Micro heaters cut 87% off my electric heat bill • Hacker News Board

Re: Micro heaters cut 87% off my electric heat bill

#211

Post by dredmorbius » Thu, Jul 31, 2014, 10:16 AM UTC

mhandley wrote: post #184

jwmerrill wrote: post #114

Earlier quoted context omitted.

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…

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…

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

Re: Micro heaters cut 87% off my electric heat bill

#212

Post by benjamincburns » Thu, Jul 31, 2014, 10:22 AM UTC

The idea of a heated keyboard is a little gross to me. Seems like it'd encourage bacteria growth.

Re: Micro heaters cut 87% off my electric heat bill

#213

Post by dredmorbius » Thu, Jul 31, 2014, 10:26 AM UTC

snicker wrote: post #124

jwmerrill wrote: post #114

Earlier quoted context omitted.

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…

...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 you…

"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...

Re: Micro heaters cut 87% off my electric heat bill

#214

Post by dredmorbius » Thu, Jul 31, 2014, 10:30 AM UTC

japhyr wrote: post #95

dredmorbius wrote: post #46

Spot-heating, especially within a leaky building envelope utilizing expensive heat sources (electric resistance) is one option. But it's hardly the only one, or the best. For northern climates, taking a whole-system approach to home/structure design gets you a tremendously greater payoff in terms of energy savings. Among the most powerful demonstrations of this I've seen are Thorsten Chlupp / Reina LLC's experiences…

I live in southeast Alaska, where heating issues can make or break your ability to pay rent or afford a mortgage. Most homes are heated using fuel oil. We moved into an awesome old house last summer, and we were worried about heating costs. The landlord had a heat pump installed last fall. I'm sure we would have had some $300-$500 heating bills (per month) last winter if we were using oil, but the heat pump never cos…

Take a look at the presentations I listed above. Chlupp eliminates the furnace / heating system entirely.

It's one of his design goals. Minimizing your furnace requirements (but not eliminating them) gives you progress, but it remains a major expense (for installation and operation). Eliminating it entirely addresses a huge cost element for northern construction.

I don't live in Alaska myself, and can't speak to the full validity and credibility of his work, though it seems pretty solid. But I'm absolutely impressed with his attention to detail and the specifics of his methods and approaches.

Re: Micro heaters cut 87% off my electric heat bill

#215

Post by dredmorbius » Thu, Jul 31, 2014, 10:31 AM UTC

jtheory wrote: post #131

XorNot wrote: post #93

Earlier quoted context omitted.

On hot water bottles: the energy spent boiling that water is likely comparable to the energy an electric blanket on low power would use.

Don't use boiling water with hot water bottles; the ones you can buy have warnings about this (it can damage them). Just use hot water from the tap (which hopefully should be more efficiently heated than however you boil water as well). Don't worry about energy consumption; it's negligible for the hot water bottle, and for the electric blanket as well, especially compared to keeping your house heat up high at night.…

If the bottles are dedicated to this use, it's fine.

The soft-walled ones may discolour slightly.

Re: Micro heaters cut 87% off my electric heat bill

#216

Post by gone35 » Thu, Jul 31, 2014, 10:34 AM UTC

tungwaiyip wrote: post #116

nostromo wrote: post #36

People should check out WattVision. It's a little device that attaches to your electrical meter so you can monitor your electrical usage. What I've found is that almost all of our home's electrical usage comes from heating (heaters, clothes dryers, and hot water). Everything else is almost a rounding error. For example: I saw almost no change when we switched from incandescents to LED lighting (even in the summer whe…

Totally heating dominate energy use. I'm rather disturb to hear people pay much attention to so call "phantom power" from idle electronics and adapters. Adapters plugged in the socket do not consume measurable power. Wifi router uses about a few watts. The worst offender is a set-top box that uses 18W irrespective of power on or off. It is piece of crap anyway so I unplugged it. All the electronics in the house proba…

Not really. It used to be true in the 1970s; but today appliances consume roughly about the same as space heating, despite decades of energy efficiency improvements --and the culprit is, you guessed it, the rise of consumer electronics [1,2].

And it's not just standby-mode "phantom consumption", as the EPA used to think. Today there are more devices with larger, sharper screens and hungrier batteries to charge, all left on for longer than ever before [2]. And it all adds up.

[1] http://www.eia.gov/consumption/residential/reports/2009/elec...

[2] http://www.energystar.gov/ia/partners/prod_development/downl...

Re: Micro heaters cut 87% off my electric heat bill

#217

Post by dredmorbius » Thu, Jul 31, 2014, 10:35 AM UTC

XorNot wrote: post #93

dredmorbius wrote: post #46

Spot-heating, especially within a leaky building envelope utilizing expensive heat sources (electric resistance) is one option. But it's hardly the only one, or the best. For northern climates, taking a whole-system approach to home/structure design gets you a tremendously greater payoff in terms of energy savings. Among the most powerful demonstrations of this I've seen are Thorsten Chlupp / Reina LLC's experiences…

On hot water bottles: the energy spent boiling that water is likely comparable to the energy an electric blanket on low power would use.

The thing about bottles is that they're cheap, multi-purpose, very specific, don't have cords, pose no fire / tripping / electrocution risk, can be moved about, etc., etc.

A highly effective low-tech solution.

Yes, the efficiency of heating the water in the first place may vary, though that's energy that's almost certainly delivered to your residence space, so in that regard it's pretty much a wash.

Re: Micro heaters cut 87% off my electric heat bill

#218

Post by dredmorbius » Thu, Jul 31, 2014, 10:43 AM UTC

_delirium wrote: post #58

Swizec wrote: post #6

This is where I really prefer the European model[1] - you heat the whole house with hot water. You make hot water with cheap natural gas. If you're lucky enough, you can even use geothermal. Then you use insulation. This especially is something that feels nobody in the US has heard of. At least in California. As a result, your radiators are turned down to 2 out of 10 all day (heating turns off automatically overnight…

> You make hot water with cheap natural gas. If you're lucky enough, you can even use geothermal. In Denmark it's getting pretty common to produce it via trash incinerators. They produce a kind of heat that is apparently that not great for electricity generation, but pretty good for district-heat production. So much so that the price of unsorted municipal garbage in both Denmark and Sweden has risen in the past few y…

CHP (combined heat and power) or district heating, typically, though I'm not sure which applies in your case.

Most areas which have implemented a trash-to-energy program pretty soon run out of high-grade suitable trash and resort to imports. Entropy always wins.

Re: Micro heaters cut 87% off my electric heat bill

#219

Post by dredmorbius » Thu, Jul 31, 2014, 10:47 AM UTC

dirkf wrote: post #197

alricb wrote: post #86

Earlier quoted context omitted.

Not to mention: In a well-insulated, well-designed house, you won't have any worries about frozen pipes, even with a low thermostat temperature, since all pipes will be inside the insulated enclosure. There's a whole community of people trying to build better buildings out there. The Passivhaus people, the PERSIST (Saskatchewan) and REMOTE (Alaska) approaches ( http://www.greenbuildingadvisor.com/blogs/dept/musings/g…

I'm currently living in a passive house (in Belgium, Europe). We've only moved in two months ago so we haven't experienced a winter yet but even in the current summer climate it has already been an order of magnitude more comfortable than any of the other places I've lived in. When the temperature hit 30 degrees Celsius for a couple of days last week the house was at a comfortable 23 degrees. Everywhere. Key design p…

Chlupp's design borrows very, very heavily on passive house concepts.

And he uses massive amounts of insulation. He creates a large (big enough to walk in) wall cavity, and fills that with blown cellulose. I think it's on the order of twenty tons of cellulose. Not only does it insulate, but it forms a thermal mass.

Similarly for his foundation: a large sand base as I recall, then a slab, which create yet more thermal mass. The masonry stove at the building core is another, and finally a 5,000 gallon vertically-arranged water tank. This has a perforated distributor, and the idea is that he circulates water through his masonry stove and solar thermal panels whenever the output of those is warmer than the water at some point in the tank -- it tends to be ~120F near the surface, and ~40F near the bottom. The water stratifies according to temperature, and he banks his BTUs.

Re: Micro heaters cut 87% off my electric heat bill

#220

Post by dredmorbius » Thu, Jul 31, 2014, 10:49 AM UTC

mmikeff wrote: post #192

XorNot wrote: post #93

Earlier quoted context omitted.

On hot water bottles: the energy spent boiling that water is likely comparable to the energy an electric blanket on low power would use.

There are gel filled alternatives that can be heated in the microwave. No idea if they retain their heat as long.

Any thermal mass will do.

What I've found is that a liter bottle packs a goodly amount of heat, the sock regulates the release just about perfectly (and keeps you from scalding yourself against it), and the result is quite toasty. There's also nothing quite like having warm toes in bed.

The bottles are still warm in the morning under a good duvet.