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Update to fan control on my Harman P38

Post in 'The Pellet Mill - Pellet and Multifuel Stoves' started by MCPO, May 19, 2008.

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  1. MCPO

    MCPO Minister of Fire

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    In two other related threads I talked about the distribution blower fan cycling on/off , mostly off , when I set the stove on the lowest setting. I figured the stove is plenty hot enough that the blower should be circulating that heat even on the lowest settings but the fan wouldn`t run for long so I added/installed a separate manual speed control @ $13 plus a double pole double throw toggle switch @ $7 so I could return it to it`s original factory mode of operation at the flick of the switch.
    It`s working absolutely perfect doing what it should be doing .

    OK, here`s what it looks like: http://www.pbase.com/johnd1/image/104829389.jpg

    BTW, the stove was running at the pictured feed rate setting , slightly less than #1 and blowing constant heat @130 degrees coming directly out of the stove.
    34 hrs is the most I can get from a bag. However 34 hrs doesn`t hardly equate to the literature that says .75 lbs per hr at low speed. I`m burning more like 1.12 lbs per hr. Like most other competitive manufacturers , Harman must exaggerate their burn rate claims.

    John

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  2. JDenyer232

    JDenyer232 Member

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    They really are not exagerating their feed rate claims, feed rates can be variable based on density, and actual size of the pellets being fed. There can be a range of feed rates using different brands of pellets. Most brochures note this fact and state that actual feed rates, and btus delivered will vary based on density of the pellet, quality of the pellet, and size of the pellet. By the way, nice looking job with the switch.
  3. MCPO

    MCPO Minister of Fire

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    OK I can see how certain pellets would vary the burn rate. So far I`ve tried 2 premium brands , "green supreme" and "dry creek" and they were about equal.
    Thanks, John
  4. Wet1

    Wet1 Minister of Fire

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    John,

    I'm currently trying to see how long I can run on one bag of pellets. I agree with your comments regarding not having the blower kick on while at this lowest setting. It looks like the link to your wiring diagram appears to be dead... could you repost it so I can look over what you did and see if this is something I might want to consider doing myself?

    Thanks,
    Scot
  5. MCPO

    MCPO Minister of Fire

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    I don`t recall posting a wiring schematic but I did post the side of the stove with the two new additional manual controls mounted. There was little interest last spring but I can tell you it works great and I`m not having to burn extra pellets to get a larger fire just to keep the distr fan running these fall evenings and mornings.
    Give me a few minutes and I`ll draw something up and post it for you.
    Of course you must pull the blk/wh (distr fan) leads off the control board to start with .This is actually the only change to the board. (no other mods required)
  6. MCPO

    MCPO Minister of Fire

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  7. Wet1

    Wet1 Minister of Fire

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    Thanks for the info John (as well as the PM), that's exactly what I was looking for! I have a TON of things I need to get done over the next couple of months, but I'll probably do these mods this winter as it would be nice to have the blower functioning at the lower setting(s). If it's not a problem, please leave your pictures posted for a while...

    BTW, I'm getting well over 34 hours out of a bag of pellets with my stove set at its lowest setting. I'll let you know shortly what the final outcome is...
  8. Wet1

    Wet1 Minister of Fire

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    John,

    It looks like I got about 47 hours out of one 40# bag of pellets with the stove on the lowest setting (turning the burner control pot. until the stove just turns on)... This works out to .85 lbs/hr., I think the factory claims .75 lbs/hr at the lowest setting. Not too far off on this stove, but I can't imagine even less heat output would be of much real world benefit. Even at .85 lbs/hr, it's just barely enough to keep the place warm on a cool day.

    I might try this again, but it will have to be some other time as the house is currently a little cooler than I'd like to have it.

    I can tell you there's a world of difference between the P38 and my Englander pellet stove when it comes to running at low output... the Harman does it w/o any issue at all, the glass is even remaining clean. The Englander just makes clunkers and covers the interior in heavy layer of blackness at these low outputs...
  9. MCPO

    MCPO Minister of Fire

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    Thats great that you are getting 47 hrs from a bag. I`m gonna retime mine now since I turned the pot down . Maybe I`ll get more burn time from a bag too but as you already know there isn`t much heat at less than a lb per hour.
  10. Wet1

    Wet1 Minister of Fire

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    John,

    I let my P38 run at the lowest setting again, this time I ran it nonstop. I got 50 hours out of this last bag. Surprisingly, this has been enough to keep ~950 sq ft at 57* to 63* F over the last couple of days (with a fan effectively circulating the air) and the outside temps have been a low of 32* and a high of 48*. I'm beginning to think there might not be much to gain by rewiring the stove blower to run at these low feed rates, my guess is it would probably run up the electric bill more so than adding much heat to the living area. The stove seems to do a half way decent job of radiating the heat as it is, but I should mention that air is being moved across its surface due to my unique circulation fan setup that I'm using (see link below). I know with the wood stove it feels a lot warmer with the warm air blowing out from the blower while it's at a low idle, but it doesn't seem to heat the room much more, although it clearly does when the fire is ripping.
    http://www.hearth.com/econtent/index.php/forums/viewthread/18734/

    I'm absolutely thrilled to be able to stretch the pellets out this much and amazed the stove is holding this living area at these temps with no other source of heat. I'm starting to think I'll be able to cover a large portion of our heating needs (in this part of the house anyway) with fewer pellets than I initially thought I'd need. I had the Englander in the basement (the Harmon is on the 1st floor) and not only could I not get it to run at this low of a feed rate, but it required a lot more pellet usage to heat the first floor to this same level of warmth... Having the P38 and placing it in the actual living area is obviously much more efficient than the previous configuration, but I guess that should come as no surprise.


    So what's your take on running the blower at low heat output levels?
  11. MCPO

    MCPO Minister of Fire

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    Sounds excellent!
    I`m only getting 36 hrs from a bag but I am heating (900sq ft) which is most of the finished basement from 63* to 74*
    with some of that heat moving up the stairs and/or thru the floor vent fan when running. I do believe that running the stove fan on low continuously does indeed extract more heat because the heat exchanger was designed just for that purpose. I don`t see much radiant heat being given off with the stove set on #1 . If your stove is getting that hot you are not utilizing it`s best potential and that is to extract heat directly off the tubes using the integral fan system.
    I can understand your excitement getting 48-50 hrs from a bag but if you do the math you will see that the BTU`s derived from .85 lbs per hr is the reason you are only getting a 6* differential and I`m getting 12*. Come colder temps we will no doubt see those figures decrease . There`s just so much heat in a lb of pellets period.
    I think one the best benefits/advantage of the Harman P38 is this wonderful and useful ability to burn so low and so clean for lengthy periods as we are doing in this intermediate weather.(spring/fall seasons). And for that sole reason why I felt the need to modify my Harman to allow the fan to run continuously on the #1 setting.
    Good luck.
  12. Wet1

    Wet1 Minister of Fire

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    I'm sure the reason you're seeing your temps 10 to 15 degrees warmer is because you're burning more pellets/hour, but also because your insulated basement is is not exposed to the colder outside temps my first floor is exposed to (my finished basement is still in the mid to upper 50's right now with no heat).

    You're undoubtedly correct that running the blower will pull more heat out through the heat exchanger... I just wonder if it's enough to offset the increase in electrical usage. I can tell you my stove doesn't feel like it's giving off much radiant heat, but it's obviously transferring a decent amount of heat based on the interior temps. I suspect the fact that the stove is placed in the path of moving air significantly impacts this. I guess I'll have to rewire my stove one of these days (when I get a chance) and see how much difference it makes. I'd also like to stick a meter on that blower and find out what it's pulling on a low setting...

    I wonder why Harmon decided to wire the stove the way they did in the first place?
  13. MCPO

    MCPO Minister of Fire

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    Yes Wet 1 , We both pretty much understand just how much heat we are able to get out of a certain amount of pellets and you obviously have extended the burn times for a bag of pellets to the max which is something I can`t match.
    Harman designers (engineering) had to be pretty much satisfied with the preset temperature at which the exhaust probe cuts the fan off when burning low . I wouldn`t even attempt to dispute that they were wrong since I`m not qualified to assess that design or reason. I just feel that the fan cutoff temp could have been lowered so I just came up with a simple inexpensive modification to circumvent the fan from shutting off too early and yet leaves the original wiring nearly 100% intact . I have been through approxiametely 8-10 bags of pellets since doing the mod and have experienced no noticeable effects whatsoever.
    BTW, I did read somewhere that the total stove draws 75 watts with the blower on low. Of course with the auger motor intervals it might draw another 50 watts.
    It`s been an enjoyable experience learning this stove`s nuances and so much about pellet burning in general.
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