With cooling season upon us down south, what's the ideal temp drop (delta?) per hour? I heard 2F, buy want to make sure. I think mine is a little more than that and why I can never get my inside humidity below 50%. Air leaks aside, I also have roughly 400ft/sq of conditioned space mostly closed off, so I'm thinking I may need to slow my fan speed one setting to rebalance the system. Am o thinking correctly?
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proper temp drop per hour?
- Thread starter pablo94sc
- Start date
mygarageone
Well-known member
Not enough information
How many sq feet of home , how many ton's cooling , how old is the system .. Just buy the amount of humidity you say you still have make for a lot more info needed.
Coil clean , filter clean , condenser clean ?
We need a lot more .
How many sq feet of home , how many ton's cooling , how old is the system .. Just buy the amount of humidity you say you still have make for a lot more info needed.
Coil clean , filter clean , condenser clean ?
We need a lot more .
1908sqft, everything is clean, everything is in great working order, pressure levels are fine, no leaks, etc.
It's a 29yr old Carrier 58ssb095-cc series 120 air handler/furnace inside (SN: 4687C84116), and a 35yr old Carrier 38gs042310 condenser outside (SN: R176736). I'd wager it's a 3.5-ton based on the 042 of the model number (42/12 = 3.5).
So back to the question, since I have restricted air flow to ~400sqft of the conditioned space, is my thinking correct that I should probably lower the blower fan speed from 4 to 3?
It's a 29yr old Carrier 58ssb095-cc series 120 air handler/furnace inside (SN: 4687C84116), and a 35yr old Carrier 38gs042310 condenser outside (SN: R176736). I'd wager it's a 3.5-ton based on the 042 of the model number (42/12 = 3.5).
So back to the question, since I have restricted air flow to ~400sqft of the conditioned space, is my thinking correct that I should probably lower the blower fan speed from 4 to 3?
JimRB
Well-known member
Many utility companies will help you evaluate your house for insulation and air infiltration issues. Current code in some areas is a new house has to pass a test for air infiltration and insulation levels.
Air balance is another test.
Air balance is another test.
coonhunter
Active member
- Joined
- Apr 13, 2014
- Messages
- 25
temp drop per hour is going to depend on a bunch of things, are you talking about walking into a shop that is 100+ and turning the a/c on? If your unit is keeping up, your temp drop would be 0/hr. Also, the temp drop per hour is going to decrease as you get closer to your set point.
What is the temperature difference between your supply and your return?
What is the temperature difference between your supply and your return?
Dick in Wisconsin
Well-known member
temp drop per hour is going to depend on a bunch of things, are you talking about walking into a shop that is 100+ and turning the a/c on? If your unit is keeping up, your temp drop would be 0/hr. Also, the temp drop per hour is going to decrease as you get closer to your set point.
I agree. If you're cooling a small space with a huge A/C machine, the temp drop will be HUGE. If you're cooling a large space with a small A/C machine, you might be lucky just to hold steady. Different between outside and inside temps, amount of insulation (R-value), windows, daytime vs nighttime, amount or lack of air infiltration ... all factor in.
That said, I don't think 2 degrees F per hour is very much. If I walk into my house and its 90 out, the inside temp is 84 degrees, at 2 degrees/hour its going to take ten hours to get to 74, eight hours to 76 ... that's TOO LONG in my book.
What is the temperature difference between your supply and your return?
I'm not an HVAC expert or an engineer, just a DIYer who thinks he has a reasonable amount of common sense. This is what I would use to gauge the effectiveness of an A/C unit. You should get a pretty good temp drop if the A/C machine is working right.
temp drop per hour is going to depend on a bunch of things, are you talking about walking into a shop that is 100+ and turning the a/c on? If your unit is keeping up, your temp drop would be 0/hr. Also, the temp drop per hour is going to decrease as you get closer to your set point.
What is the temperature difference between your supply and your return?
delta T was a little high last fall around 18*F if I recall, but I do know it was within range. I haven't tested this year yet because of how few cooling days we've had. I use a high-flow pleated filter (MERV 7, IIRC) instead of the factory reusable style due to allergies and dog hair. Replace that every month or two depending on how dirty it gets.
I do know I need to replace the fan motor sooner than later. Long story short, I haven't changed that yet (it's an older oiled bearing type) because I need a puller to safely remove the squirrel cage from the shaft. Until then, I just oil the bearings every season.
I agree. If you're cooling a small space with a huge A/C machine, the temp drop will be HUGE. If you're cooling a large space with a small A/C machine, you might be lucky just to hold steady. Different between outside and inside temps, amount of insulation (R-value), windows, daytime vs nighttime, amount or lack of air infiltration ... all factor in.
That said, I don't think 2 degrees F per hour is very much. If I walk into my house and its 90 out, the inside temp is 84 degrees, at 2 degrees/hour its going to take ten hours to get to 74, eight hours to 76 ... that's TOO LONG in my book.
I'm not an HVAC expert or an engineer, just a DIYer who thinks he has a reasonable amount of common sense. This is what I would use to gauge the effectiveness of an A/C unit. You should get a pretty good temp drop if the A/C machine is working right.
An A/C system conditions the air. The first part of that is removing humidity. You typically won't notice a large reduction in temp, if any, until the relative humidity in the space is reduced.
Edit for technical speak: "A coil does two things: It cools and it dehumidifies. The cooling shows up in the form of increased dT (lower SA temperature), but the dehumidification does not. When a higher proportion of the refrigerating effect is used to dehumidify, the dT is lower (more dehumidification means less cooling)."
Depending on the relative humidity in the area, 1-2 degrees per hour isn't all that bad and why my utility says to raise your temperature to 78F when you leave for work in the summer. By the time you get home from work, a system that's been on an hour should have you back to a comfortable 76F.
In my case, 76F is warm/muggy instead of comfortable/cool as my inside humidity seems to stay around 55% instead of 40-45%. That 10-15% makes a huge difference in feel. At my uncle-in-laws house, 76F makes me want to wear a jacket because the relative humidity is around 35%! He's an HVAC guy, but I hate bothering him for stuff like this because he'll want to drive the 200+ miles to check on it himself instead of walking me through it on the phone, and at 70 with Dr's orders not to work because of his heart... well you get it.
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pseudorealityx
Well-known member
You don't get any dehumidification until you get the air below dew point.
Degree per hour is a useless unit of measure because of the variables involved. Especially for a house that is typically maintained within a few degrees of ideal. A shop that sits dormant for days at a time between work sessions would make it a more applicable unit, but it's still not something you can 'calculate' without a lot of detail that nobody has time to mess with unless they're writing an academic research paper.
Degree per hour is a useless unit of measure because of the variables involved. Especially for a house that is typically maintained within a few degrees of ideal. A shop that sits dormant for days at a time between work sessions would make it a more applicable unit, but it's still not something you can 'calculate' without a lot of detail that nobody has time to mess with unless they're writing an academic research paper.
LS6 Tommy
Well-known member
I haven't done a heat transfer calculation that complex in 20+ years. Not enough of the required variables are known. All you can do is actually monitor the temperature drop in the space. Realisitically, the unit should have a 20* delta across the evaporator if it's working well.
Tommy
Tommy
going4speed
Active member
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- Mar 6, 2016
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Well I'll tell you what happened in my current house. We could not get the humidity down in this house when we first moved in. Same thing in that the temp would say 78 but it felt like 90. So we called an hvac company and found that the return duct to the air handler had a massive hole in it 24" by 2" from when the house was built. All those years (26) sucking in humid attic air. So we got all the duct work replaced, units serviced/cleaned and that massive hole fixed. Now we can get the house down to 75 from 82 in about 15 minutes. We have 2400 sq.ft with 6 tons of cooling. I'm guessing they oversized to compensate for the hole they never found.
Have you checked all your returns and delivery duct work? An 18 degree delta is pretty good in my book especially for equipment with that age. I dont think fan speed has anything to do with the issue at hand.
Edit: the other thing we did was add multiple returns all over the house as we have only 1 per unit. This helped keep the temperature the same all over the house instead of warm/cold rooms based on door closures.
Have you checked all your returns and delivery duct work? An 18 degree delta is pretty good in my book especially for equipment with that age. I dont think fan speed has anything to do with the issue at hand.
Edit: the other thing we did was add multiple returns all over the house as we have only 1 per unit. This helped keep the temperature the same all over the house instead of warm/cold rooms based on door closures.
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That could be. I didn't feel any air leaks, but that doesn't mean I don't have one somewhere. I'll give that a go. Probably need to replace it all anyway - a lot of grey jacketed flex duct, and that's the bad stuff that was recalled for failure.
Edit - didn't even think of that because I have two return ducts, one in the shut off room. The main return is under three sides of the mechanical closet.
Edit - didn't even think of that because I have two return ducts, one in the shut off room. The main return is under three sides of the mechanical closet.
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pseudorealityx
Well-known member
So I have proper delta, but not enough dehumidification. So, lower fan speed?
If everything else is working (including the duct leakage mentioned), then yes, you will trade some sensible heating for additional dehumidification if you slow down the fan. Typically, standard single stage equipment can deal with ~325 cfm/ton on the lower end before you start to run into issues. Depends on multiple variables, rule of thumb, YMMV,etc.
If everything else is working (including the duct leakage mentioned), then yes, you will trade some sensible heating for additional dehumidification if you slow down the fan. Typically, standard single stage equipment can deal with ~325 cfm/ton on the lower end before you start to run into issues. Depends on multiple variables, rule of thumb, YMMV,etc.
I don't have the tool to measure cfm (anometer?), but I think I'll be able to move enough air. I'll test it and report back when I have some solid results.
truckman5000
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- Mar 11, 2008
- Messages
- 1,440
Do a real good cleaning of the condenser and indoor coil, clean the underside of the coil that you cant see (depending on style).
Like said the electric company in your area may check insulation values? For instance my state offers a 75% rebate for insulation, the company's estimate high..so my home is 2700 sq ft. they insulated the whole home to a r-38. The attic has about an r-56 ish. I bought the guys lunch and they spent 3 hours spray foaming every joint from sheetrock seams in the attic and about 1ft in from the exterior in the basement. This ran $650 ish
I replaced all the windows prior to this with a rebate also. All of this cut utility bills by 30%
Like said the electric company in your area may check insulation values? For instance my state offers a 75% rebate for insulation, the company's estimate high..so my home is 2700 sq ft. they insulated the whole home to a r-38. The attic has about an r-56 ish. I bought the guys lunch and they spent 3 hours spray foaming every joint from sheetrock seams in the attic and about 1ft in from the exterior in the basement. This ran $650 ish
I replaced all the windows prior to this with a rebate also. All of this cut utility bills by 30%
LS6 Tommy
Well-known member
So I have proper delta, but not enough dehumidification. So, lower fan speed?
How do you know what your RH is? You said it's 50%. That's not really all that bad. A desert is normally 45% max. What RH do you want to achieve?
You may just have an oversized system. That's the single biggest mistake that people make. The system satisfies the t-stat too quickly. Your superheat may be too low, subcooling may be too high and there's all kinds of variables involved to guess without knowing the entire system's operating conditions.
Tommy
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LS6 Tommy
Well-known member
I think I got it. You said you're 3.5T, 1900 square feet with 400 square feet closed off. Open the 400 back up. You're using 3.5 T to cool 1500 square feet. You're probably oversized and are short cycling on the T-stat before the RH is reduced.
Tommy
Tommy
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I think I got it. You said you're 3.5T, 1900 square feet with 400 square feet closed off. Open the 400 back up. You're using 3.5 T to cool 1500 square feet. You're probably oversized and are short cycling on the T-stat before the RH is reduced.
Tommy
No can do right now, but you're probably right.
I lowered the fan speed and it dropped the RH in the house a good 3-4%. It recently creeped up to 58% inside with outside hovering around 83% at 78F. Of course, once it cooled the house the humidity started going back up so I need to find out where my air infiltration is coming from.
I'm going to go back over the ductwork soon and also check that my new window weather-stripping didn't cause any new air leaks. Also need to rehang my doors as the hinges are starting to sag. Always something. Lol
So I lowered the blower speed and noticed for significant temp reductions it'd run long enough to start forming frost on the lines by the condenser, even with the extra space opened up, so I returned the blower speed back to high. It did lower the RH in the house an extra 2-3% though, so that was good. However, that's not the problem apparently.
Yesterday the outside RH was in the 36-38% range at 68F, so I had all the windows open and the used the ceiling fans and furnace blower to help draw in outside air. Knocked the humidity level down to ~45% according to a few hygrometers I have throughout the house, but as soon as I closed up the house it quickly rose back up to the lower 50% range (about 2 hours), and then back up to 54% overnight!
So I'm pretty sure it's not from typical air leaks around the windows/doors since the house was opened up, but where it's actually coming from...
- Not enough airflow in the attic and it's being sucked out of the wood up there. No signs of moisture damage or mold though.
- Coming up from under the slab.
- It's in the wall cavities (again no signs of moisture/mold) and seeping in?
Any ideas? I really want to get this solved before the Memphis summer gets into full swing. Thanks!
Yesterday the outside RH was in the 36-38% range at 68F, so I had all the windows open and the used the ceiling fans and furnace blower to help draw in outside air. Knocked the humidity level down to ~45% according to a few hygrometers I have throughout the house, but as soon as I closed up the house it quickly rose back up to the lower 50% range (about 2 hours), and then back up to 54% overnight!
So I'm pretty sure it's not from typical air leaks around the windows/doors since the house was opened up, but where it's actually coming from...
- Not enough airflow in the attic and it's being sucked out of the wood up there. No signs of moisture damage or mold though.
- Coming up from under the slab.
- It's in the wall cavities (again no signs of moisture/mold) and seeping in?
Any ideas? I really want to get this solved before the Memphis summer gets into full swing. Thanks!
pseudorealityx
Well-known member
What temperature are those RH% readings at?
Assuming it was 68 degrees and 45% RH in the evening.... the unit would not have run overnight, but the RH climbed... this is to be expected. The occupants are adding moisture, but nothing is removing it. Even if the unit ran all night long, it's not going to be able to pull the RH% down below ~50% if you've got the temperature set to 68.
Assuming it was 68 degrees and 45% RH in the evening.... the unit would not have run overnight, but the RH climbed... this is to be expected. The occupants are adding moisture, but nothing is removing it. Even if the unit ran all night long, it's not going to be able to pull the RH% down below ~50% if you've got the temperature set to 68.