Honeywell R8182D control explainedHeating Boiler Aquastat Selection
How to select the right boiler aquastat control

  • AQUASTAT SELECTION GUIDE - CONTENTS: Guide choosing the right heating boiler combination control aquastat or water heater aquastat control, or to replace obsolete aquastats such as the Honeywell L8024B,D with the L8124-series controllers. Honeywell L7224U1002
  • POST a QUESTION or READ FAQs about Aquastats controls used on Heating Boilers

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Guide to aquastat control selection for heating boilers or water heaters:

How to figure out which aquastat controller your heating boiler or water heater wants and needs.

This article series explains how aquastats work and what the different aquastat controls are, what they do, and how they are set. We define the HI LO and DIFF controls on heating boiler aquastats and explains what they do and how they work. We explain the location and use of the heating boiler reset button found on aquastats.

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Aquastat Selection Guides

Honeywell L8148 Aquastat description (C) The photo at page top shows a Honeywell R8182D combination heating control, also called an "aquastat". Shown in in our illustration here is the Honeywell L8148 Aquastat. So what's th difference among the many types of aquastats used on heating boilers and water heaters or calorifiers or geysers and how do we know which aquastat should be installed?

[Click to enlarge any image]

Question: how do I know which aquastat controller to use on my heater

Iva said:

How do I know if the aquastat controller is the right one for my oil burner furnance.


Let's start by being sure we're talking about the same type of heating equipment, Iva. I call forced air or warm air heating systems "furnaces", hot water heating systems "boilers" or "hydronic heating systems", and steam heating systems or "steam boilers". If you're not sure what kind of heat you have, see HEATING SYSTEM TYPES.

That out of the way, aquastats are controllers used on hot water or hydronic heating system boilers to control the boiler's heating burner by turning it on or off in response to the boiler temperature, and often to also switch a heating zone circulator pump on or off. Aquastats also may have additional controls and features as I'll describe below.

Features & Properties of a Heating Boiler Aquastat

Basic aquastat features are few and simple:

An aquastat just has a few basic functions to perform: control the high limit that will turn on and off the burner in a response to a call for heat, and to keep heat in the boiler to support a tankless coil if one is installed. If your boiler doesn't use a tankless coil then you don't need those features, so you would either choose a simpler aquastat without support for a coil or you'd choose one that has those features but turn the features off in the control.

Aquastat models vary by physical orientation

Some confusion among aquastat models has to do with the need to physically fit the aquastat in position on the boiler where other pipes and controls may be in its way. So the same aquastat might be sold designed to be mounted with its rectangular body oriented vertically versus horizontally.

Aquastats vary by using a temperature sensor integral with the aquastat versus a temperature sensor at the end of a flexible coil

Honeywell L8148 Aquastat description (C)

Most aquastats mount directly to the body of the boiler, with a temperature sensor on the back of the aquastat designed to insert into a temperature sensor well on the boiler (along with some heat conductive grease). You can see that type of aquastat by looking at the back of the Honeywell L8148 aquastat shown just above.

Other aquastats may have a temperature sensor fixed directly to the aquastat back - shown below using a White Rodgers Aquastat #1131-102 produced by Emerson Controls - White Rodgers whose website, unlike their competitor Honeywell, make it almost impossible to find a chart or table listing all of the company's boiler and water heater controls: they prefer that you give White Rodgers/Emerson a call.

White Rodgers Aquastat selection guide (C)

But if there is not enough room around the temperature sensing well on a particular boiler, then an aquastat might mount somewhere nearby while its temperature probe is at the end of a flexible coil of tubing so that the sensor itself can be inserted into the sensor well.

Honeywell L4008 Aquastat controller (C)

Use a Universal Replacement Aquastat to Simplify Control Selection

1. To simplify aquastat replacement across a very wide range of boilers, manufacturers like Honeywell make a "universal"replacement unit.

If you choose a universal type aquastat to put on your heating boiler then you're pretty safe that the
Device and be made to work correctly for your needs. That's because the specific features that need to be tailored, such as a presence or absence of a tankless coil for domestic hot water, or the actual temperature settings, can be set or chosen using the universal aquastats built-in features.

2. Typically the installation manual for your boiler will specify what type of controller it needs.

3 the manufacturers of aquastats also include or provide a substitution chart so that if you are replacing an existing control that is no longer sold you can find a model that replaces it

So How do I Choose an Aquastat - "which aquastat do I need" ?

I agree that the number of aquastat controller models can be overwhelming; one needs to consult both the boiler manufacturer and the aquastat manufacturer to see what heating boiler features need to be controlled by the aquastat, what sort of burner and burner electrical wiring need to be controlled by the aquastat, the placement of the aquastat's temperature sensing device, and also the physical mounting space for the aquastat.

If you Do Know the Brand & Model of Aquastat that you are Replacing Use One of These Cross Reference Charts

Then use a manufacturer's aquastat cross reference or replacement chart or select a universal replacement aquastat model.

If you Don't Know the Previous Aquastat Brand & Model Find the Controller Features Your Heater Needs

Basically, if you do not know the brand and model of the aquastat that you are replacing, then you choose an aquaststat by finding or specifying both boiler features and physical installation space for your specific boiler - which is why I said start with the boiler's installation manual.

Examples of Features on Aquastats

Multi-purpose aquastats are controls that manage both heating boiler on and off temperatures or maintenance temperatures and also other features or services. Here are things you'll consider when choosing an aquastat of this type:

Here is an example of aquastat sub-models and their differences using the Honeywell L6006 and L6007 Aquastat Controller as a model:

These Aquastat® Controllers operate in response to temperature changes in hydronic heating systems. They provide spdt switching for three-wire applications. The L6006A,B, and L6007A combine low or high limit and circulator control; L6006C combines circulator control with low and high limit.

The L6006A,B are for horizontal insertion; the A model uses an immersion well; the B model uses a capsule compression fitting for direct immersion. The L6006C is for horizontal or vertical surface mounting. The L6007A is for horizontal or vertical insertion using an immersion well .

Aquastats including a Differential Control

Aquastats in "combination" mode include a feature to support the presence of a tankless coil, by providing a LOW-DIFF control pair; the number of degrees in the DIFF or "differential" setting may vary, but in general the DIFF setting controls the number of degrees of temperature range to be maintained in the boiler to support a tankless coil used to make domestic hot water (washing and bathing).

At AQUASTAT CONTROLS we explain and define the HI LO and DIFF

At AQUASTAT HI LO DIFF SETTINGS we describe the best HI LO DIFF settings to use

At AQUASTAT LO & DIFF DISABLED we explain how to disable the LO and DIFF if you are installing a combination aquastat on a boiler that doesn't sport a tankless coil used for making domestic hot water (for washing and bathing)

Single-purpose Aquastats

Single purpose aquastats are used on older heating equipment and on some water heaters. So if your heating appliance is a water heater that needs only a HI limit and LO limit it may use an aquastat like the Honeywell L6006C1018 offering a boiler temperature range of 65 to 200 deg F while the single purpose HI LO aquastat Honeywell L4006A2007 controls temperatures from 100 to 240 degF and adds a DIFF adjustable between 5 and 30 degF.

You can see that the L4006A2007 would be suitable for a water heater but not a home heating boiler.

Aquastat & Boiler Controls & Information Sources

We provide links just below to several aquastat installation, setting, and adjustment documents in response to reader requests and comments that people sometimes have difficulty finding this information. But readers looking for specific aquastat control information should always first try the control manufacturer.


Continue reading at AQUASTAT CONTROLS - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see MANUALS for HEATING SYSTEM CONTROLS for a list of aquastat installation & repair guides

Or see these

Article Series Contents

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