Viessmann 4-Way Mixing Valve

theowwm

Hi Everyone, I'm getting ready to do a boiler replacement. Existing unit is an Atola AR 199k BTU (circa 1991). The heating load on the existing boiler is a mixture of baseboard and radiant under-floor loops. The new boiler is a Vitocrossal CU3A 199k BTU. This unit is essentially a drop-in replacement. There is an existing Viessmann 4-way mixing valve on the radiant load. The controller on this valve is not compatible with the CU3A, so I had to contact Viessmann to get the part number for the add-on module. During that conversation, the factory rep questioned the use of a 4-way valve. His concerns were vague, but he thought it should be a 3-way. I'm attaching a diagram of the existing system for reference. I'd appreciate any thoughts on this.

GGross

The original 4-way valve provided return temp protection for the old boiler and mixed down for the radiant citcuits. The new boiler does not need return protection, it is condensing and will run more efficiently with lower return water temperature. Due to the mass of the boiler, and the fact that you are making DHW with it (high temp application generally) you will want a 3-way valve to correctly control the outgoing temperature to the radiant floor.

hot_rod

do you even need a mixing valve, what temperature does the fin tube need to meet the load? Run the boiler on reset for the required radiant temperature, DHW on priority

If you could run just one lower temperature, below 130 the boiler efficiency jumps. Beef up the fin tube to run at the radiant SWT

I think you can plug a port on a 4 way, but it will require some repiping. Doesn’t that boiler have multiple temperature output in the control?

theowwm

hot_rod said:

do you even need a mixing valve, what temperature does the fin tube need to meet the load? Run the boiler on reset for the required radiant temperature, DHW on priority

If you could run just one lower temperature, below 130 the boiler efficiency jumps. Beef up the fin tube to run at the radiant SWT

I think you can plug a port on a 4 way, but it will require some repiping. Doesn’t that boiler have multiple temperature output in the control?

The existing valve is connected with unions on every port. It wouldn't take too much to re-pipe it. Removing the valve altogether or using a different type of valve is pretty appealing. The Atola supported the 4-way with no add-on. The CU3A requires an add-on module that is hard to get (long lead time), and very expensive. The current water temp for the radiant floors is 135, and the baseboard is 120. This seems backwards to me, but the radiant load is 3 times that of the baseboard, so the temp at the manifold might be higher. I updated the diagram with the 3 way valve. I assume this is the way it would be configured. Thanks.

hot_rod

Gosh it seems just running the baseboard at 133, same as the radiant simplifies all this?

Why does the condensing Vitocrossal even need a mix valve? Seems the boiler is ideal for direct pipe, low temperature applications.

theowwm

Viessmann pointed me back to the Application Guide as well. Specifically to Application 6, which includes mixing valves where radiant loads are combined with baseboard units. This diagram from the application guide is essentially what's in place, except that all of the radiant loads are serviced by a single pump and mixing valve.

hot_rod

That is the correct drawing if you need higher temperature for the baseboard.