Cold Room? Its not a furnace problem!
That cold corner room has nothing to do with your furnace!
Well kind of….
We field several calls each month related to a room in a house that is cold in the winter and hot in the summer. The assumption is we can do something with the furnace or a/c unit to make it better. Unfortunately, it's not that simple.
The underlying issue is the network of ducts in the house is likely less than stellar, resulting in poor air distribution throughout the house. That is why the little hallway bath in the center of the house gets air blasted out of the vent while the bedroom in the corner gets a mere trickle of air.
Oftentimes we hear “let's put a bigger furnace with a bigger fan in and it will push more air to the far away rooms”. I cry a little inside when I hear this, as it almost always makes the problem worse. (read here for more on that https://www.jbhenergy.com/bens-blog/new-heating-and-cooling-system) There is a little truth to the bigger fan portion, but never the bigger furnace portion.
So let us go over the most likely reason the room is colder than the others:
Can you believe it, I am going to talk about ACCA manual J load calculation again. It's funny how it keeps popping up in my writings, yet the vast majority of heating and cooling companies will look at you with deer in the headlights eyes when you say “load calculation”. Alright enough beating that horse already. So we start with our load calc and it tells us to properly heat the room, we need 2500 btu/hr. Knowing the btu/hr need, we can use the temperature rise (commonly called delta T) to calculate the number of CFM we need coming out of the ducts to deliver that 2500 btu/hr. If our gas furnace has a temperature rise of 55*, we take the 2500/55 to get 45 cfm needed in heat (please note we are simplifying the formulas for the sake of the article). Now that we know how many cfm is needed to the room we can figure the size duct needed to move those 45 cfm.
All we do now is go to Google and search “airflow in duct chart” and a easy to read chart pops up and gives us the answer…………. If only it was that simple. What is scary, is that there are people out there using those charts, and it is resulting in those hot and cold rooms.
One of the best methods for sizing ducts is using the Friction Rate method. In order to calculate the FR, we must first find the available static pressure of the blower, and the total equivalent length of pipe. Here is where most people throw in the towel.
Available static pressure (ASP) is easy to calculate, just take a little research and time. We start with the pressure available from the blower. Many furnaces are rated with a max blower pressure of .5”. Newer units with more powerful ECM motors are coming out rated at .8”, but for this example we will use .5”. From the .5” we must subtract the filter, coil, registers, dampers and grills. All this information is readily available in the manufacturer's documentation. We will assume our coil has a pressure drop of .22”, our filter .17” grills registers and dampers each .01”. Added together we get .42”, which we then subtract from the .5” the blower is rated at. We have left .08” of pressure to design our duct around. (Allergan filters can have a pressure of .45”!!!)
Next we need the Total Equivalent Length (TEL) of our duct. To do this we add together all the fittings and piping of the longest supply and longest return. This portion takes into account the fittings above the furnace, elbows, branch take offs and so forth. Depending on what fittings were used, it is possible to see up to 250’ of equivalent pipe in the furnace closet alone. Installing turning vanes in a fitting can take the TEL from 120’ down to 15’, which can really add up throughout a house. Ill use the numbers from a recent project that we calculate the existing TEL to 596’. We are now able to calculate the FR. FR=(ASPx100)/TEL. In this case the FR is .013.
We now use a duct calculator to find that in order to move 45 cfm at a friction rate of .013 we need a 7” pipe.
So what if they only installed a 5” duct pipe, how do we fix it? The first step is to evaluate if the we can save some TEL of duct nearest the furnace since this is typically the easiest to access. Lets say we use turning vanes, radius corners etc and we can get the TEL down to 429. We now calculate the FR to .0186. We now can move the same 45 cfm in a 6” pipe. How is that possible? Friction rate is exactly as it sounds. As we encounter more friction, the less air that will come out the other end of the pipe.
And that is why it is important to hire contractors that understand airflow. And remember, a bigger furnace with bigger motors is not the answer.
