Boiler Temperature

I've been experimenting a bit with my boiler temperature settings, what is the lowest I can run it at without causing problems? I think I heard that below a certain temp the gases will condense and rot out the steel? I know that the heat from the registers and the domestic hot water is just as hot when the boiler is at 150 degrees as it is when it's up around 200, I think it would use less coal maintaining a lower temp, what do you think?

I run mine at 150*, unless it is very cold out. Then around 160* or so. The determining factor is the heat output needed by your baseboard units or radiators. The hotter the water, the higher the BTU output of a baseboard or radiator. When it is really cold out, and the wind is howling, I need the extra BTU output from the higher water temp.

There won't be any condensation with the water temp at 150* +.

Greg L

e.alleg wrote:I think I heard that below a certain temp the gases will condense and rot out the steel? I know that the heat from the registers and the domestic hot water is just as hot when the boiler is at 150 degrees as it is when it's up around 200

I think what you are talking about here is oxygen. At 150*, there will still be quite a bit of oxygen entrained in the water. This can eat your boiler's metals. If you run closer to 200*, it will drive the oxygen out of the water and reduce your corrosion considerabley.

However, this applies to commercial/industrial applications more so than residential systems as the oxygen comes in with the feedwater. Since your home system may take on a quart or gallon of water every year, incoming oxygen is hardly a problem. In a commercial application, say a laundry, where massive steam losses are the norm, thousands of gallons of fresh water daily can bring in many pounds of entrained oxygen. Untreated, the boiler would dissappear quickly.

I wouldn't worry about it as long as the waters temperature is heating your home, your probably safe from the evils of oxygen.

Thanks. I have a problem though, the gauge on the front of the boiler doesn't correspond to the aquastat settings. I set the aquastat high limit at 150 and the boiler runs up to 180 before shutting off on high limit. I took it out and put some grease on the sensing bulb but it still doesn't match. The aquastat works fine, it's just the numbers don't match the gauge very accurately. It registers 30 degrees too low.

I would be more suspicious of the thermometer as the culprit here (does it read temp & press?), unless the small tube to your aquastat bulb is kinked or pinched. If it repeats consistently, I would not worry about whichever is causing the problem. Just adjust yourself to its innaccuracy or replace the offender if is really annoying you. Keep in mind that these two things are probably reading temperatures at different spots on the boiler, this along with some error in the instruments may be part of it. My aquastat is on the face of the boiler and the thermo is on the top and the temps are vaugely close to the setpoints.

The temperature/pressure gauge is on the front of the boiler, the aquastat is on the back at the same height. I might try to score a new aquastat off of ebay because mine might be bad. The circulator runs sometimes when it isn't supposed to, when there is no call for heat the circulator shouldn't run but sometimes it does. It's not so bad that it's worth buying one at retail prices though.

You may want to just replace the temp sensing bulb, the aquastat is pretty pricey in comparison, it is usually the bulb anyway.

I'm running mine at 140* right now. Like Greg, when it gets colder, I'll crank it up some. The first year I ran it at 180*-190* and all that did was burn alot more coal. My system works just fine with the cooler water and since I'm a penny pincher when it comes to energy, I like the coal savings.

If your worried about the flue gasses condensing because of the cooler temperature don't. Coal is not a hydrocarbon, and does not produce any significant water vapor when burned, so there is nothing to condense. Ever see steam or vapor coming out of your coal chimney on a very cold day?

Gas boilers that condense the water vapor out of the flue gas gain a significant amount of heat back due to the heat of vaporization, the additional heat reqired to change water from a liquid to a vapor. When it condenses, you get the heat back.

E.Alleg,

You mentioned that your circulator pump sometimes runs when it isn't suppose to. Just thought I'd throw out another possibility. Depending on the aquastat, and how everything is wired, sometimes there can be an emergency setting which will engage the circulator pump in a last ditch effort to radiate some heat before you blow the pressure relief valve if things get too hot.

I believe he does have an Honeywell overtemp controller on that unit. Generally set around 200-220*, its function is what you describe. Perhaps e. needs an adjustment as it may be set to low or there is another issue with it.

There does seem to be a problem with the aquastat. I studied the manual for it and also called Honeywell tech line and it seems like the control is bad. The part that is weird is on the low limit setting when there is no call for heat the circulator is energized. All other times it works fine. It doesn't matter when it's cold out because I want the circulator running to keep the heat exchanger warm. On a call for heat from the thermostat the stoker comes on and raises the water temperature enough so that the fan limit turns the fan on. I might experiment with a transformer relay which will allow the room thermostat to control the circulator pump directly. The funny thing is the outdoor wood boiler people should have figured this out years ago and come up with an automated system but I can't find any information on it on all the major manufacturer's sites. It seems that they are happy with an inefficient system that works "good enough". It's better than buying propane for sure, but there is no reason that my coal boiler can't be fully automated to the point where it can maintain a constant house temp. without constantly readjusting the controls. My hobby is starting to become full time.

If you need to buy a new aquastat consider the "Honeywell AQ475A Aquatrol Outdoor Temperature Compensator for Boilers". Product specs are on the Honeywell web site. It measures outside air temperature and adjusts the boiler water temperature accordingly. Hotter when cold, cooler when warm. Makes it possible to run your boiler during those mild days and reduces coal usage. Some boilers may require other controls to solve the fire going out problem at low usage.

On my AA 260m I have a HW 8124A on the front and the low is at 165 and the High at 190, on the rear I use a 4081 Low/high limit switch that is wired to be a outfire and the setting are, low 120 and high 230.... the low is if the boiler goes below 120 it stops unit from feeding coal to a dead fire and the high is there if the timer goes to run the unit for a 1min every hour and the temp is over 230 it won't run.

Thanks for the relies. I think I had the low limit set too high, or too close to the high limit. The low limit was firing the stoker and the high limit was running the circulator. It seems to be working great now, I set the limits farther apart.

On another note I'm doing much better than I thought with coal, the propane guy came to fill the tank the other day and I only burned through 1200 pounds of coal and basically no propane usage except the cook stove. I would have needed 230 gallons of LP minimum as he doesn't ever come unless the tank is down to 10% left based on degree day science (I was at 75% so I didn't take any) 230 gallons @ $2.699 (truck price on delivery day 11/14/07) = $620.77 + 4% tax = $645.60 vs. 24 fifty # bags @ $5.63 each. (tax and delivery inclusive)= $135.12. That's $510.48 in my pocket! and it's warm in here. I don't touch the thermostat, it is set at 67 degrees and this old house stays a very even 66-68 degrees.