Letter to the editor
LETTER TO THE EDITOR:
COMMENTS ON A
PUBLISHED ARTICLE
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have been replaced by companies, staff and customers who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 – nonexistence,
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 underceiling
and hide away units.
AIR DISTRIBUTION
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!
NOISE LEVELS
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.
46
RACA Journal I October 2020
www.hvacronline.co.za