Temp overshoot

Still having trouble with my Harman sf-260 boiler the setpoint is at 165* and overshoots about 30*. The maximum air automatic draft is set to open only about 1/2 in. Mininum air is set to half (two holes about 3/4 in. on the ashpan door) Manual drafts on the loading door just cracked loose, Baro damper opening about half way with a connector pipe temp. around 250*-300*. Can anybody tell me if a firebox reducer would help prevent this? I have exhausted all other ideas.

Is the overshoot of 30* over your setpoint of 165* causing some form of problem?? Most solid fuel burning appliances will have some temperature overshoot or control issues, it is the nature of lighting a pile of fuel, then trying to dampen it down. It takes time to light the fire, and takes time to reduce the heat from the fuel once burning.

Have you checked the draft over the fire?? maybe you need a bigger barometric damper. A high draft will make it hard to control overshooting the target temp.

You may be able to make a firebox reducer, but I'd only recommend it if you have a way too large boiler for your heat use.

Is the 165* + 30*=195* causing problems??

Greg L

two questions for you other then what Greg said, does this happen after the unit has run because of a call for heat or does it just creep to that even if there hasn't been a call for heat? As Greg stated solid fuel boilers (Coal in our case) are just that always on and the stoker case they burn the fuel at very high rates and then all of sudden the stop or so we think. because this takes time and if you have a stong draft even a baro or bigger baro may not help. If it happens after a call for heat and you have baro on it then one thing I would look into is a outdoor reset controller with "post purge" honeywell makes a few so does tekmar. Now here is the thing you have to think about most small units are fixed whether it be 10sec's (good for a small volume boiler like a munchin), 30 sec (good for most oil gas boilers) and then Adj (good for large volume or big boilers). the whole idea of the post purge is to contine to circulate water after the reset controller shuts the boiler down to move the building heat out of the boiler and into the zone were it is needed. for me and my setup I have to run a 2:30min post purge to get ride of the heat or I have a 20-40F temp over shoot. plus with the reset controller it will adjust boiler temp automatic and save fuel. if you have anymore questions let me know because I've been thru the mill with the honeywell units out there.

The temp over shoot happens after a call for heat, usually after a heavy use of domestic hot water. The boiler has a overheat safety, which is set to start the circ. pump at 20* over setpoint (165*) Eventually the temp begins to drop and everything is normal again. I was just hoping there would be a better way controlling this. Under normal use (heating the home) the temp holds to 160*-175*. Thanks again for all the help.

U235a4 wrote:if you have anymore questions let me know because I've been thru the mill with the honeywell units out there.

Do you have experience with the Honeywell AQ475A Aquatrol Outdoor Temperature Compensator for Boilers? How well might it work on an AHS S130 for year round use with a indirect hot water heater?

Yanche wrote:

U235a4 wrote:if you have anymore questions let me know because I've been thru the mill with the honeywell units out there.

Do you have experience with the Honeywell AQ475A Aquatrol Outdoor Temperature Compensator for Boilers? How well might it work on an AHS S130 for year round use with a indirect hot water heater?

Question for you, what controls do you have on the boiler now? And you answer your question yes it will but I would like to know more about your install aka # of zones, valves or pumps if you have a good picture just send it my way. just to let you know here are "SOME" of the differences of the units and prices

AQ475A1004 $154.04
• Adjustable settings for boiler maximum temperature, boiler minimum temperature and outdoor low (design) temperature.
• Selectable Warm Weather Shut Down (WWSD) at 70 F (21 C) to prevent summer boiler operation.
• Domestic Hot Water (DHW) priority.
• Replaceable circulator relay common to other
• Supply and outdoor sensors included.
• Automatic boiler operating differential.
• Automatic system circulator exercising. Long life DC relay drive control technology.
• Troubleshooting LEDs.
• Push-to-test button.
• Full size screw terminals.
• Functional replacement for Honeywell T475A Outdoor Reset.
• Controller and W964F Aquatrol (on-off mode).
• Post Purge 10sec

AQ2504B2 $219.83 - MUST USE AQ1000TN2 Thermo
• Factory configured for control of up to four zones plus domestic hot water.
• Expandable up to 16 zones with expansion zoning modules (sold separately).
• Single stage boiler control synchronized to zone-of-greatest-demand.
• Reduced boiler short-cycling.
• Integral Domestic Hot Water control.
• Installer selectable DHW Priority.
• Installer selectable DHW Priority Override
• Post Purge 30sec

AQ2514B2 $431.39 - MUST USE AQ1000TN2 Thermo
• Factory configured for control of up to four zones plus domestic hot water.
• Expandable up to 16 zones with expansion zoning modules (sold separately).
• Menu-based 3-line backlit LCD display.
• Separate user and installer configuration modes.
• Outdoor reset of boiler temperature.
• Optional Load reset of boiler temperature based on thermostat demand.
• Warm weather shut down. Installer program backup.
• Centralized 7-day setback program with warm-up boost.
• Single stage boiler control synchronized to zone-of-greatest-demand.
• Reduced boiler short-cycling. Integral Domestic Hot Water control.
• Installer selectable DHW Priority.
• Installer selectable DHW Priority Override.
• Freeze protection.
• Boiler shock protection.
• Burner short cycle protection.
• Boiler heat post purge. Self-test mode.
• Post Purge 0-30mins

AQ1000TN2 $70.62
2-WIRE NON-PROGRAMMABLE COMMUNICATING THERMOSTAT for use with AQ 250/251 controls

Zach [u235], did you write all that from memory???

I think the temp overshoot is from a long strong burn after as described a call for heat and hot water. So there is a LOT of heat in the firebox, and it is slow to drop down to a heat level that won't cause an overshoot.

Greg L

LsFarm wrote:Zach [u235], did you write all that from memory???

Greg L

Sure did, well ummmm some of it.... it's much easier to copy and paste then to type.

one more thing is that the R8888A-B zone panels have been made Obsolete and the replacement of those units are the AQ2504B2 (pumps or 2wire zone valves) or the AQ2504B4 (4 wire zone valves). or the fully adjustable units AQ2514B2/B4.

Although I appreciate A235a4,s detalied post, I think this is going way above my skill level or expertise. If things get anymore complicated, the next site I visit will be Ebay.. You also asked if the temp. creeps up. Depends on how hot I burn the fire between cycles. A nice orange glowing fire the temp creeps up. A not so hot fire (dark red deep in the bed) the temp holds around setpoint. Which way should I be burning the coal hot or not so hot? Another problem I am having is, the bed is not burning evenly. hot toward the back, next day hot in the front, sometimes the side is burning hot. Usually opening the ashpan door evens out the fire but eventually reverts back to the uneven burn. Back to my question about the firebox reducer, The Harman sf-260 is 130,000 btu designed to heat 2200sqft to 3200sqft. My home is 2 yrs old 2300sqft. with not alot of heat loss. the heat is being transferred through a 60,000btu water to air coil placed in the duct work of my propane furnace. Is this unit to large for my home? would a firebox reducer correct the problems I am having if the unit is to large. Thanks again for the help

Yanche, I did forgot to mention that the AQ250, AQ251 and the 475 don't work like a triple aquastat meaning they will only control temp when there is call for heat or DHW. IF you don't don't want to go for the bigger panels and can still get a R8888 (pump) or R8889 (zone valve) relay panel to run your zones on. Those panels would get hooked to the AQ475 to tell it there is a call for heat and then the 475 would mange boiler heat at the reset level. But you'll still need either a timer to run the unit like a cold start boiler setup or a triple aqua stat to maintain at least a low water temp. When you get the AQ475 and a R888? panel you pretty much have the price of the bigger AQ250 or AQ251. One other nice thing about the AQ251 is you can set you amount of times the boiler is allowed to run an HR, 2 4 or 6 mine is set to 2 which helps at saving fuel even more by only running the boiler twice and running more zones at once.

kmarsh wrote:Although I appreciate A235a4,s detalied post, I think this is going way above my skill level or expertise. If things get anymore complicated, the next site I visit will be Ebay.. You also asked if the temp. creeps up. Depends on how hot I burn the fire between cycles. A nice orange glowing fire the temp creeps up. A not so hot fire (dark red deep in the bed) the temp holds around setpoint. Which way should I be burning the coal hot or not so hot? Another problem I am having is, the bed is not burning evenly. hot toward the back, next day hot in the front, sometimes the side is burning hot. Usually opening the ashpan door evens out the fire but eventually reverts back to the uneven burn. Back to my question about the firebox reducer, The Harman sf-260 is 130,000 btu designed to heat 2200sqft to 3200sqft. My home is 2 yrs old 2300sqft. with not alot of heat loss. the heat is being transferred through a 60,000btu water to air coil placed in the duct work of my propane furnace. Is this unit to large for my home? would a firebox reducer correct the problems I am having if the unit is to large. Thanks again for the help

Thanks but if you have alot of over heating problems and the air/coal feed is right then a AQ251 would help you in several ways

Temp reset ( adjust boiler water temp base on your design temp to outside temp)
heating cycles ( this helps stop a boiler from short cycling meaning one zone calls for heat then stops then 5-10 mins later another zone calls for heat. instead it makes the first zone wait a little longer before turn on and then would run two zones)
post purge (basicly this feature continues to run the pump even after the the call for heat has ended and the boiler shuts down, it moves the hot water out of the boiler for X mins moving cooler water back into the boiler to be heat by the excess heat of the cooling boiler)

post purge? Where does this water go? Does it just continue to circulate through a closed loop sys. I have a over heat safety that circulates water at preset temp over setpoint. Again, filled to capacity with coal, max output is 130,000 btu and I am only using 60,000btu. isn't this excessive? is the unit to large?

Can you scan a drawing from the owners manual that shows the arrangement of the grates, the firebrick firebrick retaining structure in the firebox?? That is the only way I can see if you can make up a firebox reducer.

Greg L

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