Correct BTU and tonnage but low CFM
I am finishing a walk-out basement and have been getting HVAC quotes. I have learned that the furnace BTU and AC tonnage is more than enough for the first floor and basement combined, but the blower moter is a bit under-sized for the first floor without even considering the first floor. Both companies that have quoted me have suggested a new additional split system. After researching I understand that just boosting the RPM of the motor is problematic, but what about adding 1 or 2 in-line duct fans and use the existing equipment to handle both floors?
Other pertinent facts:
It’s a straight duct-board install with flex duct (centipede).
There are two systems – one for the second floor and one first floor. There is a large 2-story room and both thermostats are in this large space.
I will be spray-foaming the entire basement.
Location is Charlotte, NC metro area.
Replies
The problem with that idea is that you would have to put any fans in branches BEFORE they split off, otherwise you would just be sucking, or blowing, air into/out of another branch instead of into/out of the air handler.
The problem with THAT is your main branches are likely so large that it would be impossible to afford an inline fan of the correct size.
In addition to the points in my response to your other post:
I really, really doubt that a system with more than enough capacity on the furnace and A/C would truly be equipped with an undersized blower.
Far more likely that the duct system is improperly designed (if it was, in fact, actually designed). In other words, the blower is probably capable of delivering enough CFM; it's just that your duct system is not designed and installed to proportion the air delivery to each space. The problem could be on the supply side, the return side, or both. Return is the first suspect in most houses.
Also, it may be that the system has no balancing dampers in the runouts that supply each register. This means that certain runs "hog" the air while others starve. If the t-stats are located in rooms that hog the air, the t-stat will shut down the system before other rooms have a chance to heat or cool.
You are justified in questioning the motives of the contractors whose immediate solution is to sell you more equipment. Not only is that a big expense, you would still have the problem of an inefficient original system that may well be operating below 60% as a system--regardless of the rated efficiency of the equipment by itself. Equipment efficiency is very different than system efficiency.
Duct board.
I think you hit it squarely, after seeing that it is a straight forward ductboard and flex duct system.
He needs to get a contractor tocheck the cfm output at each of his supplies, look for balancing dampers and check his return air runs while they are at it. Even money say neither of the two contractors did a load calculation for each room of the house, much less the whole house.
I have questions.
So, both companies have identified that the capacity of the current lower level system is "more than enough" but yet opt to add more to it by an additional system? Red flag here.
You have learned that "the blower motor is a bit under-sized ....." How so? Told to you by an HVAC contractor? Measured in the actual installation?
Just boosting the fan RPM is problematic? The unit is most likely factory provided with four speeds, none of which will be "problematic". If a higher speed is available, use it, as necessary.
What research?
I ask these questions because the collection of statements do not make sense. Something is not right here (besides the ductboard/flex install, which is cheap and quick and worth every cent).
What tonnages and air flow rates are we talking about here? How were the loads calculated? Who did it? You can (and most likely will) get advice from many people that have no experience in evaluatimg or designing HVAC systems. That includes most on this forum.
I would want to see a room by room calculation with the required air flows determined before I would consider any changes.
Because of diversity, a system is sometimes designed for more air flow than can be acheived, but because of zoning (manual and/or automatic dampers and registers being opened and closed depending upon occupancy and load), the system can handle the load at any given point in time. If a unit serves east and west cooling loads, and was ducted to provide the design cooling air flow at the maximum loads for each scenario, it would appear to be improperly sized because the unit could never meet the deamand that its "hooked" to. Of course, the east and west maximum cooling loads occur at different times of the day and will never coincide.
A better solution than duct booster fans (always a patch, never a cure) would be some zone controls ( and rigid, galvanized duct work, sized correctly). Not real common in residential systems, but common and simple enough.