Arkansas HVACR NewsMagazine January 2020

S tate, National, Chapter News Tech News

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 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 is going to 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 and could shorten the life of it 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 and not designed to shut the furnace on and off regularly and will eventually fail. This is also a sign that you are over-heating the furnace and will cause problems with the heat exchanger and well as other components. When the rise is too high you are not moving enough air over the heat exchanger. This 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… Like usual, when you see signs of low airflow like high-temperature rise, look for the obvious maintenance related issues first.

— Bryan

Bryan Orr ( Founder / Podcaster / Dad of 10 ) Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade