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The question is: how effective is the thermostat control in which the fan is on only when the outside air temp < grain temp. Let’s compare the thermostat control to running the fan continuously with essentially no control.

In August 2009 a 2200 bushel hopper bottom steel grain bin, was run for 117 hours, continuously; the amount of water removed each hour was recorded and shown in Figure 1. The temperature of the peas at hour one was 27 deg. C and the moisture content was 16%.

The cost to run the 3 HP fan for 117 hours @ $0.15 kWhr, where 1 HP is 0.7457 kW; 117 x 3 x 0.15 x 0.7457 = $ 39.26

At the end the temperature of the peas was 18.8 deg C. the moisture content was 14.65% and the Safe Days Index was 3.6%.

Now let’s apply the thermostat controller, such that the fan will only run while the outside air temperature is less than the pea temperature. Using this control strategy the fan will only run for the first 17 hours to reduce the temperature of peas to 12.7 deg C and reduces the moisture content to 14.33%. The safe days index under this control turns out to be 1.9%. If the Safe Days Index is 100% the grain would have deteriorated to have a reduced germination rate of 95%. The power to run the fan for 17 hours would be: $5.70

Clearly the thermostat control is better in terms of cost ( $39.26 to $5.70); the peas would be cooler ( 18.8 to 12.71 deg); more moisture is taken out so the peas are drier (14.65% –> 14.33%) and consequently the peas are safer with less spoilage with the spoilage index (3.6% –> 1.9%). Clearly the thermostat control is superior on all accounts compared to running the fan continuously for 117 hours.

The reason that the thermostat control is more effective is that it only runs when water is removed and does not pump water into the bin, back and forth needlessly.

OK, so you want to: “measure the moisture of grain using only a sensor in the plenum and headspace.”
To determine the moisture content of the grain, you would use EMC equations that would require the parameters of the air surrounding the grain in question. These parameters would be temperature and relative humidity.  So you need sensors, that would sense the temp and RH of the air surrounding the grain.  Now we need to make some reasonable assumptions here.  I will assume that you have just been running your fan and as such the top layer of grain will probably be wetter and warmer than the bottom layer.  But it is the top layer that we are interested in, why? Because it is the last layer that the air goes through before exiting. So the temp of the exhaust air will be the same as the temp of the air in the top layer.  The same can be said about the RH.  So whether you have the sensors in the top layer, or in the stream of air exiting the bin, it will be the same.  The second assumption is that the top layer of grain in the bin is consistent in moisture content, and as such the air surrounding it will be consistent in temp and RH and that the air exiting the bin would be the same in both temp and RH.  And assuming this is the case, then it would not matter if the bin has exhaust vents, because the air exiting the bin, whether by vent or main portal, the exhaust air would be the same in both temp and RH.  Now, to get to the question of where to place the sensors.  In the experiments we did, we had the temp and RH sensors mounted under the roof, just inside and down a couple feet from the main portal.   In your case, I will assume that you will be using a moisture cable with temp/RH sensors every few feet.  I would hang this string right from the collar of the main portal.  It should be a bit to the side so as not to interfere with the auger’s loading chute.  In our experiments, we used as many as 9 moisture cables per bin; but, I think you only need one — one in the center.  The layers going across the bin were fairly consistent; but there was a difference in the top to the bottom layers.  So, there you have it, my recommendation would be to hang one moisture cable from the collar of the main portal.  That all being said, I am not a real fan of the moisture cables because:1. They are expensive2.The RH sensor is not reliable and not that accurate.3. One must use EMC equations by inputting the T and RH to get moisture content, and they are ugly equations, with questionable accuracy.4. EMC equations give moisture content on a dry basis, and this must be converted to wet basis. (This conversion is often over looked)
I like the idea of using only a temperature cable.  To determine the actual moisture content, one would sample and measuring it directly. The top layer’s moisture content does not change quickly and is the last to dry.

You asked me how long it would take to dry your grain.  I don’t think I gave you a very good answer, so I will try to explain this.  I will use the WGRF final report as a reference.
1. On page 6 I explain the black box approach to drying. If one measures the amount of water going in, and out of the bin, the difference would indicate the amount of drying taking place. The Absolute humidity is the amount of water in the air. The absolute humidity can be determined from the psychrometric graph of Fig 4 or from an equation or from an online calculator. www.planetcalc.com/2167/  One simply enters the Temperature  and Relative Humidity of the air, and out pops the Abs Humidity. We can get the Temp and RH of the outside air from any weather station.  However getting the temp and RH of the air exiting a bin is a little more involved.   It will be assumed that the temp of the exiting air will be the same as the temp of the top layer of grain.  I think that’s a reasonable assumption; but what about the RH of the exiting air — where do we get it from?   We could have a RH sensor at the exiting port; but most of us don’t have that.   So, we will again assume that the exiting air will have the same temperature and the same RH of the air in the top layer of grain. 
2. What is the RH of the air in the top layer of grain.  We will use EMC (Equilibrium Moisture Equations) equations to determine the RH. If we know the moisture content and the temp of a specific type of grain; an EMC equation will give what the RH of the air will equalize to. We can use my calculator to do this at www.planetcalc.com/4959/   Enter the moisture content of the grain, as well as the grain temp.  For the outside air temp, DO NOT enter the outside temp, but, rather the grain temp.  Then the EMC RH will be the resulting threshold RH for your specific grain.   Use this RH and grain temp of the top layer to calculate the absolute humiidity of the exiting air.3. The measurements made on the air entering and leaving the bin are all that is necessary to determine the amount of drying that is taking place.  If the absolute humidity of the air entering is 20 gr./m^3 and the abs hum of the air exiting was 25 gr./m^3; then for every cubic meter of air that flows through the bin there will be 5 grams of water removed.4. Let’s say that the fan is pushing 3,000 CFM.  For every cubic meter of air that goes through the bin, there are 5 grams of water removed, as above. There are 35.31 cubic feet in a cubic meter, so there are  3000 / 35.31 = 85 m^3 /min or 5098 m^3/hour x 5 gr/m*3 =25,488 gr/hr  or 25.488 kg/hr  or 56.17 pounds per hour of water removed. (There are 2.204 lbs in a kg)
5. So, how long would it take to remove 1% of moisture.   Let’s assume that we have 5000 bushels of wheat at 16.5% moisture and 60 lbs. per bushel. Above we see that we are removing 56.17 lbs/hr from the bin of 5000 bushels.   or  56.17/5000 = 0.01235 lbs/bu/hr.    But we want to remove  1%  or 0.01 x 60 = 0.6 lbs. Therefore, it will, with the conditions and the rate above, will be  0.6 / 0.01235 = 48 hours  or two days.  The problem is that the outside air is changing in both temp and RH and therefore to give an accurate time, this calculation must be done every hour.
Let’s go through an arbitrary example: The question is:  How much drying, in terms of % moisture content per bushel, occurs in one hour given that we have a bin of 5000 bushels of wheat at 16.5% moisture.  The aeration fan puts out 3500 CFM.  The outside temp. is 0 deg C and the relative humidity, RH, is 66%. The top layer of grain in the bin is 6 C, the middle is 5 C, and the bottom is 4 C.
1. What’s the abs. humidity of the air entering the bin?   Using the abs humidity cal  www.planetcalc.com/2167/   and enter 0 C for temp and 66% RH  yields  3 gr./m^32. What is the RH of top layer? using grain drying calc, www.planetcalc.com/4959/  enter 16.5% for grain moisture, and 6 for both grain and air temp. gives RH of 75.1%3 What is absolute humidity of exiting air? Using the abs humidity cal  www.planetcalc.com/2167/   and enter 6 C for temp and 75.1% RH  yields  5 gr./m^34. For every cubic meter of air that passes through the bin,  5 – 3 gr = 2 grams of water are removed.5  3500 CFM /35.31 = 99.12 cubic meters per min     x 60  = 5,947 cubic meters per hour     x 2 grams = 11,894 gr/hr   0r  11.894 kg/hr   x 2.204 = 26.22 lbs of water removed per hour6. How much water are we removing from one bushel of wheat per hour?    26.22 / 5000 = 0.005243 lbs of water removed per bu. per hour7. In terms of % moisture, how much is it reduced?   Assuming wheat is 60 lbs/bu.   (  0.005243 / 60 ) x100 = 0.00873%  At this rate it will take 114 hours ( ~ 5 days) to remove 1% MC ( 16.5- 1  to 15.5%)
I hope this example will help to give one the ability to calculate how long it will take to dry grain.

A farmer asked me if it was possible to determine the moisture content of his grain, by knowing the relative humidity (RH) of the air being expelled from the grain bin. Here is what I told him:

OK Kelsey, I am behind my laptop and I now can give you an in depth reply.  Your question: “a way to determine the moisture content of the grain inside a aeration bin? By measuring the relative humidity of the air exiting the top of a bin?”   You need to know two things, the Relative Humidity  RH and the Temperature T.  Knowing only the relative humidity, is not enough, in fact without knowing the temperature, the relative humidity is meaningless.  The amount of water that the air can hold is very much dependent on the temperature.  The relative humidity only tells you the percentage of that capacity, at that particular temperature.   OK, so let’s assume that you do have knowledge of both the temperature and the relative humidity of the air leaving the bin. There are EMC (equilibrium moisture content) equations in which you can plug in the Temp and RH and out pops the moisture content (MC) of that particular grain.  These equations were made by trial and error.  Scientist would take grain at a certain MC and Temp; put it into a sealed container and let it equalize with the air for an hour or so and then measure the RH of the air. They would do this hundreds of times for various temperatures and MC and then plot the results on a graph.  They then tried to define the points with the best fitting equation. These equations have been around for a long time and are published in ASAE (American Society of Ag Engineering) journal D245.5 “Moisture Relationship of Plant Based Agricultural Products”    However they are not for the faint of heart — they are ugly equations.   Yes they relate Temperature, RH and MC and you kind of have to know what you are doing with conversions etc.   For example we are used to MC given on a WB or wet basis.  Whereas the EMC use a DB dry basis for MC.    I tried to help some farmers through these calculations, and then it dawned on me that we have computers and smart phones that are really good and fast at doing calculations and conversions.  So, I made a sort of app for your smart phone that does these calculations for you.  You can find it at planetcalc.com/4959/ called the grain drying calculator.  It is normally used to determine if you have conditions for drying; but we can use it also the other way to determine the MC if you know the Temp and Relative Humidity. The calculator asks for inputs of the Temp of Grain, the Temp of the Outside Air, the MC and it gives you the thresholld RH to which any RH below this RH thres — you will have drying conditions.  I uploaded this calculator to my Iphone, so that I can use it anywhere.
    Let’s make up an example to show how we can get the MC.  Assume we have Canola, the RH of the air leaving the bin is 70 % and the Garin Tempi is 20, and Outside Air is 15 deg C. The first thing to do is to set the Grain Temp and Air Temp to 20. We don’t know the MC, but we will guess — put in 10% in the MC; and press calculate. Under Canola we get an RH of 74.  This is too high,  So guess 9 for the MC and that gives us 68.6.  A bit too low.  8 gives us 61% and 11 gives us 79.9%.  But we want 70%  So, I try 9.2% for the MC and it gives a thres RH 69,96.  This is very close to 70, so I will claim that your Canola is at a MC of 9.2%   — by a little bit of trial and error.  Just make sure that the outside air is set to the grain temp when you are doing this (even though it isn’t)I hope this helps.
//Ron Palmer  Ph.D. P.Eng.

  OK I am back, and yes it is that simple:
Turn the fan on if:  grain temperature > outside air temperature
I have gone through all the control strategies, and as for a controller, this would be my choice because of its simplicity and ease of use.  It does not guarantee the fan will only be on when you have drying conditions, but it does guarantee that you will have the safest , most secure storage with the least spoilage. It keeps your grain cold.  The control strategy for only running the fan when you have drying conditions would be what I call the Absolute Humidity Controller.
It calculates the actual water content of the air inside the bin and the air outside the bin.  It turns the fan on when the outside air contains less water than the air in the bin.  It is much more involved and gives even a micro a hard time with Psychrometric equations, and EMC equations.  It also requires the user to input the moisture content and type of grain. It is the ultimate in a control strategy that only runs the fan when there are drying conditions.  But it is way more complex, and not as convenient for the user and therefore not as reliable and more expensive.
The strategy of:
Turn the fan on if:  grain temperature > outside air temperature also dries the grain except for when the grain temperature is greater than, but only slightly greater than, the temperature  of the grain AND when the outside relative humidity is close to 100%.   But once the difference in temperature of the outside air and the grain becomes more than a few degrees, then even with the RH being 100%, you will still get drying.  What is the chance of having an RH of 100% and only a slight difference in air/grain temp? Very very small.  We could condition our strategy above by saying that we would only turn the fan on if:
grain temperature > outside air temperature, AND the RH < 85% — but is it worth it?  This would require a humidistat etc.  I don’t think it is worth it because of the probability of having a small differential temp and an RH > 85%
We can achieve almost the same thing by putting in an offset:only turn the fan on if:
grain temperature > (outside air temperature + 2 degrees)
So, what do we need to automate your fans — Are you ready for this??   All you need is a thermostat that is used to control baseboard heaters. You can find these at Lowes or Home Depot.  It has a rotary knob that is set to a specific temperature. It is all mechanical ( I believe a bi-metalic strip), that closes a contact (than can handle a pile of current at 110 or 220 volts) and the contact closes when the air temperature is less than the indicated knob temperature.  In our case we will set the knob temperature to that of the grain temperature and connect the contacts to the actuator’s (relay) coil. Most aeration fans are wired to have a latching actuator coil.  This will need to be modified.   Assuming we will have 220 volts with L1 and L2 power leads.   The following would be wired in series, ignoring the Start and Stop switches.         L1  – actuator or relay Coil – Thermostat – L2 The relay’s coil will only be activated when the contacts on the thermostat are closed, and that is when the temperature of the air is less than the temperature of the grain that is indicated on the knob.  Your fan will only run when the air temp < grain temp. 
  Your grain temp will not change that quickly and maybe once a week you will need to adjust the temperature on the knob down to match the grain temp.
A more automated approach would adjust the temperature of the grain automatically. Thermistors are used to measure the temperature of the grain and the outside air.  I used 10 K ohm thermistors.   These are run into a comparator such as LM311, through a pulse integrated circuit and then into a solid state, high voltage relays to simulate the Start and Stop pulses on the fan.  I have built these circuits and have wired up a couple of aeration fans with it.  I called the system “Cool It”  I still have some of these boards around, but I can not sell or even give these boards away as they would have to be certified by CSA or ULC and to go through all that hassle is not worth it.   I suppose I would be willing to share the schematic, but I still would be a bit nervous about the liability.
I hope this gives you a start, with a cost effective, simple solution to automating your fans.

I wanted to ask about using the terms “aeration” and “natural air-drying.” I’m trying to avoid confusion – I found in a grain aeration spreadsheet by PAMI the following definition:Aeration = grain conditioning/cooling low airflow rate (0.1-0.2 cfm/bu) Natural air Drying = removing moisture from grain high airflow rate (1-2 cfm/bu) 
   Natural aeration or Natural Air Drying would be using the natural ambient air (no supplemental heat) to condition the grain.  I think it is generally accepted that the fan is pumping air into a steel bin.    Our research has shown that Cooling is Drying and whether you have an air flow of 1 CFM/bu  or 0.1 CFM/bu, you will be cooling and drying the grain. I am not sure where the distinction between airflows for cooling and drying came about, but in fact they are related.  Even at very low air flows, you will still be drying — albeit somewhat slower, but; there are advantages to drying slower.  1. Because the higher flows make for more pressure on the bottom of the bin, they also will create more of a difference in top to bottom of the bin drying.  Slower flows have a more even distribution of top/bottom drying.  2. You will use more of the heat energy in the grain to push the water out of the grain.  3. You will use much less electrical energy with a smaller fan — even if it takes a bit longer.    We found the sweet spot for flow to be about 0.4 CFM/bu.  You get the advantages, as mentioned above, and you still get the grain dried in a reasonable time.   But back to answering your question: Natural Air Drying and Natural Aeration are really the same thing.

Do you agree with these definitions? Natural air-drying seems to be both a somewhat general term for the two methods and also a specific term for the high airflow rate method – thus my confusion. 

Also, would you have any figures on the costs to run a grain aeration fan? You said it would be pennies on the dollar – do you know where I can find more specific numbers?
   Here is the math that you can use to calculate the cost.   In Sask the cost of a KiloWatt Hour is about $  0.13   1 HP requires  0.74 KW  So to run a 1 HP fan for 1 hour cost  9.62 cents — lets round it off to 10 cents.  A 5 HP fan would cost 50 cents per hour  or 12 dollars a day.   A 10 HP fan would cost  $24/day.   You can take it from there.

I was looking at your presentation and the diurnal drying cycle graph – it shows drying starting at 6:00 pm and wetting starting at 9:30 am (with best drying happening at 2 am). So, after that initial 24 hour drying period during/after harvest, are those the times farmers should be following for natural aeration?
   After the initial 24 hours, the fan should only be run if the outside air temperature is less than the grain temperature. This will probably be at night.   And as the temperature of the grain goes down, there will be warm nights that the fan should not be run at all.

I got an interesting email from Tom in Ohio, wandering how he could dry his cold tough corn. It went something like this:

Hello Ron Palmer, hoping you can offer suggestion. South West Ohio U.S.A. 5000 bu. bin with 5 hp fan, 2500 bu of corn harvested Feb with air temp 20 f. and mc of 20.5. a cooking thermometer was inserted from top of corn today that read 50 f, probably colder farther down. Spring temps are fluctuating into 70’s f. with off and on rain in the 10 day forecast. Your trials involved grain that was much warmer than this corn, should it be warmed up with daytime air before starting the night time drying and to run or not during nights with rain??   Thanks you for any help/strategy to dry down corn. 

And my reply:

Tom:   You have some pretty tough corn at 20.5 % MC; however, the good news is that it is not heating at 50 deg. F.   So, the  question is: how are we going to get some of that moisture out.   And you are right in thinking that we need to get some heat into the corn, so that it has some energy to push the moisture out.  Actually most of the energy is used to change the water in the grain from liquid state to a gas state.   We have found that it takes about a 15 deg C (27 deg F) reduction in  temperature to evaporate the water to get a one point reduction in moisture content.  For example, let’s say that we have corn at 70 deg F @ 20% MC, and we run the aeration fan to get the corn down in temperature of 43 deg F  (70 – 27).  We should have reduced the MC by one point, 19%.   But how do we get energy into the corn??   Yes you can use the outside temp of the air to heat the corn; but, as you heat the grain this way, there is a good chance that you will be wetting the corn down.  So, yes use the heat of the day to get your your corn warmer, and then use the cool night air to cool it down and for every 27 deg F that you drag the corn down, you should be able to take the grain down one point.  We want to get this cycle going of getting energy into the corn during the day, and then cooling it at night to remove the water.   My first reaction would be for you to run the fan continuously; heat the corn during the day, and cool it down at night.  If you can get a 27 deg swing you could take out as much as a point a day.  However when the corn get closer to being dry, you might find that you are adding as much water during the day when you are heating the grain as you take out at night.   How will you know?   I suggest you use my Grain Drying Calculator. You can find it at  planetcalc/4959/ and it will tell you when the conditions are OK for drying.  I ported this website calculator to my cell phone, and I can use it anywhere.  Unfortunately the temperature entries are in Celsius, and I know you like F, so you have to convert.  To convert F to C, subtract 32, and then divide by 1.8.  So 32 F is 0 C.  70 F is 21 C,   50 F is 10 C.   I entered the following into my calculator.
Grain Moisture Content %     20.5  
Grain Temperature C    10         (this is 50 deg F, — this is what you told me the temperature of your corn was)Air Temperature     C     20        (this would be 70 F, the daytime high)
Now push calculate and scroll down to shelled corn, and you will see that you get a threshold relative humidity of 48.4%.  In other words if the relative humidity of the air outside is below this you will be drying.   I doubt this is the case for you, when you said there were showers around.  It is probably more like 70 or 80%, in which case you will be wetting you corn down.
OK let’s run the fan when it is 10 C or 50 F outside — what does the calculator give us — it says the threshold relative humidity is 93%.  So even though the air temp outside is the same as your corn temp; as long as the RH outside is less than 93%, you will be doing some decent drying.  Not bad. 

If we run the fan when the outside temp is just above the corn temp. say 15 C (59 F), and we run that through the calculator we get a threshold RH of 66.6%.  You might get this on a dryer day — but certainly if it is not raining.So, roughly speaking, it looks like we can run the fan without wetting down too much provided the temperature of the air does not exceed the temperature of the corn by more than 5 C or about 9 deg F.    But if you want to be more precise, use the calculator.    As the corn gets drier, you will find that the temperature difference will get less and less, and it will be more difficult to get some heat into the corn.I hope this helps//Ron

The following report was submitted last year and its represents a culmination of what was learned with this research since 2007.

Last year we measured the pressure drop across the perforated pipe in the bin when the aeration fan was one. We typically saw a one inch (of water) drop across this perforated pipe, (used a manometer). My thoughts were that an open bottom pipe or plenum would preclude this drop. I sketched out what such a thing could be and I ended up with design that had an open bottom pipe leading to a Christmas Tree looking rocket in the middle of the bin. louvers placed on the outside at 45 degrees provided direct grain exposure to the air from the fan.

The 18 ” and 6″ pipes could also be open bottom to provide more direct exposure. Being open bottom would preclude any fine buildup which plugs holes and also is a wonderful breeding ground for pathogens.

Open bottom pipes, would add more exposure for a total of 35 sq feet. If the material used for the louvers are 10% perforated, this will increase the exposure to 43 sq. feet. Remember that a 10% perforated pipe (18″) provides only 10.3 sq. feet. With this much more exposure, 4 X, it is obvious that this would be a better air delivery system.

have been following your work the last few years and it is good stuff.  I do have a few questions though on how best to use the info based on a normal year and secondly advice for this year.  I farm close to Drumheller, AB and normally we get enough heat to harvest a dry crop.  This year not quite as nice, though it may turn warm yet.

Question 1.

How best to use natural air drying in a normal year for me.

In a normal harvest here, the crop gets preharvest roundup and the immature plants get desicated and are dry to ambient conditions when my combine arrives 2 weeks later.  It sounds like farmers might lose the ability to use preharvest roundup in the future.  Previous to preharvest roundup my crops would come in with varying moisture levels due to immature seeds.  The grain will average dry.  I have aeration in most of my bins, but they are all spread out in various fields and right now I use a generator to cool the grain overnight after storage and then again a night 2 weeks later when it is cooler and once more in December when it is cold.  Fuel for the generator is expensive and I would like to keep my costs down.

So my question is how the immature kernels affect natural air drying?  Looks like the grain will lose 1.5% moisture by the time it is cooled to final storage temperature.  So with immature kernels, I assume that it is safe to store when averaging dry if the fan runs somewhat to make sure the moisture equalizes in the bin.  Because some of the kernels are high moisture, should the bin sit for 24 hours before turning the fan on the first time to allow for moisture migration from the tougher kernels to the surrounding dryer kernels?  No, turn your fan on immediately, even while it is being filled — get the grain cooled as soon as possible to prevent spoilage, especially if it is tough and has green seeds in it.

Also, if by final storage the moisture is going to drop by say 1.5%, does that mean I could bin grain that is 1.5% average higher than dry even with uncure/immature kernels in it and the bin will average dry after final storage and the grain condition will keep for 6-8 months until sold?  I would say yes, it will keep without spoilage provided it is kept cold — really cold, below freezing.

I am just trying to come up with a workable system in case preharvest glyphosate is removed as a harvest tool.  I really dont want to go back to swathing as a way to kill the plants.  I would certainly get the jump on harvest by starting when the crop is a couple of points above dry, and then use your aeration to get it cold and dry. Aeration is way cheaper than swathing!

Question 2.

How to deal with grain that might get harvested tough this fall.

I have read most of your articles and it sounds like if the grain is 20 degrees C or more and only 2% above dry that it will dry during the cooling process by running the fans at night.  Once the core grain temperature drops close to freezing the amount of drying that occurs is minimal.  So assume grain will still be too tough to sell after cooling it (and hopefully drying it 1-2% in cooling).  I see 2 options and perhaps you can expand on these please or inject some ideas that I have not thought of short of getting a grain dryer.

Option A.  Dry in Spring

Cool the tough grain at night and if the moisture is low enough for safe storage, it could be dried in the spring.  Just not sure what proper procedure would be in the spring and what the economic numbers would look like.  Outside air would be warm, so if supplemental heat is added I assume the condensation at the roof thing would  not be an issue.  I assume that the grain mass would have to be warmed up with supplemental heat, but perhaps it could be warmed up during the day, then shut fan off, resume heating the following day and once the grain temp is warm enough you start cooling at night and removing moisture.  Just not sure how warming up a mass of tough grain slowly would work as spoilage might be an issue.  Your work showing you add moisture during the day and removing moisture at night is going to come into play here, but not sure how the grain can get warmed up with ambient air and not add moisture to the grain during warm up.   If you get your grain as cold as possible by running your fan in late December for one cold night, we might get the grain down to -20 C.  Even if it is tough, it will not be spoiling.  Now seal the bin up the best you can, for two reasons: 1. We want to keep the grain as cold as possible, for as long as possible, and 2. We don’t want warm spring breezes, containing lots of water, from hitting your cold grain and condensation forming.  During the winter the grain will warm about 1 C per week with the bin kind of acting like a solar collector, and heat coming in through the walls and roof.   Monitor the temperature of the grain, and when it naturally gets to 15 to 20 C, start pulling it back down as cold as you can.  This will probably be in August or Sept.  and continue with the practice of getting the grain as cold as possible.  Let’s say we pull it down again from 20 to 0 — we will have taken another point out and it should be close to dry by now, but even if it isn’t, keep the grain temp as low as possible; seal the bin up at Xmas and repeat yearly.  Your grain should end up dry, and more importantly with no spoilage and no expensive supplemental heat.

Option B.

Add supplemental heat to the natural air drying fans.  I see your work in the blogs about condensation at the roof being a problem.   Is it possible that you are missing the effect that heating may have on the roof panels themselves?  If enough heat is passing through the grain, could it be heating the roof panels and changing the condensation point to allow more humid air to escape the bin and condensate outside.  Would higher airflow rates, higher plenum temperatures, much fuller or shallower grain depths make the condensation different?  I am thinking there is a strategy here that is being overlooked, much like people were overlooking the effect grain temperature was having on running the aeration fans at night to dry grain (which you proved).  Type of aeration system (full floor, pit, rocket, round tube, inverted v) probably all work different. Biggest downfall that I see is severely overdry grain in the bottom of the bin by adding enough heat and airflow to keep condensation at the roof minimized.  Not sure if the whole bin could be over dried and then mixed with untouched tough grain and be mixed together, moved to a different aeration bin and run the fan a little to equalize?  Your blog showing the 50,000 btu heater and removing 0.3% moisture every night is good, but too slow for what I need to do.  I would require a huge power service to run that many fans, which I do not have, so trying to explore other options.    I would only use supplemental heat if you were in a hurry to sell your grain as dry.   As my blog says: heat the grain during the day, and cool it at night.  and you don’t have to worry about over heating the grain, it takes 12 hours to heat 5000 bushels, see

Supplemental Heat: Act III How Long to Heat Grain 5 C