Here's one of them. After I get back from the store I'll post some others. I really hope one of you answers them.
I have sheetmetal @8" round ductwork in the attic supplying the upstairs bedrooms. Right now, I definitely have major airflow issues so I'm going to do something about it. Would I be better off having the existing ductwork cleaned and sealed, and reinsulated as necessary ,or, buying new ductwork which will most likely be flex duct because that is what it seems many guys want to install.
Be better off having a company find out what the problem is.
Don't just through money at guess work.
My Ruud mod is probably oversized one size, but it's zoned. It's set up to only run at 40% on the two smaller zones and the third (large) zone modulates. The smaller zones are a perfect match for the heat loss at the 40% fire at 15-20 degree OAT. Extremely comfortable! Temp never varies by 1/2 degree.
OK, that seems to make sense. There is general reluctance in the industry (at least in my area) to do Manual J calculations. My current set up is as follows:
In the summer, the AC runs almost constantly during the hottest and will not keep up in the afternoon if there are many people in the house. Temperature creeps up.
In the winter, on cold, windy days, the furnace runs non stop and it “feels” a little chilly. Recovery time from a set back is almost not possible. Maintenance of temperature only. I have to turn the gas fireplace on as a supplement.
One contractor commented that it was odd to have a 64,000 BTU furnace matched with a 1600 cfm blower and 4 ton coil and AC. I was also told that with older furnaces, the 64,000 was output spec and that input is 80,000 BTU at 80% AFUE.
So as a starting point, is this far off?
4 ton 2 stage AC with matching coil
100,000 BTU (input) modulating furnace
Since the modulating furnace draws combustion air directly from outside, I was also planning to close the 6” fresh air intake that goes into the cold air return.
I’m still working on finding someone to do the manual J and will try to buy the HVAC calc program to do it myself as a check.
Would those specs be that out of whack?
100,000 @ 90% might handle ~ 3,000+ Sq. Ft residence in a cold climate
Need REAListIc sizing ... see my profile.
Windows and infiltration/tightness need to be well defined for
results to match actual operation.
Guess that open 6" outside air duct may provide as much as 140 CFM.
140 CFM * ( 120'F - 20'F) * 1.08 = 15,120 BTUh Minimum
- 20 Outside
In other words, the 6" duct should at least have a damper to restrict air to 1/3 to 1/2 ... 50 to 70 CFM
It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
I've seen this formula quoted several times on this forum, and I'm guessing it's what beenthere has in his spreadsheet. The thing to note about it is the multiplier 1.08 is highly dependent on altitude. The following table shows what should be used instead of 1.08 (unless you're at, or near, sea level - 0 ft. altitude):