Heat gain from furnace blower
Frequent reader, firts-time poster - just registered.
I'm hoping that one of you professionals can help me determine if it's normal to pick up heat in the blower section/furnace in AC mode.
I have a 2-story home and the furnace/air handler/evap coil are located in the blazingly-hot attic space. It's a Bryant 376CAV installed horizontally with the return air on the right as you look at the control-panel side. The Carrier 3KCB coil is on the left.
I've been told that the AC for the second floor is a tad undersized, so I'm making sure that it is running as efficiently as possible. I have a 2.5 ton Bryant 561C on a 2200sf second-floor.
I recently moved into the home. It was built in 1999 and the AC is all-original.
The installer, or subsequent servicer, made test holes on each side of the evap coil. Using a digital thermocouple two-probe thermometer I borrowed from work, I see a 20-degree drop across the coil. 77 degree air after the blower, just in front of the coil, and 57-degree air after the coil. From what I've read here, that's probably good.
Using the same thermometer, the temp in the test hole in thereturn plenum right in front of the furnace reads 74. I took this reading several times and used both probes to make sure I had a good number. If I'm at 74 entering the 376CAV and 77 leaving it, where is that heat coming from?
Does the blower motor introduce that much heat? Dies it pull in some fresh air by design? Any input in greatly appreciated.
My father spent 21 years in your industry working in hot attics and hot supermarket roofs. I know it's hard work.
Thanks in advance for your assistance.
The blower motor does create heat while running and depending on how close the probe is to the motor and how restricted the return air passage is will influence the reading.
Originally Posted by RichmondDan
I'ts defnitely not using 1000Watts... which is about what 3F temp rise would be at 1000CFM. Probably around 250-350 Watts depending on how restrictive your ductwork is would be about right.
No reason 2.5 tons can't maintain temp on a 2200sqft 2nd floor in most locations and with typcial tightness and insulation. However, keep in mind that the last few weeks we've regularly bee nwell above desing conditions. In many areas, about 10F above design. So in a location where desing temperature is 90F, and indoor design is 75F, now at 100F, you have a 25F instead of 15F diffrence form inside and out. A properly sized system will fall behind.
You not using afternoon setbacks are you. It amazing how many system seem undersized, then you stop using setbacks and amazingly they maintain temprature even well above design tempratures.
In my home, if I keep it 74 all day and 73F at night. Its' handled the 100F temps (design is only 93F where I am) just fine, and the temp rises up to 75F but it catches up by 10PM. If it set it to lets say 78F in the afternoon, it would still be 77F at 10PM. Most homeowners would think the system is undersized or not working right.
Further, I've doen the math. With a 2 stage system in particular, using setbacks will is either break even or you may even use up ot 5% more energy. IF you oversize a systme so it can "catch up" quickly, then you'll use an additional 15-25% from he systme being oversized and not running as long at each cycle.
Being undersized a tad (you're probably sized perfectly), could be saving you about 10-20% in energy.
Your blower will generate heat, with your size AC, your blower is probably a 1/2 to 3/4 hp blower which adds about 1,272 to 1,908 btu/hr of heat to your air respectively.
You can call me Sam
It should be a crime to be a mechanical engineer in San Diego
Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7
1260BTUs per 1000CFM of air flow
Thank you all for your comments.
I couldn't agree more on how setback will truly "set you back" on keeping the house cool. We tried that function last year, and when that AC is 8 degree above where you want it to be when it kicks in, you'll never cool down. We simply set it at 72 and it runs all day and can maintain 72-74 on a 100+ Virginia day. Anything below 95 will maintain 72. Running all day really helps with keeping the humidity down.
It's nice to know that my AC is properly sized and more efficient that way. There's a 3.5 ton downstairs and a 2-ton on the third floor. I keep the third floor at 84 to keep the load down on the second floor. I keep the first floor at 72 as well (1750 sf on first floor) and my electric bill was only $240 last month.
Never thought that the probe could be close to the blower motor - good point.
I just wanted to make sure that my AC wasn't picking up some heat somehow in the furnace blower. Even 1200 btu/hr is significant; never realized that much heat was thrown off.
Thanks for the help,
2nd floor is undersized to me.
Why do you say that? I have 2 tons on 1500sqft 2nd floor of a 1925 home with 22 original windows covering 27% of the wall area and no wall cavity insulation. If I had a more typical 15-20% window area and 6" of fiberglass in the wall cavity, I'd easily be at 1.5 tons.
Originally Posted by hewitt
THen again, I do radiacal things in mid summer like keep the shades closed. I also don't use setbacks on my AC since that uses more energy anyway... especially with a 2 stage unit.
Even more importantly, the OP never states his location. If he's in a northern climate with lets say a 87F, 70F wet bulb, and insulated to code for a northern climate, he's probably oversized.
Remember, as you increase in square footage, the surface area to volume/floor area ratio decreases. So a 3000sqft space will only need a unit perhaps 2/3rds larger than a 1500sqft space, not twice the size.
2.5 tons on 2200 sq ft would be grossly undersized here, especially upstairs. But yall arent here so what do I know
Originally Posted by ar_hvac_man
NW AR only has a 1F high dry bulb and 2F higher wet bulb than where I'm at. It still depends greatly on the home construction. Air leakage rates and featurs like spray foam, especially unvented attics can have a big impact on heat gain.
But I would agree, on a typically tract home, with average air leakage, open concept floor plan and large open stairwells, minimal shading or soffit overhangs, and a hot attic, yes, you probably would need closer to 3 tons.
OTOH, consider this. My 3200sqft home only needs 4 tons. I have a summer electric rate of $0.13 and with a 16 SEER unit upstairs and 10 SEER unit downstairs, my total electric bill even in this likely to be a record setting July, will be right around $240-270. So if you think a 4400sqft 2 story home needs 5.5 tons, then if sized right, I'd expect a electric bill around $350-400.
Simialrly a home that actually needs 100k BTU furnace should have a natural gas bill over $300 in mid winter.
100 degrees used to be an extreme here. Now we are seeing 105+ for weeks or MONTHS on end. The design temp is a freaking joke. Go by the book here and you will not have a happy customer.