Mixing Valve Questions

[emesine]

I have two questions concerning mixing valves:

1. Will a mixing valve function if there is approx 5 feet of head pressure difference between the cold and hot sides? During one state of my boiler schematic the hot port of my mixing valve is 5 feet of head pressue above the cold port.

First off, will the mixing valve function in this state?

Second off, will hot water leak through the mixing valve to heat my basement zones when my circulators are turned off?

Thanks!

 

[sgschwend]

A mixing valve is much like a automobile thermostat. A temperature sensitive spring holds a valve closed, as the set point is reached the spring shortens and the valve opens. In a mixing valve the hot port is thru and the cold port is off. As the temperature increases in the hot port the spring begins to shrink allowing the cold port access to the output.

Taco 5000 specifications:
Maximum differential pressure between hot and cold 10:1
Minimum pressure 1.5 psi

In your questions you want to know if the hot port can overdrive the cold port. Since the two ports are connected together during mixing it would seem that the port with the higher pressure would drive into the lowest pressure port, but my guess is that the mix port is the lowest pressure port. So my guess is it should work as long as the mixing port is the lowest pressure (suction side of pump).

If the pumps are off the heat from the hot water can only move by conduction, so you second question should be that the hot water will not leak into the basement zones.

 

[jpowell1979]

I have a similar setup and I get some hot water bypassing into the cold pipe but very little circulating through the zones when the circulator is off.

 

[Medman]

I have a Caleffi mixing valve on the output of my DHW tank and I had to install a check valve on the cold side to prevent reverse flow of hot water into the cold side. Once the spring is hot, flow can move from the hot side to the cold side. A swing check on the cold side will prevent this without introducing much additional head pressure.

 

[emesine]

Thanks for the info so far. To clairify, the total pressures are as follows for the two states of the system:

State one:

Hot- 30 ft
Cold- 25 ft
Mix- 20 ft



State two:

Hot 25 ft
Cold 30 ft
Mix 20 ft


Thanks!

Andrew

 

[emesine]

Steve,

Thanks for the information. "the hot port is through, and the cold port is off." According to TACO, their mixing valves need a 20F differential between hot and mix to work properly. If the differential is less than this, what happens? According to what you described, I would guess that the cold port would be stuck shut, and the hot port would supply all of the output.

Therefore, as my storage temperature drops, the mixing valve will allow hot water from storage to go straight through the mxing valve. Is this correct?

Thanks!

Andrew