Upper floor duct changes
Hello all -
1.5 story house with two systems - one does the main level and basement, the other does the upstairs 1/2 story.
The upstairs system is very noisy - too much fan, not enough duct. When we first bought the house, there wasn't a filter in the unit. We put one in and realized why - the return plenum started vibrating. Luckily, we had a piece of styrofoam that was the same size as the space between the blower and the return plenum, and that got rid of the vibration.
However, the unit is still quite noisy - presumably because of the load on the fan to blow the required air (and the resulting velocity). I also have one room that gets too cold in the summer (several degrees difference between room and hallway). I tried to adjust the register damper to reduce the air flow to that room, but I could not make much adjustment before it started whistling.
The blower unit is in a closet. From the blower unit, the supply and return plenums go up 8-10 feet into the attic area. Once in the attic, the supply is routed to one 6" flex to a bathroom, and three 8" flex ducts - one to each bedroom, and one over the living room atrium (18' high walls). For returns, three 10" flex - one in each bedroom, and one in the hallway between the bedrooms. No kinks or hard bends in the ducts, and all are well sealed to the plenum.
Based on some load calculations, here's what I've got:
Bedroom 1: 165 sq ft, 2900 cooling (2425 sens), 3500 heating, 230 cfm
Bedroom 2: 185 sq ft, 5450 cooling (4675 sens), 5925 heating, 115 cfm
Bathroom/Hall: 95 sq ft, 2275 cooling (1650 sens), 900 heating, 80 cfm
Total: 450 sqft, 10625 cooling (8950 sens), 10325 heating, 425 cfm
Keep in mind - I've got a register over the living room/foyer to dump the extra air - it's 560 sqft x 18 feet tall, but is primarily served by the main unit.
The system is all 5yo Comfortmaker. Outdoor unit is 1.5 ton 10 SEER. Indoor unit is 2 ton blower (MF08B1500A2) and coil (EPA24B15B1). 4.8kW heat. According to the manual, on low tap it provides 850 cfm at .7" (with .5 of that going to filter, coil, and heat element).
My plan is to increase the supply flow to both Bedroom 2 and the Living Room, as well as increase the return duct in the hallway (which is directly outside the door to Bedroom 2). I believe this will help with some of the stack effect as well, since it will encourage more bedroom air to mix with the hall air where the thermostat is.
I was thinking changing to 10" into Bedroom 2, and 12" into the living room, with the hall return 14". But would this be going too far?
Sorry for the long-winded question, but I know some of it would be asked if not provided.
Your calculations (assuming they are correct) show you need 425 CFM and your blower is rated at 850 CFM in the lowest speed? That is 100% too much air flow. If the indoor air handler is in fact a 2 ton unit then I would think the low tap would be lower than 850. What is the CFM rating on the other tap speeds?
At 850 CFM you will not get much, if any, dehumidification out of that unit. In fact, it is probably be blowing the water off the indoor coil. Instead of trying to figure out how to get all that air out of the system, I would be figuring out how to reduce the CFM output of the air handler down to the load needs and the capacity of the condenser. Your ducts should be able to handle the CFM you have calculated that you need.
If you're gonna stick with a two ton drive you'll be looking at installing balancing dampers (at each branch take-off from the plenum, not at the register neck), upsizing the supply run diameters, or both. You have not reported an insufficient cooling issue - in fact one room is too cold - you have a noise issue, and that is typically the result of trying to shove too much air through too small of a duct. The feet per minute velocity rises to form objectionable noise when this is the case. Typically you want to shoot for around 600 feet per minute for branch runs...go much higher and the registers will be loud.
If you upside the ducts you should also upsize the supply boots...if you neck down at the boot you may recreate some of the velocity noise issues you're trying to get away from by upsizing duct.
Stack effect is from natural convective currents that set up in a house when warmer air rises and cooler air is brought in to replace the rising warmer air. It is much more pronounced in winter than in summer. One key aspect for offsetting stack effect is to tighten up the house. In summer, it is important to have as few air leaks between unconditioned attic spaces and the interior upper level rooms as possible. This means sealing can lights, supply boots in the ceiling, electrical j-boxes in the ceiling, etc.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.
You stated:"The system is all 5yo Comfortmaker. Outdoor unit is 1.5 ton 10 SEER. Indoor unit is 2 ton blower (MF08B1500A2) and coil (EPA24B15B1). 4.8kW heat. According to the manual, on low tap it provides 850 cfm at .7" (with .5 of that going to filter, coil, and heat element)."
I don't think this is accurate ,can you post the mfrs data ?
The 425 is just for the upstairs rooms. I also have a register over my atrium living room where the excess can go (which is really nice in the summer!).
Originally Posted by mchild
I'll post tonight the mfr data from their book for you and Dash.
Thanks shophound -
Originally Posted by shophound
I planned to upsize the register and boot based on the final new size of the duct. Otherwise, I'd just be shooting myself in the foot.
As for the stack effect - I've been pleasantly surprised how even the air temperature is. We do run a ceiling fan on low above the living room 24/7/365, so I'm sure that helps keep things moving. I'd say it's no more than 1-2 degrees difference. I blew in more insulation a couple of years ago, and took the opportunity to use 6-8 tubes of caulk to get around all of the boxes, registers, and wall joints.
I notice the stack effect most during the afternoon setback, when I go from 83 to 77. The upstairs rooms cool off a lot quicker than the hallway. I guess I could alter the 'stat program to cool down in two steps instead of one.
Airflow Based on no coil, no filter, no electric heat. Deduct heater static ... coil static.
Originally Posted by dash
The following numbers are for .3 to 1.0", 230V
Low: 1053, 1013, 965, 911, 856, 761, 654, 552
Med: 1333, 1281, 1221, 1150, 1060, 990, 857, 743
High: 1553, 1486, 1411, 1328, 1227, 1106, 1032, 865
The other blowers in this model line are MF12, MF16 (which is my main unit), and MF20.
Air Coil EPA24B:
600 CFM: .11 Dry, .12 Wet
700 CFM: .14 Dry, .15 Wet
800 CFM: .18 Dry, .19 Wet
900 CFM: .23 Dry, .23 Wet
1000 CFM: .27 Dry, .29 West
Electric Heat is .01 for all CFMs in this range.
As far as I know, there's no way to reduce the blower speed on a Comfortmaker.
For the money and effort you would be putting into trying to get all that air out of the system, and since the blower and coil are two separate units, I would look into having another blower matched to that coil that would deliver the CFM you need.
With a 1.5 ton condenser, and assuming a good latent removing 350 CFM per ton, then all you need is 525 CFM. The 850 CFM (are you assumig that the static is .7?) is doing almost nothing for removing humidity and may be blowing water off the evaporator.
With the coil being separate, then replacing the AH should be able to be done without having to touch the refrigerant. I don't know what other AH will match to that coil, but it may be worth looking into. Maybe just a newer blower motor (not the AH) with less CFM could be installed.
Originally Posted by Waterloo
Sorry,didn't catch that it's a blower and coil,as we never see that here,I was thinking air handler ,where the coil is included in the fan chart.
Why not have a Pro check the static ,add static(damper or obstruction0,to raise it to .7 ,.8 or .9 ,depending on the air flow required.
I'd double check the wet coil data,are you saying that's the wet coil or the adder for wet? Either way seems off to me.
Some manufacturers have ECM variable speed adapter kits that may work to reduce the air flow, help with dehumidification, and operate at a lower cost.