Air To Water Heat Pump Manufacturer Chiltrix
Dynamic Outdoor Reset Control

Automatic Heating Reset Curve

All air to water heat pumps used in heating mode work more efficiently and have higher capacity when the heating loop operating temperature is lower, including the AHRI-Certifed record-setting Chiltrix CX34. That's because when the loop operating temperature is lower, there is less "lift" or delta between outdoor ambient and the loop operating temperature, resulting in less work for the compressor to do and this produces gains in both efficiency and capacity.

air to water heat pump manufacturer chiltrix - outdoor reset curve

Accordingly, indoor equipment, fan coils, air handlers, or radiant installations, should be sized to run at the most efficient (coolest possible) heating temperature. A cooler supply temperature means you will use a slightly larger fan coil, air handler, or more radiant material.

As mentioned, with fan coils or air handlers, when using a lower operating temperature, just use a larger size unit.
Chiltrix fan coil units and air handlers are rated to deliver performance at low operating temperatures. For radiant systems, using a lower loop temperature means a design using more PEX, or larger panels, etc.

Normally, a radiant floor system would be designed to handle the design heating load at a 95F loop operating temperature. But for some buildings in certain climates, at design-day conditions the loop either may need to be warmer than that, or else the loop needs to have higher capacity. A larger capacity radiant design is always the better choice for COP and this may mean closer PEX spacing, or even adding some PEX in walls, or ceiling.

From an efficiency or capacity standpoint, increasing the indoor load handling capacity is vastly preferable to increasing the operating temperature of an air to water heat pump

However, in some cases the only option is to raise the operating temperature when it gets colder outdoors. And even when a system is designed to meet the coldest-day design heating load at a low operating temperature, why not take advantage of an *even lower* temperature on a mild day? These are the areas where "outdoor reset" can be beneficial.

Following shows four examples from the 400+ possible curves available from the Chiltrix automatic reset curve configuration tool.

Programmable Dynamic Outdoor Reset Curve For Maximum Efficiency!

Note the "orange" curve above. In this example the loop operating setpoint will be 100F at or below 5F outdoor temp. But on a milder heating day of 53F, the loop setpoint will be around 85F, saving a large amount of energy used per BTU of heating compared to simply setting it at 100F all of the time.

As outdoor temperature falls, the loop operating temperature is automatically increased. As outdoor temperature rises, operating temperatures are automatically decreased. The orange curve is designed for extra energy savings on milder days. This, or any of the above curves, can also be used to work around indoor-side design limitations.

The other curves shown are not necessarily valid or recommended and are only presented as examples. Once you create and input your curve, it will work automatically and adjust the temperature in 1c increments. And it is easily configurable so you can tweak it to be perfect for your application.

Download the outdoor reset curve development tool (.xls file) to create a custom curve:

For CX34-1, CX34-2, CX34-3 Use this tool

For CX34-4 & CX50-1 Use this tool

For CX35 & CX50-2 Use this tool

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Advantages & Misconceptions of Using Outdoor Reset

Most people in the radiant heating industry will tell you that outdoor reset is used to get more BTU delivery from the radiant system on a day when the radiant system can't deliver enough BTUs to keep up with the load, and they use outdoor reset to crank up the radiant heat operating temperature to 120F, or higher.

While that's true, it's also true that the best designed air to water radiant systems will be able to deliver what's needed at a fairly low temperature even with severe outdoor design conditions.

So if you can't get enough radiant capacity into the floor, then if possible, add some to the walls or ceiling. Note that radiant heat works just as well from the ceiling or walls as it does from a floor. And in some cases, it works better from the ceiling.

Why does keeping the operating temperature low matter so much?

With a fossil fuel burning boiler, it doesn't matter. Gas, propane, and oil efficiency does not vary with outdoor temperature or the amount of lift. But that's not the case with a heat pump.

Here's why:

Carnot efficiency equation of a heat pump The above is the Carnot efficiency equation for a heat pump. The numerator Q¹(T¹- T²) ) tells the story of how lift affects efficiency.

For a real-world example, at 0F outdoor temperature, an air to water heat pump such as the CX34 will have >20% higher capacity when used with an operating temperature of 95F compared to operating at 122F. And COP at 95F will be >30% higher at 95F than at 122F.

So why use dynamic outdoor reset?

You can use dynamic outdoor reset when it is simply not possible to design the indoor side of a system to handle the peak BTU load using a low operating temperature.

But the best use of dynamic reset is as a strategy for extra energy savings, by designing the indoor side of the system to handle the peak load at the lowest possible operating temperature, and then letting the system automatically reset to an even lower and more efficient temperature (less lift) at times when weather is milder.

Air To Water Heat Pump Manufacturer Chiltrix Dynamic Outdoor Reset Control