PID controller for operating limit control?

I recently installed an AHS-130 boiler. I observe a substantial overshoot on the boiler water temperature to the point that the circulator override is activated. This occurs even though the override is set 35 degrees above the operating limit.

Is overshoot like this normal? If it is not normal what should I be checking to correct the problem?

If it is normal, would a PID controller for the operating limit help get more accurate regulation of the boiler water temperature? Has anyone experimented with a PID controller for operating temperature control?

Bob, I'm moving this to the stoker forum.

Bob wrote:I recently installed an AHS-130 boiler. I observe a substantial overshoot on the boiler water temperature to the point that the circulator override is activated. This occurs even though the override is set 35 degrees above the operating limit.

Is overshoot like this normal? If it is not normal what should I be checking to correct the problem?

If it is normal, would a PID controller for the operating limit help get more accurate regulation of the boiler water temperature? Has anyone experimented with a PID controller for operating temperature control?

Do you have a barometric damper on the chimney ? What is your draft ? Dave

A PID type of control uses logic to analyze the temperature signal and adjust the output in proportion to the rate of change of the temperature signal, as well as the difference between the actual and setpoint temperatures. That being said, it is not a suitable subistitute for a limit alarm when used as an overtemperature monitoring device. Most programmable PID temperature controls can be set up for on/off control, but they are not typically listed as limit devices and should not be used as such.

Without a barometric damper, your flue draft might be drafting the fire a little too well resulting in the overtemperature condition.

As the other said you could have to much draft and it's pulling someair thru the coal during off periods, two other things are possiably your ashing the unit too much and it burning too hot which also leads me this this unlike a oil or other type of boiler that when they turn off their off. Ours of the other hand still burn for sometime after the fan shuts down and the heat has to go somewhere which is in the water till a equalibriam is met. now that is where a pid or other unit that has a feature call "POST PURGE" which runs the pump after the call for heat stops and the unit shuts down. what it does is continues to move water thru the boiler pulling the heat out and into the house where you want it. I use a pid unit which is a honeywell AQ251 which allows me to adjust the purge time from 30 seconds to 30 mins. do a search on the board for aq251. another unit is the AQ250 which doesn't have outdoor reset but maintains the zc,zr control funtion to the boiler aquastat.

I have a barometric damper installed. The AHS literature calls for an over the fire draft of .04. I have been unable to attain that--rather I run .02 to .03 with the outside temperature in the high 30s or low 40s. I have sealed the stove pipe joints.

The boiler is in a 1600 sq ft outbuilding and the building is fairly tight and reasonably well insulated. I have checked the draft with a door open and with the building closed and it really doesn't change the measured over fire draft.

The boiler has an operating limit control, a high limit, and a circulater override. If I were to install a PID controller it would replace the operating limit but be wired in series with the high limit and the circulator override would still be in the circuit.

The boiler has the AHS thermograte system installed. The AHS recommendation is for the set point for the ash temperature setting to be 140 during cold weather and 130 during times of lessor demand. I have the control set at 130--the causes the grate to operate when the sensed ash temperature drops to 120 and to stop operation when the sensed ash temperature rises to 130.

and for the wiring depends a little on what controller you get but I'll tell you both but first , what aquastat do you have on your unit? honeywell ? model aka L8124A ? and what are low, high, and operating limit settings? how about diff 10,15 or 20 on the low limit setting?

The operating limit is a Honeywell L4006A. A Honeywell L4081B provides the high limit and circulater override. There is no adjustable differential on either aquastat.

I have the operating limit set at 170, the high limit set at 190 and the circulater override at 205.

Bob wrote:The operating limit is a Honeywell L4006A. A Honeywell L4081B provides the high limit and circulater override. There is no adjustable differential on either aquastat.

I have the operating limit set at 170, the high limit set at 190 and the circulater override at 205.

how many circulators do you have? if more then one are they on relays? do you have zone valves? I'm looking at those controls at the moment....

Bob wrote:I have a barometric damper installed. The AHS literature calls for an over the fire draft of .04. I have been unable to attain that--rather I run .02 to .03 with the outside temperature in the high 30s or low 40s. I have sealed the stove pipe joints.

The boiler is in a 1600 sq ft outbuilding and the building is fairly tight and reasonably well insulated. I have checked the draft with a door open and with the building closed and it really doesn't change the measured over fire draft.

The boiler has an operating limit control, a high limit, and a circulater override. If I were to install a PID controller it would replace the operating limit but be wired in series with the high limit and the circulator override would still be in the circuit.

The boiler has the AHS thermograte system installed. The AHS recommendation is for the set point for the ash temperature setting to be 140 during cold weather and 130 during times of lessor demand. I have the control set at 130--the causes the grate to operate when the sensed ash temperature drops to 120 and to stop operation when the sensed ash temperature rises to 130.

I too can not get an over the fire draft greater than 0.02 to 0.03. My flue is short run, about 35 inches, with an 90 deg El out of the breach and a 45 deg El turn to the thimble. Includes a 5 inch Fields RC barometric damper. Chimney draft is the same if the combustion blower is running or not. Less than 0.04. Joints are taped. Chimney is 1-2/2 stories clay tile lined and is clear. Chimney clean out is taped shut. The only thing left to do is the insulate the stovepipe. When the combustion blower is running the inspection port door gets sucked shut. I've measured the over the fire draft during this time. It measures 0.10 to greater than 0.25. Pegs my draft gauge. Higher numbers occur when is see a lot of unburnt coal in the inspection flap. Low numbers when there is a good burning fire visible. I believe there is some correlation with grate operation. Just what I haven't figured out yet.

U235a4 wrote:how many circulators do you have? if more then one are they on relays? do you have zone valves? I'm looking at those controls at the moment....

Perhaps I need to describe the set up a bit more before answering your specific questions. The AHS 130 is located in an outbuilding and connected in parallel with an oil fired boiler in the house. The connection piping is 1 inch and approximately 135 feet each way. In the outbuilding there is one zone for an air handler in addition to the connection to the boiler in the house. Inside the house there are 4 high temperature zones (indirect water heater, upper level air handler, flat plate heat exchanger for low temperature zones, and pool heater). There are 4 low temperature radiant heating zones supplied from the flat plate exchanger. There are no zone valves. All zones, both high and low temperature, operate from relays except the indirect fired water heater.

As to a circulator count--4 for the low temperature radiant heat zones, 1 for each the flat plate heat exchanger, indirect water heater, pool heater, upper level air handler, outbuilding air handler, and circuit between coal boiler and oil boiler--a total of 10.

I guessing the relays are R832A's and the boilers are connected in series with the cold return water entering the coal boiler first and then the oil? I'm wrong let me know. there are number of things you could do if you want to goto a reset controller wether it be honeywell, tekmar or some other brand you know. I'm going to PM my number to you if would rather talk via phone or we can continue on here not a problem. As for myself I went with a Honeywell AQ251 on a A-A 260m because after a big heating cycle my boiler would shutdown at 190 then creep to 230F-240F. last year I saw temps between 265-270 at times. with the reset controller I'm still testing weather the reset feature will help on coal useage or not but I do use the post purge feature and have that set at 3mins. basicly after the zone stops calling for heat and the boiler stops the control contunies to run my pump for 3mins pulling the hot water out and moving cooler water in to cool down the boiler. Now I do want to add that last year my ashing was at 140degrees and now I have it at 130F and that i believe accounted for the 20-30 temp drop in the override.

U235a4 wrote:I guessing the relays are R832A's and the boilers are connected in series with the cold return water entering the coal boiler first and then the oil? I'm wrong let me know.

The relays are R845A. The connection between the boilers has the output side of the coal boiler connected to the output side of the oil boiler. The return side of the coal boiler is connected to a plug in the base of the oil boiler. I think this is a parallel connection.

For the circulater in the piping between the boilers I have two alternatives for operation:
1. It operates only when a heat zone in the house demands; or
2. It operates continuously.

If I operate the circulator between the boilers continuously I get very little overshoot. If I operate with option one then I get the overshoot I described.

ok I think I may have figured a few things out but let me go thru the running order with how I think your units running.

your unit runs untill it reaches the operating limit of 170 on the L4006a unit and if your using a timer the high limit of 190 won't let the unit run until it falls below that. if the temp goes above 205 the circulator won't run and your dead in the water till the unit cools.... if this is the case being wired this way are you trying to stop hot water from going out to the zones? if not I would rewire the unit with the operating limit on the high side of the L4081 at 170 then use the L4006 as a high limit controller at 190. connect the circulator to the low limit side of the L4081 and set that at 140/150 or what ever you want that way the no matter how high the temp is the circulator will run and only shut down if the unit cools down too much to give the water time to heat up the the circulator turns back on at that setting... hope this helps as far as overrides are concerned with a stoker like these it is normal to some point if your ashing is right and you don't have excessive draft then the only thing that will help that is a unit with post purge feature. now also if you use a timer on the unit to run it 1min every hour that also will allow the temp to creep up over long periods of not running.

I wanted to seprate post on how i'm wired. My main feed goes to a L8124A with the operating limit at 200 (I'll touch on why it is so high further down) then my low limit is at 150 ( this maintains water temp for domestic hot water but doesn't shut down my circulators) then the boiler outputs from that go thru a junction box where the timer is hooked in on the hot wire and from there to the high limit/out fire control which is a L4081 wired so that is the unit goes above 230 the timer can't run the unit and the low limit is there for if some reason the water temp would hit 100 it will shut the unit down most likely because of a out fire. after that the unit goes thru a low water unit that if there is no water in my main supply pipe for some reason the unit won't be aloud to run, after that it's too the motor/grate switch/anthrastat. Now for as too why my high limit is at 200 is because the AQ251 reads return temp and supply temp if supply temp drops below 180 then it makes a connection on the T T terms of the L8124 and turns the boiler on which is why the temp is at 200 so that it doesn't interfer with the aq251. now for the low limit the Aq251 watches the return temp and if it goes below 140 then it turns a bypass pump on to circulate water from the supply to the return side heating the cold return water and once a 150 return water is met then the bypass pump shuts down. this not only makes for hotter water going to the zones sooner but helps the boiler heat up faster since it doesn't cool down as much..... sorry so long winded I hope you understand what goes on.