Where do we start? Comfort? Energy economy? The potential for degradation of a structure and its contents? Can anyone think too long or too hard about the literal atmosphere inside a house without hearing the whistle of a head spinning?
We should start with the smaller issues before moving ahead to the really big pieces, or to the nuances of getting everything gelling into symbiosis.
We’re covering all this on one long page, so here are the jumps:
When you think of a lavatory, how many of your senses come to mind? For the moment, we can concentrate on two: your sense of smell and your sense of hearing. The first may drive you to turn the fan in the loo on; the second may drive you to turn it off. We, of course, were driven in even more ways. We've had a few Panasonic exhaust fans through our review evaluations; they perform well, they're quiet and they're excellent choices.
Then Broan came into our orbits and amazed us. Their Z80M fan (right) has a larger, deeper squirrel cage fan in its blower housing than we've seen in a long time. It runs almost dead-silent at its slowest speeds (truly, it takes effort to hear it) and doesn't get much louder at its highest speeds. We especially like that it has a PIR motion detector tied into a timer, factory set to 20 minutes (but you can adjust it across a 5-60-minute range). When somebody comes into the lavatory, the fan turns on and stays on, and any new movement restarts the timing, so the fan stays on until 20 minutes after you leave. In our book, it doesn't need a switch; well, not a wall switch anyway. Every fan in the house is controlled through two relays so automation can force it to be on or force it to be off, as you'll soon see.
The exhaust fans for the showers could more appropriately be automated through a different trigger, humidity. We had reviewed a very cool Panasonic wall switch with a built-in humidity sensor. Broan topped that with a humidity sensor (left) in their ZB110H fan (right) that lets you set a trigger point, but more significantly, doesn’t wait to reach it if there’s a rapid rise in humidity, like when you first turn on a shower. Its quiet blower fan is an enormous 7 inches in diameter and 5 inches deep..
We know there will be a vent fan over the cooktop, automated to respond to the cooktop temperature, but it’s too early to provide details.
There is one more place where we want the house to be exhausting air - at the same time as it inhales air - to give it a chance to breathe. This, broadly, is called recovery ventilation; if the air moving through HVAC ducts is a wind, this air exchange is less than a breeze, but it helps significantly. One reason is people - actually, most reasons are people. In a tight, well-insulated house, every time we breathe, we leave a little less oxygen and a little more carbon dioxide in the air. CO2 is normally only 0.4% of the air; if that climbs too much, we feel some fatigue. That may be one reason for what we call winter doldrums.
We also cook, which can introduce all manner of vapors, aromas and odor factors into the air. And after we eat, some of us may surrender to a biological compunction to add other gases, obnoxious if not noxious, into the sealed box that a house manifests. All of the nice things we do to make our homes more energy efficient tend to seal that box ever more tightly, And no matter where you live, there will be times of the year when airing out the house by opening windows is not a comfortable option.
That’s where recovery ventilation plays a role. In South Carolina, where outdoor air tends to be warm and moist, our choice is energy recovery ventilation, (ERV, diagrammed on the left). The modicum of cooled air it exhausts helps remove moisture from the air it draws in. A Broan ERV140TE (right) goes into the attic to add that constant flow that can help keep our oxygen levels normal and, to borrow a phrase from Mrs. Winston, to blow the stink out.
Our image of dry ice (right) cheats the point a little, but it’s a good mnemonic: warm air rises while cold air falls. (In the case of dry ice, it has more to do with the relative density of carbon dioxide, so this one time, skip the science and remember the image.
Or maybe this image of a hot air balloon (left) can help make the same point. The point is convection, that natural flow of air that happens simply because different layers or pockets of air are at different temperatures.
In a Southern climate, comfort systems have to do considerably more cooling than warming. Knowing that, would you put the cent registers on the floor to blow upward, on the wall to blow inward or on the ceiling to blow downward? And where would you put the return registers? OK, the return register placement choice is easy: you want to exhaust the hottest air, so the return goes in the ceiling.
If the return goes on the ceiling, where should the supply be? Putting it on the floor might seem to make sense but pragmatic second thoughts arise: the room isn’t empty so furnishings may mean incomplete air flow... the cooled air pool near the floor may start to feel a little too cold... you create an element you want to avoid walking on... and you limit your choices for rearranging furniture. The optimum position for floor registers seems to be under the footprint of an open door but that door can’t be good for airflow.
You could mount on a wall (well, on an interior wall; ducts in exterior walls compromise both the insulation of those walls and the thermal integrity of the duct path). The theory is that the blow will force mixing. Low placement on the wall faces many of the same practical issues as floor vents. Middle placement tends to pool a bottom cool.
High placement on a wall could work but any wall placement means there’s a far wall, and the blow pressure dissipates with distance, so any wall placement is likely to leave a room with a side-to-side imbalance in temperature.
We decided on a ceiling placement, but that’s not the only reason. We had already decided to insulate the attic at its ceiling, not its floor, and to place the HVAC gear up there. That yields a shorter, more direct path to ceiling-placed ventilation. And by placing the inlets and returns diagonally opposite in a room, we can help even out the room’s temperature spread.
Also remember, the cold air that ventilation blows downward is colder than the cooler air below so it will seek bottom, but as it does so, it also exchanges thermal energy with the air in its path. That helps cool the air at every level while also helping urge warmer air upward.
Many rooms have something else to help that. Airflow within rooms can potentially involve two sources, both the forced air ventilation system and the stirring of moving or stationary air with ceiling fans. We’ll get to those in a moment.
The comfort system system blower sends warmed or heated air (as needed) through the ductwork and into a room using the ventilation registers we just discussed, and for as long as that blower keeps running, the companion return draws airflow back, through separate ductwork, to get cycled through and brought to the desired temperature. We’d like your attention to go with the flow, but not one room at a time.
Few homes have any identical rooms; that’s even more true when you consider, beyond their geometry, where they are relative to the various outside walls, which ways their windows face, what furniture is in the room and so on. When a comfort system has just one blower blowing, the best most people can do is to adjust the little metal damper flap to cut back on a room that’s consistently too hot or too cold.
That’s potentially dangerous, because cutting too many off creates a back-pressure in the ductwork that can damage the comfort system.
We found a genius solution from Ecovent Systems. A smart hub (above left) wirelessly collects temperature and humidity readings from sensor beds in every room (below left, designed to plug into a wall outlet while offering 2 power outlets plus a USB charging port) and, based on that information and its own initial fine tuning, perform the magic you see on the right.
The registers equipped with more than just motorized shutters; each also has an air pressure sensors (part of its monitoring for dangerous back-pressure conditions) and its own separate temperature sensors for monitoring the air stream.
These measures help safeguard the health of the HVAC system.
The intelligence embedded in the system hub is remarkable. After a few days of initially determining the thermal nature of each room and refining its understanding of the whole house, it trims those shutters to keep the rooms in a perfect balance so there’s no heating or cooling overkill in any room..
It’s compatible with the Emerson Sensi WiFi thermostat (left) and when they work together, they make some major changes in the way the comfort system operates.
It eliminates the usual short on and off operations that are stressful to comfort systems; instead there are longer on times. Lab results interpret this as a direct contributor to longer maintenance-free HVAC life times.
From Emerson Residential Advanced Technologies Lab:
Now, the register unit’s batteries - and noting, that if you do what we do, you may void your warranty:
We tested and confirmed that the two pads marked VCC are connected to each other and the two marked GND are similarly connected to each other. We drilled one hole below where the batteries normally go and another in the long side of the back housing that slips into the duct boot. We soldered two small wires to VCC and GND, fished them through, sealed them down and crimped on connectors.
Since the duct boot has a foam surround, we can provide 3.3 Volts from our DC feed run to a mating connector and, since that power system is backed up by a UPS, never have to worry about changing batteries.
So far, we have cooler air entering the room from the ceiling, convection pushing it toward the floor and returns elsewhere on the ceiling looping the room’s warmest air back to the comfort system. But what happens during that brief part of the year when you may want to heat rather than cool a South Carolina home?
The warm air blows out of the ceiling, pushes into the cooler lower levels a little but what goes back through the returns is still the warmest air in the room. That’s one way to look at it, but remember, the Ecovent sensor on the wall is governing its operation, so the air at its level will reach the temperature you want there with the air above being warmer and the air below being cooler.
We don’t yet know exactly what kind of ceiling fan we’ll have in that room, but we do know a lot about what its chores will be and chief among them is stirring the air when the air needs stirring.
Whether heating or cooling, this helps even temperatures across a room. In warmer weather, it encourages skin evaporation to help people feel cooler. In cooler weather, it can run at faster speed when a room that isn’t flagged as vacant is, for the moment, not occupied, and slower when people are present.
The lateral dissipation of the swirl of air from even a slower-moving fan is enough to mix more warm air downward.
And note, with ceiling-mounted vents and returns, all of the fans motion is in one direction, to push air downward. The whole point is to help even out the mix of warmer and cooler air, to stir it up. It is, in some ways, like cooking an omelet in an uneven pan, leaving some parts of it overcooked and browned and other parts still goopy and wet. To get it right, stirring makes all the difference.
We were feeling pretty cocky about our crawl space design with a water-shedding buried skin on the up-slope side, a sheet under the base and ICF construction when a friendly HVAC guy told us some horror stories about water incursion into spaces built, it seems, just about as well. A crawl space is not usually a conditioned space and its extra volume would present an unnecessary load on a comfort system. And we know how detrimental water can be to the longevity of a structure.
This is a job for a dehumidifier. The Honeywell TrueDry DR90A (right) is capable of pulling up to 90 pints of water per day out of moist air. Its manual shows three ways of connecting it to work cooperatively with an HVAC system. We chose the fourth way:
In this “orphan” installation, there is no connection to the main HVAC system, only to local ductwork within the crawl space, but no connection doesn’t mean no impact. Flooring is not designed to block the passage of moisture, so some of the humidity in the main house can pass through the flooring and into the crawl space, where the DR90 will condense and eject it. That reduces the amount of dehumidification the maincomfort system has to address.
We also learned that surprisingly often, air conditioning will run primarily to dehumidify, not so much to cool. This reduces the odds of that and, in so doing, helps extend the healthy longevity of the main comfort system.
We grew up with sheet metal ductwork and never gave it a second thought. United McGill gives theirs a second wall, and double-wall ductwork makes sense for several reasons. There’s less direct-contact conductive heat transfer along its run, so less loss of the desired delivery temperature. There’s also less acoustic coupling, so you don’t hear as much of the airflow or fan noise in the pipes. Accidental damage is unlikely to penetrate both walls. And it’s available with an anti-microbial coating.
The ducts connect to the registers through a boot (not exactly the one shown at the left, but it gives you a good idea of the transition it has to accomplish).
All of the ductwork joining points get wrapped with a purpose-built Shurtape HVAC foil tape. Air leaks are the enemy of efficiency in HVAC and this kind of tape can seal the tiny junction gaps that air (or sometimes insects) can otherwise find ways to sneak through.
Did you think we were done?
We also bury the ducts in a thick coating of Foamsulate closed cell spray foam insulation, further reducing noise transmission, energy loss and susceptibility to damage.
The centerpieces of our comfort system - the HVAC elements at the heart of all this are...
We don’t know yet. When we do, we’ll have details here.
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