Air Conditioning - A/C size ?

Sorry if this is the second time I posted this.....but I'm new

I was given a 3 ton A/C unit that works excellent. I want to have it installed in a 12X 40 mobile home, but everytime I call a service center to come out and install it, they tell me that it is too large and that it will cause mold.
Can someone please explain this to me, or are they just trying to sell me a new unit? ( Seems to me that an oversized unit would just be more cost efficient would it not?)
Thanks Rosemary Stewart

Okay, here's the short version. In order for your living space to be properly dehumidified, the ac unit (and blower, of course) have to be running. That is the only time the humid air flows over the cooling coils and condenses out the moisture. When the temp in the living space drops to where the thermostat tells the unit to turn on, it needs to run for a while to do its job. If you put a hugely over sized unit in what will happen is thatn when the thermostat says "ON!", the unit will run for about a couple of minutes and quickly lower the temperature to where the thermostat says "OFF!". So, although you will get the cooling capability to turn you living space into a meat locker, your humidity level will be too high, thereby encouraging mold growth.

A three ton unit would cool a house (depending on other factors) of 1500-2000 square feet or so and you are talking about a space of only roughly 1/3-1/4 of that.

Looks like Tow pretty much said it.


i agree, cooling a 480 sq ft area with 3tons is WAY oversized.

I don't blame the company for not putting it in. They don't want you go be calling them every week saying, "house is too humid, My power bill out out of control, I feel cold and clamy, the system is too loud, the coil is frozen"... The list can go on and on.

I have a 1,700 sq ft home, and I have a 2 ton system.

I would kindly return the unit who gave it to you, or sell it and get a system that i sized for your home.

Air conditioners take time to build up the liquid refrigerant. You don't get any cooling out of this and it costs money to build up too. With a larger unit, it runs for short period then shuts off. The unit is forced to build up the liquid each time and you pay for the unit to do it. A larger unit takes more energy to build up than a smaller unit compounding the problem. Another problem is that it only removes moisture for a short period so it's not an effective dehumidifier. This means that it could be freezing but you will still feel uncomfortable because of humidity. Of course cool moist air means mold and mildew. Don't forget that a big unit uses big power and is harder to find a hookup that can supply that much power.

An underpowered unit will cost the least since it has the smallest build up costs and does this the least. During the day it probably won't ever turn off. It works fine at night and early and later in the day. During hotter days, it will get too warm in the living space. This gets worse with everything you do to put heat into the area. Lights, microwave, dishwasher, TV.. everything will make things worse. In your case parking in a spot that gives shade from 1-3PM may make this unit work ok.

A well suited unit is simular to the underpowered unit but slightly more costly to run overall. When it gets hot out, it won't be able to keep the temperature down low (70F or less) but will keep it reasonable (mid to upper 70'sF). Don't forget that the dehumidification of the unit running continuously will make it seem cooler than it really is. Turning off lights, not cooking, listening to music instead of watching TV and anything else that lowers heat will keep things cool. Parking in shade will work better too.

I have a 1,700 sq ft home, and I have a 2 ton system.



Unless you live in a very mild low humidty climate with exceptional insulation and storm windows, your unit is undersized by a ton.

Either get a contractor or go here www.hvaccomputer.com and have a load calc done. You guys would be surprised at how big of a unit is required for a mobile home as compaired to a conventional home.

Unless you live in a very mild low humidty climate with exceptional insulation and storm windows, your unit is undersized by a ton.

Its amazing you were able to do a load calculation on a house you've never seen before with the only details being the square footage. You should come work for me. The money my customers could save from not needing service calls would be astounding. :thinker:

Unless you live in a very mild low humidty climate with exceptional insulation and storm windows, your unit is undersized by a ton.


We do get very warm humid days up here in Minnesota. Highs near 100 at times, but mostly in the mid-upper 80's. Dew points 60 to 70 degrees. Home was built in 1968, still got the orginal windows. Our warmest day, my A/C is able to keep temp of 76 degress in the afternoon.

Did a load clac and came shy close to 2 tons.

Tht's why it's very important to do a load clac in each and every home.