Hey all, I went from R-30 to R-60 in my attic. This summer my AC practically never shuts off. It is able to maintain 72F when it's 92F and sunny. 1400SF ranch, 2 ton AC unit. Even when it's a low of 67F, my AC occasionally runs throughout the night. The lines outside feel moderately cold, air coming out of the vents inside is moderately cold as well. Never really checked how cold they were prior to this summer. Should I have my AC refrigerant checked? Anything else I can do?
AC and attic insulation
- Thread starter tadmaz
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johneh
Minister of Fire
brenndatomu
Minister of Fire
Checked the furnace filter lately? How about the coils of the condenser, (the outside unit) are they clean?
Sounds like something is up with your AC system...if you can't find anything yourself, having a tech check it over may be a good call.
Sounds like something is up with your AC system...if you can't find anything yourself, having a tech check it over may be a good call.
Outside coils were cleaned a year ago. Filter is good, blue fiberglass MERV 2 should have best airflow. House is fairly tight, built in 92. Exterior walls are 2x6 (R-19), windows are original, some had a vacuum issue so I drill a hole to release and resealed, so they are double pane but air filled.
I did a test, between 3 digital sensors at the vent directly above the furnace I am getting between 61-63F. Much better than what I thought.
I did a test, between 3 digital sensors at the vent directly above the furnace I am getting between 61-63F. Much better than what I thought.
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Highbeam
Minister of Fire
Is this your first summer in the house? If not, did the AC perform better before? Maybe the insulation crew crushed or separated your ducts.
heat seeker
Minister of Fire
One way to measure the effectiveness of the AC is to determine the temperature difference between the return air and the chilled air. I check mine periodically. The difference should be about 12 to 15 degrees. So, if the return air is 75º, and the air from the vents is 65º, you have a 10º Delta T.
Checking it once in a while will let you know what's going on.
Checking it once in a while will let you know what's going on.
Is you ductwork inside or outside the conditioned space?. If its in the attic between the roof and the insulation then its likely it got damaged during the insulation work.
That is my plan today, note the outside temperature and temp coming out of the vents at various points of the day. I did the insulation myself, no ductwork in attic.
BTW, a lot of folks forget about latent heat which is related to water vapor in the air. if you have raised the humidity level in the house, it can eat up a lot of HVAC capacity. Hot tubs used to be pretty notorious for messing up AC systems.
semipro
Minister of Fire
The temp difference you need to measure is between the HVAC return and the registers as @heat seeker described. This will give you some idea of AC performance.
Couple of additional thoughts which maybe or maybe not already mentioned:
When you added insulation, did you make sure to keep all eve vents open? ...or be sure to open them / install eve vents if you don't have them? Getting the hot air out during the summer with adequate eve/ridge venting is as important, or more so than just a layer of insulation.
Coil drop or deltaT - good to look at, but I was thinking it should be 15-20 degrees for a well functioning system working within moderate humidity levels. Yes, if you are trying to drain a swamp of 95% RH air, or pulling in arctic 55° air in the return, then the temp difference will be less. But pulling in ~75° return air at 60-70% RH, I'd think you could easily spit out 58-60ºF supply air.
A-frame coils can get plugged over the years...especially running those 'see-through' fiberglass filters as you mention.
This an orifice tube or expansion valve system? Possibly the orifice is plugged or the expansion valve is stuck?
Any evidence of ice/frost on any of the lines, especially near the evaporator (inside) coil? Or signs of condensate leaking out at 'unintended' locations? All these can point to an undercharged system.
The 'real judge' would be getting a set of gauges on it and looking at pressures/temps. Superheat / subcooling will give great info on the health of the system and help localize any issues to specific components.
If you DIY, you can probably 'get by' with a $30-$50 set of gauges, though you can also find fairly decent ones for a few hundred. Over the years, my $100 investment in gauges and $19.99 investment in EPA certification to buy freon has undoubtedly saved multiple thousands $$ in home, auto and refrigerator AC repair. Plus, if I do need a freon top-off, just go out and squirt! No waiting around for a service call / time off work to meet the tech, etc.
When you added insulation, did you make sure to keep all eve vents open? ...or be sure to open them / install eve vents if you don't have them? Getting the hot air out during the summer with adequate eve/ridge venting is as important, or more so than just a layer of insulation.
Coil drop or deltaT - good to look at, but I was thinking it should be 15-20 degrees for a well functioning system working within moderate humidity levels. Yes, if you are trying to drain a swamp of 95% RH air, or pulling in arctic 55° air in the return, then the temp difference will be less. But pulling in ~75° return air at 60-70% RH, I'd think you could easily spit out 58-60ºF supply air.
A-frame coils can get plugged over the years...especially running those 'see-through' fiberglass filters as you mention.
This an orifice tube or expansion valve system? Possibly the orifice is plugged or the expansion valve is stuck?
Any evidence of ice/frost on any of the lines, especially near the evaporator (inside) coil? Or signs of condensate leaking out at 'unintended' locations? All these can point to an undercharged system.
The 'real judge' would be getting a set of gauges on it and looking at pressures/temps. Superheat / subcooling will give great info on the health of the system and help localize any issues to specific components.
If you DIY, you can probably 'get by' with a $30-$50 set of gauges, though you can also find fairly decent ones for a few hundred. Over the years, my $100 investment in gauges and $19.99 investment in EPA certification to buy freon has undoubtedly saved multiple thousands $$ in home, auto and refrigerator AC repair. Plus, if I do need a freon top-off, just go out and squirt! No waiting around for a service call / time off work to meet the tech, etc.
Sawset
Minister of Fire
It seems to me that delta T across the coil is a function of air flow across the coil. A plugged coil, fan blades, or filter can reduce air flow. Also fan speed will alter air flow. For AC, fan speeds are generally set to high. For heating, fan speed is set to obtain proper delta T, generally less than high.
The delta T required for determining if a system is charged with refrigerant is that across the high and low pressure lines at the compressor.
Going by memory, correct me if I'm wrong.
The delta T required for determining if a system is charged with refrigerant is that across the high and low pressure lines at the compressor.
Going by memory, correct me if I'm wrong.
semipro
Minister of Fire
Is it possible you've just been having warmer weather than you have in the past?
Is your thermostat set point lower than it was in previous years?
Are there any additional thermal sources in your house?
72 seems pretty low to me. Keep in mind too that going from 74 to 72 takes a lot more cooling capacity than going from 76 to 74. The relationship between setpoint and load is not linear.
Many ranch-style houses have a brick veneer that has provides a significant thermal mass. If you have south-facing brick walls it wouldn't surprise me if the AC had to work at night as that heat moves inward.
Is your thermostat set point lower than it was in previous years?
Are there any additional thermal sources in your house?
72 seems pretty low to me. Keep in mind too that going from 74 to 72 takes a lot more cooling capacity than going from 76 to 74. The relationship between setpoint and load is not linear.
Many ranch-style houses have a brick veneer that has provides a significant thermal mass. If you have south-facing brick walls it wouldn't surprise me if the AC had to work at night as that heat moves inward.
brenndatomu
Minister of Fire
Very good stuff guys. When it was 85F outside the temp coming out of the vents was 59F, tested two days in a row, seems really good. When insulating I did not push insulation too far to the edges of the house. Every other rafter there are attic baffle vents. When having a roofer look at my house a while back he suggested ridge vents, currently there is nothing except an electric fan that does run all day. I'm not sure if I have an a-frame coil inside the furnace. There is some black putty that I'd have to remove to take off the sheet metal. Not sure I want to mess with it if my delta T is good?
brenndatomu
Minister of Fire
If there is a coil in there, its almost certainly an A coil...and just cut a hole in the side of the plenum for access...its just sheet metal...really easy to screw a larger piece over the hole, then tape it with actual HVAC tape (not "duct" tape!) to seal it up
brenndatomu
Minister of Fire
How does that work out? I was thinking about adding an attic fan...it gets SOOO hot up there...makes the AC for the upper floor work pretty hard! Beefing up the attic insulation is in the works too...but that won't be done for a bit... the fan can be done now.
I've gone up on the roof in the heat of the day and you can feel the heat billowing out of the fan. Gotta be doing something. It's temperature and humidity activated. The attic is 120F for a good 5 hours on a hot day, since I can hear/feel the fan running.
semipro
Minister of Fire
Continuous ridge and soffit vents are the best way to go for attic ventilation.
it creates a flowing layer of cooling, drying air under the roof.
It works even better if you seal up gable vents.
it creates a flowing layer of cooling, drying air under the roof.
It works even better if you seal up gable vents.
brenndatomu
Minister of Fire
If you are replying to me, I have a brick Cape Cod style home with no soffits...it has 4 gable vents, that's it. 1 each in the front and rear "dormers", and one at each end of the main upper attic. I wish I had soffits with about 2' of overhang!
I did install an roof mounted attic fan then...need to finish the wiring tonight...we'll see how much it helps then! (I did plug it into an extension cord just to see what its like...seems like it gonna help a lot)
DuaeGuttae
Minister of Fire
I don’t have a lot of advice about the OP’s problem, but I did want to mention that powered attic fans are not always a great solution for home energy savings. They certainly do work to exhaust hot air but are not always an energy saver. They use energy to run, of course, but can increase air conditioning expenses as well. If the attic is not well sealed off from the rest of the house (more than just insulation, actual air sealing), then the attic fan can pull conditioned air from your living space to help replace what is being ventilated, and the living space then pulls more unconditioned air in as well. It actually results in more air conditioner usage in the long run. Soffit and ridge vents are what’s recommended by energy experts, even down here in Texas (as well as radiant barrier roof sheathing). If the attic is really well sealed off from the rest of the house, it‘s different, but that level of sealing is not the norm. Insulating (and air sealing) is the best way to keep that heat out of the home.
semipro
Minister of Fire
brenndatomu
Minister of Fire
Yeah soffit and ridge venting would be ideal, but not how this house was built...and it would be a major project to change now...the attic fan is not ideal, but at 195 watts, the fan draws less than 1/3 the power of the air conditioner, and I think it will in fact reduce how much the AC needs to run, as the difference in a sunny day and a cloudy day on how the upstairs room feels is huge. The house is pretty well air sealed as far as to the attic is concerned...just not insulated well...that is in the process of being fixed though. Even once the insulation issue is fixed I think the fan may still run some, just because of the less than ideal gable vents that the house has...I'll probably just set the thermostat on 120* and let it do its thing at that point. I have it wired to run on temp, or timer.
Got my power bill, $140, no worse than it's ever been. I think vacuuming the outside coils and a fresh filter helped a little, and there is nothing really wrong. When it's 90F+ out, the AC is just going to run a lot.
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