Please explain 3-way versus 4-way mixing valves

Brad White_185

A 3-way valve blends in returning water from a secondary/radiator circuit and primary (boiler) HW from, well, the boiler... sending out a mixed-down custom temperature to the radiation. The remaining return water goes back to the boiler in proportion.

(This presumes that the 3-way valve is used as a mixing valve is on the suction side of the circulator. If on the discharge it often is a diverting valve, another topic.)

When the 3-way valve is fully closed to the boiler side, all water recirculates within the secondary or radiator circuit. None leaves the boiler to that circuit and none returns.

Now, a 4-way valve is similar in function, but that the valve never really closes.

When the 4-way valve is "fully closed" to the boiler-side, it really means that it is stopping boiler water from entering the radiation loop. The boiler-side ports remain open to the boiler (100% flow) and the radiation side also sees 100% flow, independently of one-another.

The radiation circuit continues to recirculate but boiler supply water, -which is not being used-, passes right back to the boiler as hot as it left. This will help both protect the boiler if conventional cast iron or higher temperature types and in any case elevate the HWR temperature.

The best distinction I can make is that you want to use 3-way valves on condensing boiler setups and 4-way valves on conventional cast iron/high temperature setups.

The 4-way valves afford more positive boiler protection (can be set up that way to give priority to boiler return temperature too) and as mentioned, elevates the boiler return temperature at all modulating positions.

But if installed on a condensing boiler, the recirculated boiler hot water would raise the HWR temperature to the boiler and defeat efficiency.

The 3-way at least passes "used" water back to the boiler which is cooled somewhat by the radiation first.

That is a "quick/short" answer, by no means complete, nor the only opinion.

Steve Garson_2

Please explain 3-way versus 4-way mixing valves

I understand how these two different valves function. What I don't understand is why you would use one versus the other when feeding a radiant heating system.

Can some pros help us understand this?

Thanks!

Steve