"Understanding Your Aquastat"

Aquastat Basics

The aquastat controller on your outdoor boiler serves some very important purposes. Mostly, it controls the blower fan and draft, turning the draft on and off to keep the outdoor boiler water in a desired temp range.

If it weren't for your aquastat, your boiler would require constant attention so that your fire does not burn too hot (and boil out your water) or too cold.(and you freeze!).


The standard aquastat has a high limit setting, and a differential. The "High Limit" is the setting at which the aquastat cuts the power to the blower fan and the draft solenoid, which allows air to draft to the fire. This is normally set at 180 degrees F.  The "Differential" is a setting between 5 and 25 degrees, and this determines the temp at which the fan will be turned back on. So an aquastat set at 180, with a differential of 20, will turn ON power to the blower fan at 160, and turn OFF the power at 180 degrees, with the draft correspondingly opening and closing as needed.

Regardless of the outdoor temp, that setting will always keep the water jacket temp between 160 and 180 - this saves us from having to continually babysit the boiler, like most people have to do with an indoor wood stove.

Factory Settings

The traditional boiler models - GH and HE Series - are shipped with thefactory settings at 180 with a 20 degree differential. Here again, these settings will ensure that the boiler always maintains a water jacket temp between 160 and 180. Do not change these settings under normal operating conditions.

GX Model furnaces are set at the factory to operate up to 182 with only a10 degree differentialDo not attempt to change those settings without speaking to the OutdoorBoiler.com technical team first. This cannot be adjusted without special training guide, and there is almost never a need to change this.

GX Aquastat Controllers - Different Features

GX Aquastat Controllers are much different. They are electronic, digital, and they control other things such as the low-water shut off. If the "Water" light turns on, this indicates that your boiler is low on water. Just add water and this will enable the boiler to function as normal.  This is an added safety feature required by our industry safety standards.

How To Lose Money?

If your aquastat setting is set below 180, you are losing efficiency, and therefore losing money. There is almost never a reason to set your aquastat at any temp other than 180. Also, if your differential is set at anything other than 20, you are losing efficiency there also. If your custom needs vary, you may only try changing the differential from 20 to 15, but we do not recommend any other changes than that.

Calibration Error?

These aquastats are manufactured in Mexico. There have been a few reported incidences where the aquastat was set at 180, but the white dial thermometer was reading a different temp when the fan shuts off. This could be for two reasons:

1. The white dial is not properly seated in its well, or is out of calibration(more often the case, even though rare). Or,
2. The aquastat itself is not properly calibrated. If this is the case, and you set the aquastat at, say, 185 to get the fan to shut off at 180, that is fine. This was a common occurrence about 8 years ago with a batch of aquastats and Honeywell simply asked us to make that adjustment. This is not a big deal and does not affect the operation of any other part of the boiler.

Be Careful!

Please always be careful to never allow the boiler to operate with the door open, except for brief periods when loading wood or removing ash.

Also, never allow the boiler to operate at temps below 140 degrees F, unless you are first starting up the boiler in the fall, or shutting it down in the spring. At temps below 140, you will get huge amounts of creosote buildup.

How to Kill an Outdoor Boiler in Two Years

If you have an infloor radiant system, you must NOT allow outdoor boiler water to run through your infloor tubes. In-floor heating systems require much lower temps: 120F in cement, 115F for hardwood floors. Cement and hardwood floors can get damaged at higher temps, and I doubt most homeowners would be comfortable with a 180 degree floor

Some homeowners with infloor systems believe therefore that they can just turn down the temp on their aquastat to 120F  NEVER attempt to operate the outdoor boiler at these low temps as creosote and condensation will destroy your boiler. Take it from me - many years ago I tried heating  my pool with a test boiler we were testing for R&D purposes, and the boiler was not big enough to heat the pool properly. This boiler almost always ran at temps below 140...that boiler lasted less than two years before it had a catastrophic failure! We had a good laugh about that, but it was a valuable lesson learned.

Failed Aquastat?

Aquastats normally last a long time. But if you notice that your blower fan is not turning on at the same temp, you may need a new aquastat soon. These components normally die a slow death, so if you notice that youraquastat is powering the blower on and off at unpredictable temperatures, you are due for a new aquastat.

The aquastat is a cool little component in an outdoor boiler that allows us all to enjoy life without worrying about our heat, except once or twice per day when we need to add wood. Not only do we get to save lots of money on our heating bills, we also get to have a life in the meantime!  After all, we need lots of time to enjoy spending all that money we are saving on our heating bills!