Differential Setting

lewglick · Post by **lewglick** » Sun. Nov. 02, 2008 10:04 pm

If I don't use my boiler for domestic hot water, does it make a difference what the differential is set to?? Or what should it be set to??
Lew

traderfjp · Post by **traderfjp** » Sun. Nov. 02, 2008 10:10 pm

Are u leaving your boiler on for a backup in case the stove goes out and you're not home? I do this myself. I also heat my basement with coils in my stove. So I turn down the boiler's temp to 120/140. I would think you could do the same. That way if the stove goes out the boiler will have enough heat to keep everything from freezing. There is no reason to keep 160/180.

gambler · Post by **gambler** » Sun. Nov. 02, 2008 10:23 pm

I am no boiler expert, but I have read that you should not go below 140 as it is bad for the boiler ( rust and corosion)
I am sure one of the boiler dudes will give you the low down.

traderfjp · Post by **traderfjp** » Sun. Nov. 02, 2008 10:25 pm

I've heard that too but I've been doing it for several years will no ill effects and in the summer I turn it down either further. It uses a lot of oil to keep the boiler at 16/180.

Freddy · Post by **Freddy** » Mon. Nov. 03, 2008 6:42 am

Set the differential to 10*.

Oil boilers should not be kept below 140, but I think coal could go lower...if you can get them to stay running that cool and still burn properly.

traderfjp · Post by **traderfjp** » Mon. Nov. 03, 2008 6:52 am

I've been doing it for the last several years in the winter and for 15 years in the summer with no ill effects. You could try it one season and then drain the water in the boiler and see if there is any rust/corossion.

Post by **coaledsweat** » Mon. Nov. 03, 2008 8:39 am

Water temps over 150* drive the Oxygen out of the water, no OX, no corrosion. Corrosion isn't a serious issue for the homeowner unless you have leaks or other problems. The amount of makeup water in the average home is minuscule.

U235a4 · Post by **U235a4** » Mon. Nov. 03, 2008 9:00 am

Ok there is a difference between an oil boiler and coal boiler when below 140F. When a coil boiler is at idle even with the water temp say 135F your fire is still burning and flue gasses around say 170F-220F at least on the Axeman-Anderson units and alternate heating units so if any condensation would show up it would quickly evaporate from the heat and the water temp of the boiler is slowly going to climb to meet flue gas temps as the BTU off the coal is more than the heat loss off the outside the unit. Now with a oil boiler at 135F when that unit shuts off it's off and condensation can form on the inside of the boiler. so for a oil boiler if you’re using a Honeywell L8124 I would keep the low limit set to at least 150F because with a 10 diff the boiler will kick on @ 140F and kick off @ 150F if you would use 140F and a 10 diff it would kick on @ 130F and off @ 140F but to me that is too close because of cooling over time you would end up below 140F. Now you could set it at 140F with a 25 diff at which the boiler would kick on @ 130F and off @ 155F plus with the longer run time you would get better efficiency.

Post by **Rob R.** » Mon. Nov. 03, 2008 10:13 am

lewglick wrote:If I don't use my boiler for domestic hot water, does it make a difference what the differential is set to?? Or what should it be set to??
Lew

Are you talking about a coal boiler or an oil boiler? Are you using an oil boiler for backup? In either case the differential has a function, but depending on the type of fuel/boiler the settings can be different. I have an oil boiler used for backup heat, I decided to try and make it a "cold boiler" to elimiate standby fuel consumption. I will attempt to explain my setup and what I learned by reading by boiler manuals.

My boiler is an 11 year old oil-fired unit and has a Honeywell triple aquastat with a Hi, Low, and a differential setting. Mine was set as follows, Hi: 180F, Low: 160F, Differential: 10F. I have an electric hot water heater, so the boiler doesn't have a coil. The system heats the house fine, and behaves as follows: When nothing is calling for heat the boiler will sit idle until the jacket temperature reaches 160F (low limit), at that point the gun fires until the jacket gets to 180F (high limit), then the gun shuts off and stays off until the jacket cools to 160F again.

Lets say that the thermostat calls for heat, if the boiler is between 180-170F the circulator will turn on and the gun will not fire. This range is determined by the high limit and the differential setting, in my case the differential was 10F so the gun will kick on at 170F while there is a call for heat. Once the gun fires, it will run until the high limit is reached or the call for heat is satisfied, whichever comes first. If the boiler temperature happened to be below 160F when the call for heat was satisfied, (e.g. the thermostat called for heat and the circulator started to circulate cold water from the radiators back into the boiler then I turned the thermostat down below the room temperature) the gun will stay lit until the high limit of 180F is reached.

My coal stove keeps the house warm most of the time, but when it gets really cold or really windy the upstairs of the house can get chilly. I like to leave my boiler "on" for backup heat, but hate the fuel it uses to maintain the jacket temperature while my coal stove keeps the house toasty. This past weekend I sat down and read the manuals that came with my boiler and found an interesting paragraph. It basically said that if the boiler is not used to produce domestic hot water then it is pointless to maintain a minimum jacket temperature. It recommended disabling the lower temperature limit by moving a jumper wire and provided instructions. I followed the wiring directions, set the differential at 15F and turned the boiler on for a test run.

My boiler had been shut off for almost two days since I had to replace a leaky air vent, so the jacket temperature was down to room temperature. The room thermostat was set below the current temperature, so I knew it wouldn’t call for heat when I turned the boiler on. I turned the boiler on and to my delight the gun did not fire. I turned the thermostat up and the gun fired. The gun stayed on until the high limit was reached then shut off, the circulator kept running and when the boiler got down to 165F (180F minus the 15F differential) the gun fired again. This cycle repeated until the thermostat was satisfied and everything shut off. Now I have the best of both worlds, warmth from the coal stove and the oil boiler providing a silent backup, only firing if the coal stove can’t keep up.

Flue gas condensation becomes an issue if your oil boiler routinely operates with a jacket temperature below ~140F. When the jacket temps are this low, corrosive vapors in the flue gas condense on the cool boiler internal surfaces and corrode them. Having the aquastat settings too low (or too close) can cause this.

Flue gas condensation isn’t really an issue with coal boilers because there is little to no water vapor produced by burning anthracite.

-Robert

traderfjp · Post by **traderfjp** » Mon. Nov. 03, 2008 12:14 pm

There are also many boilers now that only turn on when there is a call for heat to save energy so you are never heating water just to maintain temp. These boiler are super effecient and don't have a lot of water in them. It probably takes longer to get heat but you save dollars. Somehow these boilers don't corrode.

U235a4 · Post by **U235a4** » Mon. Nov. 03, 2008 12:26 pm

traderfjp wrote:There are also many boilers now that only turn on when there is a call for heat to save energy so you are never heating water just to maintain temp. These boiler are super effecient and don't have a lot of water in them. It probably takes longer to get heat but you save dollars. Somehow these boilers don't corrode.

These boilers are called low-mass boilers and are normaly made of stainless steel, and have no problem being setup as a cold-start boiler (meaning only fire when there is a call for heat). older oil boiler and coal boilers are what we call high-mass boiler being made of cast-iron or a lot of normal steel which don't normaly fair the best when dealing with condensation. Also remember that cast-iron boilers can suffer from thermal shock and crack.

lewglick · Post by **lewglick** » Mon. Nov. 03, 2008 7:01 pm

markviii wrote:
lewglick wrote:If I don't use my boiler for domestic hot water, does it make a difference what the differential is set to?? Or what should it be set to??
Lew

Are you talking about a coal boiler or an oil boiler? Are you using an oil boiler for backup? In either case the differential has a function, but depending on the type of fuel/boiler the settings can be different. I have an oil boiler used for backup heat, I decided to try and make it a "cold boiler" to elimiate standby fuel consumption. I will attempt to explain my setup and what I learned by reading by boiler manuals.

My boiler is an 11 year old oil-fired unit and has a Honeywell triple aquastat with a Hi, Low, and a differential setting. Mine was set as follows, Hi: 180F, Low: 160F, Differential: 10F. I have an electric hot water heater, so the boiler doesn't have a coil. The system heats the house fine, and behaves as follows: When nothing is calling for heat the boiler will sit idle until the jacket temperature reaches 160F (low limit), at that point the gun fires until the jacket gets to 180F (high limit), then the gun shuts off and stays off until the jacket cools to 160F again.

Lets say that the thermostat calls for heat, if the boiler is between 180-170F the circulator will turn on and the gun will not fire. This range is determined by the high limit and the differential setting, in my case the differential was 10F so the gun will kick on at 170F while there is a call for heat. Once the gun fires, it will run until the high limit is reached or the call for heat is satisfied, whichever comes first. If the boiler temperature happened to be below 160F when the call for heat was satisfied, (e.g. the thermostat called for heat and the circulator started to circulate cold water from the radiators back into the boiler then I turned the thermostat down below the room temperature) the gun will stay lit until the high limit of 180F is reached.

My coal stove keeps the house warm most of the time, but when it gets really cold or really windy the upstairs of the house can get chilly. I like to leave my boiler "on" for backup heat, but hate the fuel it uses to maintain the jacket temperature while my coal stove keeps the house toasty. This past weekend I sat down and read the manuals that came with my boiler and found an interesting paragraph. It basically said that if the boiler is not used to produce domestic hot water then it is pointless to maintain a minimum jacket temperature. It recommended disabling the lower temperature limit by moving a jumper wire and provided instructions. I followed the wiring directions, set the differential at 15F and turned the boiler on for a test run.

My boiler had been shut off for almost two days since I had to replace a leaky air vent, so the jacket temperature was down to room temperature. The room thermostat was set below the current temperature, so I knew it wouldn’t call for heat when I turned the boiler on. I turned the boiler on and to my delight the gun did not fire. I turned the thermostat up and the gun fired. The gun stayed on until the high limit was reached then shut off, the circulator kept running and when the boiler got down to 165F (180F minus the 15F differential) the gun fired again. This cycle repeated until the thermostat was satisfied and everything shut off. Now I have the best of both worlds, warmth from the coal stove and the oil boiler providing a silent backup, only firing if the coal stove can’t keep up.

Flue gas condensation becomes an issue if your oil boiler routinely operates with a jacket temperature below ~140F. When the jacket temps are this low, corrosive vapors in the flue gas condense on the cool boiler internal surfaces and corrode them. Having the aquastat settings too low (or too close) can cause this.

Flue gas condensation isn’t really an issue with coal boilers because there is little to no water vapor produced by burning anthracite.

-Robert

This is a coal burner. Great info, could you relate it to coal?
Lew

Post by **stoker-man** » Mon. Nov. 03, 2008 7:20 pm

The differential setting is for the circulator. Since you don't need to stop the circulator to preserve the domestic hot water, turn it to 25. In fact, you don't need a triple aquastat at all.