Sizing a wall air conditioner for a split-level house

[Pickngrin]

My house is approximately 1600 square feet (1950s split-level house with cathedral ceiling for the middle and top floor). My 19,000 BTU Friedrich wall unit just died (perfect timing!). I'm having trouble picking out a new unit with all of the choices (there is no exact retrofit). I'm wondering if it's advantageous to go with a higher BTU unit - I found a 25K BTU Frigidaire unit. Is there a possibility that it could cool the upper level without freezing us out of the middle level, keeping in mind that the ceiling of the middle level is also the ceiling of the top level? The house is open concept.

Thanks.

[jadnashua]

Bigger isn't always better, especially when it comes to a/c units. the worst thing you can have is one too big (no idea if what you are looking for is too big). You'll be cold but clammy. Under ideal conditions, the a/c runs all the time and keeps the house perfect temp...you get maximum benefit of dehumidifiaction (which only happens when the a/c is actually cooling). Otherwise, you get a cold blast, it shuts off without passing the air over the evaporator (to wring the moisture out and cool) enough and dry the air. It may be that some rework of the air distribution is what is really needed, not a bigger unit. You won't know without a proper heat loss calculation.

[jimbo]

Don't know what climate you are in, but 18,000 BTU is pretty small to cool 1600 square feet. Without a formal load calculation, we are guessing, but I know a bunch of HVAC guys who "wing it" on the load, and they all use about 550 sq ft per ton, meaning you need more like 36,000 BTU.
But the bigger issue is try to cool a large, multi room, multi floor area with a single unit. No matter how big that puppy is, the blower can not properly circulate the cold air throughout the area. Multilevel is another whole problem because of the basic physics....hot air rises, cold air settles to the lowest floor. Even a central forced air unit may perform poorly in multi level houses unless you use multiple units, or sophisticated zoning.

Any way you can put another unit, perhaps mini split, on the upper floor??

[Rat]

Was the old unit keeping the home comfortable when it was operating; If so it is probably sized right. Go back with the same size unit and all will be well. If it did not keep the space comfortable you will need to perform a load calc like the others have stated and maybe consider other, more efficient (both in comfort and energy), options. The new Carrier PTAC (Package Terminal Air Conditioner) units are about the best I have seen. They have excellent heat pump models that are extremely quiet and use larger gauge sheet metal (lasts longer). I have no experience with Frigidaire models, only with Trane and Carrier.

[Pickngrin]

Thanks for the input. I should have clarified - the current (read: deceased) unit is in the living room. We use small (5K or 6K BTU) units in some of the bedrooms. We aren't expecting to cool the entire house with the living room unit, but weren't sure if it made sense to go a bit larger. Fact is, the old unit did cool that space well. We contacted a local, independent appliance retailer and arranged to have an 18K Friedrich unit installed. It sounds like their contracted installer is very familiar with the types of houses in the area and hopefully will do a high-quality job. I'm a DIYer but don't want to take the chance of butchering the wall etc. I'm thinking that a new 18K BTU unit won't be much less effective than a 20 year old 19K BTU unit.