When one room in a hydronic (hot water) heating system goes cold, or one zone won't shut off, the zone valve is often the culprit. The good news is that many zone valve failures are in the power head (actuator), not the valve body, so the repair can be simpler than it looks. This guide explains how zone valves work, how to tell 2-wire, 3-wire and end-switch valves apart, and how the two most common residential brands, Taco and Honeywell (Resideo), compare. It also covers the general replacement steps.
How a zone valve works
A zone valve is an electrically operated valve on the supply or return pipe of one heating zone. When the thermostat for that zone calls for heat, it sends low voltage (usually 24 VAC from a transformer) to the valve's actuator. The valve opens, hot water flows through that zone's baseboard, radiators or radiant tubing, and the room warms up. When the thermostat is satisfied, the valve closes again.
Most residential zone valves have two parts:
- The valve body: The brass part piped into the system with sweat (solder), threaded (NPT), flare or press connections.
- The power head or actuator: The electrical part that opens and closes the valve. On most Taco and Honeywell residential valves, you can replace the head without draining the system or cutting pipe.
Heat motor vs motorized actuators
The two brands use different ways to move the valve:
- Taco 570 series (heat motor): The head contains a wax-filled element wrapped with a small heater. When 24 VAC is applied, the heater warms the wax, which expands and pushes the valve open. Heat motor valves open gradually, so a short delay after the thermostat calls is normal.
- Honeywell V8043 / V8044 (motorized, spring return): A small electric motor drives the valve open, and a spring closes it when power is removed. Honeywell lists about 30 seconds of valve timing on the V8044A.
2-wire vs 3-wire vs end switch
Wiring is where most replacement mistakes happen, so identify your valve's wiring before you order parts.
- 2-wire (no end switch): The thermostat circuit powers the actuator, and that's it. The valve doesn't send any signal back to the boiler or circulator. These valves are used where a zone control panel or another device handles boiler and pump control.
-
End switch valves: An end switch is a small switch inside the head that closes when the valve is fully open. It is typically wired to start the boiler or circulator, so the pump only runs once a zone is actually open. This is the most common setup in residential systems that don't use a zone control panel.
- 3-wire: Some end switch valves share one terminal between the actuator and the end switch. Taco's 570 series uses a three-terminal block.
- 4-wire: Others keep the end switch completely separate. A Honeywell V8043E, for example, has motor leads plus separate end switch leads.
- Terminal block vs lead wires: Some heads have screw terminals, and others come with pre-attached lead wires. Honeywell's V8043F, for example, uses a terminal block, while the V8043E has 18" lead wires.
Tip: Before you disconnect anything, take a clear photo of the existing wiring and write down the full model number printed on the head and on the valve body.
Common Taco zone valve models
Taco 570 series (Gold Series heat motor valves)
The 570 family (570, 571, 572 and 573) covers different pipe sizes. It is one of the most widely installed residential zone valve lines in the US. The Taco 571-2 is a 3/4" sweat, 2-way, normally closed heat motor valve with an end switch and a 3-wire terminal block. It is the model people search for most when they replace a Taco zone valve on 3/4" copper.
If the valve body is fine and only the head has failed, the Taco 555-050RP replacement power head fits 570 series valves. It lets you service the valve without removing the body from the pipe.
Taco Zone Sentry
Zone Sentry is a separate Taco line. It is a motorized ball valve design (model numbers such as Z075C2 for 3/4" sweat), available in 2-way and 3-way versions with an end switch. It has a manual override on top that also shows the valve position. Zone Sentry heads aren't interchangeable with 570 series heads, so match the replacement to the line you have.
Common Honeywell (Resideo) zone valve models
Honeywell Home zone valves are now made by Resideo, so you may see either name on the label and packaging.
-
V8043 (2-way, 24 VAC): The most common Honeywell residential zone valve.
- V8043A (normally closed) and V8043B (normally open) are the basic versions.
- V8043E adds an end switch with 18" lead wires.
- V8043F adds an end switch with a terminal block.
- V8044 (3-way diverting, 24 VAC): Used where flow has to be diverted between two paths instead of simply turned on and off.
- V4043 / V4044: The 120 VAC (line-voltage) versions of the V8043 and V8044. Never install a 24 VAC head on a 120 VAC circuit, or the reverse.
- 1000 series (V8043C/D/F/G): Newer valve bodies that use their own replacement actuators. Match the actuator part number to the valve letter.
Taco vs Honeywell: how they compare
| Taco 570 series | Honeywell V8043 | |
|---|---|---|
| Actuator type | Heat motor (wax element) | Electric motor with spring return |
| Opening speed | Gradual | Faster |
| Typical voltage | 24 VAC | 24 VAC (V8043); 120 VAC (V4043) |
| Wiring | 3-wire terminal block with end switch | Lead wire or terminal block; with or without end switch, depending on letter |
| Head replaceable without draining | Yes (555-050RP for 570 series) | Yes, on most models (with a matching replacement head or actuator) |
Neither brand is "better" across the board. In most cases, the right answer is to replace like with like, so the wiring, controls and pipe connections stay the same. If you switch brands, you'll usually need to replace the whole valve and confirm the new valve's wiring works with your thermostat, transformer and boiler or zone control.
Power head or full valve: which do you need?
Replace only the power head if:
- The valve won't open (zone stays cold) or won't close (zone overheats), and the body isn't leaking.
- Your controls show 24 VAC at the head during a call for heat, but nothing happens. On a motorized valve, there's no motor movement. On a heat motor valve, the head doesn't open after a reasonable delay.
- The manual lever moves freely, which usually means the valve body isn't seized.
Replace the full valve if:
- The body or stem is leaking.
- The valve is stuck open or closed even with a working head, for example because of debris or a worn seat.
- You're changing brands, pipe size or connection type, or the original head is no longer available.
Browse heads and actuators for both brands in our zone valve replacement heads collection.
General replacement steps
Safety first: Zone valves connect to your boiler's electrical and control circuits, and full valve replacement means working on a pressurized hot water system. Turn off power to the boiler and zone controls before you touch any wiring. Always hire a licensed HVAC or hydronic professional for any work on gas appliances, line-voltage (120V) wiring, soldering, or draining and refilling a boiler system. If you're unsure, stop and call a pro.
Replacing a power head (low-voltage, no draining)
- Turn off power to the boiler and to the zone control or transformer.
- Photograph the wiring and label each wire.
- Disconnect the wires from the terminals or wire nuts.
- Remove the head from the valve body, following the manufacturer's instructions for your model.
- Fit the new head and make sure it seats fully on the valve body.
- Reconnect the wires exactly as before, following the wiring diagram supplied with the new head.
- Restore power, raise the zone thermostat, and confirm the valve opens, the end switch starts the boiler or circulator (if used), and the zone heats up.
Replacing the full valve
This job involves isolating or draining the zone, cutting out the old valve, and installing the new body (soldering, threading or press fitting). After that, you refill, purge air and check for leaks. Leave this to a licensed professional unless you're experienced with hydronic systems.
Before you order
- Copy the full model number from the head and the valve body.
- Confirm the voltage (24 VAC vs 120 VAC), normally open vs normally closed, and whether you need an end switch.
- Confirm the pipe size and connection type (sweat, NPT, flare or press) if you're replacing the full valve.
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