HVAC on 16x80 v. 18x80?

Friends -

I’m brining-in a few new 16x80 and 18x80 mobile homes into our property in Oklahoma (moderate temperature swings). What size centralized AC units do you recommend for each size home? 3 ton? 3.5 ton? 4 ton?



The manufacturer of the homes should be sizing the HVAC units based on the sqft of the home and other design aspects they build into it. If they are asking you what size you want, I’d be concerned.

First of all you should be clear about the total sq ft of the rooms of your house, becaues HVAC is according to that size. And once you will be clear in relation to the size of your rooms thereafter only the capacity of the AC will be known.

Jefferson, a general rule of thumb for sizing AC is 400 to 500 Sq ft per ton, T. Of course, there are many factors to consider, but one important one is what size is the existing evaporator coil in the furnace. If none there, then you will want to get the furnace MN and BTU / hour and look it up or call a contractor. Or send me those numbers and I will do the calcs. Please note that a lot of southern exposure on a building may bump the calcs 1/2 to 1 T, all other things the same. If I were having the work done, I would likely bump the size 1/2 T anyway. However, it is possible to oversize a system which may cause the AC to not run long enough to remove the water vapor from the air leaving the air feeling damp or muggy which, until lately, we haven’t had to worry about in SoCal.
Jim Allen

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Jim, this was a 2 1/2 year old post someone revived.

Thanks Coach, I realized that later while I was looking thru other postings. I’m using a new tablet and it’s confusing me. Next time i’ll look at the date as well as the subject. I forgot to mention above that when sizing the evaporator coil the outside cabinet dimensions of the furnace are needed as well since some manufacturers will put the same BTU rating in more than one cabinet size.
Jim Allen

You should see the way they oversize AC’s down here in SWFL. I inspected a 2/2 this week that had a 4 ton package unit.


Your answer is extremely valuable and this thread deserved to be bumped.

What is the “furnace MN” you refer to?

By “size” do you mean physical dimensions of the A-coil or something else?


Brandon, MN stands for model number, SN stands for serial number which is generally not as important as MN. Various manufactures will put the same size BTU rating in different size furnace cabinets or housings. Generally, the larger the BTU rating of a furnace (heating rating, ie 80,000 BTU/hr), the larger the evaporator coil that can be put in. The larger the evaporator coil, then the larger the tonnage. For reference, 1 T of cooling is equal to 12,000 BTU of cooling. One T is generally good for 400 to 500 Sq ft of floor area, but there are many factors that can change that. Yes on the physical size of the A coil or N coil, depending on refrigerant used and cabinet size.
Coach above notes that they oversize package units in some parts of the country. This would be due to the very high humidity levels. Keep in mind that in a given volume there is air and there is water. The water in the form of humidity has a good deal of heat in it. This heat must be taken out to cool the air. Thus, in some areas AC units are oversized, seemingly, when in reality that is what is required to get the job done.
Jim Allen

@FandT -

Thank you for your insight! Don’t worry about it coming late.


Well, as long as it’s timely now, I have an A/C question.

We run into a lot of expenses for A/C capacitor replacements. What is causing this?

Is water condensation in the furnace cabinet a sign of mismatched “tonnage” between the condenser unit and the A-coil (is that even possible)? It pools and then the breaker gets ruined (and costs $50 to replace?) Or due to a mismatch between the condenser unit and the home generally?

We also see a lot of thermostat replacements, and fan motor (condenser unit fan) replacement. I have always assumed the latter is just from ordinary outdoor wear-and-tear that is not preventable.

Thoughts on these and related problems?

For 16x80 size mobile homes AC of 2 ton is sufficient and for 18x80 size mobile homes AC of 2.5 ton is sufficient. These ACs should be of good company.