Cool down speed

[perrybucsdad]

Just curious. Is there a general rule of thumb of how fast a properly working and sized central AC unit should cool a house down x degrees?

For example, if the outside temp is 89 degrees and the inside of the house (because the windows have been open) is the same temp, how long should it take to cool it down, say 10 degrees? 1 hour, 2 hours, 3 hours, etc?

I'm sure there are a ton of variables in the equation, but I am just trying to get a general idea.

[jared]

Depends on how humid it is inside the house....if it's very humid it will take longer, if it's extremely dry it won't take long..

[beenthere]

If its 89 and humid, 12 to 14 hours.
If its 89, and not very humid, 5 to 8 hours.

[jeff520]

When you talk of the temperature in the house that generally means the temperature of the air around the thermostat.

So, when you first run the air conditioning and get the air cooled ten degrees the thermostat will stop calling for cooling and at first glance you have met your need to "cool the house ten degrees".

BUT, at that moment the contents of the house (furniture, walls, etc) are probably still more than five degrees warmer than the air and they will quickly heat the air so that the thermostat is calling for more cooling quite soon after it first reaches the setpoint.

You will have truly cooled the house ten degrees when all the contents (air, furniture, walls, etc.) are at the same temperature.

Also, remember that Manual J calculates the cooling needed to maintain a defined temperature in the house given a defined outdoor temperature and a calculated heat gain from air infiltration and radiant heat. In theory then, it would be impossible for your AC system to ever reach a lower temperature if the outside temperatures and other conditions were as entered into the Manual J calculation.

Right now I am experimenting with my system (a 3 ton Trane XL19i) to see just how cold I can get the house when the outside temperature is above 100 degrees and I only allow the thermostat to call for first stage cooling (1.5 tons). The answer looks to be somewhere in the mid 70's, and cooling from 80 degrees to 76 degrees takes less than two hours in the morning as the outside temperature climbs from 70 to 90 degrees.

My house is not well insulated and the thick adobe walls retain a lot of heat from the day and release it during the night. The AC is still cycling on at 3 in the morning trying to keep indoors at 80 degrees when it is in the 60's outside! That is why I am painfully aware that cooling the entire house is a very different proposition than just cooling the air in the house.

[perrybucsdad]

Thanks Jeff, that makes sense.

So I pose another question to you then. In the summer time when in cooling mode, is it better to set the stat and leave it at a specified temp (say 75) and not use a setback stat? It would just seem logical that once you get the house cooled and all its contents, it would seem more energy efficient to just keep it at the same level (or am I leaving something out of the equation).

[DanW13]

IMO yes it is best to just set it and forget it, I would also run your fan on low to circulate the air in the house will help also. I live in Wisconsin and have kept my door's and windows closed and have my fan running on low and I have been able to keep the house warm at night when the OD temps drop and cooler during the day when the OD temps rise during the day and still have not needed to run my AC even when it has been alittle humid here with all the rain we have had. Just the last couple of days we have been in the high 80's and still able to retain ID temps of 72*, at night the home retains ID temps of 75 in the evening. If the house has decent insualtion and is somewhat sealed with good doors, and windows and door's and windows closed you should be able to do pretty much the same as me. Oh my indoor RH stays right around 50% with all the rain and on less humid days it keeps right at 45%.

[jeff520]

Perry, Still trying to figure out the economics/comfort issues around that question!

These things I know:

The A/C system does not reach its peak efficiency until it has been running for ten minutes or so. Short runtimes are bad from both an efficiency point of view and also for wear/stress on the system as it starts up.

The colder the interior of the house then the more heat will enter from the outside because the temperature differential is greater.

Based on the above, the optimum arrangement would seem to be to have the smallest possible system, sized so that it has to run continuously just to keep the interior at the desired temperature. That appears to be the best you can do from the A/C side - for economy insulate against outside heat or unconditioned air ever getting in.

Setback for short periods of time are probably not going to result in any savings since the A/C will end up running extra time just to recover (although that might be a good thing for efficiency and the life of the system). Setting back for a week while you are out of town obviously makes sense, just for half an hour while you run to the store is crazy. Don't know what time period makes the best balance between comfort and savings, would depend a lot on how well your home is insulated.

I suspect that there is an excellent way to do a quick check on the Manual J results and the sizing of the equipment installed by looking at the rate of temperature change both with and without the A/C running. The only variables that would need to be considered would be the temperature differential between indoors and outdoors and the amount of solar radiant heat. Indoors would have to be stable (no cold air/hot furniture situation), but I think that would make for an interesting project for some aspiring thermodynamics student.

Surprised that the professionals here have not offered more rules of thumb. It was a little difficult to answer your original question since you used X as your desired temperature drop and got back specific answers as to the time it would take.

[green jumper]

Its go time jeff! work work work!

The A/C system does not reach its peak efficiency until it has been running for ten minutes or so. Short runtimes are bad from both an efficiency point of view and also for wear/stress on the system as it starts up.

I will go into the office with a smile on my face because of this though, which will be quickly removed from my face as I face the horror of the office.

[perrybucsdad]

I guess I was more thinking of setback from 9am - 4pm (at work... but also peak heat time of the day) and 11pm - 5am.

In the winter time, the setback works just fine and we can heat the house up quite quickly, but the AC is a different animal. My Honeywell stat has a nice feature which will "learn" how long it takes to reach a desired temp (at least on the heat side I have seen this). On the cooling side, I don't think it has yet figured out how long it will take.

On a positive note, I have started to add heat reflective tint to my west windows. We have 4 windows in the family room, a slider in the kitchen, a back door, and a bathroom window on the first floor that all face west. On the 2nd floor, we have 6 windows that all face west that will all be treated as well. Hopefully this will help at reducing the heating of the house in the afternoon.

After that is all done, time to do the east side of the house.

[beenthere]

How much money you save using set back varies with how long the set back is. The outdoor temp during the set back period. The amount you set the temp back. Weather your area is a high or low humidity area. And if you try to recover during the high or low solar gain period of the day.

[perrybucsdad]

Okay, so from the sounds of it, in the summer, set it and forget it should be the best. Winter time is a different story (at least in my house).

[pstu]

I am a homeowner, rather than HVAC schooling my qualifications are from other sources. But that includes a basic knowledge of physics and math, and working 15 years for an electric utility. I firmly believe that setbacks save energy, although in some cases the amount may be too small to care about. It would be comfort reasons IMO, not economic reasons, which tell you to leave it at one stat setting 24/7. Still good reason, just not dollars and cents.

It is very hard for me to believe the amount of energy spent in pulldown mode, will exceed the amount *not* spent in setback mode. I believe the utility claims of a few percent savings are well researched and truthful -- these claims are usually overseen by the PUC and not just salesmans' jabber. You can look for 3rd-party verification via FSEC publications (Florida Solar Energy Center, a very good resource for all hot-humid states), or possibly Building Science Corporation or other sources.

In my own house I find myself using setbacks of just 1-2 degrees, and the pulldown process in my house is highly variable but may take a couple hours (any oversizing is relevant to pulldown). Last year I had some wider setbacks from day to night, and found to my surprise there were quite long runtimes after midnight. This seemed to teach me there were a lot of daytime BTUs stored in all the building structure.

If you can find any attic duct leakage (assuming not a sealed attic), that would possibly outweigh benefits from using large thermostat setbacks. Let me suggest solar screens on the west facing windows rather than tint -- screens are inexpensive and they work very well for me. In my house there were several east facing windows which led to an uncomfortable room in the morning sun, and solar screens were an effective solution to that comfort problem.

Hope this helps -- Pstu

P.S. I misspoke a bit, forgot that my setback on one system is 3 degrees, 78 day and 75 night. Overstates my case some.

[perrybucsdad]

Well, my setback degrees were more than just 1 - 2 degrees. We are talking 75 degrees while at home and 85 when away (or at night).

I think the other problem is that my system is not working efficiently yet, so the cool down takes days, not hours. I am waiting for the installer to come back out and take care of that.

[Shophound]

Well, my setback degrees were more than just 1 - 2 degrees. We are talking 75 degrees while at home and 85 when away (or at night).

I think the other problem is that my system is not working efficiently yet, so the cool down takes days, not hours. I am waiting for the installer to come back out and take care of that.

That's a really steep setback...you may be negating any savings such a strategy is hoped to provide.

At my own house, during the week, I set up to 78 during the day, then down to 75 in the evenings, 76 overnight, 74 for 2 hours in the morning to help remove humidity from showering, etc. The system seems to consistently pull the house down to 75 by the time I arrive, and then cycle.

Rough rule of thumb regarding pulldown time is one degree per hour. Tight ducts and insulated ducts run through an attic with radiant barrier installed will improve on that time frame.

[perrybucsdad]

I guess part of my problem is I always went by what the stat was pre programmed with. For winter that may be fine, but for summer, it seems like a tighter offset would be better. I'll take a look at that and play around with it.

[pstu]

That's a really steep setback...you may be negating any savings such a strategy is hoped to provide.

At my own house, during the week, I set up to 78 during the day, then down to 75 in the evenings, 76 overnight, 74 for 2 hours in the morning to help remove humidity from showering, etc. The system seems to consistently pull the house down to 75 by the time I arrive, and then cycle.
...

Shophound, is there any reason why an AC would be less energy efficient in pulldown mode than at a constant thermostat setting? If there is then I may have to take back my words on energy efficiency. If not then I wonder how efficiency is being negated.

I think we already agree on comfort being compromised with a setback. Another negative on comfort during pulldowns, some houses such as mine have a tendency to overheat or overcool with the system as installed. It is exceedingly hard to fine tune the dampers to eliminate this (the summer problem and the winter problem are just not the same, one solution is not always possible).

I am completely envious of your success with humidity control. Does this or does this not argue contrary to the classic HVAC theory that you need Manual J sizing for humidity removal and efficiency? You did say your system cycles during a design day, right? If so that would argue you actually have a certain amount of oversizing.

Best wishes -- Pstu

[Shophound]

Shophound, is there any reason why an AC would be less energy efficient in pulldown mode than at a constant thermostat setting? If there is then I may have to take back my words on energy efficiency. If not then I wonder how efficiency is being negated.

I think we already agree on comfort being compromised with a setback. Another negative on comfort during pulldowns, some houses such as mine have a tendency to overheat or overcool with the system as installed. It is exceedingly hard to fine tune the dampers to eliminate this (the summer problem and the winter problem are just not the same, one solution is not always possible).

I am completely envious of your success with humidity control. Does this or does this not argue contrary to the classic HVAC theory that you need Manual J sizing for humidity removal and efficiency? You did say your system cycles during a design day, right? If so that would argue you actually have a certain amount of oversizing.

Best wishes -- Pstu

It's important to differentiate between long or constant run times at indoor and outdoor design conditions vs. long or constant run times for a pulldown where outdoor is at or above design and indoor is above, perhaps considerably above, design conditions.

Let a house warm up to 85 while outdoor ambient heats up to 100+...it is true there's less heat transfer from outdoors to dwelling at this state. But...to then come home and kick the setpoint down to 75...you have 85 degree air entering the evaporator vs. 75. That's more heat load, that's a more loaded compressor, that translates to more watts at the meter. While the system may not have run much or at all during the setback period, now it has to run well into the evening or night to get indoor temperatures to design conditons. While it's true that much of this period will occur post peak day heating, the attic is still superheated and is radiating into the conditioned space. The pulldown is slow because the heat gain is great, and the outdoor ambients are still somewhat high. The system runs and runs, trying to catch up. Interior materials also must cool down, which when warmer than indoor ambient air will make a person feel warmer than the air around his skin would otherwise indicate.

This is not to say "set it and forget it", only to say setback spreads should be modest vs. radical.

Regarding my own dwelling, I have yet to run a Manual J on it (just bought house late last year). Carnak and I have been discussing various aspects of building science/HVAC system interaction in TB's thread...we both appear in agreement that oversizing does not always result in loss of humidity control.One thing to bear in mind...we get our heads wrapped around nominal tonnage ratings but glibly overlook actual functioning capacity of an installed system. Nominal means nothing when my design conditions are not what ARI rated my system at.

I'm eager to get data loggers in both my supply and return plenums to record entering and leaving dry and wet bulb temperatures, and measure ESP, so I can know under various condtions what my actual operating capacity of the system is vs. ARI nominal. I also want to perform a Manual J on the house with its current thermal envelope improvements, then compare the Manual J outcome with the system's performance on a design day. I've done this at my father-in-law's house and it was an eye opener, and yet more bolstering for how wary we should remain regarding rules of thumb.

One last thing...my design condtions in Fort Worth are 99 degrees dry bulb with coincident wet bulb of 74. That yields an outdoor design dew point of 63. On a design day I should expect relative humidity levels to be around 30-31%, which is exactly what I saw yesterday when my own mini weather station thermometer/hygrometer hit 99 degrees. If my design indoor condition is 75 dry bulb and 50% relative humidity, that's a 21 grain difference between indoor design and outdoor design.
For Houston, recalling from memory, your design day conditions are 94 dry bulb with 78 wet bulb, yielding a dew point of 72 degrees and a relative humidity condition of nearly 50%. Using same indoor design as above, your grains difference is 54! You definitely have more of a dehumidifying challenge under your design day conditions than I do.

So, I wouldn't be overly envious, Pstu. I don't have to wring out as much water as you do, and probably would have to wring out more water had I not taken some steps to reduce infiltration. If I lived in Houston I'd make that aspect first priority before I did anything else to a house. Reduce the load and watch the performance rise!

[udarrell]

When you talk of the temperature in the house that generally means the temperature of the air around the thermostat.

So, when you first run the air conditioning and get the air cooled ten degrees the thermostat will stop calling for cooling and at first glance you have met your need to "cool the house ten degrees".

BUT, at that moment the contents of the house (furniture, walls, etc) are probably still more than five degrees warmer than the air and they will quickly heat the air so that the thermostat is calling for more cooling quite soon after it first reaches the setpoint.

You will have truly cooled the house ten degrees when all the contents (air, furniture, walls, etc.) are at the same temperature.

Also, remember that Manual J calculates the cooling needed to maintain a defined temperature in the house given a defined outdoor temperature and a calculated heat gain from air infiltration and radiant heat. In theory then, it would be impossible for your AC system to ever reach a lower temperature if the outside temperatures and other conditions were as entered into the Manual J calculation.

Right now I am experimenting with my system (a 3 ton Trane XL19i) to see just how cold I can get the house when the outside temperature is above 100 degrees and I only allow the thermostat to call for first stage cooling (1.5 tons). The answer looks to be somewhere in the mid 70's, and cooling from 80 degrees to 76 degrees takes less than two hours in the morning as the outside temperature climbs from 70 to 90 degrees.

My house is not well insulated and the thick adobe walls retain a lot of heat from the day and release it during the night. The AC is still cycling on at 3 in the morning trying to keep indoors at 80 degrees when it is in the 60's outside! That is why I am painfully aware that cooling the entire house is a very different proposition than just cooling the air in the house.

This entire thread & your experiment are very interesting!

My bother has a 1.5-ton 12-SEER on his single story 1000sf home with two dehumidifiers in the basement. Even on mid 85-F days with the indoor humidity around 50%, it runs all the time & can't get the temp below 75.

The unit has a scroll compressor & a TXV metering device. You can hear the liquid R-22 surging in the line by the coil. The condenser split ranges from only 10 to 12.5-F.

His home is fairly well insulated & tight, however, that A/C system is probably doing less than a nominal ton of cooling.

I leave in an older 2-story home with the same outdoor conditions & NO dehumidifiers in the basement. I have a little dinky Half-Ton window unit that cools the 1st floor close to 900sf & under those same conditions will pull it to 72-F at 50% RH. With 104 Heat Index it pulls it to 76-F & around 55% RH.

The installing contractor is my brother's buddy & he doesn't want to hurt his feelings. He finally called them to check the unit, the tech(?) checks the suction pressure & says it's okay.

I said, aren't you going to check the subcooling & superheat, his eyes glazed over. He did not have anything to check SC & SH with, he found a dial air probe TH & tried to get a reading with it, ha. Well, he was going to get their top guy to come out but to this day he has never showed up.

Jeff520, I know you live in a very dry climate, but was wondering what size your home is, windows, etc. I would like to hear more about your experiment as it unfolds.

Shophound made some excellent comments.
"Rough rule of thumb regarding pull-down-time is one degree per hour."

If they were to do an accurate M.J. on his home, it would probably be about halfway between a ton & 1.5-ton.

We need a lot more threads like this one! - udarrell

[perrybucsdad]

Very interesting information guys. I have learned a lot today.

I know DIY questions are off limits, and I hope my next question does not fall into that category. If so, I apologize and will understand if nobody answers or corrects me.

Is it possible, to do a manual J on my own to see some of the numbers my house should have, or is this something that I should leave to a professional to do?

[pstu]

One technical point...

>>Let a house warm up to 85 while outdoor ambient heats up to 100+
>>...it is true there's less heat transfer from outdoors to dwelling at this state. But
>>...to then come home and kick the setpoint down to 75
>>...you have 85 degree air entering the evaporator vs. 75.


My knowledge of this is only from perusing a couple of Trane manuals. Let me just quote something they say:

2TTZ9036A1 WITH TWE040E13 AT 700 CFM 1ST STAGE

O.D. I.D. TOT. ---- SENS. CAP. AT ENTERING D.B. TEMP. TOTAL
D.B. W.B. CAP. ---- 72F 75F 78F 80F ---- KW
85F 59F 18.2 ---- 16.5 18.3 19.1 19.5 -- 1.04kw
85F 63F 19.6 ---- 13.7 15.9 18.0 19.4 -- 1.03kw
85F 67F 21.0 ---- 10.6 12.8 14.9 16.4 -- 1.02kw
85F 71F 22.5 ---- 07.5 09.6 11.8 13.2 -- 1.01kw

95F 59F 17.4 ---- 16.2 17.7 18.4 18.8 -- 1.23kw
95F 63F 18.7 ---- 13.3 15.5 17.6 18.8 -- 1.20kw
95F 67F 20.0 ---- 10.2 12.4 14.5 16.0 -- 1.18kw
95F 71F 21.4 ---- 07.1 09.3 11.4 12.8 -- 1.15kw

It seems this source states that total capacity is essentially unchanged with respect to indoor dry bulb temperature. As indoor temperature rises the latent capacity goes down but sensible goes up the same amount... just noticed the constant is wet bulb temperature, that won't assure constant dewpoint will it???

Those figures are from Trane publication 22-1746-01-0103, just so any of you can dig up the entire book and see I have copied correctly. There *are* dry coil conditions where the sensible capacity is stated higher than total capacity, I made no typo but am not qualified to talk about that.

Best wishes -- Pstu


P.S. added after msg #24: I can see now that holding wet bulb temperature constant and raising dry bulb temp, changes the absolute humidity quite a lot. For purposes of thermostat setback, the desired table would hold absolute humidity constant... if only Trane would publish such a table.