    Maybe.

    But as I understand it, the stove sets the combustion blower speed based on settings, so it may just be that at lower (ie slower combustion fan) settings, the fine ash is not getting blown out the chimney, and is instead being deposited on the window & other surfaces
  18. Shane

    Shane Minister of Fire

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    Theres a difference between the ash which is grayish brown and the soot which is black and obviously unspent fuel.
  19. Shane

    Shane Minister of Fire

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    I called the tech department and they said that my stove gets 82% overall efficiency whether I burn it on high or low. It's the same either way.
  20. MountainStoveGuy

    MountainStoveGuy New Member

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    Ash on windows has nothing to do with efficiency, it has to do with the air wash, the harder the stove is pushing out flue gas, the harder the air wash is working. I have never seen any pellet stove on low not have a sooty, ash coverd glass.
  21. stovemanken

    stovemanken New Member

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    Methinks we are far off the original question.

    There have been numerous posts, some of them mine, some from HarryBack concerning the Feed Rate knob on a Harman. You WILL NOT save any fuel by turning down the feed rate! In ROOM TEMP mode, the control board will automatically decrease the feed rate to match the thermal loss in the room. The feed rate should be set at the maximum that the pellet quality, the venting configuration and the cleanliness of the stove will permit.

    I tell customers if they see a hot pellet roll off the front of the pot, either the stove needs to be cleaned or the feed rate is too high. (Usually the former)

    The owners manual, which every stove owners reads religiously, outlines the correct procedure for setting the maximum feed rate.

    Back to the topic, thermister technology is used in many cutting edge heating appliances, including high efficiency furnaces. It's expensive, which is why cheaper stoves don't use them.

    It you must use a programmable, it can be done, although Harman will tell you that it will void your warranty.

    SMK
  22. Retired_Ted

    Retired_Ted New Member

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    Just wondering why thermisters are used. Whereas a thermostat is simply a switch (I know, an oversimplification) , can we assume the use of a thermister produces data (in ohms) that a computerized circuit can utilize to better advantage? Although I don't own my Accentra as yet, I am quite interested in learning how the thing works. What parts of the stove are actually regulated by the thermister? Feed rate, ......?
  23. stovemanken

    stovemanken New Member

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    A thermister is a thermal resister, working on the principle that the lower the temperature, the lower the resistance. (Absolute zero = no electrical resistance).

    Actually, Harmans have two thermisters, the ESP and the Room Temp sensor.

    In ROOM TEMP mode, the thermister is reading changes in the room temperature to control ALL stove functions: Distribution air volume, pellet feed rate and combustion air volume. Additionally, the board reads the previous 7-minute trends in the room to anticipate what might be happening, i.e. sun warming the room, doors left open while unloading groceries and the like.

    The goal is to maintain a room temperature plus or minus 1 degree of the customer's selected set point. By not overshooting the set point, you avoid burning pellets that it would obviously take to make the room 3 degrees hotter than the set point. 3 degree variations are fairly typical in most thermostats.

    The thermister is capable of reading 1/4 degree variations in the room's temperature, so it is very sensitive. Location is important to avoid direct sunlight, windows, drafts, etc. With that being said, 1/2 of all owners leave them coiled up behind the stove, where they work just fine, although the stove will be set at a lower temperature because it's colder at the rear of the stove.

    It's very nice technology and works very effectively.

    SMK
  24. Retired_Ted

    Retired_Ted New Member

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    Very nice explanation - makes a lot of sense - installation is set for next week. First time I can remember being anxious for colder weather. :)
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