Aquastats: High Limit, Low Limit, and How to Choose a Replacement

Ask what keeps a hot water boiler from running away, and most people point to the relief valve. The relief valve is the last line of defense. The aquastat is the control that's supposed to make sure it never gets that far. It watches the boiler water temperature and switches a circuit on or off when the water crosses a setpoint, which makes it one of the most common replacement parts on any hydronic system.

How an Aquastat Works

Most aquastats use a temperature-sensing element, either a bulb on a capillary tube or a probe, that sits in an immersion well in the boiler or in a strap-on position on the pipe. As the water heats or cools, the element moves a switch. That switch opens or closes a low-voltage or line-voltage circuit, depending on the control and what it's wired to. Electronic versions do the same job with a thermistor and a circuit board instead of a mechanical element, and they usually add a digital display and more precise setpoints.

High Limit, Low Limit, or Both

• High limit. This cuts the burner when the boiler water reaches the maximum allowable temperature. It's a safety function, and on many boilers it's a manual-reset control, so someone has to look at the boiler before it fires again.
• Low limit. This keeps the boiler water above a minimum temperature, or holds off a circulator until the water is warm enough. On a cold-start boiler it helps keep flue gases from condensing inside the boiler.
• Combination controls. Many aquastats pair a high limit and a low limit in one housing, with separate switches for each. The Weil-McLain 381-300-000 electronic kit is built this way, with both high and low limit functions.

What to Match on a Replacement

Aquastats look alike and are easy to confuse. Before ordering, match these against the control you're replacing:

• Temperature range. Listings in our catalog span ranges such as 100 to 240°F, 120 to 180°F, and 40 to 180°F. A control whose range doesn't cover your intended setpoint isn't a substitute, however similar it looks.
• Switching action. SPST and SPDT switches wire differently. The wrong one won't do what the original did.
• Reset type. Manual reset and automatic reset behave differently after a trip. Swapping one for the other changes how the boiler responds to an overtemperature event, so match what the manufacturer specified.
• Sensing method and size. Capillary length, bulb size, and well or connection size (the Armstrong Astro line, for example, comes in 1/2" and 3/4" connections) all have to fit the existing installation.

Aquastats Paired With Circulators

Some aquastats are made specifically to run a pump rather than protect a boiler. The Bell & Gossett AQS-1/2 and AQS-3/4 are sold for use with NBF bronze circulators, switching the pump by water temperature in a recirculation setup. They solve a different problem from a boiler limit control, so don't treat the two types as interchangeable.

A Safety Control, Not a Convenience

A failed low limit usually shows up as a boiler that won't hold temperature. A failed high limit can go unnoticed because nothing seems wrong until the day it's needed. If a boiler has a manual-reset high limit that keeps tripping, treat that as a symptom to investigate, not a nuisance to reset. It works together with the low water cut-off and the safety relief valve as part of the layered protection on a boiler, and replacing one of them with a close-enough part weakens the whole arrangement.

National Boiler Supply carries aquastats from Weil-McLain, Lochinvar, Bell & Gossett, and Armstrong. If you have the model number off the old control, send it over and we'll match it.