Upstair HVAC System in Dallas Cooling at Only 1 Degree Per Hour. Is it working?

[ganny]

I have 3 HVAC units serving my 4,000 SF home in Dallas (two units serving the down stairs, and one 3.5 ton unit serving the approx. 1400 SF upstairs).

The downstairs units will cool the floor by 5 degrees (from 82 degrees to 77 degrees) in about 25 minutes when it is 95 degrees outside.

However, the upstairs unit takes between 4 and 5 hours to cool the floor by 5 degrees (from 82 degrees to 77 degrees).

The units are only 2 years old, and the contractor that installed the system as come by on two occassions to work on it. After the last visit he stated that he is getting a 17 degree temp drop, and that the unit appears to be working. When asked why its takes so long to cool, he said that the time to cool is not the correct measure, but rather how long the systems needs to run to "maintain" the desires temperature. That explanation does not work for me as I need to be able to cool the upstairs down in a reasonable periord of time and do so efficiently. I am beginning to suspect that there is a real problem with he system, but that the contractor doesn't want to tell me b/c I will require that he fix it at no charge (since he would have installed it incorrectly).

So, is it normal and expected that a new system would take 4 - 5 hours to drop the room temperature by 5 degrees? Could inadequate return air cause this problem?

[saylor240]

i would call out another company ask them if they do free second opionions in this situation, and you will find out what going on...4 to 5 hours does seem a bit to much...

[DeAngel]

On most units with an outdoor temperatures between 80 and 90 degrees with an indoor relative humidity between 40 and 50 percent, the temperature drop across the evaporator should be between 17 and 20 degrees. Many factors can contribute to an extended pull down period, especially in an upstairs zone where you have heat rising from the lower level and heat coming from the attic space.

[leander311]

There are far too many variables to give you an adequate answer, but given the outdoor temperatures lately that are above the temperatures your system should have been designed to cool in, the "pull-down" time you mention is not unreasonable if you were trying to do this in the heat of the afternoon.

In other words, if you are using a setback thermostat where it kicks on at 5pm to pull down from 82-77F, it requiring several hours to do this is not unreasonable at all. If these are your circumstances, I would actually be concerned your downstairs units are substantially oversized.

If you still have concerns about the installation, have a competing contractor come out and provide a 2nd opinion. The competing contractor would hopefully be able to point out any deficiencies in the original install and/or suggest cheap improvements (more attic insulation, for example) to aide in faster pull-down. Hope this helps! (PS - I'm a registered professional HVAC/facilities engineer and work over in Arlington. Shoot me a note at leander311 @ aol.com if you'd like to discuss further.)

[udarrell]

First, the tech has to collect some data to determine what the 3.5-ton units operating BTUH is with those conditions.

Ductwork & airflow are always suspect.

Then he would have to know what the 2nd floor heat-gain rate is to determine why it is taking that long to pull the temp down.

The indoor temp-split alone is not enough data to make the necessary determinations.

Many things can affect both the units performance & the rate of heat-gain.

Were I you, I'd address both issues.

Click udarrell above, or go to: twitter.com/udarrell

[wraujr]

First, when you say it takes 4 -5 hours, what exactly is the A/C unit doing
(1) Is the A/C a one-stage or two-stage unit.
(2) Is it running (i.e outdoor compressor) non-stop OR is it cycling on/off.

a. A single-stage running NON-STOP for 4-5 hours might be a problem.

b. A single-stage cycling but still taking 4 -5 hours could be due to all the built up thermal energy in the walls and attic. Here's the thought, your AC cools down the room's air and cycles off. But thermal energy radiates from the walls and ceiling re-heating the room air and promptly requiring A/C to run again. So its cycling on and off, eventually removing this stored heat.....

c. A two-stage is a funny beast when compared to a single-stage. During high heat load it spends a lot of time in first stage and if it can't keep up it goes into 2nd stage but only to play catch-up. On my Trane, once it catches up it drops back to 1st stage and continues running. But, if the heat load is high it falls behind again and eventually calls for 2nd stage. Under high heat load it essentially runs non-stop (which does have comfort benefits) mostly in 1st stage and occasionally bumping into 2nd. This is how my Trane xL16i 2-stage appears to work . Once we get thru the first summer with this two-stage I'll be able to say whether I like it or not....

[john_ertw]

a. A single-stage running NON-STOP for 4-5 hours might be a problem.

b. A single-stage cycling but still taking 4 -5 hours could be due to all the built up thermal energy in the walls and attic. Here's the thought, your AC cools down the room's air and cycles off. But thermal energy radiates from the walls and ceiling re-heating the room air and promptly requiring A/C to run again. So its cycling on and off, eventually removing this stored heat.....

What is wrong with a single-stage unit running NON-STOP for 4-5 hours if the outdoor temperature is at or above design temperature?

Is residential a/c equipment not designed to run non-stop for long periods of time?

Or are you saying there is an equipment sizing problem since it is not achieving the desired temperature?

[ganny]

I have a two stage unit, and it runs non-stop for the entire time to cool 5 degrees over 4-5 hours. The only reason we go upstairs is to use the Media Room for movies, tv, etc., and we only do this about twice a week. We normally keep the upstairs AC off (or set to 85 degrees) b/c we don't go upstairs very often, and we don't want to waste electricity to cool an area that is not being used. The problem is when we want to use the area, we have to plan 4 to 5 hours in advance in order to cool it to a reasonably comfortable temp (77-78 degrees). My downstair cools in less than 1/8th time (in 25 minutes). If my upstairs could cool in a similar amount of time my problems would be solved.

[wraujr]

In his situation it might be an indicator of a problem since he can't acheive his desired temp, yet his downstairs can in 1/8th the amount of time. My comments are about HIS situation and issue, not equipment in general.

The blower in my office space runs 24/7 and is doing quite fine. Homeownwers on the other hand are not used to nor have they experienced equip. running 24/7 so when it does they perceive it as a problem. If it is 95 deg out my new 2-stage looks to be running 24/7 while cycling between 1st and 2nd stage. Many comfort benefits come from this but this type of operation is new to many homeowners who are accustomed to hearing the A/C cycle on and off.......and at first glance it might appear as if unit is undersized, etc. With high tech comes more complex operation which will confuse the average homeowner's and result in many "but thats not the way it used to work" comments..

[texsn95]

That does seem extremely excessive, and I'm in Houston so I know about this GD heat. I have an upstairs 3.5 ton unit, 20 years old, cooling approximately 1200 sq ft that we keep warmer than downstairs. To go from the low 80s to high 70s only takes about 20-25 minutes.

[wraujr]

Now can you tell if its cycling between first and 2nd stage???

I suspect with the stored thermal load (attic, walls, etc.) the unit is cycling between first and 2nd.

Try this test. Let rooms heat up with unit off. Turn on A/C and set tstat to 64 deg. Unit should go into and stay in 2nd stage. With room thermometer, see how long for temp to drop. What I'm suggesting is first eliminate the 1/2 stage algorithm from play and just see how well 2nd stage can drop temp. This should give an idea if equipment sizing is adequete.

2nd, you are asking this unit to remove a lot of stored thermal energy from your walls and attic and this energy radiates back into the room and it doesn't do it instantaneously.....

[motoguy128]

I have a two stage unit, and it runs non-stop for the entire time to cool 5 degrees over 4-5 hours. The only reason we go upstairs is to use the Media Room for movies, tv, etc., and we only do this about twice a week. We normally keep the upstairs AC off (or set to 85 degrees) b/c we don't go upstairs very often, and we don't want to waste electricity to cool an area that is not being used. The problem is when we want to use the area, we have to plan 4 to 5 hours in advance in order to cool it to a reasonably comfortable temp (77-78 degrees). My downstair cools in less than 1/8th time (in 25 minutes). If my upstairs could cool in a similar amount of time my problems would be solved.


well... it does sound like the downstairs is possibly a little oversized and the upstairs is just suffering from a high heat load, heat rising from downstairs and probably some leaky ductwork. Keep in mind when you let a room sit at 85 degrees... when you go to cool it down, you also need to cool off the mass of the walls, floor, ceiling, and interior furnishings.

You home design and layout is actually a good place for a zoned system with a 2 stage compressor. A single 4 ton 2 stage unit would probably do a better job of cooling off the upstairs space in less .. especially since it sounds like the donstairs systme is a little oversized. I think it was sized based mainyl of square footage. I'm guessing by how you describe your home that hte downstairs is more like 2500 sqft and the upstairs is closer to 1500 sqft.

[leander311]

The way the industry standards dictate professionals to size residential systems (for efficiency, proper dehumidification, etc), they should actually be "undersized" for 2.5% of the yearly hours, which is basically 220 hours a year. So for the hottest 220 hours a year and everything close to it, your unit should be running non-stop just to maintain a comfortable temperature. Pulling down quickly from 85F setback right when you get home from work (which is incidentally usually during said 2.5% time period) is above and beyond for a right-sized system.

That's all to say it's possible there is nothing wrong with your upstairs unit. On the otherhand, if it is improperly oversized like 70% of HVAC installed residential applications, there may be a problem.

Either way, improving the thermal envelope in your upstairs will go a long way to solve your problem. It's possible ~$500 of added attic insulation could cut your pull-down time from 5 hours to 1 hour.

[wraujr]

Another way to look at it. If your design temp is 95 and your 3 ton is the proper size, then at 95 deg ALL the BTUs are going to remove the thermal gain from outside/sun. Which would be excellent, EXCEPT you have 0 BTUs left over to remove the heat load that has accumulated in the walls, ceiling, drywall, studs, furnishings, etc. (even the insulation is holding heat) while the unit was off. Think of a red hot frying pan on the stove, turn off the gas, and stand there for the next 5 -10 minutes with you hand out feeling the heat radiated back. Thats whats happening upstairs. To handle the stored up heat, my guess is you would need a greatly oversized unit, but thats just asking for trouble......

[Sleuth]

I have a two stage unit, and it runs non-stop for the entire time to cool 5 degrees over 4-5 hours. The only reason we go upstairs is to use the Media Room for movies, tv, etc., and we only do this about twice a week. We normally keep the upstairs AC off (or set to 85 degrees) b/c we don't go upstairs very often, and we don't want to waste electricity to cool an area that is not being used. The problem is when we want to use the area, we have to plan 4 to 5 hours in advance in order to cool it to a reasonably comfortable temp (77-78 degrees). My downstair cools in less than 1/8th time (in 25 minutes). If my upstairs could cool in a similar amount of time my problems would be solved.


Considering the temperatures and humidity we're having at this time the performance you describe is reasonable for a good system.

It's about the latent load or humidity.

Programmable t-stat set to pre condition the area on the days you want it to "Pull Down". Let the system have a running start (5 hours) during the SUMMER.

[sktn77a]

Was a heat gain (sensible and latent) calculation done upstairs? If the temperature drop is within spec, then the new unit may have been undersized. It shouldn't take 5 hours to lower the temperature 5 degrees with a properly sized and installed system.

[beenthere]

1° an hour, when its 95 outside, isn't a bad amount of time.

Are you setting all of your A/C's back to 82 at the same time, and then turning them all down to 77 at the same time?

[ganny]

Was a heat gain (sensible and latent) calculation done upstairs? If the temperature drop is within spec, then the new unit may have been undersized. It shouldn't take 5 hours to lower the temperature 5 degrees with a properly sized and installed system.

I don't know whether this calculation was performed, and I agree that it shouldn't take this long. I don't understand the notion that retained heat in the walls, furniture, etc. is causing the delay in cooling the upstairs because the downstairs also has retained heat (when the unit has been off for a while), and the downstairs is able to cool quickly despite such retained heat.

I do agree that perhaps it should take a little longer to cool the upstairs b/c heat rises from downstairs and b/c of the proximity to the atic, but 4 hours longer just seems like way too much time.

If the unit was undersized by the contractor, perhaps I can get the contractor to replace the unit.

[ganny]

1° an hour, when its 95 outside, isn't a bad amount of time.

Are you setting all of your A/C's back to 82 at the same time, and then turning them all down to 77 at the same time?

Yes, I did this is order to compare drop down times. I caused both the upstairs and downstairs to reach and maintain 82 degrees all day long. Then, in the evening, I set both units to 77 degrees at the same time. Down stairs took 25 minutes, upstairs took more than 4 hours.

[beenthere]

No wonder.

Next time. Just leave the upstairs at 82, and see how long it takes teh downstair to get to 77.
It will take longer bcause the cool air from teh second floor won't be helping them.

Your test is not realistic.

Do you live in a part of Texas, that doesn't have humidity?


My system can maintain 72 when its 98 outside. it ill have 5 and 6 hour long run times.

If I left it get to 77 and then ran it. It would take probably 1 hours to cool the house back down to 72.

If your upstairs is bringing the temp down 5 degrees in 5 hours when its 95 or more outside. Its doing fine.

[leander311]

I don't understand the notion that retained heat in the walls, furniture, etc. is causing the delay in cooling the upstairs because the downstairs also has retained heat (when the unit has been off for a while), and the downstairs is able to cool quickly despite such retained heat.

If the unit was undersized by the contractor, perhaps I can get the contractor to replace the unit.

It's called "thermal inertia". Same reason you can't throw a few gallons of water in a pot and expect to immediately boil pasta... The water has to heat up first. Same concept applies to HVAC. Now, again, according to industry standards, "properly-sized" HVAC is actually supposed to be a tad undersized for a number of very good reasons, and thus your downstairs A/C is likely oversized if it's pulling down in only 25 mins. The rest of the time, it's cycling on for a couple minutes, and off for an hour or so to meet the load. In this way, it won't wring moisture out of the air very well, and you'll get a cool but relatively humid environment...and during the short time it is running, it achieves nowhere near the efficiency its rated... just a couple of good reasons proper sizing is to slightly under-size. If your need to pull down a warm room at above-record outside temperatures was not captured prior to design/install, the contractor is not necessarily at fault, and certainly not negligent. A 2nd opinion from another contractor would at least confirm proper installation/setup, and if confirmed, adding insulation or something similar will be the cheapest and most effective way to obtain the results you desire.

[ganny]

No wonder.

Next time. Just leave the upstairs at 82, and see how long it takes teh downstair to get to 77.
It will take longer bcause the cool air from teh second floor won't be helping them.

Your test is not realistic.

Do you live in a part of Texas, that doesn't have humidity?


My system can maintain 72 when its 98 outside. it ill have 5 and 6 hour long run times.

If I left it get to 77 and then ran it. It would take probably 1 hours to cool the house back down to 72.

If your upstairs is bringing the temp down 5 degrees in 5 hours when its 95 or more outside. Its doing fine.

I'll try leaving the upstairs at 82 per your suggestion, and then test the downstairs. We do have quite a bit of humidity in Dallas (although its less than Houston, which is closer to the coast)

[beenthere]

If the upstairs cooled down in 5 minutes under those conditions. You would have probably 70%RH on the days it was only 85 outside.

[Shophound]

What is wrong with a single-stage unit running NON-STOP for 4-5 hours if the outdoor temperature is at or above design temperature?

Is residential a/c equipment not designed to run non-stop for long periods of time?

Or are you saying there is an equipment sizing problem since it is not achieving the desired temperature?

A residential system undergoing a pulldown (especially from 82 degrees to 77 degrees) will not cycle until the 77 degree setpoint is reached.

Upstairs areas in most residential construction has a higher heat gain than downstairs areas of equivalent square footage.

For Ganny in Dallas...try 80 degrees as your higher set point, so your pulldown spread is 3 degrees vs. five. Setting the temperature higher when the house is unoccupied is a good idea, and can be a money saver, but if the spread is too far apart between occupied and unoccupied set points, the savings can be diminished to erased.

[leander311]

If your upstairs is bringing the temp down 5 degrees in 5 hours when its 95 or more outside. Its doing fine.

Most of DFW has consistently hit highs in the low 100's for the past several weeks... and it hasn't hardly dropped below 86F at all over the past week in Dallas proper: http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KTXDALLA78&day=25&year=2009&month=6&graphspan=week

Isn't it nice to know the industry is being incidentally sponsored by global warming? :D

[leander311]

...but if the spread is too far apart between occupied and unoccupied set points, the savings can be diminished to erased.

Unless I'm misunderstanding you, this doesn't sound right. It reads parallel to "programmable t-stats don't save any energy because the AC has to run FOREVER at the end of the day to pull all that heat out of the space", which is one of the biggest myths in our industry.

[beenthere]

Isn't it nice to know the industry is being incidentally sponsored by global warming? :D


Can't prove it by here.

[skizot]

What is the typical indoor design temperature you guys use when sizing A/C?

[udarrell]

Unless I'm misunderstanding you, this doesn't sound right. It reads parallel to "programmable t-stats don't save any energy because the AC has to run FOREVER at the end of the day to pull all that heat out of the space", which is one of the biggest myths in our industry.

The reason operating costs have a possibility of being saved with a set-back t-stat is that temp differential is a factor in the rate of heat transfer; equal temps NO more heat transfer during that length of time. Your statement concerning myths has weight.

I don't have any problem pulling the heat build-up with a little half-ton room unit that cools he entire first floor area. Another half-ton'er cools the bedroom, bathroom, hall & stairwell to very cold conditions.

I doubt that his A/C is performing up to its Nominal Rating, & list the test data. My brother's 1.5-ton system is still delivering half its Nominal Rating & I made that determination with a slim digital pocket TH used on the copper lines, etc., & a self-made make-shift velocity instrument.

Don't doubt me fellas, I'm on solid ground with my tests; & the service guys are just checking the suction pressure on TXV system! It has one degree Subcooling & not enough airflow for a one ton system.

The condenser has a 10-F split, & with those conditions using mfg'ers data it should have between 18 & 20-F condenser delta T. The condenser where both the sensible & the latent can be measured with a TH, or a manifold gauge.
- Darrell

[motoguy128]

Isn't it nice to know the industry is being incidentally sponsored by global warming? :D


Ya, but it's a killer on furnace sales.:rolleyes:

[beenthere]

Here, 70 for most homes.

[mbmi8802]

Quick question for you all. I want to add zoning to a system to newly installed XL20i heat pump system. It is currently running in data format. Im adding a new 3h 2 cool zoning system by EWC. Im gonna run it with VP8000 stats. Question is once I wire over to 24 vac, it is possible to still run data communication between hp and air handler. 4TEE3c10. vs??? Eliminating attempting to pull more wire to hp in difficult location.

[skizot]

Here, 70 for most homes.
I guess it's time to call an HVAC tech then. :( Although it's been in the upper 90s, and hit 100 a couple of times in the last week, my A/C is unable to cool it down below 77-78 here. It just runs non-stop, no cycling, and after it's ran for a while the RH is around 50% inside (dew points have been in the upper 70s). The temp split is around 17-18 *F measured at the closest supply, and averaged among the returns. From those numbers, I assume the system is running correctly, but it seems undersized. Upper 90s are not at all unusual around here during the Summer. I should be able to get it done to at least 74, even at these temperatures.

Edit: The house is fairly well-sealed with a Natural ACH of 0.37 (not the tightest, but much, much better than it was when I bought the home).

[mbmi8802]

I have 3 HVAC units serving my 4,000 SF home in Dallas (two units serving the down stairs, and one 3.5 ton unit serving the approx. 1400 SF upstairs).

The downstairs units will cool the floor by 5 degrees (from 82 degrees to 77 degrees) in about 25 minutes when it is 95 degrees outside.

However, the upstairs unit takes between 4 and 5 hours to cool the floor by 5 degrees (from 82 degrees to 77 degrees).

The units are only 2 years old, and the contractor that installed the system as come by on two occassions to work on it. After the last visit he stated that he is getting a 17 degree temp drop, and that the unit appears to be working. When asked why its takes so long to cool, he said that the time to cool is not the correct measure, but rather how long the systems needs to run to "maintain" the desires temperature. That explanation does not work for me as I need to be able to cool the upstairs down in a reasonable periord of time and do so efficiently. I am beginning to suspect that there is a real problem with he system, but that the contractor doesn't want to tell me b/c I will require that he fix it at no charge (since he would have installed it incorrectly).

So, is it normal and expected that a new system would take 4 - 5 hours to drop the room temperature by 5 degrees? Could inadequate return air cause this problem?

I work in Dallas homes every day. I can tell you that the homes layouts make our job very difficult with air migrating up and down. Most of these homes will have main floor cooling quicker then the 2nd floor. Ducting here in the Dallas area is always an issue. People size ducts without consideration of length and other restrictions. Many ducts are supported poorly leading to airflow issues. Many homeowners neglect filtration systems as well. I have seen countless coils with 50 to 70 percent air restrictions after 1 year. I would have to see your entire layout and duct system to know if improvement can be made. The rate of temp drop will very according to humidity level. Start with holding temp on your t stat at same temp. Your utility and comfort level should improve if you set back is stretched to far.

[Daltex]

Ganny- My Dallas house has a similar layout as yours. Upstairs media room had to have extra ductwork run to it even when the upstairs unit was running great. Mine has a projector which along with the other electonics creates some serious heat. Get the unit running right and then see about adjusting the airflow to the media room to make it comfortable.

Is the upstairs thermostat inside the media room or is it in a hallway/landing that is open to the first floor?