By Mike Schaefer
I read with great interest the article on indoor ACs in the April 2020 RACA Journal. Since the introduction of outside air is not the only drawback;
I would like to add more factors on this matter.
In my opinion, the extensive use of the unitary/split system has lowered the image of, and has also incurred great damage to, the air-conditioning industry.
We had engineers, technicians and artisans who understood the subject of air conditioning, and a lot of this knowledge has disappeared with the retired people who have been replaced by companies, staff and customers who see this more affordable solution to every air-conditioning requirement.
The article focused on the lack of, or rather – non-existence, of outside air introduction. This is not the only shortcoming when using split systems, so, here are a few more factors. There are models where it is possible, such as under-ceiling and hide away units.
Using mid wall splits, return air enters on top and the supply air is discharged at the bottom via a single outlet below, sweeping low temperature air into mostly unwanted directions. Unless the return air is at a high temperature, the low temperature air blown at occupants defeats all purpose of proper air distribution.
Ceiling cassettes – if installed correctly, provide a somewhat better distribution, yet many of them have an undesirable sweeping action. Surely the reason for this type of air distribution is that, in most cases, the equipment is unable to maintain overall room comfort – and relies on the supply air blowing into your face?
Hide away provides the best results as far as air distribution is concerned – ducting supplying air to correctly-sized diffusers in the correct areas will do a good job. As for outside air introduction and the right air quantities, this is possible with the correct engineering knowledge.
Correct air quantity
To achieve the correct desired space conditions related to the load and capacity, the system has to have the correct air leaving temperature and air quantity. Herein lies the biggest problem in the use of this type of equipment – it is designed with low noise levels in mind which can be achieved with low air quantities.
Low air quantity results in higher DT, lower leaving temperature and unnecessary de-humidification. Rating conditions quoted as indoor 27°dB/19wB are test conditions with continuous heat and moisture being added. In reality these conditions indicate common international ratings and are not possible unless you introduce outside air into the return air.
Unless the space has a very high moisture gain (unlikely in most applications) the resulting room and air entering conditions will always end in low humidity and temperature which in turn reduces the system capacity with a high-power consumption.
To achieve a more correct room condition, oversized systems with higher air quantities could do the job, provided the compressor capacity is reduced (inverter technology). The load calculations are a joke – they are done with a tape measure in two dimensions. The sizing of the equipment is in BTU. If it is too expensive, use a smaller unit; after all you don’t want it too cold!
While indoor sound levels are often acceptable, the literature however never states it correctly, always at the lowest airspeed and an unknown distance from the unit. The same applies for the outdoor units, depending on their positioning. Units above 65dBA are not uncommon and don’t comply with by-laws in South Africa.
Heat pumps/reverse cycle systems
I don’t want to go into details of reverse cycle systems and restrict my comments to the use of cooling only functions. Just to mention here, the biggest sales pitch rubbish is that the systems are advertised with power savings, which are only possible when heating is unlikely to be needed. Outside conditions are never quoted in wet-bulb temperature (WBT) and so on – another subject for another day.
It is interesting to note that VRV systems became very popular using indoor units as discussed above, but they have the same shortcomings. The advantage lies in the outdoor unit having better power factors and that they can be installed at distances where noise and serviceability is improved. Zoning is possible when typically interior zones require cooling while exterior zones require heating with excellent power savings in such cases.
The only advantage of split systems could be zoning? In short, the engineering science has left the industry and has been replaced by the ‘bakkie and ladder brigade science’.
The tools and quality of the ‘bakkie and ladder brigade (technicians)’ arise as a result of the split system dilemma. How and when the systems are installed can or can’t make it possible for them to be serviced or repaired.
Condensing or outdoor units are sometimes installed at 10 or more metres above the ground and indoor units at heights over permanent seating arrangements like conference centres or cinemas, where the floor can be at an angle. Such installations are a nightmare to get to.
Outdoor units installed inside are sometimes hidden away in spaces with very limited ventilation. I could go on and on with examples of crazy installations – enough to fill up books. Pipe sizing does no longer consider oil return, distance, height of indoor unit above the condensing unit, or pipe wall thickness when using R410.
Lifetime of equipment depends on the usage, installation and environment it’s operating in. If it fails or it was bought in a supermarket, obtaining spares or lack of knowledge of how to repair can be greatly problematic too.
These above points only touch on the subject, which is in fact enormous, leading to the question – “What has become of our industry?”