Float & Thermostatic Steam Traps: Continuous Drainage for Modulating Systems

If you read the first post in this series on inverted bucket steam traps, you'll remember that one of the design's core limitations is intermittent drainage. Condensate accumulates in the trap body between discharge cycles rather than being removed the moment it forms. For stable-pressure steam mains and steady industrial loads, that's workable. For modulating systems, heat exchangers, and process equipment where steam pressure fluctuates and condensate loads vary continuously, it isn't.

That's where the float and thermostatic trap (commonly abbreviated F&T) earns its place. It is the workhorse of HVAC steam systems and light-to-medium industrial process service, and for good reason: it handles the operating conditions that account for the majority of steam-using equipment in commercial and institutional buildings.

This post covers how F&T traps work, where they perform best, where their limitations matter, and how to choose across the Mepco 42, Mepco 44, Spirax Sarco FT, and Watson McDaniel WFT series available at Athena Supply.

How a Float & Thermostatic Steam Trap Works

The F&T trap combines two distinct operating mechanisms in a single body (one for condensate, one for air), which is what gives it its name and its versatility.

The float mechanism handles condensate. A sealed spherical float sits inside the trap body, submerged in condensate. As condensate enters and the level rises, the float rises with it, opening a valve connected by a lever to the float. Condensate discharges continuously through that valve at exactly the rate it enters the trap. When the condensate level drops, because steam pressure is high and condensate formation is slow, the float drops, partially closing the valve. The modulation is continuous and proportional to load, which means no condensate backup and no waterlogging of the steam space.

The thermostatic air vent handles air and non-condensable gases. Located in the upper portion of the trap body, in the steam space above the condensate level, it uses a temperature-sensitive element (typically a balanced pressure bellows or capsule) to detect the cooler temperature of air or non-condensables relative to steam. When air is present, the vent opens and purges it. When live steam reaches the vent, the element responds to the higher temperature and closes. This operates entirely independently of the float mechanism, which means the trap can vent air aggressively on startup while simultaneously draining condensate.

That combination of continuous modulating condensate drainage plus independent active air venting is what makes the F&T trap the right choice for systems that the inverted bucket cannot handle well.

Where F&T Traps Excel

Modulating systems. Any application where a control valve regulates steam pressure to maintain a setpoint temperature (heat exchangers, unit heaters, coil heating systems, air handlers) creates a constantly varying condensate load. The F&T float responds instantly and proportionally. There is no cycling lag, no prime loss risk, and no erratic behavior when steam pressure swings from maximum to near-vacuum. This is the application the F&T was designed for.

Air venting on startup. Large steam spaces, coil systems, and heat exchangers can accumulate significant volumes of air during shutdowns. The thermostatic air vent in an F&T trap purges that air quickly and independently on startup, allowing steam to reach the heating surface without delay. This is a meaningful operational advantage over the inverted bucket, which vents air slowly and only through a small orifice constrained by the need to retain the bucket's water seal.

Continuous heat transfer applications. In applications where the rate of heat transfer depends on keeping the steam space free of condensate at all times (shell and tube heat exchangers, jacketed process vessels, unit heaters) the F&T's continuous drainage is a direct performance advantage. Condensate that backs up into a steam space reduces effective heating surface area and lowers heat transfer rates. The F&T prevents that.

Light and variable condensate loads. The float modulates down to near-zero condensate flow without losing function. Applications that cycle between heavy and light loads, or that operate at reduced capacity for portions of a shift, are handled smoothly.

Backpressure tolerance. F&T traps operate against backpressure as long as a differential pressure exists across the trap. The float responds to the net differential, not absolute inlet pressure, which means moderate backpressure in a condensate return line reduces capacity but does not cause the trap to fail or flood.

Where F&T Traps Fall Short

The F&T is the right trap for a wide range of applications, but it has documented limitations that are worth understanding before specifying.

Water hammer vulnerability. The thermostatic air vent element (typically a bellows or capsule) is the weakest point in the trap under hydraulic shock. A slug of condensate hitting the vent at high velocity can collapse or damage the element, leaving the trap unable to vent air. Modern designs use more robust all-stainless steel capsule elements that handle moderate water hammer better than older designs, but the F&T remains more susceptible to shock damage than the inverted bucket. On steam mains where slugging is a known risk, additional protection such as a separator upstream is worth considering.

Freeze susceptibility. Like the inverted bucket, the F&T trap contains standing condensate and is vulnerable to freeze damage in exposed or uninsulated locations. The float and valve mechanism can be damaged by ice formation. In outdoor or unheated spaces subject to freezing temperatures, the trap needs to be drained when not in service, or a more freeze-tolerant design should be considered.

Dirt sensitivity. The float valve and seat operate on precise geometry; dirt, scale, or debris that reaches the valve seat can prevent a tight shutoff and cause the trap to pass live steam. Upstream straining is important with F&T traps, more so than with the inverted bucket's self-scrubbing mechanism. In systems with older piping or poor water quality, keeping a clean strainer upstream of the trap is not optional.

Superheat service. The standard balanced pressure thermostatic air vent element is sensitive to temperature. In superheated steam applications, the element can be damaged by temperatures beyond its design range. When specifying an F&T trap for superheated service, confirm that a bimetallic or live-orifice air vent is available for the model selected.

Mepco 42 Series: In-Line for Space-Constrained Applications

The Mepco 42 series is an in-line float and thermostatic trap with a single inlet/single outlet configuration; steam enters and condensate discharges from opposite ends of the trap in a straight-through piping arrangement. This makes it well-suited to tight mechanical spaces where a side-port or H-pattern trap would require additional piping offsets.

Available in sizes ¾", 1", 1-¼", 1-½", and 2", with pressure ratings of 15, 30, 75, and 125 psig, the 42 series covers the standard HVAC and light industrial pressure range. It is applicable to all types of steam heating and steam process equipment where continuous condensate drainage with active air venting is required.

The in-line configuration also simplifies retrofits in existing piping where the original trap had a straight-through connection. If you're replacing an older in-line trap and want to stay in the same footprint, the 42 series is a natural fit.

Reference: Mepco 42 Series In-Line Float & Thermostatic Steam Trap — Mepco LLC

Mepco 44 Series: Double Inlet/Double Outlet for Flexible Installation

The Mepco 44 series uses an H-pattern double inlet/double outlet configuration; steam can enter from either side of the trap, and condensate can discharge from either the same side or the opposite side. This piping flexibility is a practical advantage during installation and when repurposing existing pipework, as the trap can be oriented to suit whatever connections are available without adding elbows or offsets.

Available in sizes ¾", 1", 1-¼", and 1-½" with pressure ratings of 15, 30, 75, and 125 psig, the 44 series covers the same service envelope as the 42 series. Standard construction is cast iron with stainless steel internals throughout. Capacity ranges from 300 lb/hr at the low end up to 4,300 lb/hr depending on size and pressure differential.

The 44 series is specifically recommended for modulating pressure applications where loads vary widely and pressures can swing from maximum to near-vacuum, precisely the conditions where the inverted bucket struggles and the F&T excels. It handles startup air loads well and operates reliably against backpressure.

Repair kits for the 44 series are available as either a full head assembly (cover, gasket, float mechanism, thermostatic element cage, and valve seat) or an internal mechanism only (float, linkage, and seat without disturbing the cover). Both allow complete in-line repair without disconnecting the trap from piping.

Reference: Mepco 44 Series Float & Thermostatic Steam Trap — Mepco LLC

Spirax Sarco FT Series: Parallel Configuration with a Wide Pressure Range

The Spirax Sarco FT series is a cast iron F&T trap with parallel (same-side) port connections, with inlet and outlet on the same side of the body, available in sizes ¾" through 2" and pressure ratings from 15 through 125 psig on the standard FT, extending to 150 and 200 psig on the FT-150 and FT-200 variants. Condensate capacity reaches up to 10,900 lb/hr depending on size and differential.

A notable feature of the Spirax Sarco FT line is the balanced pressure stainless steel thermostatic air vent, which is designed to resist water hammer damage, a meaningful durability improvement over older bellows designs in systems where slugging cannot be entirely ruled out.

For installations requiring higher pressure ratings beyond what the standard 44 series covers, particularly in the 150 to 200 psig range, the Spirax Sarco FT-150 and FT-200 variants extend the usable pressure envelope while keeping the same cast iron body and stainless internals. The FTB series further extends this into super-capacity territory, with sizes up to 2-½" and capacities to 100,000 lb/hr for large process applications.

Reference: Spirax Sarco FT Series Float & Thermostatic Steam Trap — Spirax Sarco

Watson McDaniel WFT Series: The Widest Pressure Range in the Group

The Watson McDaniel WFT series stands out in this comparison for one specification that the other series don't match: a maximum operating pressure of 250 psig. Available in sizes ¾" through 2" with pressure ratings of 15, 30, 75, 125, 175, and 250 psig, the WFT covers the full HVAC and light industrial range and extends into medium-pressure process service that would require a different trap entirely from the other series covered here.

Construction is cast iron with all-stainless steel internals including a welded stainless steel thermostatic air vent, a design that improves durability over brazed or mechanically assembled vent elements. All internal components are attached to the cover for simplified in-line repair; removing the cover gives complete access to the float, valve, and vent assembly without disturbing the trap body or piping connections.

The WFT is specifically designed for HVAC and industrial process applications (unit heaters, pressing machines, heat exchangers, and coils) and is rated to 450°F maximum allowable temperature across the full pressure range. For facilities running higher-pressure steam distribution that feeds into process or HVAC equipment, the WFT's 250 psig rating means a single trap series can cover the full system rather than requiring a different trap type at the distribution level.

One installation note: like all F&T traps, the WFT must be installed upright and level for the float mechanism to operate properly. Unlike the inverted bucket, which is tolerant of slight misalignment, the float depends on gravity to modulate correctly; a trap installed at even a slight tilt can exhibit erratic drainage or fail to fully close.

Reference: Watson McDaniel WFT Series Float & Thermostatic Steam Trap — Watson McDaniel

Choosing Between These Series

Unlike the inverted bucket comparison in P1, where Armstrong and Mepco were spec-equivalent and the decision came down to availability and price, the F&T lines covered here have genuine specification differences that matter at the selection stage.

Series Comparison at a Glance

On pressure range: If your application runs above 125 psig, the Mepco 42 and 44 series are no longer options. The Spirax Sarco FT-150 and FT-200 extend to 150 and 200 psig respectively. The Watson McDaniel WFT reaches 250 psig and is the only series in this group that does. For medium-pressure process service, the WFT should be the starting point.

On connection configuration: In-line piping favors the Mepco 42. Side-port H-pattern piping favors the Mepco 44. The Spirax Sarco FT and Watson McDaniel WFT both use parallel same-side connections. Confirm your piping layout before specifying; forcing a trap into an incompatible connection pattern adds fittings, potential leak points, and installation cost.

On capacity: The Mepco 42 tops out at 6,830 lb/hr (2", 125 psig differential). The Mepco 44 tops out at 4,300 lb/hr (1-½", 15 psig differential). The Spirax Sarco FT reaches 10,900 lb/hr on the standard series, with the FTB extending to 100,000 lb/hr for large applications. The Watson McDaniel WFT reaches 10,930 lb/hr (2", 15 psig differential). If your condensate load calculation (with the recommended 1.5–2.5× safety factor applied) exceeds what the Mepco series can pass, the Spirax Sarco or Watson McDaniel lines are the next step.

On water hammer risk: If slugging or hydraulic shock is a known risk in the system, the Spirax Sarco FT's water hammer-resistant stainless steel vent element and the Watson McDaniel WFT's welded stainless vent both offer better durability than a standard bellows design. Upstream separation or straining should still be addressed regardless of which trap is selected.

Sizing Checklist

Before specifying any float and thermostatic steam trap, confirm the following:

1.     Maximum inlet steam pressure (psig): determines which series are eligible; the trap's PMO must exceed maximum operating pressure

2.     Condensate load: lb/hr at maximum operating conditions, with a safety factor of 1.5–2.5× applied for F&T traps per standard practice

3.     Backpressure (psig): net differential pressure across the trap determines actual capacity; size to differential, not inlet pressure alone

4.     Connection size and configuration: in-line, H-pattern, or parallel; confirm before selecting a series

5.     Installation orientation: F&T traps must be installed upright and level; horizontal or tilted installation impairs float operation

6.     Water hammer risk: if slugging is possible, specify a trap with a robust stainless steel or welded vent element and address upstream separation

7.     Superheat: if superheated steam is present, confirm air vent element suitability or specify a live-orifice vent

8.     Ambient conditions: assess freeze risk for exposed or uninsulated locations; drain trap when not in service if freezing temperatures are possible

Summary

The float and thermostatic trap is the right choice for the majority of HVAC steam systems and process applications with modulating loads, variable pressures, or high air venting requirements on startup. Its continuous drainage, independent thermostatic air venting, and proportional response to load make it a more capable trap than the inverted bucket for these conditions, though it requires more attention to dirt, freeze protection, and level installation.

The Mepco 42 and 44 series are well-suited to standard HVAC and light industrial service up to 125 psig, with the 42 offering in-line simplicity and the 44 offering piping flexibility. The Spirax Sarco FT series extends the pressure ceiling and capacity range for larger or higher-pressure applications. The Watson McDaniel WFT reaches 250 psig and is the strongest option when medium-pressure process service is in scope.

All four series are available at Athena Supply.

Part of The Agora's Steam Trap Mastery series. Previously: Inverted Bucket Steam Traps. Next: Thermostatic Radiator Traps, the workhorse of low-pressure HVAC steam systems.