HVAC

(Heating, ventilation, air conditioning)

As I have already mentioned, this is the aspect of studio design that is most complex and time consuming (at least for me). I recently realised the way I have been thinking about it is mostly wrong!! More about that in a paragraph or two but this is a subject that gets complicated very quickly. In essence, what you are trying to do is ensure that:

  • Sufficient fresh air is brought in to your insulated space
  • Humidity is controlled in order to make heating & cooling efficient and
  • Exhaust air is vented properly
  • Transmission loss is maintained throughout the ventilation system

Broadly speaking, there are two types of HVAC systems you can choose from. The first type is a ducted, forced-air system that circulates via a heat exchanger and can include a dehumidifier unit. The total volume of air in the system is cycled 6-8 times an hour and is ‘topped up’ with 30% fresh air. The boiler will need to be housed in a separate room, the ducting will need to be large and the silencers even larger!

The second type is a ‘ductless’* mini – split system. This is where air circulation and heating / cooling are separate and only needs to move 30% volume of air compared to a forced air system (the fresh air you need to breath). This type of HVAC is less efficient and will require a drain pipe to be installed from the AC condenser unit through the inner & outer leaves to the outside, but has the significant advantage of being cheaper to install and requiring smaller silencers.

My studio is a single room, so adhering to the KISS maxim (“keep it simple stupid”), I have opted to build a ductless HVAC system. Please note that when I say “cheaper to install” it’s still actually fairly costly – it’s a relative term to describe money well spent.

The first step is to calculate the volume of your room or rooms. The quantity of supply air needed to ventilate the room adequately in cubic feet per minute (CFM) is: 30% of 6 x room volume (in cu feet) ÷ 60. (6 = the minimum number of air changes per hour).

In my instance this works out to be: 30% of 6 x 1600 ÷ 60 = 48 CFM (It took me 3 years to properly understand this and I’m eternally grateful to Gregwor from Alberta, Canada [designer of the MSM calculator and moderator of the John Sayers’ Recording Studio Design Forum] for making the penny drop with one of his many clear, concise posts on the subject).

Once you have this figure, you have to consider the velocity of the air entering the room. Why? – because if the air velocity is too great when it arrives at the supply register for the inner leaf, you will hear an audible ‘whoosh’ due to turbulence which, obviously defeats the whole project. Conventional wisdom has it that air velocity as it comes into the inner leaf must be no more than 300 cubic feet per minute, ideally it should be even less. The only way to control this is by adjusting the dimensions (specifically the cross sectional area) of the duct and silencer boxes you are going to build. *(Yes, ‘ductless’ mini-split systems do, confusingly, often have an amount of ducting in them).

You will be intuitively familiar with the general principles involved here. What helped me was to visualise using a garden hose. We all know that if you partially block the flow of water with your thumb, the speed of the jet (velocity) increases. When you un-block the hose (increase the cross sectional area or CSA) the speed of the water falls. This is the effect we’re looking to achieve in our HVAC silencers.

Making sure the dimensions of all the elements of your HVAC system are correct is critical! We already know our desired air volume (in my case 48 CFM) and we also know that we have to keep the velocity to under 300 feet per minute. The maths for calculating the minimum CSA of your supply is:

CSA = CFM ÷ 300 [48 ÷ 300 = 0.16 ft (1.92 inches)]

Remember, we really want the air flow to be slower than this, so let’s metaphorically take our thumb further away from the flow and increase this to 4 inches (100mm).

Great! I know the minimum CSA of my system and with that figure, now I can scale all the other parts of the path. Starting at the beginning, I’m going to need an air intake something like this – often referred to as a ‘louvre’ (nothing to do with the Parisian museum).

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Something to keep in mind is that louvres have a defined ‘open’ or ‘free’ area. This refers to the unobstructed region of the grille which is typically between 35% – 60% of the total area. I’m going to connect my louvre to a filter box and then a short length of round steel ducting – so the maths goes like this:

50mm x 50mm (duct radius) = 2500mm²

x 3.142 = 7855 mm²

multiply by 2 to give double the surface area (assuming 50% open area):

= 15710mm² (square root of this will give the louvre size)

= 125.339539mm

Filter Box without lid. undefined

Filter Box with latched lid.undefined

So let’s round that up to 150mm. This is connected to the filter box which in turn, connects via a circular flange to a short run of round steel duct where it feeds into the outer leaf silencer. In order to slow the airspeed, the CSA of the silencer must be twice the CSA of the supply duct. For round duct this area = 3.142 x r(adius)² (Pi x r squared).

My duct has a 5cm radius: 5 x 5 = 25cm² 25 x 3.142= 78.55cm² (CSA)

So doubling and rounding up gives us a silencer CSA of 160cm². Remember that this is the space for the air to flow through, not the over all dimensions of the silencer. The silencer has to be made of materials that match the surface density of the leaf that it’s attached to so they’ll need to be fairly massive and include three or four internal baffles for adequate insulation. At the point that the silencer goes through the outer leaf it should be via a sleeve of identical density. Inside the void, the connection to the inner leaf silencer can be made with conventional ducting.

The CSA of an inner leaf silencer should be greater again than the outer leaf silencer (thumb well and truly away from the end of our imaginary hose). So I intend to build mine with a cross section of 324 cm² (18cm x18cm). The attached sleeve goes through the inner leaf and into the room through a diffuser grille directly above the intake of the AC condenser unit. This will help to mix the fresh air with room air and maintain an even temperature.

Outer leaf silencer (open).

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The exhaust side of the ventilation system is almost identical – only in reverse. It should be located on the opposite side or end of the room to avoid the re-circulation of stale air. The one extra component needed is a fan to pull air through the path. The fan can be fitted on the supply side to ‘push’ instead. The main difference is that this will create positive air pressure in the room and locating the fan on the exhaust side will cause negative pressure. The advantage of having negative pressure in your room is that it acts on the doors (I’ll cover doors and windows in another post) and helps to increase insulation by pulling them into the seals.

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The fan has to be powerful enough to meet the airflow requirements of your room and also overcome the static pressure inherent in any HVAC system. Every component added to the path will add a small amount of resistance to airflow or static pressure. This can be cumulatively significant and is tricky to work out although there is a very helpful app available from ASRHAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) which helps to detail static pressure for specific HVAC components. Static pressure is usually measured in inches of water column, which is often shown as an abbreviation such as “in. wc,” “in. wg” or “in. H2O”. Even experienced designers will concede there is at least a small element of educated guessing involved in calculating the static pressure of a sound insulated HVAC system. This is partly because of the requirement to maintain transmission loss. If you were simply installing ventilation and AC, you would keep the ducting as straight as possible to lower static pressure and turbulence, but we need to include baffled silencers which are made up of 90° & 180° turns. The method I used to estimate the static pressure of my silencer design is called “equivalent duct length”. This asserts that a 90° turn in ducting is equivalent to the duct diameter x 60. If you are using a combination of round and square ducts, you can use this online calculator to work out the equivalent diameters:

https://www.engineeringtoolbox.com/equivalent-diameter-d_205.html

For my design I calculated that the small silencers would have an equivalent round duct diameter of 14.2cm and the large ones 19.9cm.

Equation for converting a sharp 90 degree bend into an equivalent length of straight duct is:

Duct Diameter x 60

14.2 x 60 = or 852cm for each 90 degree turn x 8 for each silencer = 68.16 metres (223.622ft) of straight duct for each outer leaf silencer

and

20 x 60 = or 1200 cm (3.937008 ft) for each 90 degree turn x 11 for each inner silencer = 132 metres (433.0709 ft) of straight duct for each outer leaf silencer.

Using the engineeringtoolbox.com friction or head loss calculator for air ducting this results in:

Friction Loss (inH2O): 0.072
Friction Loss (inH2O/100 ft): 0.0302
Air velocity (ft/min): 282
Air velocity (ft/sec): 4.6899999999999995
(Outer leaf silencer)

and

Friction Loss (inH2O): 0.0216
Friction Loss (inH2O/100 ft): 0.005
Air velocity (ft/min): 138
Air velocity (ft/sec): 2.29

(Inner leaf silencer)

Giving a combined loss for all 4 silencers of 0.14882 (inH2O) or 37 pascals and apparently achieving an air velocity of 138 ft/min at the point it enters the inner leaf.
Using the ASHRAE app I added in the losses for 2 outer louvres with filters [2x 0.05 in.wg], 2 plenums[2 x 0.01in.wg] and 1 metre of additional 4” ducting[0.02 in.wg]
Total = 0.28882 in.wg or 71.9 pascals.

So, in short, I know that a fan that will move up to 300 CFM through 100m duct against a static pressure of 0.3 in.wg will be fine.

© John Steel 2020

Published by johnmsteel

Musician, editor and now studio builder.

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