In a few weeks I'll be buying and installing my long-delayed air conditioner, and I am finding it not so easy to determine just what capacity I need.

Too small and it runs all the time and costs a lot to operate. Too big and it doesn't run enough to dehumidify the air (although I don't think that would be a big problem here, this is high desert with low humidity.)

The best source I have come across is this one where they take into account a lot of variables that the simple rule-of-thumb charts seem to ignore. Things like ceiling height, hallway widths, window area relative to wall area, wall construction, home location, SEER rating, etc.

I am distrustful of the results, because the annual operating expenses seem unreasonably low. Attached is a rough layout of the area that I am trying to air condition. The calculator linked above said I would need 26,300 BTU/hour. Does this seem reasonable? There is only about six weeks a year that the A/C would operate, daytime temperatures seldom exceed 90 degrees (F), evening temperatures usually drop down into the high 60's. We would be cooling the house to about 75-80 degrees. All walls (interior and exterior) are 8" masonry, as is the ceiling, all uninsulated.

I know some of you have a lot of experience with air conditioned homes. Does 26K BTU seem right? How many hours a day do you think it might need to operate?

tanstaafl.

Is there anything you can do in terms of external shading or insulation to moderate the cooling requirement? Seems a shame to install a huge AC system for 6 weeks a year

Looking at the plan a bit of a roof overhang on the W & E would make a huge difference to the amount of morning and evening sun, S would deal with the rest of the day. A bit of external insulation on the roof would help too.

Even here in the UK our architect was worried about summer overheating (lot of glazing) so we have around 1 mtr overhang all the way around the house.

External shading... no. Insulation on the roof... yes. BUT the cost of insulating the roof is greater than the cost of installing the air conditioning, the operating cost of the A/C is projected to be about $100 per year, and the unit I plan to install is a heat pump system which means it can heat the house as well as cool it.

tanstaafl.

As a matter of act, there is about a 1-meter wide soffit above both the East and the West windows, but by the time the sun has climbed high enough that the soffits would offer shade, it has swung around enough that the awnings on the South windows have taken over the job of sun blockage.

Insulating the roof is not easy. The ceilings are domed, and thus the roof (outside) is domed as well. It isn't just a matter of laying down sheets of styrofoam insulation, it has to be cut and shaped to fit the contours of the roof. I got an estimate, don't remember exactly how much it was, but the A/C was cheaper.

tanstaafl.

OK, sorry can't help you. Sounds like a cool roof.

My office is 1/2 of that size, but similar in that it's mostly glass. You're lucky 'cos in the summer months the sun will be north of you and away from that big south glass wall.

I had a 12K btu air con in my office and it wasn't nearly enough. I had to start it early in the morning to give it a chance to keep the office just mildly hot in the afternoon. This is with outside temps in the 90s.

So, you've got double the air con, and double the space and windows. Even with the solid north wall, you might consider the next larger size air con. And at least two of those heads.

Can you get someone to foam the roof? Also not cheap, but deals with strange shapes well. Our roof is a Dura-foam one - not a very thick layer of closed-cell foam but very very good insulation....

Probably... but a cost/benefit analysis suggests that it might not be feasible.

Yesterday, in the process of getting 220V power up to where the compressors* will sit, I learned a bit more about the construction of this house. Carlos (the engineer/contractor who built the house, and still owns the two bottom floors) went somewhat overboard in its construction, by local standards at least. The roof is about fourteen inches thick, made of bricks set edgewise on the inside, and outside covered with concrete painted a painfully bright white** to reflect heat. On a sunny day you really need sunglasses to be up there. The ceiling inside is 14 feet above the floor at the highest point of the dome. I would guess that not a lot of heat is getting through that roof, but I have no real data to back up that assertion.

I do have data to estimate what my air conditioning costs will run me, however. Making the hopefully not unwarranted assumption that my A/C will run for four hours per day in April and May only, 36,000 BTU with a SEER of 10.3, at a cost of about 21 cents/kWh***, my annual operating cost should run about $180.

Insulating the roof would not obviate the need for the A/C, because we have discovered to our dismay that SWMBO simply cannot tolerate high heat. If the temperature in the house is in excess of 85 degrees F (29C) she is literally incapacitated. Not just uncomfortable, but completely unable to function. If insulating the roof saved (arbitrarily) 40% on my A/C operating costs, it would take decades to recover the cost, and I don't know how well the foam would stand up under the intense sun. Probably they have that part figured out, though...

So, for better or worse, the A/C contractor will be here later today to do the installation. A pair of 18,000 BTU mini-splits, heat-pump systems for cooling and heating, operating independently in each room. We will keep the inside temperature between 75-80 degrees to minimize usage, we're both comfortable at that temperature. About $1800 including installation.

Let the adventure begin!

tanstaafl.

*Two 18,000 BTU (1.5 Ton) units, one for the bedroom, one for the kitchen/dining room.

**When flying over Texas desert on the way down here one time, I was absolutely astonished to see whole subdivisions, hundreds (thousands?) of houses, all lined up in rows with black roofs.

***Electricity pricing here is amazingly complicated. Our electrical usage is low enough that most of it is at the "Basico" rate of about 6 cents per kWh, some of it at the "Intermedio" rate of about 10 cents. But on the months that the A/C runs, most of that additional usage will fall into the "Exceso" rate, of about 21 cents per kWh. If I ran the A/C all year long, I would fall into the dreaded "DAC" rate ("Domestico Alto Consunción") in which every single kWh I used would be billed at the Exceso rate of 21 cents per kWh. My neighbors across the street with a big house and swimming pool are paying eight times as much as I am per year for electricity, but are probably using only 50% more kWh. They're on the DAC rate, I'm not. I have made a fantastically complex spreadsheet that accommodates all the variables involved in projecting an electric bill: the three tiered rate structure, the DAC penalty, the cost effectiveness of solar panels with amortization times, sales tax, projected rate increases based on past performance, pesos vs dollars, number of meters, months per billing cycle, wattage of appliances, etc. Interestingly (to me, at least, probably not to you guys) our computers (my PC, SWMBO's iMac) are using exactly as much electricity as the rest of the house combined. After 10 months of operation, the computers have consumed 1167 kWh, the rest of the house is at 1166. The A/C will be split across the two meters so the "meter parity" will continue.

db

One benefit of the trend towards using portable notebook computers as replacement for traditional desktop/tower computers is the generally much lower power consumption of the portable designs.

Recent and upcoming Intel low power chipsets, including the soon to appear Ivy Bridge versions provide fairly significant computer power with relatively low electrical power requirements. Along with reduced power consumption you generally get quieter cooling fan noise.

Apple uses the same portable chipsets in the Mac Mini 'desktop' models with similar low electrical power, low noise results.

My current generation (late 2011) MacBook Air 1.8Ghz drives 8 megapixels of displays (three external screens plus the 13 inch laptop itself) yet the automatic variable speed Macbook cooling fan generally runs so slowly that I do not hear it at al.

I didn't try to estimate what portion of your overall computing electrical consumption was due to the computer itself, but I imagine you have.

Are they inverter models? I couldn't tell from the link. They will be much more efficient if they are.

I don't believe they are. There is apparently only one model in the Mirage lineup that is inverter, but the combination of Inverter plus Heat Pump design is not available. Mirage is the only Mexican manufacturer of A/C units, and import duties are so dear that foreign units become prohibitively expensive.

The SEER of the model I am getting (900 Xeries) is 10.3, not extraordinarily high, but adequate to my [hopefully] minimal needs. The SEER of the one model I know to be inverter (the "Magnum") is claimed to be 21, which I find difficult to believe. There are some subtle qualifications to that claim: It is only the 12K BTU model, it uses R410 coolant, and the SEER is "...up to 21" which suggests that only at the minimum speed is it that efficient. However, that model is disqualified because it has cooling only, no heating.

tanstaafl.

It is indeed very cool. We looked at a timbrel vaulted roof for our new house, the engineer we spoke to had already completed this one and went on to do this one

One of the reasons the architect was keen on the vaulted ceiling was how much weight it would carry, ours was designed to carry 6ft of soil above.

We've ended up with a much more traditional (but still super insulated) construction, the roof will be planted over autumn.

This may be something you know already, but if you want to run an air conditioner economically, you need to understand something called "latent heat of vaporization". Humidity is the big energy sucker when running air conditioning. I seem to recall you live in a very humid climate, so I thought I'd mention this.

Many people in humid climates make the hugely expensive mistake of opening their windows when the temperature drops but while the "wet bulb" (dewpoint) temperature remains high.

In order for the airconditioner to do it's thing, it needs to condense the water vapor out of the air passing over the evaporator. Since the evaporator coil is going to be down in the 40-50 degree range, it's going to condense water vapor out until the dew point reaches this level. This takes an extraordinary amount of energy because water has a very high latent heat of vaporization (which is the heat required to move from liquid to vapor state, or vice versa). This is one reason why steam engines and steam turbines are so effective -- steam holds a shitload of energy.

When you turn on the air conditioner on a hot & humid day, it will run pretty much until it dehumidifies the air close to the dewpoint corresponding to the evaporator temperature. Then it will cool down the air to the temperature you set (these things actually happen simultaneously). After the air is dry, it takes relatively little energy to keep it cool. It's condensing out all that water vapor that takes a ton of energy.

So the worst thing you can do is close the windows at 85 degrees and 70 degree dewpoint, and turn on the AC, then at night when the temperature drops to 75 degrees (but the dewpoint is still in the high 60's) open the windows and let all that water vapor in. Now the AC has to dehumidify that air again.

If you turn it on and leave it on, then you don't need a very big air conditioner. You only need one that can keep up with the heat entering the building. It may take a long time to cool down if you have a smaller unit, but it will be able to maintain the temperature *unless* you open the windows and let in all the humidity. Then the poor little AC unit is going to work itself to death constantly condensing water vapor.

I turn on my AC in early summer and turn it off in the fall because I live in a very humid climate.

If you're only worried about 6 weeks, I'd strongly encourage you to read up on latent heat, then turn it on and LEAVE it on for the whole 6 weeks. It's much cheaper to do it this way.

Temperature is not the measurement that matters in most heat removal -- enthalpy is, and water vapor content is the critical factor in enthalpy of air.

Jim

No, actually, I live in Central Mexico up in the mountains (5200 feet) and it is pretty much high desert as far as climate goes.

June-July-August is the rainy season. September through May we might have a total of 10 days with rain. As I write this the relative humidity is less than 20%.

I do understand very well about heat of fusion (80 calories per gram) and heat of vaporization (540 calories per gram) but you really opened my eyes by pointing out the energy costs of dehumidification. It was perfectly obvious once you pointed it out, but I had not given it a thought. An excellent post, as is your usual. Unless you're talking about global warming, but we won't go there tonight.

I feel sorry for the people installing my A/C. They came by the house, took a cursory glance, and decided it would be an easy installation, $150 flat rate. Then... when they got started they found out just how over-built this house is. The roof is 14" thick, concrete, steel and bricks (edgewise). The ONLY place they can put the evaporator is above the South-facing window-wall, which has glass 9 feet tall, going up to a 2.5 foot tall steel beam the encircles the entire house. They were just able to make the necessary hole through the wall (for the coolant pipes, water drain, and wiring) by drilling (hammering and chiseling, actually) at an upward 45 degree angle between the top of the steel beam and the bottom of the domed boveda ceiling. The poor guy had a four-man crew working their butts off in 90 degree heat for eight hours, and managed to almost get the first of the two units installed and operational. I think if he hadn't already unboxed the compressors and dragged them up to the roof he would have backed out of the whole deal. Since this project is coming in about $1500 under (!) my projected budget, I can afford to be generous and I'll give him an extra $100 as a "hardship bonus". He'll be back on Wednesday to [hopefully] finish the job.

tanstaafl.

I'm probably the one living in a humid climate. Mean relative humidity for an average year is recorded as 87.8%. It's middle of dry season right now, so today it's an astoundingly low 48% (temp 96 F). Balmy!