All Mixed Up Questions & Answers on Mixing Methods Used in Hydronic Heating

What is Mixing?
Mixing is when a portion of return water from the system is “mixed” with a portion of hot water from the boiler to supply a specific water temperature that is lower than the boiler temperature but warmer than the return temperature.

What methods are used for mixing?
There are two basic mixing methods: mixing valves which consist of three-way and four-way valves, and injection mixing which uses either two-way valves or injection pumps.

What is the difference between each method?
A three-way mixing valve has three ports and a four-way valve has four ports. Mixing blends cooler return water into one of the valve’s ports with hot water that is entering another port. The two temperatures blend and exit the supply port. With a four-way valve, any of the return water that isn’t used to mix with the hot water is returned back to the boiler. Injection mixing injects bursts of hot water into a constantly circulating loop. A two-way valve opens and closes, or a pump’s speed is changed, to introduce the right amount of heat.

Why would someone use mixing in a hydronic system?
Three major uses, for:

  • Radiant heating that requires lower water temperatures than most boilers can produce without experiencing flue gas condensation.
  • Outdoor reset. By matching the supply water temperature to the load on the building, the heating system will operate more efficiently. Unfortunately, the required water temperatures are lower than most boilers are designed to handle.
  • Hydronic systems that incorporate different types of heat emitters such as in-floor heating, panel radiation, cast-iron radiation and hydroair coils. Each type requires a different supply temperature but all receive their water from the same boiler.

What happens if I use only one pump with my mixing device?
There will be only one mixing point. This will control the supply water temperature for that particular zone, but not the temperature of the water returning to the boiler. Also, the flow rate through the boiler will vary, decreasing the boiler’s efficiency.

Why should I use two pumps?
Using two pumps, with a mixing device, establishes two mixing points. This protects the boiler by controlling the temperature of the returning water. The second pump also provides constant flow through the boiler, improving the boiler’s efficiency. How should I pipe the mixing device and the two pumps? Use primary/secondary pumping so the two pumps will not operate in series with each other. Another benefit of primary/secondary pumping: you can efficiently size the mixing device.

Why should I be concerned with the temperature of the water returning to the boiler?
If you are using a non-condensing style boiler, it is important that the flue gases released from the combustion process be vented out of the boiler. When the water in the boiler is at a temperature below the dewpoint of the flue gases, these gases will condense back to water inside the boiler. The results can be very damaging. Boiler thermal shock is another reason for controlling the return temperature.

What is flue gas condensation?
During combustion of the fuel, many byproducts are formed including carbon dioxide, sul- phur compounds and water vapor. Low return water temperatures will cause the compounds to condense, forming corrosive liquids in the boiler stack and heat exchangers. The amount of damage that will occur depends on the design and materials of construction used in the boiler, as well as the specific compounds in the flue gas. Always check with the boiler manufacturer to find the minimum recommended return water temperature.

When using a mixing device, how do I calculate the flow rates to achieve the desired mixed temperature?
The answer can be found in this example:

  • Radiant zone load = 50,000 Btu/h designed at 20° temperature drop.
  • Design radiant zone flow rate = 5gpm
  • Radiant design supply temperature = 120°F (based upon 20°F temp. drop, return temp. of 100°F)
  • Boiler loop supply temperature = 180°F
  • The design temperature difference between the two loops is 80°F so  50,000 : ( 80°F  x 500 ) = 1.25 gpm

This is the amount of 180°F boiler water needed to “mix” with 3.75 gpm of 100°F return water from the radiant zone to supply 5 gpm of 120°F water.

For answers to any hydronic heating question, contact your Bell & Gossett representative.