Comment by jwmerrill | Hacker News Reader

Comment by jwmerrill | Hacker News Reader

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Your theoretical analysis misses an important physical fact: houses are able to exchange air with the outside world, not just heat. As you heat a normal room, it will expel air in order to maintain constant pressure, and similarly, as you allow a room to cool, outside air will enter again to keep the pressure constant.

There was an article about this in the American Journal of Physics [1, 2] a couple years back, and the authors calculated that heating a room from 273K to 300K (32F to 80F) causes it to expel about 10 percent of the air inside it.

Somewhat surprisingly, the authors don't comment on how relevant this is to the question of when to turn your heater off. The fact that allowing a room to cool draws in cold air from the outside means that the practice is worse than you would naively think, but I don't know when if ever that actually suggests leaving the heater on a constant setting. I'd be very curious to see someone extend the calculation to give an answer.

[1] http://scitation.aip.org/content/aapt/journal/ajp/79/1/10.11...

[2] http://www.stat.physik.uni-potsdam.de/~pikovsky/teaching/stu... [PDF]

This effect isn't going to be very significant. A constant exchange of air will have a significant effect, but a one-off exchange of 10% of the air will be negligible. The reason for this is that the air has a very small thermal mass compared to the building structure.

I've got mote sensors in every room in my house, including the hall. When my kids leave the front door open in winter for a few minutes, as kids tend to do, I can see the temperature in the hall dive on the graphs. But close the door again, and the temperature is right back up very close to where it started in just a few minutes - even when the heating is off.

That's not true of all houses and especially not true of cheap apartments, whose residents would benefit the most from saving on heating.

cheap apartments are also poorly air-sealed, which means they're already losing a great deal of their heat through air leaks. An extra one-time 10% turnover won't make a puddle of difference.

In winter in particular, the rate of through-roof air leaks is also greater when temperatures are warmer due to the stack effect. The author is completely wrong on this front - it's far better to let your house cool down and then re-heat.

The only time this isn't true is if you have a two-mechanism heating system such as a heat pump with resistance heat backup, where a large temperature swing invokes the more expensive resistance heater. The author doesn't.

The heat loss due to thermal expansion and contraction of air will be quite minimal.

Thermal density (specific heat) of air is going to be ~1000x less than that of solid objects.

The specific heat of gypsum (the primary constituent of drywall) is 1.09 kJ/kg.K

For dry air it's 1.0 kJ/kg.K

Air's density is 1.225 kg/m3.

A 6m x 9m x 2.3m (20' x 30' x 7.5') room has a volume of about 130m^3, so a 10% exchange would be 13m^3, or 16kg.

That's about 15 kJ of heat energy per degree C, or roughly 0.0004 liter (0.0001 gallon) of heating oil equivalent.

The drywall would be (in feet) 20x7.5x2 + 30x7.5x2 + 20x30 ft^2 (I'll assume the floor is some perfect insulator for now, and that the room has no doorways), and 1/2 inch thick, or 1.6 m^3. That's about 3600 kg of gypsum, which has a heat capacity of about 4000 kJ per degree C, or about 0.1 liter (0.027 gallons) of heating oil equivalent.

If I'm doing my maths right.

Sources:

Specific heat of gypsum: http://www.engineeringtoolbox.com/specific-heat-solids-d_154...

Specific heat of dry air: http://www.engineeringtoolbox.com/air-specific-heat-capacity...

Density of gypsum: https://en.wikipedia.org/wiki/Gypsum

I have been told by multiple HVAC professionals recently that one should keep their fan on their system always running. They have in fact adopted this practice at our offices and it greatly help with keeping temperatures in consistent from room to room. The arguments they provided for this were: [1] Constant air flow creates more consistent temperatures (less highs and lows from interior to exterior or floor to floor. [2] Allows filtration system to work continuously cleaning the air. [3] (This is the one I am skeptical of) Starting the fan motor has a large current draw, but leaving it running is very low current on modern high efficiency HVAC systems. Also apparently when the fan starts i the most likely time for fan failure due to high current and torque on the motor. These fan motors are typically not repairable in small commercial and residential units and are quite expensive.

We adopted the fan always on with scheduled temperature adjustment here at our office, it is more comfortable for sure and less dusty. I do not know if it affected energy costs. When I tried this at home it was more comfortable but my electric bill went up by ~$20

[3] is bunk. Those blower motors are immense; if they didn't use a lot of energy as they ran they would be a lot smaller.

In my limited experience the run/start capacitors fail more often than the motor itself.

About 3, the idea is right, but they're probably overstating the difference between starting power and running power.

Starting a motor is not something you do lightly for bigger motors (but big like a subway or a car) but I doubt they're that heavy for it to be a big concern.

My bet is that it'll spend the starting power in around 5s of running, tops.

For offices, it's likely more a matter of keeping pockets of hot/cold air (or other quality issues not related to temperature) from forming. Even just with lighting and office equipment, there's a fairly considerable energy flux into many office spaces, and without mixing you'll tend to get uncomfortable pockets forming.

Your #3 point strikes be as fairly valid: large electric motors do impose a very high load, and the current draw could be harmful to the motor (and switching / control circuits), while the imposed load might also be hard on other equipment. I doubt the energy usage argument (for cycling the motors off) really carries much weight, and it should be possible to idle such systems at low power. For most commercial buildings, you've got systems for heating, chilling, and air handling which are pretty much all operating simultaneously: you have a need for hot and cold air in different places, there are mixers which will deliver what's needed where it's needed (at least in theory), and the fans blow all the time.

Though I've only a pretty glancing familiarity with HVAC in general, not particularly my area of expertise.

I run the circ fan all the time as well, but the bearings on the motor do have a limited lifetime. Ask me how I know this.

A new motor will cost hundreds of dollars, more so if the blower and fan cage are a single assembly. You'll need to amortize that cost into your comfort level.

It will also quit at the least convenient time, following a combination of Tuttle's and Murphy's Laws. Again, ask me how I know this.

...but the ideal structure is not supposed to exchange air with the outside particularly because of this. That is why windows and doors have seals on them. I'm no mechanical engineer, but I've worked with enough of them in my career to understand that their goal is usually keeping a structure tightly sealed when ingress/egress paths are closed (of course opening windows/doors is a choice of the occupant, which if your striving to reduce heat loss/gain... you shouldn't do)

An ideal structure should minimize convection, but rooms for people really can't be allowed to undergo large changes in temperature without exchanging air with the outside environment, because that would imply a large change in pressure.

A 10 percent increase over atmospheric pressure might not seem like that much, but it's enough to notice, and would probably be uncomfortable. It's like being under 1 meter of water. It's also enough to break large windows.

I don't think comfort would be much of an issue given a slow adjustment period, but structurally it would be ludicrous. You'd be unable to open or close doors. Your walls would have to be built a hundred times stronger than the walls of a normal house. Any breach in the pressure envelope would be a miniature version of Aloha Airlines 243.

This is, by the way, why hard drives have filtered air holes rather than being completely sealed.

So if you forget to open the windows often enough, you suffocate? :-)

It's most definitely not the goal to suffocate the inhabitants, no, and the goal also isn't to have the natural humidifiers living inside create a tropical climate ripe with microorganisms and stuff. Yes, you want to avoid excessive air exchange because you'd lose a lot of heat that way, but tightly sealing a house would be a very bad idead indeed. If you do make the outer envelope essentially airtight, that gives you very low energy consumption and is part of the passive house concept, but then you need active ventilation, which obviously implies a hole in the envelope, and thus will keep the pressure inside equalized with the pressure outside (modulo a small differential caused by the ventilation itself).

>...but the ideal structure is not supposed to exchange air with the outside particularly because of this.

With regard to letting a house cool down at night, that's impossible. If you heat a quantity of air from 273 K to 300 K at constant volume, the pressure increases from 1 atm to 1.1 atm. That may not sound like much, but it's better expressed as a pressure differential of 10 kilonewtons per square meter, and the surface area of your house is such that it could severely damage the walls and blow the door open -- popping it like a balloon.

(The fundamental mathematical error made by those who minimize this effect is ignoring the surface area of the building)

"the ideal structure is not supposed to exchange air with the outside"

No, the ideal structure should minimize heat loss from air exchange.

I'm not fully up on my air exchange rates, but it's fairly typical for ranges to be in the 4-20 range, that is, the interior air is exchanged with the exterior 4-20x per hour.

In my Thorsten Chlupp references elsewhere you'll find he makes extensive references to heat exchangers which minimize thermal losses. He does this by a twofold process for his Fairbanks, AK, homes: entering air is routed first through the ground where it's heated from very cold ambient temperatures of as low as -40C / -40F to a temperature closer to freezing (~0F). It's then passed through a heat exchange where the exiting warm air transfers much of its heat to the entering cold air.

The purpose of tightly sealed windows and other possibly entry/exit points isn't to eliminate air exchange so much as to control it: you want air entering and exiting through your designated ventilation systems and transferring heat properly, not traversing the envelope arbitrarily.

See:

Chlupp discussing this issue: http://www.greenbuildingadvisor.com/community/forum/general-...

http://www.engineeringtoolbox.com/air-change-rate-room-d_867...

+this.

Another way of putting it: A well-designed structure should minimize random, unintentional air exchange, but provide sufficient deliberately-engineered ventilation to keep the air and people happy. For efficiency, that ventilation should go through a heat recovery ventilator (HRV) or energy recovery ventilator (ERV) as appropriate to the climate and budget. (An energy recovery ventilator also exchanges moisture).

Beyond Chlupp, Lstiburek is a great engineer and writer on these topics.

Perhaps I lacked the verbosity in my original comment, but this was what I meant to imply: ventilation is part of the design. Unintentional exchange is to be avoided.

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