(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).


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).


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.


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:


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


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 inner 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)


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 0.1872 (inH2O) or 46.63 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 = 0.3272 in.wg or 81.5 pascals.

So, in short, I know that a fan that will move up to 300 CFM through 100mm duct against a static pressure of 0.35 in.wg will be fine.

© John Steel 2020

Published by johnmsteel

Musician, editor and now studio builder.

13 thoughts on “HVAC

  1. Hi John
    Thanks for sharing your experience about studio design and construction!

    I’m searching about studio construction because I have a room in the basement of my building, where I’d like to create a Room in a Room to practice/play drums, and make some home recordings (if possible)

    At this moment I’m searching about HVAC. And this is a very complex aspect to deal with…

    In you post you say:
    “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).”

    Could you tell me what means the 1600, please?

    In my case I don’t need a complex HVAC system. In fact, I used this space in the last 15 years, and never used any
    type of exchanging air… because I never thought about it… But now I understand that it is important, because of the fresh air and because of the condensation.

    The room is 2,91m width, 4,25m length, and 3,45m height. When I create the inner room, it will be around 2,60 x 3,95 x 4,20 meters = 43,13 m3 (43 cubic meters).

    How should I calculate the size of the silencers?

    Do you know if the isolation of the silencer boxes is effective, I mean does it worth it? Or should I do the drum room without HVAC system?

    Thanks in advance for your time.

    Be safe
    I apologize for my English

    Paulo Carvalho


  2. Ola Paulo,
    Please don’t apologize for your English – I can’t utter a word of Portuguese.

    “Could you tell me what means the 1600, please?”

    Yes, this is the volume of my room in cubic feet (roughly 45 cubic metres).

    “How should I calculate the size of the silencers?”

    Calculate the volume of the room, multiply by 6 and divide by 60. 30% of this number represents the amount of air you need to flow into and out of the space. When you know this, you can calculate the minimum size of the ducting you need:
    CSA = CFM ÷ 300 (Cross Sectional Area [of the duct] = Cubic Feet per Minute [required airflow] ÷ Max Air Velocity).

    Round up the minimum CSA figure (I doubled up to 100mm / 4 inches) for safety. The CSA of the outer leaf silencer should be at least twice the CSA of the duct and the CSA of the inner leaf silencer should be larger than the CSA of the outer silencer.

    “Do you know if the isolation of the silencer boxes is effective, I mean does it worth it?”

    The silencers will isolate well if you build them to be as dense as the rest of the structure. They will make the difference between being able to breath and suffocation so, yes, I’d say it’s worth it!

    “Or should I do the drum room without HVAC system?”

    I wouldn’t!

    Good luck with your project Paulo!


  3. Hi John

    First of all – great idea to document your experiences

    Your posts do a great job of summarising in a logical form lots of info available elsewhere via multiple searches and I’ve picked up quite a few ideas already ..eg I’m going to need a Flitch beam for my spaces I think – I’ll talk to my engineer mate about this concept as a solution for my spans

    I am currently designing a studio down under with a live room and control room – see http://www.johnlsayers.com/phpBB2/viewtopic.php?f=1&t=22503

    In my planning I’ve spent the week-end focussing on HVAC – trawling the John Sayers site and back into the books. Your section on HVAC is great as it summarises current thinking/logic – but it has prompted a couple of questions. I apologise in advance if they are a bit naive as the penny only dropped for me late last night.


    Like you I’m looking at mini splits – in my case, one for the control room and one for the live room. However, I’m leaning towards DUCTED mini splits rather than the non ducted you’ve settled on.

    I’m trying to work out why you’ve gone that direction ie not to a ducted mini split where you could take noise out of the room without too much greater complication in design ie feeding the fresh air/stale air ducts into the supply/return ducts.

    What do you know that I don’t?


    How have you arrived at the sizing for the AC unit itself – or is that still ahead of you? Are you intending to do the Rod Gervais style of calculation with the heat generation from equipment, people, fresh air etc or have you taken another route?

    Many thanks in advance and I look forward to following your progress



    1. Hello Andrew,
      Thanks for your message and thoughts about my blog. As you mention, all this information exists elsewhere but having gathered as much of it as I needed to develop the design so far, I thought I might as well write it up. It’s a W.I.P. as I’m sure you’ve gathered and needs updating as I have a couple of months of building to detail.
      To answer your first question, the reason I choose a ductless system is twofold: It’s a small room to begin with and has a low ceiling (limited by the structural beam) so the less room volume I lose to infrastructure the better. Internal ducting would eat into this and there simply isn’t space to build ducting in the void. The HVAC noise shouldn’t be too much of a problem because I’m not building the room for critical listening – I intend to use it mainly for practise and occasional tracking.
      As for how I sized the A/C unit, I used Rod Gervais’s calculator and also consulted with a local A/C company (who also helped me to source duct liner and were very generous with advice). An important thing to remember when considering this is to make sure that heating, cooling and dehumidification are ‘right sized’ for the space. You don’t want to be constantly adjusting the settings on an over-spec’ed unit to try and find the ‘sweet spot’ or have to run an under-spec’ed unit flat out to keep up (forgive me if I’m preaching to the choir).
      You’re lucky to have a great space to work with Andrew and I will certainly follow your design and build – let me know if I can be of any help. Best wishes & good luck, John.


  4. Thanks John

    All makes sense…even since I posted this I’ve been looking at whether I could manage with a ductless unit, at least in my live room. I’ve seen plenty of people say the Daikin ductless mini split room units are “quiet” but no-one to my knowledge has posted the actual noise levels.

    I will take a closer look in my roof space re ducting – I should be OK given I have a large area but its a good flag.

    Re sizing – yes I have burned on my brain the need to not oversize…it was the first thing I got my head around re HVAC…that and the absolute need for ventilation were in fact the only things I understood for quite a while!

    I will be watching your progress with interest and I certainly intend to be stepping through mine on the forum



  5. Hi again John

    Hope you’ve dodged COVID…we sit here in OZ with all but no cases and a pretty much normal life and are amazed by stories from friends over there about how different life has been for you guys. I assume it has put you in go slow with the build.

    I do have a follow up question. While I need to do my own calcs on HVAC sizing, my gut feel is that my silencers will be very similar size to yours given your room volume vis mine (albeit I’ll need 8 for two rooms).

    I saw your pics/comments on lifting and the weight but was wondering what the external dimensions of the boxes ended up at – just juggling how to do my roof space and what goes where and wanted an approximation to work with.




    1. Hello Andrew and thanks for your message,
      Yes, we managed to avoid Covid thanks – we’re very fortunate in that regard. Don’t get me started about the way it’s been handled tho’ – I’ve just had my first vaccination, so hopefully most of the community will be developing an immunity very soon. This combined with Brexit has meant delays in getting hold of materials (2 months to order two cases of caulk) and the price of everything seems to have increased but I’m still plugging away!
      In answer to your question, the finished silencers were 84.6cm wide x 45.7cm deep x 23.4cm high (28.4cm high where they penetrate the outer leaf). I used a square cross section for my design, but I have read that you can use any shape providing no dimension is much more than twice the other. They were surprisingly heavy and the inner leaf needs bigger silencers than the outer leaf does (I’m going to build mine with 2x the CSA). Good luck & I’m looking forward to seeing your updated design. Best wishes, John.


      1. John great to hear everything is OK (aside from capacity to progress the build!)

        Thanks for the reply..I am feverishly working on a final design – so not too far away – key juggle for now is room level roof structure, hence the desire to understand the weight/size of silencers.

        Having re-read your posts today another question emerged – I’m wondering what that piece of feedback from Stuart was that allowed you to slim down your boxes…I had thought you must have gone with his dual flow boxes but that didn’t appear to be the case



  6. It wasn’t Stuart who helped with my silencer design (more about this later – it will need a post on it’s own)! I did a lot of reading and finally understood that there are basically two types of HVAC system you can install in a project studio. The forced-air type, which circulates air via a heat exchanger and the ductless mini-split type which I chose to build. The forced-air type circulates the entire volume of room air 6-8 times an hour and takes in 30% fresh air so you can breath normally. The ductless type separates the heating / cooling / de-humidification function from the ventilation and just supplies the fresh air that’s needed (30% of room volume), so the silencers can be much smaller than in a forced-air system. ATB John.


  7. Thanks for your comment Elmer – I’m happy if it’s been helpful to you. I’m sharing my experience as a guide rather than a ‘gospel’. If you’re building a studio, please be sure to do your own calculations (and triple check them). Ventilation and climate control is crucial to studio design and will make the difference between a comfortable, creative workspace & a dangerous, suffocating folly. Best wishes, John.


  8. This was very informative. HVAC is such a black art, made even more confounding by HVAC installers who know little beyond cookie cutter applications and the industry as a whole not really understanding the needs of a recording studio.

    This is the first time I’ve seen advice that the outer shell box CSA should be bigger than the inner shell box. I was under the impression that both boxes required some decent changes in area to generate insert loss but that, for example, you could use 8″ duct to feed one box, then 8″ from that box to the other (which means increasing CSA, decreasing, then increasing again). I thought the main goal would be achieved by the final opening (for the supply side), creating the biggest insert loss of all. Not sure if you have any thoughts on that, just trying to get my own ducks (ducts?) in a row.

    So for your build, did the return side go the same way, increasing size with the direction of air? Smaller inner box to bigger outer box, or were both inner boxes larger than both outer boxes?


    1. Thanks for your interest and observations. Yes , it’s very difficult to find information about HVAC systems, particularly in relation to small scale applications. I will eventually write about my experience of trying to hire professional help to design my HVAC system but the short version is that none of the designers I approached (four in total) were willing to help (even the one I paid).
      Fortunately, it is possible to gather enough information to figure it out but it takes time and effort.
      Reading your post, I’m slightly concerned that I may not have been clear enough about the dimensions of the silencers. The CSA of the inner leaf silencers should be larger than the outer leaf silencers ideally. I have read knowledgable sources that say in circumstances where space is limited, the inner and outer leaf silencers can be the same size but in this case you won’t benefit from the second reduction in air velocity. In short, the greater the increase in CSA from the outer to the inner box, the greater the insertion loss will be. The inner and outer silencers can indeed be connected with duct of the same dimensions as the supply duct to the outer silencer – I decided to use flexible duct joiners instead.
      For logistical reasons, I have yet to build my inner silencers (the inner leaf framing needs to be completed first) but they will have twice the CSA of the outer leaf silencers. Best wishes, John.


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