Continuing with this Falls posts on the 4 important aspects of heating diagnostics, this post will look at addressing temperature rise and the importance of having it correctly set.
On every heating system’s DATA plate is a setting for the temperature rise through that heating unit. It is express in a range like 30-60 degrees or 45-75 degrees. Those two numbers represent the lowest recommended temperature rise and the highest allowed temperature rise through the product. (usually a 30 degree spread). In an ideal situation, we always would like to hit the rise tight in the middle for optimum performance.
Temperature rise is the difference between the temperature of the return air and the heated supply air to 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, how do we obtain the temperature rise through a product? First of all, we need to verify that the heating unit is firing on its full input (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 the house can come to some form of equilibrium (the air and heat have had a chance to blend in the house and duct work). 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 about 6 feet away from the furnace so as to not pick up radiant heat from the A-coil or heat exchanger. You then subtract the return air temperature from the supply air temperature to obtain the temperature rise through the unit.
So great — we have a number but what does it tell us? Earlier I said there is a low-end and high-end number (45-75 degrees as an example). I you are too close to the low-end number with your measured rise, the air coming off the furnace is going to feel cool to your customer and you could 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 primary heat exchanger and will shorten the life of it due to is “rusting out”. It basically means you are moving TOO MUCH AIR over the heat exchanger and can be corrected by slowing the blower down.
The real important number is the high-end number. When we are near of over the high-end number, the furnace is going to start cycling on the limit control. the limit is a safety and not designed to be an OPERATING CONTROL. This is also t an indication that you are over-heating the furnace and will cause problems with the heat exchanger and well as other components. It basically means you are NOT MOVING ENOUGH AIR. This again can be corrected with basic fan adjustments.
Now, if making fan adjustments does not provide enough air, then we need to look at the duct work and the Static Pressure of the system. In my next post, we will look at External Static Pressure and the effects it has on proper air movement.
One last thing, though, while on the topic of Temperature rise, is you can use this number to calculate the CFM the furnace is moving.
Electric heat — CFM = (Volts x Amps x 3.41) / (1.08 x Temperature Rise)
Gas heat — CFM = (Input BTU x thermal efficiency) / (1.08 x Temperature Rise) or put another way — CFM = (BTU Output on name plate) / (1.08 x Temperature Rise)
So, hopefully you now see why Temperature Rise is som important and how you can use this as a tool to look at different things happening in the heating system.