mike_home, you make the perfect argument for why Manual J should be mandatory for all HVAC contractors. It would protect the customer from hacks and unscrupulous contractors. Smaller equipment means less cost for electricity over the life of the AC unit and less cost up front which saves the buyer money. The electricity savings is why the utility companies and many states are making it part of their mandatory requirements to have their HVAC contractors perform Manual J. Here's a good explanation from Dean Heating and Cooling on HVAC Talk Forum that explains the benefits of using a Manual J and also talking about proper duct design (Manual D). "A load calculation known as Manual J is the only correct way to determine heat loss and heat gain. Once equipment size has been determined, the duct system is designed by Manual D. These are design protocols formulated by ACCA. Duct design takes into account the location and potential gain (or loss) of having ducts in an attic, crawlspace, or even exposed on a roof. You are correct the installing a larger air conditioner on an undersized duct system is pointless. We have all seen it many times. Frequently, putting in smaller equipment delivers greater comfort when the existing equipment is too big. For example, a 4 ton unit on a 2.5 ton duct system can only deliver 2.5 tons worth of air flow. The oversized equipment still uses the electricity for 4 tons of cooling but the delivery system still limits what arrives in the space. The equipment actually uses more energy because of inadequate air flow. Plus it cycles more often than smaller equipment so its life is shortened." Here's a good discussion about Manual J calculations also from the HVAC Talk forum. There's an HVAC designer and an HVAC contractor that post over there that seem to have the most input on Manual J. Their user names are "dan sw fl" (AKA Designer Dan) and lkapigian.

Sizing can be less of an issue when you have multiple units. Because each unit is controlled independently, provided of course the unit itself is not over sized for the area in which it serves resulting in excessive short cycling. In your case 3 ton plus 2 ton isn't necessarily equal to 5 tons of cooling... unless you drive those thermostats really low. By having two systems with total capacity of 5 tons (which, most of the time this would be oversized for a 1700 - 1800 sq ft structure) has a few advantages. 1. You have the ability to use one system only during low heat days or set the opposing unit higher. The unit set higher only runs if load conditions warrant.... meaning if it's not hot, the thermostat controlling the unit does not call... the unit can not run without a call... 2. Utility bills can be much less... because you rarely use both systems at the same time. You run the bedroom system when you sleep, the other when you're awake. If you have a home office in one the bedrooms you may need to rethink things a bit. 3. Extra capacity for when it's hot out. When it's hot you have the capacity to cool the home. (even while entertaining guests --- additional people add heat) Remember heat waves come, heat waves go... a so call 'properly sized' AC system is essentially properly sized roughly 10% of the time... especially if that system is a single speed AC system. Alot of people I come across like to sleep cool. So for that reason I doubt I would change the configuration of the space. Once you realize what you have, the choices you make are mostly budget type concerns. Essentially a set up like this runs like a 2 stage system. This kind of subject is above the pay grade of many... choose your contractor wisely. I service the Katy, Texas area.

I'm disappointed that what had at first seemed a very balanced comment was followed by what seems to be misinformation and misdirection. People buy ground source heat pump equipment because they want to and can afford it. Or decide they need to in their locations. They're hugely more expensive than alternatives, which is why they're still not very common. "When’s the payback on a Tesla? People still choose to buy them for reasons other than the ‘bottom line’." Not a comparable. A false analogy. "I would not choose natural gas or propane for reasons relating to the environment, safety, IAQ & paying for yet another utility hookup for an energy source that might not even be used for months on end during the year." What effect having a gas furnace has on indoor air quality. Another hookup? Most people prefer gas cooking appliances and besides, the OP is talking about an existing home, he either has gas service or he doesn't. "Rather than spending perhaps tens of thousands of dollars on a Solar Array, I would sooner invest that same money, hopefully less, in a ground loop for a Geothermal system." Sure you would, that's the business you're in. The ground source heat pump market is not growing as expected by those in the industry, which suggests that your opinion isn't widely held or easily sellable. In many parts of the country, solar panels can have a quick payback. Not so ground source heat pumps. "I often hear about Solar Panels when geothermal is mentioned, as an alternative way to save money by creating energy." In a place like Texas, where electricity is cheap (other than during cold weather crises as they just experienced) there's likely no payback from a solar panel installation. People do it as a personal statement, call it for environmental or other personal reasons. Like buying the Tesla, not a financial decision. An HVAC system blows warm air when it's cold, cool air when it's hot. It's one of those things people don't care about - either it works or it doesn't work. Spending extra money for an exotic system doesn't equate to extra payback when the home is sold. Neither owners, nor visitors, nor people passing by, nor anyone else, concern themselves with what's doing the work. It's equipment that doesn't produce an experience for someone in the house if it's working. In the US, there are vast areas where electricity is cheap. It will remain so for some time. People don't spend money today to potentially save money 10 years out when market conditions may or may be different. I suspect you could build a house with mahogany or teak framing. Or you could paint the framing bright colors. Why bother, you can spend less money and effort and get something else that will do the same job. "We almost never use setbacks at our home - even when going on vacation for weeks on end with no one home, our geothermal just runs." You're combining two different things to produce confusion. Thermostats that have a setback feature ALL are set up to provide lower overnight temperatures to have more comfortable sleeping conditions and then a recovery in the morning before wakeup time. As sleep specialists recommend. That in demand comfort features isn't possible with ground source heat pumps because the warm up period from a setback is too slow to be convenient to use, absent triggering a secondary heating source (like a furnace) to help. THAT'S why you don't "believe" in setbacks with a heat pump - they don't work. Turning down a thermostat during heating season when the home will be unoccupied is not a "setback" as that word is most commonly used. It's done to save money, why heat a vacant house? No setback thermostat is needed, even a dumb 50 year old analog thermostat can be turned down.

I am going to give a more detailed explanation of why I typically replace old systems when I am getting ready to sell a home and the system has a problem. Houses with new units sell for more money than houses with old units. According to the NAR, replacement of old heating and cooling systems has about an 85% return on investment when selling. That seems a little high to me, but certainly buyers price in new units. So let's make some assumptions and do a little math. Let's assume that the new unit only has just over a 50% return on investment and would increase the sales price by $3,500. So, by not replacing your unit, you are paying some amount to repair it, let's say $500, and losing $3,500 in sales price, but are saving $6,750 in replacement. So you are basically saving $2,750, by repairing your old unit rather than replacing it with a new unit. However, there is one additional complicating factor. Most buyers are going to have an old unit inspected by an HVAC company and too many HVAC companies are just looking for an excuse for replacement. So you have the added concern that the buyer is going to want an allowance or to have a new unit replaced. So you could be spending the $6,750 anyway, except you are losing the advantage of pricing the home higher. You are going to have to pay the $6,750 after spending on the repair and pricing the home $3,500 less. So in the above scenario you would be betting on a $2,750 savings against a $10,750 loss ($3,500 reduced price + $500 repair + $6,750 allowance or new unit). I typically sell a home every year, sometimes two, I have been burned on that gamble too many times. --- Now in your area or at your price point the ROI when selling could be more or less than 50%, you might actually be at 85% which makes it a no brainer, or you might be at 25% which would make repair much more attractive. Good luck