15 day heat wave and air conditioning

It's been pretty hot in Melbourne too, and I'm working from home with ducted evap cooling also. I grew up with it, so I knew it was what I wanted to install... but I learnt something new this week. Running the unit on *low* all day can be more effective than running it on high! The reason being that the higher airflow rate achieves less temperature drop, I guess. Anyhow, it was definitely cooler running on low... We probably get more humid days than Adelaide, but you have to feel hot sometimes, or you forget you're alive.

I like the both the principle and the reality of evaporative aircon - one side-effect is that it makes for better-smelling houses as so much fresh air is flushed through the house. I have friends who work in the dept of the environment, and when I'm not hassling them about the dubious benefit of CFLs, I ask them what's the greater evil, the use of water in evaporative units, or the use of power in refrigerative units. Since almost all of them are lawyers, not scientists nor engineers, they can't give a reasoned answer, but in a low-humidity environment, I know which is the better solution.

The catch is, you can't buy an evaporative system at the same local Harvey's where you bought your plasma TV(s), so marketing, fashion and convenience govern what people choose.

I'm in Melbourne, and have had evap cooling for 13 years. It's very rare that it is so humid that you need to turn off the aircon, maybe

3-5 days per summer. Running costs are indeed low, mostly dominated by a 600 watt fan motor (flat out). I regard it as air conditioning since it is still thermodynamically a heat-pump of sorts, using heat of vapourisation. Something like that, can't remember my uni thermodynamics much.

Our pommy neighbours got one installed, at first believing that the summers here are not that hot and they would only use it once a week. Now they are converts too, and have it on pretty much every day like us when the temp gets over the high 20's which has been by about 9 oclock on most days since december!

Our unit has a thermostat too. I thought that would not work at first, but it is brilliant. It has a surprisingly clever control system too, adjusts motor speed and also will turn off the pump and just use the fan if it gets cool outside due to a cool change. You can go to bed and leave it running and eventually it just shuts off.

My unit has that too. I haven't got to trying the automatic operation out yet. I guess that will be a treat for next year! :)

I often used the unit just as a big fan to draw in the cool night air. All the stone, brick and concrete in my house can be used as a "heat-capacitor" to stabilise house temperature.

I'm going to try using the unit in fan-only mode to heat the house in winter during the day, again on the "heat-capacitor" principle.

It is great for pushing out cooking smells too. :)

Ross

RMD wrote: ...........

My solution was a bit more drastic - I left Adelaide 6 years ago for the Snowy Mountains. The thought of going through another Adelaide summer brings me out in a cold sweat :) Alan

$300 per week to run a domestic installation???

That means 300/168= $1.80 an hour if it was running continuously.

In Sydney I pay 14.4 cents/kw hour for top rate electricity which means the unit would have to use 12.5 kilowatts. That is a mighty big ac for domestic use.

By comparison I have a 19hp unit and a ducted system which uses 6 kw. It costs me $300 extra elec bill for the whole summer quarter( and I live in western Sydney where temps are quite high) I have the house well insulated and it would only run 50%duty cycle. It can maintains the temp at 23 if i want but I usually run it at 24.5. I think you are exaggerating on the a/c costs. Evap cooling is pathetic on humid days of which there are a lot in Sydney especially Feb.

Isn't 19hp more like 14kW?

I am not sure how the air conditioning people calculate the rating of 19 hp. I think it is something to do with the heat output, taking into account the efficiency of a heat pump. The energy used by the air conditioner is a different thing and does not contribute to the heat "pumped" The electrical requirement to run the compressor is quite a bit less than the rated heat output.

The figures I quoted are taken direct from the manufacturer. 19 hp unit which uses 6kw.

6kw is also on the plate fixed to the machine.

Power in Adelaide at summer rates is about 20cents/KWHr.

To provide equivalent performance to a large evaporative unit would require 20KW cooling power. This would require about 6.0 KW of electrical power.

Cost/hour = $1.20

Cost/day = $29

Cost week =$203

I copied the original figures from someone else without checking, but even so it is still pretty high compared to $2 to $4/day, or about 8 cents to 16 cents/hour typical of evaporative units.

I agree a running cost this high is very unlikely to be the case in a domestic situation, since people don't usually run a/c for a week straight at 100% duty cycle if only for economic reasons. Nor do they fit refrigerative a/c units this big.

But then 15 day record heat waves don't happen much either. The power companies in Adelaide are presently warning people to put aside extra money for the next power bill.

Adelaide has very low humidity in most hot weather. the winds blow off the desert from the North and are very dry. It very much suits evaporative air conditioners. Cooling of outside air by 15C+ is often achieved. I don't understand why anyone would install a refrigerative a/c here.

Obviously they don't suit places like Brisbane or Sydney with the much higher humidity.

But Perth, Adelaide, Melbourne, Alice Springs etc are very suitable locations.

Ross

>

I'll hopefully be building a new house soon so I'm also looking into cooling options.

Is there such a thing as a hybrid where evaporative cooling is used the majority of the time, with a complementary refrigerative unit kicking in on humid or really hot days? I'm talking about a ducted system here.

The other concern is that the upstairs area will be hotter, both because it's an "attic" style house and I'll also have my room heaters (currently 5 servers on 24/7) up there. I was thinking of installing a small split system unit to help remove any additional heat.

If there's such a thing as a hybrid evaporative/refrigerative ducted system with zoning then I guess that could work well in this case...

Evaporative and refrigerative are natural enemies - one raises humidity, the other removes it. Put 2 in the same place and the refrigerative's evaporator will freeze solid with ice.

I'd suggest going with evaporative, because it will cover you for 95% of the time and cost you very little to run. But more importantly, if you're building a new house, make sure it has all the passive features you can get - extra insulation, thermal mass, wide eves, double glazing, and use ventilation, forced if required, where you can.

the

Funny no one has mentioned what should be the first step IMO. Multiple roof ventilators (rare considering how cheap they are). Ceiling fans (cheap to buy and run) Window awnings (even work on 2 story houses where wide eaves are useless)

BTW too much thermal mass can work against you in a 15 day heat wave, you need to cool the house as much as possible at night, not retain the heat. In the "old days" hosing down bricks and roof tiles was a major benefit. If you have a big enough rainwater tank you can still install a proper spray system. (can double as a fire protection system too)

Pity so many cheap passive options seem to be completely ignored in favour of big refrigerated air-conditioners.

MrT.

I agree with the others, that would be a bad idea. If you absolutely must have respite on super-hot days where the evap isn't enough and you can't just escape to the beach, consider fitting a wall-mounted refrigerative unit in *just one room*, that you can keep fairly well closed off from the rest of the house. We have this, but many summers go by without that getting turned on even for one day. It's a welcome respite when absolutely needed though, and much cheaper than a full ducted system.

I think you'll find that a roof-mounted evap unit will cool your whole house very well. The upper floor needs the most cooling, and gets the air first, which naturally descends and cools the lower floor before exiting through open windows. Keep the upper windows shut.

Clifford Heath.

Would this still happen with a split system unit in the same room as an evap outlet?

Definitely. I'm considering some sort of exhaust fan which vents into the roof to cool the upper floor (drawing cooler air from below) and push hot air trapped inside the roof space outwards... kind of the reverse of an evap cooling setup. This would be switched on when the room temperature downstairs is lower than upstairs, say in the evening or after a cool change. Because it's an attic house the upstairs area is basically a box trapped inside a high pitched roof space. It is not a tiny cramped room (the total area upstairs will be nearly 50sqm / 5

1/2 squares) so it may need some extra attention with regards to cooling.

I think the display homes cheat by using refrigerative cooling. :)

I agree with Cifford on this.

Installing a one-room refrigerative a/c was my fallback plan if the evaporative unit fell down on the job enough to bother doing it.

My heavyweight consruction house would carry me through a day or two of humid weather anyway.

And just having fans improves comfort a lot too.

Ross

"Clifford Heath" wrote in message news:47edb1d6$0$26468$ snipped-for-privacy@news.optusnet.com.au...

Evaporative coolers are quite limited in the amount of cooling they provide. I doubt if the air would be still cool by the time it got to the ground floor. If you consult a psychometric chart you can find out the maximum temperature drop for a given humididy. For example, take Adelaide, which incidentally has an average humidity of

40 to 50 % in the summer months. On a day where the temperature is 37 degrees C at 50 % humidity the wet bulb temp is 28 degrees C. so this is the lowest the cooler can possibly cool the air. Less for higher humidities. With an evaporative cooler you must leave your windows open to allow a constant volume change otherwise they will not work. No recirculation means that you are constantly cooling hot outside air. Too bad if you have a hot northerly wind blowing as the wind will come in. Also there will be no heat insulation provided by shut glass windows. I have had experience with them and they are reasonably coolers in still, dry places like Broken hill and Cobar for example but they are no way comparable to decent refrigerated air. Passive solar has been mentioned as the way to go . This is good but your house will still heat up after a day or too. The way to go is passive solar AND refrigerated aircon ducted in zones. I have multiple spinning roof vents, wall and roof insulation and a concrete slab on ground with 2700 ceilings with ducted air in zones. The thermostat only comes on usually about midday . I dont bother to cool the bedrooms in the day just switch it over for a couple of hours at night which is enough to cool the rooms It is possible to fit ceiling vents also which open at night to allow the hot air to escape through the spinners. One down side of good insulation is that it keeps the heat in once the house warms up and it cools down outside. These automatic vents help with this problem. Refrigerated aircon works, thats why nearly all commercial premises , shopping centres etc use it. Evaporative air can be useful but has limited applications and in most cases is not as good as refrigerated air. This is nothing new people have found this out many years ago. In the coastal cities it is not commom. Go inland and you will see plenty of them up on roofs. If you have your house properly set up the costs aren't exorbitant for refrig air and you can take the pain out of summer

Humidity in Adelaide falls as temperature rises. Typically humidity is at 8% to 15% when temperatures are at 37C.

Humidity in Adelaide may be up at 50% (or even more) in the morning but typically temperatures are 22C to 25C at these humidities.

If it is 30C or 37C early in the morning in Adelaide then the humidity is also usually low. Even with 30C overnight the humidity is fairly low overnight and evaporative cooling still works well.

It is easy to get evaporative cooling of 15C+ in Adelaide. Evaporative coolers will cool to 90% of the difference betwen the wet bulb and dry bulb temperature.

See

which gives 9am and 3pm temperatures and humidities in March 2008 at Edinburgh AFB in the north of adelaide. Other historical temperature/humidity figures and locations are available.

Generally speaking humidity is quite low when it is hot.

Many large commercial cooling installations in Adelaide use evaporative pre-coolers too. Often they are two stage coolers to reduce the humidity in the output air.

Ross

You should also examine the design and construction of the house itself in relation to your climate conditions, rather than just plonk down an 'attic' style house where it might not be appropriate for your location. Selection of materials and construction methods also have a huge impact on thermal efficiency.

If you take the time to design it correctly to start with, you will need far less heating/cooling system requirements. However the house is likely to cost more to design and build.

The above link says the average rh in summer is 40 to 50 percent.

It is a very sweeping statement to say that the humidity is always low when the temperature is high. I find it difficult to believe that there are no humid nights in a coastal city.