Terminal Cooling Temp Rule-of-Thumb


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Old 05-09-13, 11:12 PM
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Terminal Cooling Temp Rule-of-Thumb

I needed to have two AC split systems replaced, so I got quotes from several contractors. All four recommended the same two size units, a 4-ton unit and a 3-ton unit, but I remember several of the contractors telling me while this was the correct size for my house, it is unrealistic to think that these two units will keep my house at 72 degrees through the summer.

I didn't think about it much at the time, and I did have my units replaced, so now I started wondering just how cool or warm my house will be. I should mention that this house is in Phoenix were 4 hours a day at 116+ is a pretty common experience.

I'm hoping to maintain around 80 to 81 degrees but not sure if this is realistic. Is there some rule-of-thumb that specifies the best I can get at a certain outdoor temp? What temp would the air from the vents be?
 
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Old 05-09-13, 11:55 PM
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The temp at the vents depends on capacity relative to airflow, duct work, attic temp (if system is in attic), indoor humidity, and more.

There's no rule of thumb for anything in hvac - only a heat gain calculation can tell you how much capacity is needed to maintain a certain indoor temp.

If the unit isn't installed or setup properly, it won't run at rated capacity.

Low airflow, improper charge (amount of refrigerant in system), and mismatching kill capacity.
 
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Old 05-10-13, 08:28 AM
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O.K. Let me rephrase my question. If my contractor followed all the standard sizing requirements, my ductwork is R-6, and in the attic, like ductwork is in Phoenix, and otherwise everything is setup correctly. As I said, 4 contractors all recommended the exact same size units, so I have to believe they are all following the same standards.

So my question what temperature do these standards attempt to achieve during a time that the outside temperature is 118? Assuming everything was done by the book.
 
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Old 05-10-13, 11:53 AM
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...it depends on how the system is sized.

The only way to size accurately is to do a load calculation.

Odds of having everything done properly or having proper airflow are very low with flex duct in the attic. That's probably why they do something ridiculous like 1 ton per 500 sq ft or less - to compensate for installation errors and crappy ductwork.

7 tons of capacity is a lot, but I don't know how houses are built in arizona.

A/Cs are rated at 95F outdoor -> above that capacity drops, which must be taken into consideration.
 
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Old 05-10-13, 12:09 PM
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Anog...as Muggle has said...the answer is "it depends on....". I know you are further south than me and it gets way hotter, but 7 tons is a LOT! Size of the house, construction, insulation, number and type of windows and which way they face ...all these are big factors that should have been figured in to the calculation.


As an example...I have a 20 y/o 3.5 ton (I think) package unit on the roof and even when it gets to 110-115 here it will keep the interior at 75 degrees without a problem. Of course it runs quite a bit.

House is about 1700sf 2x6 exterior walls, stucco, with about 16" of blown in fg insulation in the attic. Poor attic ventilation so it can easily hit 160 in the attic which I know is a major problem. Front of the house receives full sun all day and of course thats where the big windows (Double pane, but not tinted or LoE) are. We keep the blinds drawn on hot days to minimize solar heat gain.
 
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Old 05-10-13, 01:02 PM
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I can't comment much on the capacity, other than to say the home came with a 3 1/2 ton and a 3 ton unit, and they didn't cool enough. Even still this number can be a bit misleading. We have the typical APS electric plan where we pay dearly for high electrical peaks during any one hour. When the AC guys were installing the units they ran both at stage 2 for an hour. That 1 hour alone added $60 to our electric bill that month.

So in reality, one AC really only runs during the day, and the other runs only at night. Because of that, each is a tad bigger because its partially cooling the other space.

We put solar screens on all the windows, and that definitely helps. We also added zoning to the bigger system and that has really been working well. Sounds like putting more insulation over the ducts might be helpful. I wonder if wrapping the attic coil with some insulation could help also? I will cross my fingers that it will work in august, but I guess there is only one way to tell.
 
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Old 05-10-13, 01:08 PM
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Well...you aren't really giving any details on home size or anything....so I guess it was a roll of the dice. Did they even do the calculation required?

Guess you'll just have to wait for the hot days to find out if they hosed you or not.
 
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Old 05-10-13, 05:43 PM
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Without knowing your house's size, construction, insulation, etc... as mentioned by others, the only rule I can suggest is to make sure your average Delta T is above 18 degree.
 
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Old 05-10-13, 08:30 PM
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High delta-t can indicate insufficient airflow

----------------------

Your systems may not be delivering anywhere near rated capacity.

Also, installing ductwork in the attic in a hot climate is beyond stupid, but that can't be helped in existing homes. I'm not sure what the builders/architects are smoking down there.
 
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Old 05-10-13, 11:18 PM
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House is 3000 sq ft. single story.

I'm not a contractor, but I did have four of them give me quotes and a pretty detailed analysis was conducted by each, because this is required by our utility to receive the rebate. I also had a full energy audit including a blower door test. I've asked several times but other than a too high room pressure, they say all else came out well. Ducts had very little leakage.

My sense is these units are plenty big enough. It has only been up to maybe 100 here, and they have had a pretty easy time keeping the temp at 80. Duty cycle was never more than 20%. BUT having said that its still not much above 60 at night so the house can store some "cool." In August the low will be closer to about 90.
 
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Old 05-10-13, 11:29 PM
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> High delta-t can indicate insufficient airflow

O.K. then how high is too high?
 
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Old 05-11-13, 11:44 AM
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Duty cycle was never more than 20%.
Then you should downsize.

On the hottest day of the year, your systems should be on continuously but maintaining the setting.

It takes 10+ minutes of operation to even reach rated capacity and efficiency; that really can't happen when a system only operates 12 minutes out of every hour.
 
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Old 05-11-13, 11:53 AM
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Just for those out of area...most of the homes here are built on slabs...don't think I've seen any basements or crawls except for those homes built in to hills or similar. No other way to route ducts except in the attic unless they steal quite a bit of room from interior spaces building chases and such. Also..many homes use rooftop package units.
 
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Old 05-11-13, 12:03 PM
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^Personally I would go for mini-splits before even considering installing ducts in an attic.

That, or insulate/seal the roof deck, put in a subfloor, and use metal ducts.
 
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Old 05-12-13, 09:47 PM
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> Then you should downsize.

> On the hottest day of the year, your systems should be on continuously but maintaining the setting.

> It takes 10+ minutes of operation to even reach rated capacity and efficiency; that really can't happen when a system only operates 12 minutes out of every hour.

We certainly haven't hit the hottest day of the year yet. Today it was 100 so that is about 20 degrees short.

Outside temps can be 1 degree hotter than inside or 40 degrees, and when its only slightly hotter, than system has an easy time. Unlike 95% of the systems out there, I have set mine to have a 9 minute minimum run time, to keep efficiency up. Minimum off time is 10 minutes, so at a minimum, a cycle is 19 minutes. Seems to work fine.

I have four contractors provide me with estimates for a new system, and they all selected the same capacity units, so I can only assume they are correct. I probably should have gotten the XL20i which is 50%/100% verse the XL16i which is 75%/100% but only one contractor recommended it, the other three recommended against it.
 
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Old 05-13-13, 01:04 AM
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Oversizing defeats the purpose of purchasing two-stage equipment.

Since the existing systems still work (right), how about waiting to see how they perform in extremely hot weather?

They should be running almost all the time in extreme heat. You may not see significant electricity savings if you oversize.

Also, SEER ratings don't mean much in very hot weather (they're tested at lower outdoor temps to get better ratings); the effective seer of 18+ seer model when it's above 100F could be 13 or lower.

Pay close attention to the EER rating - derived at 95f outdoor, 80f indoor. As seer goes up, the difference between the seer rating and eer rating increases.

Example - typical ratings...

13 seer/11 eer
14-15 seer/12-12.5 eer
16+ seer/13-13.5 eer
 
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Old 05-13-13, 09:07 AM
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The test for my new system will be my August bill. Last year was $400, and this was with an inside temp of 84, which is still a bit warm, and the 84 was only maintained in the center of the house. In rooms on either side, it got 90+.

New system should maintain everywhere at 81, and if it can do that for under $400/month, it will be a big success in my mind.
 
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Old 05-13-13, 10:48 AM
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I guess you are used to it...but I can't imagine having the inside temp that high...even with <10% humidity. My 1700sf is normally kept at about 74-75 during the hottest months and my bills rarely go over $200 except in worst case. Thats with a 20 y/o rooftop package unit.

Just sounds odd. I know we are on different suppliers, but surprised their pricing might be that different.
 
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Old 05-13-13, 03:51 PM
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nog33: House is 3000 sq ft. single story.

I'm not a contractor, but I did have four of them give me quotes and a pretty detailed analysis was conducted by each, because this is required by our utility to receive the rebate.

I also had a full energy audit including a blower door test. I've asked several times but other than a too high room pressure, they say all else came out well. Ducts had very little leakage.

My sense is these units are plenty big enough. It has only been up to maybe 100 here, and they have had a pretty easy time keeping the temp at 80. Duty cycle was never more than 20%. BUT having said that its still not much above 60 at night so the house can store some "cool." In August the low will be closer to about 90.
Unless you have a lot of things adding moisture within your home, the indoor humidity ought to be relatively low; that means a higher Temp-drop.

The 'indoor relative humidity' is the biggest factor concerning how much temp-drop it delivers; low humidity high temp-drop, high humidity low temp-drop.

My college text book rates Phoenix 2.5% summer design at 107F dry bulb, 71F wet bulb or a drop of 36F, for a very low %humidity.

This load-calc rates them somewhat different: Whole House Load-Calc Print then Read & follow all the instructions; Put the air infiltration CFM number in the make-up air slot above so it figures it correctly.

If they were blowing air out of your home during the blower door test, then a too high a pressure would indicate to me there is too high an air infiltration rate. Perhaps a lot of holes need plugging; therefore I'd probably use 0.7 ACH when figuring the infiltration rate...

If they did a Home Energy Efficiency Audit, you should have a lot of information to do the input with the load-calc; it is free but you have to print the results, because you can't save the results.

The calc says 108F for Phoenix but it is using 75F indoors change that to 80F indoors before going to the next step... at that low humidity level the temp-drop on a 3-ton at 1150-CFM ought to be around 25F; couldn't find a 4-ton.
 
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Old 05-13-13, 09:23 PM
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So, have you already got the new systems installed?

I was under the assumption that you were just getting quotes and wanted advice.
 
 

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