This statement / question is tossed at me quite often, and the person's eyes normally glaze over when I'm about half way through the answer. My answer is usually followed up by the following question "So...it's 35 degrees, right?!"
Here's the short, no eye glaze, answer- Sometimes yes, sometimes no. While your eyes didn't glaze over, there really isn't an answer in the answer either. That's because the real answer is just slightly more complicated.
Let's start by clearing the cobwebs off the very old, and completely wrong, assumption that heat pumps become inefficient as the outdoor temperature drops. They do not loose efficiency when it gets cold out, they lose capacity.
Now that we cleared that up, let's add some more confusion; there are actually three changeover strategies.
- Heat loss– when the heat pumps output is inadequate for the building.
- Economic –Which fuel currently costs less.
- Comfort – When the heat pumps supply air temperature is too cool to feel comfortable to the occupants.
The good news is changeover set points are almost always base on number 1, heat loss. As the outdoor temperature drops the building requires more heat, but the heat pump provides less heat; this is the key to the heat loss change over set point. The changeover temperature should be set where the heat loss of the building, and the heat output of the heat pump cross.Example:
- The buildings heat loss at 25 degrees is 80,000btuh.
- The selected heat pumps output at 25 degrees is 78,000btuh.
- The change over temperature would be set around 30 degrees.
This example would be an "either-or" situation, meaning only one fuel can be used above or below the setting of 30 degrees. Some systems get a little fancier by allowing the systems to stage when the outdoor temperature is within a set range. An example of this would be locking the heat pump out below 30 degrees, and locking the furnace out above 50 degrees. When the outdoor temperature is between 30 and fifty degrees a call for heat starts with the heat pump and transitions to the furnace if the heat pump can't keep up. This would allow the heat pump to be the systems "first stage" of heat, and the furnace the "second stage" of heat.
In order to select the proper change over based on heat loss you need two things; the heat pump's performance chart, and the building heat loss data. The heat pump data is available from the manufacturer, the building heat loss data is on you.
If the building owner chooses an economic based set point the selection is on them. This is due to the fact that the price of different fuels, including electricity, varies day to day.
I hope this shed some light on selecting dual fuel change over settings.