Heat-pump vs AC ductwork? AC Only upstairs?

Question

Background: Live in a 2600 square foot colonial in the North East. We have forced hot water (baseboard) heat and no duct-work in place. Currently getting quotes to do one of the following:

1. AC and ductwork just upstairs
2. AC and ductwork both upstairs and downstairs
3. Heat pump with ducted installs both upstairs and downstairs

If we just do AC now we want to be "future proofed" for if we want to transition entirely to a heat pump for heating and cooling in the future. We've gotten 5 quotes from HVAC companies so far and I've been told some very different answers two the following questions:

Question 1: Is option 1 even feasible without supplemental air conditioning downstairs?

• Ans 1: No, the system won't be warrantied by the manufacturer without AC downstairs. The system will fail in single digit years (assuming 2.5-3 ton upstairs).
• Ans 2: Will stress the system more but won't damage it more than just wear and tear.

Question 2: Is the ducting different for AC vs heat-pump?

• Ans 1: Yes, it's different.
• Ans 2: No, It's the exact same (the majority)

Any guidance would be very much appreciated.

Answer 1

An air conditioner already is a heat pump

You'll be getting a heat pump with every option you are considering.

The only difference is, an "Air conditioning only" heat pump is simply a normal heat pump with the reversing valve deleted. And with crud 15 SEER efficiency, which makes no sense because you'd expect them to be optimized if they only go one-way. It really makes no sense, but these things are simply continuations of a 1960s design. They are as cheap as they can be made, hence no reversing valve.

Unfortunately most HVAC contractors are conservative folks who dislike change (and so are the manufacturers who they are licensed dealers of)... and most of the industry would be perfectly happy if their great grandchildren in the year 2100 were selling forced-air gas furnaces with 15 SEER non-reversible A/Cs. So it's no surprise that HVAC contractors are not leading the parade and banging the drum for next-gen technology. Change costs money.

Ducts are very, very bulky and will affect your home

A duct is a 6 to 10 inch diameter pipe, or a 4 x 14" box. These are huge passages and they are difficult to add to a house without affecting its aesthetics. Even if you have basement and attic in all the right places, the "Duct-o-pus" will dominate it - and that will also force you to have the intakes and outlets on the same level, which is not ideal.

Ducts are better built into the home at time of construction. It is possible to achieve your goals and avoid ducts, so you might consider it.

The A/C units you're shopping for are designed to work with ducted, forced-air furnaces.

That is, they are designed to piggyback on top of the forced-air furnace. Above the furnace is an "air handling stack" where things like humidifiers, HEPA filters and air conditioner evaporators can be stacked before the airflow goes into the massive ducts.

And so, A/C is a real smooth "bolt-on" on a fully ducted house, where the ducting is already in place for forced-air heating. In the North American market, every A/C contractor wants to sell you this system because it's cheap and well-understood. (well, it's cheap if ducts don't have to be added).

And this one type of system has dominated the industry for 60 years. Historically you are told "if you want A/C in your home, either use window or wall units per-room, or tear the house apart to add monstrous ducts all over the house so your heating can be converted to forced-air". Those were the options available to you prior to 2010.

The controls on forced-air systems are terrible.

It is difficult to fine-tune temperature balance from room to room. And much more difficult to the system so it is respectable both in winter and summer.

Remember, solar gain is a big part of heat management in your house. By winter, rooms that get sun are warmer and need less heat. But by summer, rooms and roofs that get pounded by sun are hotter and need more A/C, so the ducting requirements are completely different! This is why people who have forced-air systems often have window A/Cs also in those troublesome rooms. Trying to have a balanced system is just hopeless.

The state of the art has moved on

The newest units are of course reversible heat pumps, and also feature variable speed drive, to run quieter but more continuously, and get away from the inefficient "BANG-on BANG-off" simple thermostat controls of the past with their several degrees of temperature swing.

The other huge change for you is they've gotten rid of ducts. Now one popular option is to bring the refrigerant lines to head units in each room. And each head unit gets to be controlled separately, which nicely solves the balance predicament I mentioned earlier.

Further, the best new heat pumps work all seasons anywhere in America, without need for "emergency heat". Most of the time they are more efficient than gas, in two meanings: First they are carbon-better, because it is more carbon-efficient to burn natural gas in a power plant to make electricity to run the heat pump, than it is to burn it locally at the house (except in the lowest of temperatures, e.g. 100 hours during Chicago's cold 2019 season). And second, they are almost always more efficient than that, which means they may be cheaper to run versus gas. Certainly the ability to use the heat pump allows you to arbitrage the cost of the two fuels.

Also, they make heat pumps which interchange with water - so a heat pump could feed your existing hot water radiators if you desired. We're all in the realm of air-sourced heat pumps here... ground-sourced units are more efficient but the gap has closed to where it's not worth digging up yards anymore for refrigerant loops.

Answer 2

A duct system should be designed for its primary function (heating or cooling). You're in the north east -- and the home so far has only a heating system -- so it's clear that you're in a heating-dominated climate.

Let's think about how the air should move during heating season. Warmer air rises naturally. Unfortunately, we tend to be more comfortable when our feet are warm and our heads are cooler. The blower in a forced air system can work to overcome physics and improve our comfort by:

• drawing return air near the ceiling, especially on the upper level
• supplying air at floor level, from which the force of the blower and also natural convection will cause the warm air to rise while mixing with and warming the cooler air near the floor

A cooling-focused system would do the opposite: supply air from the ceiling and draw return air near the floor, especially in a lower/basement level. This arrangement would be more common in the south west where cooling is important and heating is seldom required.

Frequently, though, a system needs to work well in both modes and architectural and construction concerns also factor in. In my heating-dominant climate air returns are usually at floor level on both the upper and basement levels, while air supplies are in the floor of above-ground levels and in the ceiling of the basement.

Answer 2: If you were going AC-only it could make sense to do the ducting optimized for that. If you go with the heat pump, as would make sense for your climate, then it could also make more sense to arrange the ducting to serve the heating mode better. So yes, the ducting could be different for an AC-only vs heat pump system.

Now, about conditioning the upper level only versus the entire house. Because of physics, if you install cooling only in the upper level, the lower level will gain some benefit too as the chilled air cascades down the stairs and warm air from the lower level rises up into the upper level. It's a tricky business though. If the cooling is sized for the whole-house load, but only ducted to the upstairs, then it will probably run in short bursts dropping the temperature quickly and then re-warming and running again soon. This is called short cycling and is undesirable both for occupant comfort and for its negative effect on equipment life. They know you'll be unhappy with the comfort it delivers, and there could be premature trouble too. This might be why you have installers instilling fear about warranty to dissuade you from that idea. (They might also simply be using fear to help sell you a bigger project.)

Answer 1: Yes it's feasible to condition only part of the house but it could be tricky to get the equipment sized just right. There's some risk that you'll be unhappy with the result, and fixing it to condition the entire house will require full equipment replacement (the outdoor condenser unit, the indoor coil, and the air handler/furnace blower).

As mentioned by another answer, a "mini split" system might suit you well. Many of them are heat pumps, and they can pump heat down to surprisingly low outdoor temperatures. The price on a multi-zone mini split system might make you cringe a bit, but when compared to the all-in cost for a central system with new duct work, drywall repair, etc it might compare favorably.

One nice thing about the central system is the opportunity to add a humidifier and air filtration. Those aren't really an option with mini splits.

Answer 3

Let's answer your questions then move to other options.

Question 1: Is option 1 [AC and ductwork just upstairs] even feasible without supplemental air conditioning downstairs?

Sure, if you only want to cool upstairs. It's a bit unorthodox, but there's nothing stopping you. If you don't have doors in your stairway, cold air will move downstairs in the summer, but not enough to actually cool your downstairs. A properly sized upstairs unit will run 100% of the time during design conditions, so you cant really make it worse. And the upstairs unit will always have a bigger load - heat rises and attics are hot.

Question 2: Is the ducting different for AC vs heat-pump?

Realistically, no. Installers will run 6, 8, 10, 12 inch flex ducts according to where it's going. Local installers will know what it takes for your house and the units you buy, it doens't really require engineering like a commercial building. You might want return ducts done differently if optimizing for heat versus cooling but I doubt you'd ever be able to tell the difference, and again, your local installer knows what works where you live.

Now, what you may be running into is a lack of expertise in heat pumps in your local area, sesulting in lots of opinions. This is pretty new for northern climes, and your local guys might not be too familiar.

Here's my recco: Put AC in everyhwere, and have it be heat pumps. You don't say how your floor heat works, but heat pumps are generally cheaper to run. They do cost marginally more than AC-only units, but not terribly more. They now work even in the coldest places (though, again, cold weather heat pumps do cost more).

The other option is, as others recommend, to look at split units. In a split unit, the air handler sits up on your wall and blows the air directly into the room without any ducts at all. The refrigerant travels outside to the outside unit. The advantage here is you can do some rooms now and others later. These may or may not be cheaper than a whole-house ducted HVAC system.

Answer 4

These days I wouldn't consider either (1) or (2). Go with heat pump. Operating costs will be lower.

No, a modern heat pump isn't just a compressor; it's a compressor/expander system which can do either at either end of the system, thus moving heat in either direction. It's much more efficient than resistive electric heating, and it can provide efficient central AC as well.

If you do not already have air ducts: a Minisplit heat pump systems do not require any ducting, just pipes mostly run through attic, basement, or on outside wall. That's a major advantage in houses which didn't previously have forced air.

If you do have air ducts, there are heat pump installations which will transfer heat into or out of the existing forced-air system. In fact those can be even more efficient since they can be ground-sourced -- and since the ground a few feet under the surface tends to settle at yearly average temperature for the area, it is both warmer than air in winter and cooler than air in summer, thus being a better source/sink than an air-sourced unit can be. In this setup, the ducting is simply your existing ducting.

Statement of bias: Installed a 5-head minisplit system a few years ago (one head each in two bedrooms and office, one in "public space" (open living room/dining room), one in basement workshop, and I've been mostly happy with it. It does have different quirks than the hydronic heating system, and fine control/network control requires an additional investment either in the company's add-on boards or some homebrewing effort (I'm working on the latter, as networked set of Pi-based IR transcievers). But I now have central AC, which I didn't before, and an electric heat option which at the very least gives my high-efficiency gas boiler a good run for its money (actual relative cost will depends on variations in utility prices). And since it was part of a town-wide group buy, the price wasn't unreasonable.