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Furnace Air Temperature Rise
Every gas furnace data plate/tag has a specification for the temperature rise through that furnace. It is shown in a range, like 50-80° or 45-75°. Those two numbers are the lowest recommended temperature rise, and the highest allowed temperature rise through the furnace is usually a 30° difference. When possible, we want to get the rise toward the center of that range through a combination of proper commissioning practices, primarily setting the airflow properly in heat.
Air temperature rise is the difference between the temperature of the return and the heated supply air from the furnace. Temperature rise must be measured during installation and must be within the range on the furnace rating plate. This is important not only for the longevity of the furnace but also for customer comfort.
So, what does temperature rise mean, and how do we calculate it? First of all, we need to verify that the furnace is firing on its full output (high fire) and that the manifold pressure is correct. Then, we need to let the appliance run for at least 15 to 20 minutes so that everything has a chance to acclimate. You then take the temperature of the return air into the furnace and then take the air temperature of the supply air leaving the furnace. The supply air temperature should be taken several feet away from the furnace so that it isn't affected by radiant heat from the heat exchanger. You then subtract the return air temperature from the supply air temperature to get the air temperature rise through the furnace.
So, what can we learn from this number? If you are too close to the low-end rating number with your rise, the air coming off the furnace will feel cool to the occupants, and you may have complaints of “drafts” or not feeling warm. If you are BELOW the low-end rise, you could even start to form condensation in the heat exchanger, which could shorten its life due to corrosion. Low rise means you are moving too much air over the heat exchanger; this can (usually) be corrected by slowing the blower down.
More important is the high end of the rated range. When we are near or over the high-end number, the furnace may begin cycling on the limit control. The limit is a safety control and is not designed to shut the furnace on and off regularly; it will eventually fail. Proximity to the high-end number is also a sign that you are overheating the furnace and will cause problems with the heat exchanger and other components. When the rise is too high, you are not moving enough air over the heat exchanger. That can (and must) be corrected with a blower adjustment or by resolving the source of low airflow in the system, such as dirty filters, blower wheel, etc.
As usual, when you see signs of low airflow, like high-temperature rise, look for the obvious maintenance-related issues first.