Something Important to understand about HVAC sytems
 

There have been several threads here on TOTV concerning HVAC equipment specifically which company to use and what equipment to buy. While everyone has their own recommendations there is some technical information that's important to understand in additon to price.

What few HVAC companies will tell you is why some of the more expensive options can potentially save you money on energy bills over the life of the system so you can make a more informed decision on what type of equipment to purchase based on your specific needs.

A TOTV member posted the following article in one of the other HVAC threads. I felt it was such a clear explanation that I wanted to re-post it so everyone could read it.

What is the Affinity Law – and what does it have to do with your HVAC system?

The article explains why variable speed fan and compressor systems such as the Carrier Infinity Greenspeed and the Trane Inverter systems use significantly less energy to run, whether you have a furnace or heatpump system vs a conventional single speed system. Most people also don't understand that this technology while recent, is used in all the mini-split systems that everyone is installing to cool their lanai's for example.

If you are considering replacing your system or will be thinking about it in the near future, this is a good read to better understand the difference between the cheapest single stage vs a more expensive variable speed HVAC system.

Interesting. Thank you for this. I just replaced my heat pump up north a couple weeks ago with a Trane variable speed, so I'm looking forward to seeing some lowering of the ol' electricity bill.

I don't have experience in the selection and sizing of residential HVAC so I checked with GPT. Are any of these responses not accurate?
What would your criteria be for selecting the right VSD compressor/condenser? Would minimum speed/size be a factor?

1. Single-Stage Units:
Less forgiving if oversized — short cycles can cause poor humidity control, uneven temps, and wear-and-tear.

2. Variable-Speed Units:
Can modulate capacity, sometimes down to 30-40% of rated tonnage.
More forgiving if slightly oversized because it runs longer at lower capacity, improving humidity control and efficiency.
Can maintain comfort better with tighter temperature swings and quieter operation.

⚖️ Design Implications:
Some HVAC pros may feel more comfortable rounding up slightly with a variable-speed unit (e.g., 3.5 tons instead of 3 tons) because it modulates.
With a single-stage unit, oversizing is more of a risk — so exact sizing or even slight undersizing is safer.

🔍 Summary:
Feature Single-Stage Variable-Speed
Sizing Sensitivity High (less forgiving) Low (more forgiving)

Mostly correct.
With the latest Carrier Infinity Greenspeed system, 27VNA3, it will modulate from 20% to 100% in 1% increments.

Sizing can be a little complicated since once you determine the size you need from your calculations, these heat pump and furnace systems only come in fixed sizes. Every 12,000 btu equals 1 ton. So, if your calculations say a 3.25 ton system is needed for example, most will choose a 3 ton unit if the equipment is a single stage system so it doesn't short cycle. For a Variable speed system they would probably choose a 3.5 ton system since it will modulate and use only the "speed/capacity" it needs at any given point in time and there is no danger of short cycling.

Where sizing even gets more "tricky" is when an undersized system won't cool down to a specific set point no matter how long it runs. Since the sizing calculation is based on a max. temperature, say 95 degrees, on a very hot day an undersized system may run for a very long time or be on most of the day trying to lower the temperature to a low set point when no matter how long it runs, will never achieve that temperature. With an oversized system, on the same day it will get the home to it's target temperature but on more normal cooler days it may "short cycle" which you absolutely don't want. Besides not running long enough to remove the humidity, there is a lot of wear and tear on the system cycling On and Off like that all day.

With a variable speed system, you don't have to worry about any of this as long as the unit is sized properly or even a little oversized because on the hottest days, it will use all it's capacity say 90-100% to reach the target temperature and all other days it will use whatever portion of it's capacity that's needed at any given time. Also, with systems like the Carrier Infinity Greenspeed, once the target temperature is reached, if the humidity target is not reached, the unit will run in "dehumidify" mode which is cooling at a very low speed to remove the humidity without decreasing the temperature more than a degree or two.

Hope this helps.

One other interesting thing to think about. Since a single stage system cycles 100% On and 100% Off, this cycling, over time wears down the mechanical components like the compressor. So no matter how well you maintain your system, the compressor, fan motor, etc, will wear out at faster rate than if the unit had a soft start or was a variable speed system because of this 100% cycling. As your unit gets "older" say 10-15 or even 20 years, although the system is still running and cooling because of the wear and tear of the 100% On and Off cycling it will not be running as efficient as when it was in it's early years. The compressor, fan motors will all be drawing more power as they age and wear out. The result is, although it's still doing it's job, it's costing you more money in energy to run than if you replaced it with a new unit at a reasonable time vs waiting for the unit to die. How much money? Someone would have to look up some studies but it's probably significant. This is also why systems get louder as the get older.

Yes it does. So I guess there are problems if you oversize the compressor due to minimum speeds limits since it may make it more difficult at times to achieve the target SHR (Sensible Heat Ratio). I had to look that term up.
Sensible Heat Ratio (SHR) - HVAC School

And with a single speed system you need to get all these calculations correct but with a Variable speed system like the Trane or Carrier Infinity Greenspeed variable speed systems, it automatically calculates all this real time, all the time. This is why you need to use the Carrier thermostat which they call a "wall unit" since it "controls" the whole system real time. Similar with the Trane.

I think, if you can afford it especially with all the Federal tax credits and instant rebates on the Variable speed systems available right now, you are better off long term with the variable speed system vs the single stage systems since they are more flexible and will adapt to the specifics of your home. What about 2 stage systems? They are good as well but I believe you will find they are not that much cheaper than the Variable speed units with the credits and rebates available right now. After 2025 when the 25C Federal Tax credits go away, then we will have to re-think what's best at that time.

I started reading this and realized "way over my head"

To keep it simple.
Single speed systems are typically not as comfortable as variable speed systems. It’s also important to have the correct size single speed systems for best comfort and humidity control.
Variable speed systems are far better at keeping comfortable, better humidity control, easier on the wear and tear parts, will cost more up front but will save in the long run.
We stayed in a Lifestyle visit single speed system house and I was bitching up a storm on its lack of humidity control and lack of comfort.
We opted for a (mid level) variable speed system on our new build and I am soooooo happy I did make that decision. Our house is more comfortable and quieter than a basic system.
Our system was a $5k upcharge when we built last year. It’s speed it variable, but in set increments.
The top of the line system that is fully variable in 1% increments, with more bells and whistles, was a $15k-20k up charge. It also had a uv light for internal mold control and other bells and whistles that IMO made the system to expensive and to difficult to maintain.
Hope this helps.

Here’s a question. Over and above creature comfort, what is the finical pay back between the more energy efficient systems and the standard systems installed?

Thanks for all the info by the way. It was helpful.

:BigApplause:

There are a lot of variables. But in my opinion, the least expensive way to go, both the initial cost and the life cycle cost, is to buy a single speed HVAC system vs a variable speed system. This may change over time because way more single stage units are sold today than variable speed systems. If variable speed systems become more popular, the cost will come down as it does for almost everything.

So, one of the reasons why I posted this is because at a high level you are correct but right now and maybe until the end of 2025 a Carrier Infinity Greenspeed system for example is not that much more than a basic single stage unit.

Here are the details:
For a 4 ton heatpump (for a 2600sqft home here in the Villages), 23 Seer2, full install, everything new, Includes upgraded air filter cabinet and surge, over and under voltage protection.

1) Cost from Sunshine is about $17K which includes 10 year parts and labor.
2) You get $1,700 instant rebate from Carrier
3) You get, if it's on, $500 instant rebate from Sunshine (this will vary during the year)
4) $2,000 federal tax credit which ends in 2025.
5) If anything major fails in the outside condenser for the first 10 years they replace the condenser vs repairing it.
6) Heat pump will work down to -13 degrees so in our colder months when you do need some heat from time to time, this unit will work more efficiently and still save you money.
7) No UV light or any other accessary.

So your net is about $12,800. For a 3 ton system it's about $1K cheaper.

For a single stage Carrier system, for example you will be spending around $8,000 - $9000 and I don't believe that includes the 10 year labor and does not qualify for the Federal Tax credits. Let's say there is about a $4-5,000 difference right now, given you should save about 35% on your electric, you should recoup the difference in cost in about 4-5 years depending on your usage.

Note Munns and other's will be more expensive since Sunshine still does new installs for the Villages so their pricing will be cheaper than those companies who do less volume with Carrier. Also, the base price may vary slightly due to the time of year.

As far as variable speed units being "popular", the Carrier Infinity has been around for 15 years and over that time has evolved to what is offered now. There are a lot sold here in the Villages but I agree not as much as a single stage because most purchase the cheapest solution for whatever reason. Carrier did stop making the 5 speed system and only offers single stage, 2 stage and variable speed systems. Will these systems come down in price. Unknown since they have gone up in price about 5% each year and with the new tarriffs and the removal of the 25C Federal Tax credits at the end of 2025, even if the price were the same, next year the same unit would be $2,000 more. That's a lot of money. So while a variable speed system which is the same technology used in the mini splits everyone is installing is the better choice if you can afford it and justify replacement this year. The other factor is energy costs which are unpredictable and will most certainly increase over time so having a more efficient unit will save you money in energy costs over the long run.

Hope this all helps understand what most HVAC companies won't always tell you.

Forgive me if I missed it in the details: if I wanted to change to a variable speed system from my current conventional one, it requires both a new exterior unit and a new air handler correct? Both the compressor speed and the house fan speed are variable?

That's correct. Since it's a communicating system where the thermostat, airhandler and condenser all "talk" to each other to determine and set the most efficient speed to run at, at any point in time, everything needs to be replaced.

Thanks.
As I think on it, the existing evaporator coil is likely quite different as well from the one that should be mated to the variable condenser/compressor.