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Florida air conditioning ??

Old 07-08-2015, 06:12 PM
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Default Florida air conditioning ??

Guys I have a small 1500sf ranch in the Keys. I just had a new central air conditioner installed. I think it is a 2.5 ton possible a 2 ton.

We don't use the house that often so we keep the temp at 85 when we are away. The old unit from the 90s vintage would keep the house 74 but would run continuously to keep it at that in the summer. The new unit seems to take forever to get the house to 78. I was just down and it took 1 full day to get from 85 to 78. It also runs almost continually.

I had the tech come out and he said everything is working as it should, but he did agree that it just did not seem to be blowing as much velocity as normal. I have a Nest thermostat and have been watching it the last week. The air runs 7-71/2 hours a day to keep the house at 85 The Nest does give me the humidity inside and it has been between 44 and 50.

The Temps down in the Keys have been in the high 80s every day. In the evening the temp will come down to 74 and the compressor will shut off. I will say even at 78 the house is fairly comfortable. Does that seem excessive run time? This is an older 1950s CBS construction, so I am sure the insulation is not the best if there even is any. The house has really no attic so insulation would be a problem. I had a new 10K roof installed a few years ago and now kick myself in the ass for not addressing the lack of insulation then.
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Old 07-08-2015, 07:29 PM
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Hit the recipe section by accident.

I'm in DE, but it has been a hot summer, prob average low 80's for highs. Set to 68, when it hits 90 out it can't hold and will go to 70 despite setting....

Is your freon charged/fresh? Changing and using good quality filters?
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Old 07-08-2015, 08:53 PM
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I'm not sure if it will help you as your house is completely different than my 1926 bungalow but I recently installed a Rand solar attic ventilation fan. Our temps have been upper 90's until the afternoon storms. It's made a huge difference in how often my ac comes on. I keep the TS around 80. It sounds like you have another issue though. Upper 80's isn't that hot.
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Old 07-08-2015, 08:57 PM
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it's cooking
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Old 07-09-2015, 03:33 AM
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I think they go by1 ton per 500 sq feet so maybe your unit may be too small.


Bet that's not the first time you've heard that..........
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Old 07-09-2015, 03:59 AM
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The current design temp in Key West is 89 degrees F. That's the temp that by law has to be plugged into the heat load calculation program.

If the load calc was done and it spit out just under 2 tons or just under 2 1/2 tons then that is the size AC you got and there is no extra capacity. When the outside temp approaches the design temp the sytem is going to run non-stop (by design).

If you got lucky enough for it to spit out 2.1 tons or 2.6 tons then you might get a 2 1/2 ton or 3 ton respectively and have some extra capacity that would allow bringing the temperature down during the heat of the day.

Most systems will simply maintain the temp during the hot part of the day. Since you have wifi turn the system down a day before you arrive.


Here's another non advertised industry secret. New R410 systems have a steeper dropoff in capacity as the temperature increases. R22 does not drop off as quickly. Google "my new ac doesn't work as good as my old one"


A few yrs ago Raleigh NC's summer design temp was 92 degrees, now it's 90 degrees. Wait, what? I thought the gov't tells us of global warming every time we turn around. Why would they tell us that and at the same time lower the design temp? Oh yeah, just another gov't lie.
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Old 07-09-2015, 04:00 AM
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It's because the house is empty. I'd be glad to go stay in it while you're away so the unit will work properly. Just let me know when to go down and what the address is.

Sounds like either too small of a unit, lack of insulation or something not hooked up or working properly. I don't know how big mine is but in a 2000sf house in the peak of summer it will cool down the house from 85 to 75 in about an hour.

AC units are like trim tabs. You should always install a set one step larger than what is recommended.
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Old 07-09-2015, 04:13 AM
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Mike, my home in Sarasota is only slightly bigger, although I have a two floor lofted area in the main room, and it has a 3-1/2 ton unit. Last year I replaced my 12 year old Trane with a Daison and while it is a higher SEER it does run much more and does a better job at keeping the humidity down. When the unit was first installed I complained as the old Trane use to drop the temperature down much quicker than the new unit, but the AC company insisted this is the way the newer units work. Even though the AC runs more my energy bills have been less for the past year, although not enough to brag about.
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Old 07-09-2015, 04:29 AM
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How is the insulation? Can't remember if it is a flat roof or not. I know it is a block house with a lot of concrete around it to reflect heat against the walls. When I stayed at the house, the A/C at the time only did a fair job cooling.
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Old 07-09-2015, 04:41 AM
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Snds like some insulation would go a long way. I'd start there 1st.
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Old 07-09-2015, 04:52 AM
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Did they replace or at least go through your old ducts? If they up sized it, is there enough return to supply the unit? Is the filter being sucked in?
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Old 07-09-2015, 04:56 AM
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I hear these issue all the time at work. I use a Lazer temperature gauge to diagnose the problem. One can be bought at home Depot. Hit the walls and ceiling the temp should be the same. If ceiling hot insulation. Next go hit all the registers with the air running. Air temp coming out should be in the low 50s. I will bet it's in the high 60s. Next go check attic temperature. I will bet it's 130. If so attic ventilation required?
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Old 07-09-2015, 04:57 AM
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With the doors not being opened/closed and no activity I would start looking at ducting 1st to make sure there were no breaks/leaks or lack of insulation. I could go on and on with examples but have found many ducts literally laying open to attics and crawl spaces over the years. If it's an older house off-grade, I would pay particular attention the intake to make sure it's sealed correctly. Also, if you have a variable speed air handler, I think it's normal for them to run more often.
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Old 07-09-2015, 05:09 AM
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44 - 50 % relative humidity = system under sized
More than once I have seen a situation like this and found the heat strip on. Running heat and A/C at the same time = dehumidifier.
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Old 07-09-2015, 05:30 AM
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Originally Posted by TheRealMacGyver View Post
Did they replace or at least go through your old ducts? If they up sized it, is there enough return to supply the unit? Is the filter being sucked in?
I suspect ductwork also..where is it? Attic? Roof? How about the return?
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Old 07-09-2015, 05:40 AM
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Originally Posted by Optimistic Angler View Post
44 - 50 % relative humidity = system under sized
More than once I have seen a situation like this and found the heat strip on. Running heat and A/C at the same time = dehumidifier.
How can you say that without knowing insulation, etc. My house is horribly insulated... I just got a new 4 Ton Trane unit and couldn't get my RH under 50%... even with slowing the blower down.

I sealed up a few of the bigger "problem" areas (under sliders, etc.) and now I can get it down to 44%... if I actually had decent (or any LOL) insulation in a few spots where it's missing, I'd be in the mid-30% range.

My unit is sized exactly perfect for my house.
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Old 07-09-2015, 05:40 AM
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Originally Posted by mikeloew View Post
Guys I have a small 1500sf ranch in the Keys. I just had a new central air conditioner installed. I think it is a 2.5 ton possible a 2 ton.
If it is a 2 ton, I think they sold you a unit that's too small. A 2.5 ton is probably the minimum especially if you have little insulation. You don't want to oversize the unit, but you want it to keep up.

The system should be sized to run constantly and keep you comfortable on the HOTTEST day... 7.5 hours to keep 85 sounds a little long to me but may not be impossibly bad. Looking at my own nest report, these last couple weeks are about as hot as we get - and I'm averaging 12-13 hours of run time, but keeping the house pretty cold - 75ish. Looks like on the days I've been mostly away, run time is down in the 8 hour range, but maintaining 79-81.

It isn't too late to blow in attic insulation and that's relatively cheap, $400-800 and FPL may cut you a check for half of that. I've also seen roof color make a big difference. My white tile roof house vs grey shingle is no comparison for attic temps.
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Old 07-09-2015, 06:20 AM
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Originally Posted by OldPete View Post
How can you say that without knowing insulation, etc. My house is horribly insulated... I just got a new 4 Ton Trane unit and couldn't get my RH under 50%... even with slowing the blower down.

I sealed up a few of the bigger "problem" areas (under sliders, etc.) and now I can get it down to 44%... if I actually had decent (or any LOL) insulation in a few spots where it's missing, I'd be in the mid-30% range.

My unit is sized exactly perfect for my house.
50% R/H is good but the lower you get below that it starts getting to dry.
In south florida A/C running non stop temp 85 R/H 50% the system is not doing its job for one reason or another.
And if a system is under sized for the current conditions then you change the conditions ie add insulation then the system happens to be sized right then good job.
I did not go into the manual j load calc like others here did because like you said i don't know any of the specs, other then OP said old unit worked better then new unit.
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Old 07-09-2015, 06:47 AM
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Thanks for all the input guys. My roof is white, and as far as the attic, the house is a single story and there is very little attic. The previous owners remodeled and dropped the ceiling in the hallway to put the duct work in. The hallway is in the middle of the house and would be the highest peak. The house has vaulted ceilings in most areas except for the hallway.

So I guess it would be possible to at least blow insulation into that area. It would only be about a 4' wide by 20' long area. So I don't know if that would really help that much? Maybe. I also think the house being CBS, heat sinks the block when we are not there and takes along time to get rid of that heat. I really wish I would have installed some insulation prior to the new roof.

But that would have been a problem as the roof is 1'' tongue and groove Dade Pine. I wonder if I could have had the contractor put rigid insulated foam under the roof first, but that's to late now.
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Old 07-09-2015, 07:27 AM
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Older home with little insulation = 1 ton per 500 square feet. I am in the keys right now and the ambient temps have been pushing the design envelope.

If you have only 2 tons your short on capacity. This is what is creating extended run times and is why your dew point inside is low. Check your supply and return air temps. They should be about an 17 to 20 temperature difference. If it is more than 20 degrees you may be low on total CFM. Fan speed if variable or multi speed may can be increased. This will add to your sensible cooling capacity.

Remember this. When you are away and have the unit set high you will have to cool the mass of the dwelling before you can expect the unit to catch up. When you set to 85 everything in the house is 85 degrees. Walls, couch, beds floors and so on. Then add the latent heat "Moisture" and recovery times can take quite a while.
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