Mixing valves, boilers and radiant panels - The Glitch and The Fix

The Fix

A four-way motorized mixing valve is specifically design to create two mixing points within itself: one to regulate system supply temperature and the other to boost boiler inlet temperature high enough to prevent sustained flue gas condensation. To accomplish the latter, the controller operating the valve’s motor must sense and react to boiler inlet temperature.

Thus, a boiler inlet temperature sensor is required and is shown in the Fix drawing.

Although a primary/secondary system would work, there are simpler and less costly methods to achieve hydraulic separation between the circulators when the boiler and header piping have low flow resistance. With relatively short and generously sized header piping between the boiler and four-way valve, sufficient flow will be created by the combined effects of buoyancy and momentum exchange within the valve.

Eliminating the primary loop circulator reduces both installation and operating cost. It also eliminates reduced water temperature to the indirect water heater coil if the space-heating subsystem is active, and the water heater is not operated as a priority load.

Did you notice that the primary loop circulator is pumping toward rather than away from the location where the expansion tank connects to the system? That’s definitely incorrect. So is the placement of the secondary circulator serving the indirect water heater. Secondary circulators should always direct water into the secondary circuit. This treats the upstream tee of the closely spaced pair as the point of no pressure change for the secondary circuit. Pressure within the secondary circuit goes up when its circulator is operating.

The supply temperature sensor is located immediately downstream of the four-way mixing valve in the Glitch drawing. Although mixing has begun by the time flow passes this sensor location, it may not be complete and the sensor may not be sensing the final blended temperature supplied to the manifold station. It’s always good practice to install the supply temperature sensor downstream of the distribution circulator to ensure complete mixing has occurred before flow passes the sensor.

Finally, purging valves have been added to the return ends of both load circuits.

Title: mixing valve
By: rjm
Posted: January 23, 2012 10:34 PM

why not use a taco 4 way with a outdoor reset?

Title: Controls for mixing
By: Todd Davis
Posted: February 1, 2012 11:32 AM

One of the concerns I have with changing the piping configuration of the mixing valve, is that in some outdoor reset controls, such as tekmar, they offer boiler protection. How they do this is by closing off the mixing valve. The problem with the "fix" would be that if its a low-mass boiler, then you are now starving the boiler with flow, and could lead to problems. This is just one thing that has come up in the field, where the fellow has piped it not as a primary/secondary. Even on high-mass boilers, the control then ends up short cycling the mixing valve.

Title: Mixing valves, boilers and radiant panels
By: c. shoff
Posted: February 17, 2012 1:31 PM

I believe this "fix" may also have a "fix" needed. The boiler loop should have a pump on its side of the 4-way valve. Not having the pump there can cause the boiler to heat too fast when the 4-way is modulating closed, especially when the temperature coming back from the low temp side is very low. During this condition, the boiler will have a reduced flow. The boiler's flow is completely dependent on the flow of the low temp pump. Not having adequate flow would cause the boiler to use its limit to cycle the boiler on and off.

There also need to be an isolation valve on the discharge side of the return manifold with a drain valve between it and the manifold. This allows for "power" purging of the radiant floor. In addition, once purged the isolation valve provides isolation when a pump or other serviceable item needs to be changed.