I am not a refrigeration engineer so I may be missing something but here is what I think is going on. To remove water from the air you cool it to below the dew point. Then the vapour condenses on the cooling side of the heat exchanger and runs away into drains etc. The temperature that this happens depends on the level of water vapour (humidity) in the air. While it does dehumidify the air it leaves two problems. One is that the temperature required to get the result may be too cold for comfort and the other is the air coming through will be saturated with water and so have no ability to evaporate sweat from your skin. This is one of the problems that you are trying to avoid, it is this lack of evaporation that makes humid weather so uncomfortable.
The solution to both of these problems is to reheat the air after it is dehumidified. This makes the temperature acceptable and reduces the relative humidity. AFAIK normally reheat is only found in large commercial aircon systems. There are other clever things that can be done mixing different streams of air but for this purpose letâs ignore that. In a non-reheat system the cooled air warms up inside the building and so we get both benefits, cooling and drier air but not immediately coming out of the vent.
My problem is that without reheat I canât see how a âdryâ setting does anything that âcoolâ settings doesnât. They may muck about with varying fan speed and whatnot but how does âdryâ get to be any less humidity than âcoolâ?
Youâre spot on with how a âdryâ mode should operate, syncretic. The controlled fan speed is what allows this to work somewhat better in high humidity than âcoolâ would normally.
As you pointed out, the air flowing over the evaporator fins must be reduced in temperature to below the dew point in order to dehumidify. In cool mode the air flow is generally kept comparatively high to ensure an even temperature throughout the room. For effective dehumidifying with this increased airflow, the average temp must be set low enough to ensure the air temp within the evaporator is below dew point. This can cause an issue of over-cooling in climates of high humidity without the high temperatures.
As a compromise in absence of a commercial dehumidifier or reheating, split systems in âdryâ mode will reduce the air flow across the evaporator to ensure the air at this point is below the dew point. This results in more effective dehumidifying without decreasing the average temp as much. Note that this is a compromise, it is not anywhere as effective as a commercial dehumidifier and is only of benefit in humid environments.
Using âdryâ mode in less humid environments will force the system to run much less efficiently than designed (due to the reduced air flow) as these heat pumps are most efficient with minimised temperature differential between the heat exchanger fins and the surrounding air, inside and out. This is why modern efficient systems have much larger heat exchanger fin areas compared to units of old.
syncretic, I use the thermostat too. I only use it on the hottest days, wear shorts and set it to a temperature that is comfortable, not cold. I have a fibro house that gets very hot/cold, but opening all windows at night and closing them by 8.00 keeps it cool till around 3.00pm.
Planting deciduous trees east and west and insulating the roof also help. I will use a ceiling fan until it gets too uncomfortable, then run at around 24C or less.
In winter I sometimes wear two sweaters and run it when necessary at 20C+, depending on how cold it is. And I never leave it on when Iâm away from the house.
Thanks for shedding more light on the DRY mode. This is how it seems to work with our Fujitsu and is probably the main the reason why power use over 2 hours is much lower in DRY than COOL mode. The difference is greatest when the machine is first switched on when in COOL mode it works very hard to get the temperature down very quickly. In DRY mode it works less hard and takes a bit longer to get to the set temperature but the overall end effect on temp and humidity is OK for usâŠ
On the rare days here when it is very hot but not humid I only use COOL mode.
When we get to summer, I hope that more people will share here their experiences with air cons in COOL only, COOL plus ECO/ECON, and (where relevant) DRY modes, in terms of both comfort levels and power consumption. We could learn a lot from that info.
This is where the information needs to include outside and inside temperatures and humidity along with unit settings and power consumption to be even somewhat useful to others. Comfort is individually subjective and may likely mean nothing to someone else as it cannot be easily quantified, even with comprehensive data.
The efficiency of a heat pump (the power put in vs the heat pumped) is easily measured in a controlled lab environment. The power consumption is relatively easy to measure at home but the efficiency is impossible to measure at home. In the real world the goal is to have enough understanding of what affects the efficiency of a heat pump to be able to know how to fine tune the settings of your unit to be most efficient in your climate and to your comfort level.
Without going too much into the thermodynamics of a heat pump, the most heat pumped vs power input (highest efficiency) will occur at minimum compressor speed and maximum fan speed. A units star rating will actually increase substantially in this condition as its official star rating is based on it operating at maximum capacity. Ordinarily a unit will be run somewhere within the extremes to effectively pump heat and limit fan noise and excessive draught.
Humid areas change the rules somewhat as reducing the humidity is subjectively more effective than temperature. Reducing the head unit fan speed and raising the temperature setting will make a heat pump operate less efficiently (less heat pumped for power in) but may use less power overall as it doesnât need to pump as much heat to dehumidify the air to a subjectively comfortable level.
Is there someway that we might all share a common template for recording observations? Would the outcome be subjective or objective. The one critical aspect that we would find hard to assess is actual power consumption.
Sort of. Except there are at least three parts to the overall performance of a split system: the thermodynamic efficiency of the heat cycle, electro mechanical efficiency of the compressor, and electo - mechanical - fluid dynamic efficiencies of the two fans. And there is a difference in the range of operating points of invertor vs older fixed speed units. The efficiency of electrical machines at low speed and power can be very poor, hence the highest efficiency operating point for heat transferred per kW of electrical energy used may not be intuitive. Commercial systems have all of this plotted on nice graphs, to be sure.
Measurable things like temperature, humidity, power use and settings are objective.
An individuals comfort will always be subjective.
This is what makes communicating anything meaningful about this sort of thing extremely complicated.
Right you are there, but this is only useful in comparing the overall efficiency between different brands/designs. Once you own a unit it becomes irrelevant and creates more confusion than necessary in understanding how to operate it under differing conditions.
I was (probably ambiguously) referring specifically to inverter type units using R410A or R32 refrigerant as these are the most prolific. There is nothing efficient about fixed speed units by todayâs standards and not much you can do with settings to improve that.
With squirrel cage induction motors, yes. With modern permanent magnet three phase inverter motors, not so much. Regardless, modern inverter vapour-compression systems are more efficient at lower speeds. Part of this has to do with the surface area of the heat exchanger fins vs the heat capacity.
There are only two different sizes (widths) of head units available across the whole split system size range. This usually changes around the 5-6kW range. As an example of efficiency vs heat exchanger area, there are same brand units in a similar model range that have the same heat capacity but different size head units. The CoP/EER/Star ratings difference between the two head sizes is significant (the larger head unit being more efficient) for the same system capacity otherwise.
Looking back, it does seem that our objective here of conveying useful, understandable, on topic information is heading awry. Hopefully readers can pick out what is meaningful to them.
Do you still need to have a way of measuring or determining power use to know if what you observe subjectively makes any difference to power consumption and if so how significant the difference may be?
How should this be done?
I can still read the pointers on our old style spinning disc meter, because they were the norm when I was doing TAFE. Comparing two consecutive days may give a representative answer. It may not if other variables have changed.
There is always benefit in this discussion. Should any feedback clarify if the results relate to older or non invertor units? There are many exisiting installs of non invertor aircons out there.
Our experience having lived both on the North Qld tropical coast and inland mirrors the numerous comments previously on humidity. It is the moisture in the air that appears to matter most. Until our current house with aircons less than two years old it was easy to note the on off load cycles of the compressor and note the bucket loads of condensate for the garden. They worked much harder on humid days irrespective of temperature.
For larger rooms the power of the fan in the head unit and itâs ability to throw or circulate air to the far side of the room may also be a factor. We found running our ceiling fans on low made a significant improvement to comfort without turning the aircons down to lower temperatures when cooling.
The trouble is without knowing what power you are consuming, you can never know true cause and effect. With power saving the goal here, you could play with settings until the cows came home and not know if you are making a positive or negative impact.
Measuring power consumption will depend on your meter type, whether the unit is plug in and can utilise a plug in power meter or if youâre lucky enough to have a modern unit that displays it. Smart meters having an LED that flashes the Watt/Hours consumed can have a wireless trending display unit attached like the Watts Clever EW4500 (available from Antique Radio & Electronics Supply) https://radioandelectronicsupply.com.au/product-category/watts-clever-ew4500-monitor/ but you need to isolate other significant power users during testing.
Old school spinning disc meters like your own require a little more nouse to make sense of. Unfortunately,
any of these options may be beyond the means of some.
Definitely. The basis of operation between them are like chalk and cheese and any results will not be able to be meaningfully crossed over without knowing the specifics.
High humidity will make any aircon work hard. It takes nearly 2.3MJ of energy to condense 1kg or 1lt of water from the air. This is equivalent to over 630 Watts per litre. The aircon has to pump that amount of energy per litre of water condensed from the air on top of what it needs to pump to reduce the temperature.
Fine tuning fan speed and temp settings to find a balance of dehumidifying vs cooling vs power consumption vs comfort is only possible with an inverter type unit.
Fixed speed units run either flat out or not at all and this makes it impossible to set them up to dehumidify effectively without over-cooling hence no net power saving is likely in this scenario.
We have a Fujitsu ASTG24CMCA aircon in the lounge room and another in the kitchen, a Fujitsu ASTG12CMCA in the master bedroom, an older Fujitsu split system in one bedroom and a Fujitsu box unit in another bedroom.
The first 3 all have an Economy mode which the manual states will operate the aircon at a couple of degrees above the selected temperature.
They also have a Human Sensor which reduces energy consumption when the room is unoccupied.
The only specific mention of the Dry Mode in the manuals states âIn DRY mode, the indoor unit operates at low fan speed to adjust the room humidity, and may stop from time to time.â
We donât use the economy mode and we normally have the first 3 aircons set to 24 degrees. The other 2 are only ever used when we have people staying with us.
We also have the ceiling fans on at any time the aircons are in use, as well as when they are not, in the rooms we are currently using.
It looks like your service expert gave you a bum steer so operating it accordingly may contribute to mould growth and blocked drains. You can dry the evaporator after use using fan only mode for 20 to 30 minutes but the only really effective way in high humidity, if it is reverse cycle, is to set it to heat for a few minutes.
To estimate the power consumption in our tests, running the hard wired air cons for 2 hours, we used a portable power use monitoring device that received info by wifi from a monitor fitted by an electrician to the switch board as part of the Qld govtâs ClimateSmart program several years ago.
For the tests we switched off almost all the other power using devices, including fridges, and recorded the indicated total power use every 30 secs. We also used an electronic device to measure the room temp and humidity and also recorded them every 30 secs.
For each mode test we had the air con set at the same temp and fan speed and made sure that the starting air temp and humidity inside and outside were the same.
Now that there are some air cons that display the current power use on the machine or make it available elsewhere, that could be an easy and accurate source of power consumption data. However, I strongly believe that industry and govts should be ensuring that consumers have access to this this sort of information, not just the star rating info from standardized testing of machines in COOL and HEAT modes.
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I think this reflects the complexity of the topic and great differences in machines and heat/humidity situations. Also, differences in acceptable comfort levels, including how quickly we require the temperature (and humidity if relevant) level to reach the target level.
I also think that the discussion on DRY mode, which I initiated, is very worthwhile for those of us who live in areas where humidity and temperature levels are usually high at the same time.
In my experience mould growth and blocked drains can be be very significant issues with air cons used in hot and humid conditions. So, it is great that you have mentioned it here as a factor to take into consideration when looking at how best to use air cons.
The part of pipe taking the waste water from our large air con to the kitchen sink waste pipe is lower than the sink pipe and mould used to grow in the water left there for several months at the end of summer and block the pipe. So now I drain the water out at the end of summer. Thinking about it though maybe I could achieve the same result by just adding some vinegar.
I can think of no reason whatsoever why running your aircon in economy mode, even for extended periods, would be detrimental to it in any way. Performance would be the only thing to suffer.
One thing that Iâve found with our aircon, or heat-pump as theyâre known in Tasmania, is that when not using it for a period of time itâs best to turn the power to the outside unit off at the switch beside it (or in the meter box if no switch next to the unit.) Found this on checking my meter readings on a weekly basis, when I found that there was power being consumed by the heat-pump even though I hadnât used it for a while. Alright, not a lot of power - maybe 1 or 2 kw a week, but why let it keep using it? The only other item on the circuit is the HWS and thatâs turned off too for 90% of the time as the solar tubes keep it plenty hot enough.
Bosky
The power consumption of the outside unit can be quite high with older machines like our 12 year old Fujitsus. I understand that the power is mainly used to keep the sump oil warm. So, I also turn ours off when I know they will not be used for a reasonably long period of time. However, to allow the oil to warm up again, I also ensure that they are not used for a few hours after being turned back on.
Yes, dry mode is a good way to increase your comfort levels without using as much power as cooling mode. I have a couple of different aircons, one of which is an LG P24AWN⊠8/7 kW aircon. It also has an energy control mode, which lets you drop the energy use to 80%, 60% or 40% of the maximum on cooling only. The interesting thing is that, it actually runs more efficiently at the lower settings. Its EER goes from roughly 4 at maximum to about 5 at 40% power. It basically turns itself into a smaller aircon. Generally, the smaller aircons are a bit more efficient. I wouldnât say that the LG is a great product but that particular feature is great in summer. I can drop the maximum power use to about 800W which gives me about 4 kW of cooling, which is good for maintaining the temperature. Itâs very useful in summer.
It would be interesting to hear some informed Choice expert advice on how this comes about. @jhook.
One question might be to explain what is observed relative to the heat or cooling load required to achieve the desired room temperature, IE comfort level. It is a constant.
The second question might consider how an EER (AEER?) approaching 5 is achieved in cooling mode for the model indicated.
Edit added Note: LGâs specification says the Cooling mode AEER is 3.59. The unit rates 2.5stars for energy consumption.
