Outdoor Furnace: Understanding Your Aquastat

Outdoor Furnace: Understanding Your Aquastat

Outdoor Furnace: Understanding Your Aquastat

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 the 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!). After all, we need plenty of time to spend all of the money we saved on heating expenses!


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 Aquastats- are shipped with the factory 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 a 10 degree differential. Do 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 your outdoor wood furnace 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. You can adjust the Aquastat to, say, 185 to get the fan to shut off at 180. This was a regular issue with a batch of Aquastats around 8 years ago, and Honeywell just requested us to make that modification. This is not a huge deal and has no effect on the operation of the boiler.

How to Kill an Outdoor Boiler in Two Years

If you have an in-floor radiant system, you must NOT allow outdoor wood boiler water to run through your in-floor tubes. In-floor heating systems require much lower temps: 120F in cement, and 115F for hardwood floors. Higher temperatures can harm cement and hardwood floors. 

Some homeowners with in-floor systems believe therefore that they can just turn down the temp on their Aquastat to 120F.

Never allow your outdoor wood stove to operate at temps below 140 degrees F, unless you are first starting up the outdoor wood burner in the fall, or shutting it down in the spring. At temps below 140, you will get huge amounts of creosote buildup.

Failed Aquastat?

Aquastats often have a lengthy lifespan. These components generally die slowly, so if you notice your Aquastat turning the blower on and off at unusual temperatures, it's time for a replacement.