A Focus on HVAC
According to JEA, HVAC is 46% of the energy consumption of the typical single family household, and the next 3 largest users are water heater at 14%, appliances at 13%, and lighting at 12%. So these are where we focus the most attention.
We use EnergyStar appliances (where applicable) and all LED Lighting, but today we are going to focus on HVAC and a little water heater - you know its out of the norm when your water heater needs to be discussed with your HVAC system!
Do you live in a house that is cold in one room and hot in another? I know I have. If you are looking for a consistent level of comfort, the TerraWise high-performance home is for you. Part of the performance is tied to the HVAC system. Our new TerraWise 2.0 homes include:
16 SEER2 HVAC System with 2-stage compressor and variable speed AHU
High quality adjustable CFM quiet bath fans with oversized duct runs and timer switches
Testing of the HVAC system to verify compliance with the design expectation
Jumper ducts or dedicated returns at all bedrooms, NEVER transfer grills
Hybrid heat-pump water heater ducted in and out of the attic
So really…. what does all that mean? It means that a tremendous amount of thought, planning and expertise has gone in to making a high-performance energy efficient home that is comfortable in every room.
What is a 2-stage system?
Why should I care?
Before we can talk about 2-stage systems, we must first discuss the standard system -single stage- and how they work.
A single stage system is binary; it is on or off; 100% or nothing.
They are very simple; A thermostat has a setpoint - a temperature at which you want your home to be, and the thermostat will ask the HVAC system to run at 100% of its capacity until that setpoint is reached and then the system turns off.
With a smart thermostat you can squeeze some extra features out, like scheduling, overcooling, and automatic setpoint adjustments, but at the end of the day the system is only designed to run 100%, or not at all.
Because of this, sizing is absolutely critical; too little capacity and you won’t be able to reach the setpoints you want, too much and your unit will short cycle - which means it will run for short periods of time, which is inefficient, bad for the longevity of the unit, and won’t pull as much humidity out of the air. Florida building code sets requirements at 80% - 115% of the required cooling load.
A 2-stage unit has the ability to be in 3 possible states, High, Low, and Off. High and Off are just like the single stage unit (100% & 0%), but Low is approximately 80% of its capacity. This allows the unit to run in what is called “Low and Slow” which is better for efficiency, the longevity of the unit, and dehumidification.
This allows us to use that “Low” setting to meet the code requirement, while having the additional capacity of the “High setting”. Effectively what this means is your system has a broader range of setpoints it can hit, and on exceptionally Hot or Cold days, your unit won’t struggle to keep your home comfortable.
Plus, we pair our systems with Ecobee Premium thermostats so you’re still getting the advantages of a smart thermostat.
High efficiency variable speed air handler unit (AHU)
Exterior 2-stage condenser
The short answer is: Because that’s all we need!
The long answer: Because our homes are so well insulated and air tight, we need approximately half the HVAC capacity (Tonnage) as a typical built house.
This means even in our largest home (3000sf 3 story) we still only have a single HVAC unit. Ultimately this means less maintenance costs as there is only 1 unit to maintain. You also can’t accidentally run zones in opposition of each other (Like having downstairs heating, while upstairs is cooling).
To use only a single set of equipment we use what is called a Zoned damper system. There are electronically controlled dampers that control the flow of air within the ductwork to different areas of the home. Each one of these dampers is controlled by its own thermostat and forms a “Zone”. In one of our homes you will see between 2 and 4 zones as each floor is a zone, and the primary suite is also its own zone.
To do this with a single unit, it must be capable of pushing enough CFM of air through the ductwork to meet demand of all zones at once, but what happens if not all zones need it? If this isn’t addressed you can over pressurize and damage your ductwork, To address this most builders use what is called a bypass duct. This is a duct with a pressure sensitive damper that will allow air from the supply side to be recycled back into the return side of the unit. While this does work, it hurts the efficiency of the unit.
Why only a single unit for the whole home? How?
Bypass duct to overpressure air from supply plenum to the return side of the AHU
We use variable dampers to address this issue, basically, the zone dampers themselves have pressure sensors in them and send that information back to the zone control board which is able to crack open closed dampers just enough to bleed that pressure off without sending it back to the unit.
This also prevents “ping ponging”. Ping ponging is something that can happen with 2-story homes where the zones are running and turning off out of sync of one another. An example is the best way to illustrate this:
The downstairs zone is asking for cooling, but the upstairs zone isn’t.
As the downstairs is cooling, the upstairs is starting to warm up but hasn’t yet warmed to the point of requesting cooling.
The downstairs zone reaches it set point and the unit turns off.
The upstairs zone has now warmed up enough that it turns on and starts cooling.
As the upstairs is cooling, the downstairs is starting to warm up but hasn’t yet warmed to the point of requesting cooling.
I think you get the idea…. This short cycles the unit and isn’t conducive to a consistent comfortable home.
In our homes, the zone board is able to see the zones that aren’t currently requesting conditioning, will be soon, and are able to preemptively send some air to that zone, helping to keep the unit from short cycling, and the temperature consistent.
Typical HVAC setup for a historic house (our 1916 office) with 2 units. The right unit is a combo AHU and Condenser which is horrible for efficiency because all of the ductwork is the crawlspace and exposed rather than in an insulated space.
Why is dehumidification important?
Earlier I stated that you want your HVAC system to run “Low and Slow” to dehumidify your home, and that’s true, but its not the whole story.
When an AC system is cooling, it is also dehumidifying, basically heat is captured in the Air Handler (AHU) and moved to the compressor unit outside where it blows off. I could go on and on about phase change cooling, but basically what you need to know is that the evaporator coil inside the AHU gets really cold and air passes over it cooling the air (moving the heat out of it and into the refrigerant). If the air is being cooled to a temperature lower than the dew point, the water will condense out of the air and onto the coil where is makes its way out of your home via the condensation drain.
If you think about that for a second you will see why having the evaporator coil cold for longer (Low and Slow) will pull more moisture out of the air…. but you will also see the flaw; if you aren’t cooling, you aren’t dehumidifying!
This means that in the cold and temperate months, you have ZERO control over the humidity in your home. For Florida this is especially an issue in the spring when the temperature is mild, but it rains nearly every day.
Moisture control is important, not just for the dreaded M-word (Mold), but for comfort as well; a drier home will be more comfortable because your body can more effectively cool itself. Because of this, you can usually keep a TerraWise Home 2 or 3 degrees warmer than a typical house and have the same level of comfort. This of course means you don’t need to cool your home as much, further reducing your electric bill.
Santa-Fe 98 Pint dehumidifier
On the left, the small duct is the fresh air and the large duct is the return from the home.
On the right is the supply coming out of the unit to the rest of the home
Humidistat showing current relative humidity (46%), setpoint (50%), and fresh air ventilation set to 15-15
Fresh air duct termination at outside of home
How do we handle dehumidification?
We use a Whole Home dehumidifier along with a paired humidistat, this gives us the ultimate control over dehumidification.
If your tech savvy, you may have noticed the Ecobee Premium Thermostats we use are capable of controlling a dehumidifier…. so why aren’t we using them?
Heating and Air Conditioning are important, but people often forget about the equally important “V” for Ventilation. We will discuss more below on ventilation, but the important part as it relates to the dehumidifier actually lies with the air tightness of our homes.
You may recall that the air tightness of a home is measured in a score called ACH50 and that anything below 3 ACH50 requires fresh air; essentially, we have made the home so airtight that we need to bring fresh air in from outside to maintain air quality.
The way this is traditionally done:
A fresh air duct is run from the exterior of the house to the return side of the AHU and wired to the compressor contactor. Anytime the AC system is cooling, fresh air is being pulled in. Like the above discussion on dehumidification, you may see the problem here: You are only bringing in fresh air when you are cooling. If it is cold or temperate out, there is NO fresh air at all, and likewise, when it’s hot and your AC is running constantly you are getting a ton of fresh air. Like with dehumidification using the AHU, there’s no control here.
The way we do it:
The fresh air duct is run to the dehumidifier, and when the dehumidifier is paired with the manufacturer humidistat, we get total control over the fresh air ventilation. The dehumidifier itself will run based on the setpoint on the humidistat, pulling air from within the home, but it also has a schedule typically set in minutes on vs. minutes off during which it will bring in fresh air and dehumidify it before distributing it through the ductwork. This may look something like 15-15 meaning that the unit will pull and dehumidify fresh air for 15 minutes, and then not pull fresh air for 15 minutes. (During the time fresh air isn’t being pulled into the home, the unit may still be running because of the setpoint on the humidistat.)
Because this is set via the humidistat, it can be adjusted to meet a home’s needs, you can choose to have more fresh air or less, pull for longer periods of time or shorter bursts, or turn it off altogether if, say, there was a fire burning outside nearby and you don’t want to pull in smoky air…. the point is you have control.
This one is simple; Smart Home readiness.
Ecobee thermostats work with basically every flavor of smart home control out there. If you want to use their app, it works, if you want to run a Home Assistant server like Jac does, it works. (Looking at you google and paying to use your API for a device I own and is in my home!)
They are also capable of running 2 stage equipment and have air quality sensors to boot!
Why Ecobee Premium thermostats?
Ventilation and bath fans
At this point you should be seeing a theme, because the answer is…. Moisture control!
The kitchen hood, whether it’s a standalone hood, or the over-the-range microwave is vented outside, but that’s pretty standard stuff. For us, its all about bath fans!
Negative Pressure
Before we can talk about bath fans, we need to touch on another building science concept, house pressure. - That’s right, we are really getting into the weeds here, but at least you know you’re working with a builder who really, really cares!
Our concern here is negative pressure; again, because our homes are so airtight, when you start pulling air out (via the range hood, or bath fan) you are pulling a negative pressure on the home and air will want to work its way in. Again, control is what’s important here.
If you think about where in our very tight homes air might be able to work its way in, ventilation ducts are the easy answer. Think of it like this, if you’re cooking and pulling 300 CFM out of your home, 300 CFM is trying to work its way in, and the easiest path is through the bath fan ducts… at least it would be if this was a typically built home. We install butterfly back draft dampers which are basically one-way valves; letting air out when the fans are running but preventing air flowing in when they aren’t.
Why are backdraft dampers important?
If humid air was allowed to backflow through a bath fan, that moist air from outside would be concentrated at that bath fan leading to moisture issues, and possible mold.
Bath fans, duct work, and timers
Part of the energy star requirements is that all bath fans must pull at least 50 CFM. In order to do this both effectively and quietly we us a larger 6 inch duct. This means we don’t have to push as hard to move that volume of air, which allows for a quieter fan and quieter operation. We actually use an adjustable fan that will go up to 100CFM allowing us to dial it in to the requirements of the home. (Bonus fact: Most bath fans don’t come with anything to prevent backflow, however the bath fans we use actually come with an integrated back draft damper, but they don’t work as well as we would like so we tear them out and install butterfly dampers)
Something that may surprise you is that running a bath fan too much can actually be worse than not running it enough. Think again about negative pressure; if you are leaving your fan on all day long, you are pulling air out of your home, and unconditioned air from outside is finding its way into your home. A single bath fan left on for 12 hours, could pull as much as the entire volume of air out of the house in that time! (50 CFM x 60 Minutes X 12 Hours = 36,000 CFM). That’s a lot of air! and that’s why we started installing timer switches as a standard.
The timer switch we use has time increments of 5, 10, 15, 30 minutes and an off switch if you want to turn it off before the timer is up, but there is no “On” so you can’t accidentally leave them running.
Adjustable 50-100CFM ultra quiet bath fan
6” bath fan duct running in the insulated attic space
Bath fan timer switch
Design, Testing, and Balancing
Of course, it all starts with design. Most builders use their HVAC company to design the HVAC system, install it, and test it…. and boy that does sound a lot like the fox watching the hen house.
We use our 3rd party energy rater to design our systems because they have a vested interest in a well-designed system, not one that’s cheap and easy to install.
Besides testing it at the end to ensure the system performs as designed, they inspect the ductwork at the Rough install stage to ensure it is installed per their layout and will function appropriately.
I’m really glossing over a lot here when it comes to testing, but the final testing is pages and pages of reports and certifications.
What is duct balancing?
Have you ever had a room that was too hot or too cold? this is a result of poor balancing!
If you tour through one of our homes you will see that every supply grill has two sets of louvers, one that directs the air flow, and another further into the guts of the grill that can restrict it.
When the system is designed, each duct has an expected amount of air that should blowing out of it, called the Design CFM.
The restrictive louvers allow a game of whack a mole where you restrict the flow of ducts where too much air is blowing out, and open ducts where you’re not getting enough, going back and forth making adjustments, until all rooms get the proper air flow. This is again done by our designer/rater to ensure to complies with their system design.
Engineered HVAC design and layout
Supply Grill with both directional and restrictive louvers
Transfer Grills vs. Jumper Ducts vs. Active Returns
More talk about pressure (room pressure this time); if air can’t get out of a room, air can’t get in. This is a problem in bedrooms when you close a door; you need a way to get air out of a room even with the door closed.
What is a transfer grill and why should I hate it?
Transfer grills are the cheapest way to address this; basically, you cut a hole in the wall (Usually over the door, though I have seen them in the door itself), slap a grill on both sides, and call it a day…. while this does allow air out of the room, it’s horrible for sound transference. I mean it’s literally a big hole in the wall. We will NEVER do this, in my opinion the bare minimum is a jumper
What is a jumper duct?
A jumper duct (or just “Jumper”) is an actual duct run with a boot and grill on each end. These are usually ceiling mounted and will make a 180 over a wall to a ceiling in a hallway or other common space. This allows the air to flow out, but not the noise. A jumper is what we will do in the event there isn’t enough space to run an active return.
What is an active return?
This one may seem a little obvious, and that’s because it is! if you need to get air out of a room, why not draw it directly to the AHU. As long as we have the ability to get a duct from a room to the return side of the AHU, we will! This makes for a better-balanced system, but most builders aren’t interested in going the extra mile because it’s more complicated; a jumper needs to be at least a certain size for a given room, but since its passive (air is being pushed through it) it can’t be too big. Since air is being sucked out by an active return, it can be too big with too much air going through it. This isn’t difficult to address, the ducts just have to be slightly oversized, and then tested, restricted, and tested again. We go the extra mile here where others don’t.
Jumper duct going from inside a bedroom (left) to the loft (right)
Return Plenum with return ducts from each upstairs bedroom and one large return duct for the rest of the home
Filtration
We use a thicker filter, but why?
Wouldn’t you know… the answer is pressure again! Really what’s relevant here is airflow. Higher filtration for better air quality means a more restrictive filer, this means your AHU isn’t pulling as much air as it should, this reduction in CFM means it will have a harder time conditioning your home, but it is also not good for the longevity if the unit.
There’s two ways to address this; less filtration (no thanks!), or more surface area.
A thicker filter means there is more surface area for air to flow through, this in turn means you can use a more restrictive filter without compromising air flow. While builders use a typical 1” filter our certifications require 2” filters, but that wasn’t good enough for us and we now spec a 4” filter!
The level of filtration of a filter is measured in MERV.
The recommended filter for a 1” is MERV 8. Using a 4” filter we are able to use filters up to MERV 13! (Here’s a chart from the EPA on the MERV Levels.)
4” filter box
Water heater… Part of the AC? What?!
Yes! our water heater is somewhat an HVAC conversation, but first you must understand heat-pumps and to understand heat-pumps you have to understand how an air conditioner works.
Through phase change via a refrigerant, we are able to capture and move heat. If you remember back to high school science class, cold is really the absence of energy or heat.
When running your air conditioner in cooling mode, you are capturing the heat from the air inside your home and moving it outside. A heat-pump is just running the same system in reverse; the outside coil becomes the evaporator, and the coil in the AHU becomes the condenser. Heat is captured outside and moved into your home… This is why heat-pump heating is so efficient; you aren’t creating heat, you’re just moving it, and the energy needed to do so is much less than creating heat with resistive elements (Think of a space heater or an electric stove).
Now with all of that said, you can probably connect the dots when I tell you we use a heat-pump water heater. That’s right! We are actually capturing heat from the air and moving it via a heat exchanger into the water. Again, this is more efficient than using the resistive heat elements. Those heating elements are still there and are used for quick recovery of the unit, but the heavy lifting is done by the heat-pump… you can even disable the elements if you so choose!
But where do we get the warm air? The answer is pretty simple, from inside your home! Air is run through the unit which captures the heat to move it into the water, and as a byproduct, creates cool air (and also dehumidifies some). Since Florida is primarily a cooling climate (meaning we are trying to cool the home) this cold air is a bonus most of the year!
Going above and beyond, we actually duct the water heater! While our attics are super insulated, since they aren’t actually conditioned, and heat rises, they will be the warmest part of your home year-round (even if it’s just a few degrees difference), so we capture warm air from the attic as well as discharge the cool air back up there. This keeps an equal pressure in the attic space as well as doesn’t add intermittent cooling to a single space as would happen if we just dumped it in a hall or laundry room.
