What is variable speed?

mbwaldropAugust 3, 2011

Hello, I'm trying to figure out exactly what a variable speed is. We just finished building and have 3700 sq ft. 2200 down and 1500 sq ft upstairs. Our downstairs HVAC is doing great, but upstairs it is struggling. We have closed cell foam throughout and I have the thermostat set at 74 upstairs and the AC runs all day and most of the evening and the temp reading is 76. I called the service tech and he adjusted the freon and he said something about it needing to a being 10 and made it an 11. I really don't understand it but we upgraded to variable speed with the understanding that it would help with cost savings and that it would help with the humid southern air. We thought we were gettying humditiy control with our variable speed, but we can read the humity levels just not adjust them. When the tech came out and I explained that my fan never blew faster or

slower and that I wanted him to adjust the variable speed he said he

could not adjust it because it needed to stay at the fixed speed, so my

question is what exactly is variable speed? Our units are to be 3.5 ton

95% gas variable speed Trane down and 3 ton 95% gas variable speed

Trane up. The tech kept mentioning yesterday that the outside unit was a

2.5 but I cannot find it on the outside unit what tonnage it is. We paid for a 3 ton. Any and all help would be great.

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What is your location?

Post mdl number of outside condenser. You can find it on the data plate.

Purchase a cheap digital thermometer/humidity gauge at a big box store.

Take several supply readings at different locations upstairs as well as return readings both morning and late afternoon. Take second floor humidity readings as well.

I am not certain what tech is measuring. On a new system like yours, you would want a temperature differential between 15-20 degrees.

I assume ductwork for upstairs system has been checked for leaks.

The two major possibilities that come to mind are bad refrigerant charge and the possibilitybthat you might be undersized.

The idea that your system runs continuous from the morning to the evening is not normal unless you are in area having extreme daytime temps.

Post back.


    Bookmark   August 3, 2011 at 3:47PM
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My Trane does this: it starts slow. It runs at a slow speed for a few minutes. Then the speed increases to a higher (high) speed. Then it turns off whereby it runs at a very slow speed for about 30 seconds. Then it quits.

    Bookmark   August 3, 2011 at 3:54PM
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We are in Alabama. It is very hot. It was just my thought that a brand new, 6 month old unit should not be running 15-16 hour days especially with closed cell foam and open cell between the 9th ceiling and attic storage. Our humidity reads 56% on the wall thermometer now and 76 degrees. I will post numbers in a few.

Tech said he adjusted the freon. I did know they still made units with freon. He said that the freon should be at a ten for the distance from furnace/blower/inside unit and he put it at a 11. I feel like I'm undersized considering our downstairs can hold at 69 and cycle on and off.

    Bookmark   August 3, 2011 at 4:20PM
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Weed made a good explanation of variable speed and what Trane calls Comfort-R which is the proper setting for ramp up/ramp down of the var speed blower.

What thermostats for new systems were installed?

Certainly install tech should return to see if Comfort R was set correctly as well as amount of airflow/CFM setting. A lower airflow setting can improve humidity level as well as Comfort R. Make certain your fan for AC mode is set at "auto" not "fan on". Fan on setting can reintroduce humidity off the coil back into the home.

Can't think of any other suggestions.


    Bookmark   August 3, 2011 at 4:51PM
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I will look up that post.

Trane thermostats were used. It is the TCONT803

The fan is set to auto.

Here are the numbers for both units.
4TTR3030D1000AA model number
103747SM3F serial number

4TTR3042D1000AA model number
10374RBP5F serial number

    Bookmark   August 3, 2011 at 5:47PM
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courtesy of weed

Posted by weedmeister (My Page) on Wed, Aug 3, 11 at 15:54

My Trane does this: it starts slow. It runs at a slow speed for a few minutes. Then the speed increases to a higher (high) speed. Then it turns off whereby it runs at a very slow speed for about 30 seconds. Then it quits.

those are 2 1/2 and 3 1/2 ton XR13 Trane AC condensers.

good thermostat.


    Bookmark   August 3, 2011 at 6:22PM
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Thank you for the information.

I'm now thinking that since we have a 2.5 ton and were to have a 3 ton that maybe we have the XR95 (multi speed) rather than the XV95 (variable speed). I would assume that is why my "system" doesn't ramp up from a slow speed fan to a high speed, it just stays at the same speed all the time.

Now what do we do if we have been sold something that is less than what we paid for and were expecting? why can't people just be honest and be forth coming with correct information? How would I ever trust this company again? Ugh!!!

    Bookmark   August 3, 2011 at 9:38PM
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Lots and lots of companies that install new systems in homes just install the equipment, turn it on, take a few temperature readings and wish the customer luck on their new system. They rely on knowing that furnaces and air handlers are rated at 400 cfm of airflow per ton of cooling. So they install a 4 ton system and automatically think the air handler or furnace is putting out 1600 cfm of air, when it might be alot less. Depending on fan speed settings, type of evaporator coil, return air size, filter size and construction and duct system size all contributes to the overall cfm of the unit. With out liquid column gauge or a manometer to actually verify the cfm output of the unit, then the installer is really just guessing. Something to think about.

    Bookmark   August 3, 2011 at 10:30PM
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Is the 2.5 ton unit connected to the upper floor? It may be undersized for your climate. What hot has it been?

I seems unusual to have a 56% humidity if your units are running for many hours. Is your fan set to auto or do you set to the on position (fan runs continuously).

    Bookmark   August 3, 2011 at 11:19PM
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We are in Alabama and the temps have been 95-105. It is a 2.5 ton, but our invoices says we paid for a 3 ton. As for the fan being variable have no idea, I just know that the installer said he could not make it go at different speeds, but variable is what is on our invoice and what we though we where getting when we did the upgrade. The 3.5 downstairs for the entire 2200 square feet with a 20x20 den with a 18 foot vault is cool as a cucumber and keep the temp at 68, 70, 72 74 and not run for more than about 40-50 mins at a given time if it was on 72 and we move it to 68. Vents are in the floor of the main level. Upstairs the vents are in the ceiling and I have to go to a vent, and see if I can feel the air moving. There are are 3 bedrooms, large playroom and heated and cooled storage. Each bedroom has an intake and and one flow vent in the bedroom and one flow vent in the bathroom. Playroom has 2 flow vents and 2 intakes and storage has two flow vents (storage seems to be the least cooled area, that is ok).

My husband just went under the house to look at the furnace numbers but said the tag is messed up, but the numbers he got are. I looked them up but they make no sense to me and I cannot find anything on any of these numbers


model # 4TXC8048BC3hCAA the first 8 could be a b

serial # 10294X0B5G

date of manufacturer 7/2010

    Bookmark   August 3, 2011 at 11:40PM
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Does your invoice give the model numbers of what was installed?

The 2.5 ton unit is 4TTR3030D. A 3-ton would have been 4TTR3036D.

I think the 4TXCB048B is a coil number, not the furnace/blower number. The coil is a 4-ton coil. I suppose it is being used with your 3.5ton condenser. Others can speak to whether this is a good combination or not.

    Bookmark   August 4, 2011 at 7:25PM
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No, the invoice just states that we purchased a 3 ton AC with a 60,000BTU variable speed furnace and a 3.5 ton AC with a 1000,000BTU variable speed furnace.

I called the company today and asked them to send an email containing all the model numbers and serial numbers for the outside units, cols, and furnaces. My husband is not sure exactly what is going on or why they have installed a 2.5 ton and not a 3 ton. I assume that this is why the AC isn't cooling upstairs because it is undersized, but chances are there is more to this story, unfortunately! The biggest thing that bothers us the most is that we upgraded both upstairs and downstairs unit to variable speed, because we "got sold" on the humidity control and the cost savings of the ramping up speeds and that isn't something we are getting. The blowers blow at a very low speed all the time. Very quiet and you actually have to feel for air to know that it is on both up and down. The company is or we thought had a good reputation and actually does a lot of commercial installation.

Hopefully it will all get figured out. Once I get all the numbers I'll post them, I'm suppose to get them tomorrow. Thank you for all the feed back. If anyone has any other information or suggestions with the situation we will gladly take them!

    Bookmark   August 4, 2011 at 9:01PM
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