The heat pump can move heat indoors to outdoors ("air condition") as well as from outdoors to indoors (heating). The cooling only unit can only air condition.
If the house already has heating or you rarely need heat, you can save some money by getting a cooling-only unit, and maybe supplement that with portable electric heaters as needed.
As to brand comparisons, I strongly recommend you review the Consumer Reports reader reliability report of air conditioners and heatpumps: that can be done online by paying a fee, or visiting a library.
As for some installers carefully measuring and the others not, the latter could be fine if your dwelling is not especially unique: an experienced installer has run the numbers so many times that they know what the result is. That experience is helped by there being a coarse choice of capacities: 2 ton, 2.5, 3, 3.5, 4, 5, 6, 7.5, 10, 12.5, etc.
On the other hand, not measuring could be a sign of an unknowledgeable installer. Do they also propose a 3.5 ton unit?
As far as questions to ask, I like to elicit being assured that if anything goes wrong for X years, they will fix it for free. If they can say so confidently, that is a very good sign that they don't think anything will go wrong.
It might be useful to ask an installer why they choose 3.5 tons and not 3 or 4. There is a certain degree of assumption about worst case and average case conditions. If the a/c can't quite keep up on the 5 hottest days that is one thing, but if it can't keep up on 30 of them, that is quite a different matter. Likewise, an oversized system can chill the air so much that it doesn't have to run much, making the air stagnant and possibly muggy. Questions asking about how they balance those factors are good for you to understand and for the installer to reflect upon.
I'm not a HVAC expert but a 5 ton unit sounds pretty large for a 3000sqft house. You can probably approach this in one of two ways. One is probably what the previous owner did and just throw more cooling capacity at the problem (two units, larger units, etc.). Alternatively, you can start looking at ways to get more out of your existing unit. Some things that might help:
- Improve attic insulation
- Improve attic airflow
- If your windows are old, consider replacing them with more efficient windows
- Find and plug sources of air leaks - doors, windows, electrical outlets and other exterior openings
- Plant trees and shrubs to help shade the house
- Install ceiling fans to help circulate the air
- Add insulation to the exterior of the house (requires replacing siding usually)
- Seal all duct work with proper HVAC tape to prevent leaks
- Insulate duct work
- Replace your air filter
- Install an air exchanger
The benefit of trying some of the above is that they also help when heating.
Best Answer
When you asked to increase the blower, what you are really asking to do is increase the fan speed (amount of air), right? You could ask the the tech to check/verify that you are getting 1600 CFM (cubic feet per minute) and if you truly are, and if that's not enough, ask them if it is possible to increase it (it might be adjustable). But I am thinking that may not really be the issue.
Your real problem is that it's not cooling the house. I'm not sure if this issue is truly the result of a weak blower (insufficient CFM); it's very possible, but honestly, a 1.5 ton unit is pretty small. For a 1,000 square foot house, I might recommend at least one 3 ton unit or preferentially, two 2 ton units, depending on your lattitude (summer temperature). Below is a map and chart. I would generally recommend double the what they recommend on the chart below, because you will only use what you need.
In case you didn't know, having more (AC tonnage) doesn't mean that you will use more. If you set the thermostat to say 70 °F, then the AC will turn off when the house is at 70 °F. If you have fifteen 3 ton units, you will use the same amount of energy to cool the house to 70 °F as one 2 ton unit (give or take maybe 5% energy).
Map source:http://www.acdirect.com/ac-package-unit-learning-center-ac-sizing-calculator
Edit based on comment that the AC is actually a dehumidifier:
Dehumidifier? Well, that's a key piece of information. The old unit may not have been well designed as a dehumidifier; if the newer unit has been set up to be more like an actual dehumidifier then that would explain the question. The older unit was probably designed (more) for cooling. The newer unit might be a more for dehumidifying.
The difference between an air conditioner and a dehumidifier is that the air in a dehumidifier first passes through the cooling coil and then back through the heating coil (basically, there will be little change in room temp) and then back into the room as dry air. In fact, the temperature might be higher.
Unfortunately, this means that the answer is to install an actual air conditioner or re-engineer the dehumidifier to make it an air conditioner.
Edit2 Okay it's not a dehumidifier. It's a regular AC and you have a separate dehumidifier. Regular AC units have protective coatings to help prevent condensation on the evaporating/cooling coil. That protective coating usually doesn't last forever which is why older AC units often collect water. Typically, you don't want the AC to collect water (that causes rust and mold issues). For a regular AC unit, having the blower on high is okay. Lower speeds (lower CFM) may cause condensation to occur (depending on all the bells and whistles).
The dehumidifier is where you do want condensation- to remove moisture. Cooling the air should help your dehumidifier to operate, because as the temperature is lowered, the relative humidity will increase. When measuring the relative humidity, the temperature is a/the factor. For example, say that you have 44% humidity at 70 °F; it has about 0.26 grams of water/vapor per cubic foot. At 50 °F the relative humidity of 0.26 grams of vapor per cubic foot would be about 92%. A dehumidifier should be able to operate more efficiently at 92% RH than 44% RH... so that's why an AC unit can help a dehumidifier.
I suspect that when you measured the RH in the basement it was higher than you expected (maybe about the same as the RH outside), but if the temperature was lower in the basement (than outside), then having the same RH means that you actually have less water vapor in the basement, where it's cooler.
To recap: The service technician might be able to increase (or decrease) the blower speed, but that would probably lower the AC's cooling efficiency. Your new AC unit is not collecting water (yet) because the anticondensation coating hasn't been compromised (yet); and that's a good thing. A 1.5 ton AC unit may not be large enough to cool the basement. Cooling the basement should help the dehumidifier. And remember, the relative humidity is based on the temperature.