Thermostat overshoot

Thermostat overshoot

PostBy: tmbm50 On: Mon Dec 10, 2007 7:41 pm

I have a LL Pioneer controlled by the coal-trol. The thermostat seems to be overshooting the desired temp by 3 or 4 degrees and wondered if there where any tweaks that can be made.

Here is my setup. I have the stove on the first floor (living space) and the thermostat upstairs. The stove is ducted up the stairs. Right now, the duct fan is not controlled by the coal-trol. It just runs full blast all day. There is no distribution blower being controlled by the coal trol.

It seems to happen when the thermostat comes off set back. So, the night temp is set at 69. Then in the morning it goes up to 72. But, when the temp hits 72, the coal trol still calls for heat. Eventually the thermostat reaches 75 or 76. This morning, when it was 76 degrees, I went into the setup and the feed rate was something like 68. Seems like it should have been much lower given the temp was over the set point. After about 3 hours the stove will eventually go way down and maintain the house at 72.

It just seems the stove should ramp down quicker than it does. But, I must confess that I'm still miles ahead of my old hand feed coal stove!
tmbm50
 

Re: Thermostat overshoot

PostBy: Matthaus On: Mon Dec 10, 2007 7:53 pm

The overshoot is a common occurrence since coal is a slow changing burn process. If you read the thread on HLF in this section it will give you some good pointers on reducing overshoot.

The bottom line is that it takes a little patience but you will be able to reduce but not totally eliminate the overshoot. Let us know if you have questions after reading the thread. :)
Matthaus
 
Stoker Coal Boiler: Leisure Line WL110 Dual Fuel
Hot Air Coal Stoker Stove: Leisure Line Lil' Heater (rental house)
Coal Size/Type: Rice and Buckwheat Anthracite

Re: Thermostat overshoot

PostBy: WNY On: Mon Dec 10, 2007 8:41 pm

Ah, your thermostat may be too far away from the stove, it takes longer to satisfy it and to get the heat to it. You say there is no blower on the stove? In the coaltrol, the blower control actually changes the speed of the blowers on the stove if so equipped. It will vary the speed depending on the situation, I notice mine run very slow most of the time to satisfy the temp and then increase speed when needed or feeding higher....Maybe get a little squirrel cage blower and hook to the coaltrol and place behind the stove or something to help move the air better.

It depends on how much difference you have in setback vs. actual temp. If more than 3-4 degrees, it will do that. I would try it with only 1-3 degree difference and see if it helps. If you setting back to 65 then back up to 70, it will get really cranking to make up the difference and then overshoot because of the feed rate increases over a period of time for that 5 degree difference and if it's not satisfied within a certain period of time, it keeps trying, then it hits the setpoint, but the stove is now REALLY cranking and it stops feeding, but now the stove is HOT and keeps pouring out the heat.....it is a slow process to satisfy the temps.

In the CoalTrol Thread about HLF (Heat Loss Factor) we discuss this extensively.
What is your HLF setting in the Setup Menu?

HLF(heat loss factor)

Try only a couple degree difference in setpoints or Change the HLF Factor to 1 or increase the Max. Feed a couple numbers.
Sometimes just changes a couple of setting may help. A little experimenting sometimes helps. I tweek mine all the time, since the weather is so unpredictable...
WNY
 
Hot Air Coal Stoker Stove: Keystoker 90K, Leisure Line Hyfire I
Coal Size/Type: Rice
Stove/Furnace Make: Keystoker, LL & CoalTrol
Stove/Furnace Model: 90K, Hyfire I, VF3000 Soon

Re: Thermostat overshoot

PostBy: ewcsretired On: Mon Dec 10, 2007 9:14 pm

As WNY stated, if there is a 3 or 4 degree difference it will overshoot, I experienced this when I retroed my Hyfire and installed the coal-trol. I have overcome that by "set it and forget it", I leave the day and night temp the same. Its a minor inconvienence compared to not having the coal-trol.
ewcsretired
 

Re: Thermostat overshoot

PostBy: WNY On: Mon Dec 10, 2007 10:30 pm

I normally just hit the Temp Hold and it maintains almost perfectly...
WNY
 
Hot Air Coal Stoker Stove: Keystoker 90K, Leisure Line Hyfire I
Coal Size/Type: Rice
Stove/Furnace Make: Keystoker, LL & CoalTrol
Stove/Furnace Model: 90K, Hyfire I, VF3000 Soon

Re: Thermostat overshoot

PostBy: wally61 On: Tue Dec 11, 2007 7:59 am

i had this problem with my alaska channing III. My stove can heat up to a 2500 sq.ft. house and my house is only 1700 sqft so when in max mode it was pummping coal to heat 2500 sqft. I turned my max dwn from 40 to 20 and this seeemed to stop over shoot. my stove is direct vent. themostat is right next to the honeywell for my furnace. house is a 2 story stove is in dining room middle of house. dwn stairs stay at temp set on coal-trol and upstairs bedroom furthest away 4 degrees different. love it
wally61
 
Stove/Furnace Make: alaska
Stove/Furnace Model: channing III dv coal-trol

Re: Thermostat overshoot

PostBy: nwaelder On: Tue Dec 11, 2007 11:41 pm

wally61 wrote:i had this problem with my alaska channing III. My stove can heat up to a 2500 sq.ft. house and my house is only 1700 sqft so when in max mode it was pummping coal to heat 2500 sqft. I turned my max dwn from 40 to 20 and this seeemed to stop over shoot. my stove is direct vent. themostat is right next to the honeywell for my furnace. house is a 2 story stove is in dining room middle of house. dwn stairs stay at temp set on coal-trol and upstairs bedroom furthest away 4 degrees different. love it


For some, Wall61's suggestion works very well. The idea is that for houses (especially "well" insulated) the stove essentially is oversized (more BTUs) than is required. By reducing the MAX setting, the "Size" of the stove is effectively reduced and this allows for tighter temperature control. For example, earlier this season I spoke to someone with overshooting issues. I asked him what his firing rate was. At the time I asked, his set-point temp and actual agreed i.e. the fire was stable and he told me FR=18% even with the outside temp in the mid 20s F. By reducing his MAX from from 85 to 65 (Keystoker ) his overshoot was eliminated.

One word of caution is in order: it really isn't much help to reduce the MAX setting so low that the stove can't keep up on the coldest days. Also, the idea is that this "Tuning" should remain a "one-time" setting.

In summary, for I'd guess 2-5% of folks with good insulation AND overshoot:
1) On a cold day when the measured temperature is within a degree of the setpoint temperature (stable condition),
a) Go to setup and look at the firing rate (FR),
b) If FR is say 65% or more, then you might want to leave MAX alone.
c) If FR is less than 65%, you may want to reduce the MAX value in small amounts, say 10-20% of it's value at a time to increase the steady state firing rate, that is to say the FR%. Remember, check the FR when in steady state, otherwise the reading and your adjustments will be meaningless.

Finally, on occasion I talk with a user whose problem can be categorized as an air distribution problem and this can exhibit symptoms of undershoot/overshoot. I'll save comment on this for another time as this post is long enough.

Thanks WNY and everyone for the thoughtful comments.
Neil
nwaelder
 

Re: Thermostat overshoot

PostBy: nwaelder On: Tue Dec 11, 2007 11:57 pm

One more comment: We implemented the HLF (Heat Loss Factor) as a way to compensate for some of the dynamics we are discussing. As a general proposition, I'd guess that for 95% of persons HLF = 0 is the correct setting. For others who may benefit from other HLF settings, at this time, I'd suggest you call or post some questions. The fact is that our implementation of HLF is less useful in practice than we had hoped. Heat balance of an unknown dwelling with only one temperature probe is a fairly complex control engineering problem.
nwaelder
 

Re: Thermostat overshoot

PostBy: tmbm50 On: Wed Dec 12, 2007 10:58 pm

Just a quick update on this.

I changed my night temp to 70 and left my day temp at 72. I thought maybe a 2 degree spread would be easier to shoot for than a 3 degree spread. I don't think I have a air distribution issue becuase you can feel the jetstream of hot air rushing down the hallway towards the thermostat

I also changed my MAX from 55 to 45. Even at 55 I had inches of ash on the grate but after reading the HLF thread i thought I would give it a try. I left my HLF at 0 since I wasn't sure how to check my version.

Now, it's only a been a day since i made the changes, but it did not overshoot today. And, I used about half the coal that I had been! Now granted, it takes a little longer to get up to the 72 mark (but not much) but becuase the nigh temp is at 70, it';s still comfortable. But I can't believe how much extra coal I saved....Even though the nigh temp is warmer it's not burning through huge amounts when it has make that first large jump in the morning.

Now, when it starts to get really cold, i may have to turn my MAX back up a bit...but we will wait and see.
tmbm50