Load calculations do look impressive, and I tip my hat to all those who can do them. However, given my (limited) experience in this industry, I am almost in the "size doesn't matter" camp. A good example of this is my own house.
I've never done a manual J on my house. But the equipment that's there is a 100,000 btu (80% afue) 3 ton (10 seer) outdoor packaged unit. When I go back over my gas bills for the most unforgiving winter months, I used slightly over 25,000 cubic feet of natural gas. This means that I theoretically could have heated my house with a 35,000 btu furnace. Sure, it would have run nonstop, but that wouldn't have been without its advantages. For example, with the continuous blower providing "constant air circulation", the heating of the house would have been extremely balanced.
But, if I were the contractor installing the unit on my house, I would have never put in a 35,000 btu furnace. Why? Well, for one thing, the house has twelve 6 inch heat runs. This is usually a good indication that the duct system can provide 1200 cfm of airflow. A typical furnace that delivers that kind of airflow is a 75-100,000 btu furnace. And I strongly suspect that if someone sneaked over to my house in the middle of the night and swapped out my old furnace for a 75,000, I would scarcely be able to tell the difference. To put this into perspective, a 75,000 btu furnace runs 2 hours to do the same job a 100,000 btu furnace does in an hour and a half. But if they swapped my furnace for a 35,000, I would probably notice that my furnace is running nonstop, and I can't turn it down when I'm away, and expect the house to heat up quickly when I return.
In sum, a rule of thumb I use for sizing equipment is this: I count the number of heat runs that are in the house. I turn on the fan to the existing furnace to check that no runs are starved. If no runs are starved, and if all runs are 6 inches, I multiply the number of runs by 100 to get my cfms. I then select a furnace that delivers that many cfms, and propose it to the customer. If the customer seems unhappy with that selection, and wants a bigger furnace, I tell them they also need a ductwork upgrade (furnaces that are oversized for their ductwork tend to fail prematurely). Usually, they just take the furnace I suggest.
So far, this method has yet to fail me.
That doesn't mean you could have heated your house with 35,000 BTUs.
100 CFM per 6" supply is a bad rule of thumb. They are usually more like 65 to 80 CFM.
A 100,000 BTU input 80% furnace, needs 1346 CFM for a 55°F temp rise.
Do an actual CFM check on your unit. I'll bet it comes ouot a lot closer to 950 CFM then it does 1200 CFM.
Originally Posted by beenthere
If what you say is true, then it carries a very interesting implication: That sizing affects the overall efficiency of a system (in terms of gas bills).
In other words, it would be possible to have a scenerio like this:
Alice and Bob live in identical but seperate houses in the same climate. Their ductwork is exactly the same, but their furnaces are are of different size. Alice has a 90% 50,000 btu furnace, and keeps her stat set at 70 degrees. Bob has an 80% 85,000 btu furnace, and keeps his stat set at 70 degrees. Last month, Alice had a higher gas bill than Bob.
If this is, in fact, possible, I would be VERY interested in knowing the whys and wherefores!
I just have my installed last week, Lennox G71P 70,000BTU and 90,000BTU for the main floor. If you want a quiet furnace and comfortable one, this will be the one to pick. The beauty of these furnaces is you don't even notice it is on, most of the time are running in the low speed, just enough to make you comfort. I don’t know how good it is in winter time, but I am quit sure I will happy about it
Originally Posted by ECIndHVAC
How do you figure you could heat your house with 35,000 BTU's from looking at a 25,000 cubic foot consumption.
Hopefully you didn't just divide the hours of a month into the CF used.
As that would only give you an average. Which would not be accurate at all.
At 2:00 in the morning, 10°F outside, you need more heat then at 2:00 in the afternoon at 10°F outside.
[QUOTE=beenthere;1980494]Hopefully you didn't just divide the hours of a month into the CF used. As that would only give you an average. Which would not be accurate at all.
Yep, that's basically what I did. But hey, I'm still young & learning! Still, I am convinced that there must be a fairly accurate method of sizing equipment based on records of previous fuel consumption. Unfortunately, I haven't yet figured out what that method is...
If you knew the true output of your furnace. You can.