Turn It Up, Turn It Down
July 14, 2013 9:15 AM   Subscribe

Extensive research into the state of the HVAC art has revealed more questions than it has answered. How do I set the thermostat in my new row house to maximize both comfort and efficiency?

My wife and I just bought (yay!) a row house in Washington, DC. While the "reclaimed swamp" thing is inaccurate, we do experience high humidity from roughly April to September, with representative dew points this week above 70° F.

The house was built in 1924 and was renovated in 2009, with a 3.5 ton, single stage heat pump installed at that time. There are two floors above grade and a fully finished basement, with three bedrooms on the top floor and a guest bedroom in the basement. Our inspector noted that airflow in the smaller bedrooms upstairs seemed restricted, but it seems likely this was caused by sloppy duct design and not just a blockage that could be cleared. We don't have a zoned system, and the single thermostat is located on the first floor above grade.

Our first change in the house was to replace the existing, non-programmable thermostat with a Nest. We've already found that the first floor is comfortable set at 75° F, but the bedrooms are a bit warm and still with the thermostat set that way. The basement, of course, is relatively cool compared to the upper floors.

The Nest offers a great range of functionality, but I'm at a loss as to how best to set it for our purposes.
  • It offers a "cool to dry" feature that will turn the AC on if the humidity passes a set point (55%, I believe), but the documentation just points out that this may be expensive, suggesting a dehumidifier might be a better choice (uh, maybe not so much). Since it says it will cool up to 5 degrees below your set temperature, I have turned it on (for now) and raised our away temperature a few more degrees, but I'm still unsure of the best advice here.
  • It offers a feature called "Airwave" that will run the fan after the compressor shuts off, which (according to several sources) could result in re-evaporation of previously removed moisture. Doesn't that effectively contradict using the AC as a dehumidifier?
  • It offers "every day" fan scheduling for set durations (e.g. run for 15 minutes every hour from 10 PM to 8PM), but they have a whole blog post pointing out how this could get expensive.
Further, the internet is a minefield of questions about closing off registers and/or doors in unused rooms, with some received wisdom stating that you should never close registers because doing so could increase back pressure in your ducts and cause leaks, but with other received wisdom stating you should switch your upstairs and downstairs registers twice a year (cool the upstairs, heat the basement, let gravity help otherwise). And also, a well designed multiple floor system will supposedly have balanced returns and registers on each floor so each one gets an equal exchange of fresh air and they are "pooled" as they pass through the air handler … but we have no indication our system was designed well.

So, AskMe: what is best?
  1. Should we close registers in unused rooms?
  2. Should we open and close registers upstairs and down on a seasonal schedule?
  3. Since an air conditioner is an effective dehumidifier that doesn't add heat back into the house, is there any reason not to turn on the "cool to dry" mode on the Nest?
  4. How much, really, will running the fan without the condensor cause the humidity in the house to increase as it evaporates moisture from idle coils?
I know I'm supposed to put these questions to a licensed HVAC tech, but a good tech would tell me what I already know (that I'd be better off with a gas furnace and standalone AC), and a bad one is responsible for the poor duct design and choice of heat pump in the first place. So how do I make the best of it?
posted by fedward to Home & Garden (10 answers total)
Your house was designed in an era when there was no AC. In summer, the house would be opened at night to cool off, then during the day a north facing basement or ground floor window would be left open and top floor windows would be cracked. All other windows would be closed and covered with heavy drapes. This would result in a slight draft pushing hot air out as the day heated up, with cool shade air coming in to replace it. This is the stack effect.

Assuming your air intake is downstairs, you'll need to circulate air in the whole house, because the hottest air rises up. That's why you upen the upstairs and push cool air up there. It will drift down driven by the intake suction.

To reduce back pressure you could get a register fan to suck more air upstairs.


We have a 3 story house in Canada built in 1929, and it handles all but the hottest days without AC by managing windows and airflow. On days like today, heading to 30C and up, we have a window AC on the top floor, sucking up the warmest air in the house and chilling it and letting it run down the open stairs the the rest of the house. It works well enough at moderating temperature and removing excess moisture.
posted by seanmpuckett at 9:53 AM on July 14, 2013 [3 favorites]

I'm not an expert on HVAC engineering but, I have to say, I don't think that running the fan without the AC causes a meaningful increase in the humidity in the conditioned space of your home from the re-evaporation of the previously condensed moisture from running the AC.

If you look at your heat exchanger/condenser that's attached to the air handler/fan, there should be a drip tray that catches the condensate and (if installed correctly) should direct it to a drain. As far as I can tell, the amount of moisture that's present on the evaporator coils after running the AC is negligible compared to the amount of moisture that's been condensed during the time the AC was running.

My understanding is that outside air (which is typically higher in humidity in the summer than the air in the conditioned space) infiltrating into your house (especially if it is older and not air-sealed) and other moisture sources (cooking, taking a shower, etc.) have a greater effect on the interior humidity than the condensate hanging out on the coils after your AC shuts off...
posted by scalespace at 9:55 AM on July 14, 2013 [1 favorite]

Re: Running the fan while the compressor is off

Unless you go around shutting all the vents except for the one pointed right at you, running the fans means you are blowing warmer and warmer air into mostly-empty rooms. Blowing uncooled air is really only helpful when blowing across a person. It would be more energy-efficient, and probably more effective, to only use the AC to cool/dehumidify air, and use portable fans for location-specific cooling.

I find that using fans instead of a dehumidifer on cool-but-humid days is also a good option. Opening doors/windows and getting a cross-breeze going is a good way to lower the apparent humidity.

If the outside temp is higher than the inside temp, you shouldn't really need a separate dehumidifying cycle. That is an indication that the AC system is oversized for the space it's operating in. I would talk to a HVAC specialist about options, especially since it sounds like you're not getting a lot of air on the second floor.
posted by muddgirl at 11:59 AM on July 14, 2013

Best answer: My 100-year old, 2 story house in the northeast US, with retrofitted AC using the floor-level heat ducts, presents some similar questions. For the past 6 years, I've been trying various strategies to try to keep the upstairs comfortably cool without turning the downstairs into a walk-in fridge. These are the strategies that have worked:

I close off about half of the register outlets downstairs during the summer. This forces a little more of the cold air to go upstairs.

For unused rooms, I keep the doors not quite closed, and pay attention to how that changes the temp inside and outside the room. I've experimented to find, for each room, the right combination of register open-or-closed and door position to keep the room at the temp I want.

For upstairs rooms that I use, I put a (very) small fan right over the register outlet, pointed upward and into the room. This forces the air to move around. These rooms stay comfortable as long as I run the fans 24/7 during heat waves. If I turn them off, the air quickly restratifies and it takes a long time to get the comfort back.

Whenever it's relentlessly hot and humid out, I set the AC fan to run 24/7, whether the compressor is on or not. Like the small fans in the upstairs rooms, this maintains good comfort and balanced air circulation throughout the house. Whatever amount of humidity is reintroduced from the coils by running the fan is utterly trivial compared to the gain in comfort.

All of this WORKS: even during long heat waves, the upstairs stays just 2 degrees (F) warmer than the downstairs, which is about as good as it gets for a single-zone system. The difference used to be 4 - 6 degrees before I fine-tuned the registers, fans and doors. I had the HVAC guy look at how I've got it set up, and he didn't think it would put any undue strain on the system.

Every house is different, so someone else's perfect strategy isn't certain to be perfect for you. Don't worry too much about other people's theories -- what matters is whatever works in your house (as long as it's not damaging your system, or costing you more than you're willing to pay). It takes patience -- it took me a few days to a few weeks to decide if each particular change was helping.

If you still need to add some more machinery, how about a portable AC in your warmest space, instead of a dehumidifier? At least it would dump the heat outside.
posted by Corvid at 12:57 PM on July 14, 2013

Best answer: The most energy efficient is as warm as you can stand it.

But that's not always good for comfort. The more humid it is, the grosser you will feel. So if your AC is overpowered for your heat load, you will end up with cold, clammy air. In this case, you would want a dehumidifier to remove the excess water. The excess heat the dehumidifier puts out won't matter too much. (Plus, a lot of the "excess" heat of the dehumidifier is just heat that has been removed from the air via the condensation process. The dehumidifier only adds as much heat as its wattage rating.)

If, on the other hand, your AC unit is underpowered, and the building is neither cooling nor dehumidifying properly, then you need to seal/insulate the building better. A dehumidifier may raise the temperature, but the building may feel more comfortable.

The third scenario is where the AC unit is underpowered, but IS dehumidifying the building well. It's just not as cold as you'd like. In this case you might benefit from running the circulator fan 24x7, getting ceiling fans and possibly even retrofitting a stronger fan into the HVAC unit. (Or adding booster fans to the longer runs.) Also things like adding awnings and radiant insulation. Your house is probably sealed pretty well; it just takes on more heat than the AC unit can remove.

One way to check if closing vents is causing inefficiencies would be to check the temperature drop in the system while it is running. I forget the actual number, but something like 20-30 degrees F is about right. If the air coming out of the HVAC unit is very cold compared to the air going into it, there isn't enough air moving through the system.

Also, make sure the filters are clean and as high flow as possible.

Last thing: many HVAC systems will have dampers inside the ducts to fine tune the airflow. Check these and make sure they are all open, and then adjust the dampers on the ducts leading to the colder rooms. Once you get it all set, mark the settings on the dampers as "summer/cooling". Then do the same thing once the heating season starts. Because of the heat rising thing, the settings may be completely different in the winter.
posted by gjc at 2:18 PM on July 14, 2013 [1 favorite]

Best answer: Re: airwave, it's just using the fact that the coils stay below ambient temperature for a while after you've turned the compressor off. But since the coil will be a bit hotter than they are when the compressor is running, they won't condense as much water. That's why they only run it when the humidity is below a certain level.

A 70 pint dehumidifier would dump around 850 W of condensation heat + 746 W for the compressor and fan running at full tilt 24/7. But in practice in will only run maybe 10 % of the time, so it would be like having a little 150 W heater running on humid days.
posted by Monday, stony Monday at 6:40 PM on July 14, 2013

Also, a dehumidifier can be more efficient as a heat pump than an AC, since it is dumping the heat into the air it just cooled. Your Carnot coefficient of performance can be fairly high, since it's Tcold/(Thot − Tcold). In a dehumidifier, the denominator is fairly small. For the AC, on a hot day, it can be pretty high.

A possibility is to add an Energy Recovery Ventilator to your system. On humid days, it will take water from the intake air and dump it into the exhaust air, thus helping keep the moisture out.
posted by Monday, stony Monday at 8:54 AM on July 15, 2013

A lot of older houses--built pre-A/C but retrofitted with central air--in my Southern city also have supplemental window units in upstairs bedrooms, which makes me wonder if it's actually more efficient to use a window unit in the bedroom at night, with the door closed, and leave the rest of the house warmer. Like you, I am comfortable in a warm room during the day, but I like to be almost cold when I sleep, and surely it is ridiculous to cool my entire house down in order to cool one room.
posted by elizeh at 7:01 PM on July 16, 2013 [1 favorite]

Response by poster: Thanks, all. After looking at our cumulative usage after the hottest week of the year, I've already learned a few things. The biggest takeaway for the Nest itself seems to have been to leave it alone. We'd been setting it to away manually when we left the house in the morning, but apparently that was a bad idea.

To address a few points:
  • Our HVAC does appear to be properly sized, going by the metric that it should essentially run full time on the hottest days. The air flow is wimpy at best, but apparently that's the way it's supposed to be (the blower is the right size for the compressor, and it's set to its highest speed). If it runs all day, the house will get and stay comfortable, but we don't really have the option of letting it sit idle while we're not there. It takes too long to recover if it is off too long.
  • There are returns on each floor. I don't really have a good way to measure if they're tuned correctly for the space, but they at least exist. I think the general problem there isn't the lack of returns, but the overall lack of air circulation from our otherwise properly sized unit.
  • If I had to guess, I'd say the basement gets a greater percentage of flow from the air handler than it should. I've closed registers down there and I'm not too concerned about increased pressure elsewhere, since the system operates at a pretty low pressure as it is (see previous comment about low air circulation).
  • We own a ceiling fan we had not yet installed, which we already know will increase our comfort in the bedroom. We now own a ladder, so we can install the ceiling fan. That should help.
  • Speaking of old houses built for ventilation, I miss the whole house fan from the house I lived in 20 years ago. That thing was awesome. 1940s engineering at its best.
Also the humidity has not been a concern. Since the unit's not too big, it effectively dehumidifies in the course of cooling the house. It just runs for longer than I'd expect, and with lower air flow than I find acceptable.

So in the end we've done two things that made a difference.
  1. We quit messing with the Nest, so it could do its thing;
  2. We placed a few Vornado fans strategically to deal with the house's severe stratification problems. There's one at the base of each stairway, blowing up, and one in the bedroom we'll be replacing with the ceiling fan we already own.
I would love it if the blower pushed more air and we didn't need to correct the stratification ourselves, but since the unit does appear to be the correct size for the house, we'll just keep using our fans.
posted by fedward at 1:44 PM on July 23, 2013

Response by poster: Also also, the Nest is doing its Airwave thing a few times a day, but not every time the compressor cycles off. I presume it's working as designed based on interior humidity levels.
posted by fedward at 1:56 PM on July 23, 2013

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