Studio HVAC: All about mini-split systems, HRV's and ERV's.

#16

If it is of any encouragement, I bought our ERV (Mitsubishi Lossnay LGH-25RVX-E), 5 years ago for 700 Euros (add 10% to get approx. US$ equivalent). With the AC (300 Euros) and extras and installation (600 Euros) it felt at the time to be an expensive risk but it turned out to be the right decision.

As a bonus, an ERV (and maybe an HRV) can recouperate enough money from you now not having to heat or cool the incoming air as much that they can recover the cost of investment in just a few years.

Watch my build topic next month (September 2020) when the HVAC company comes to install the same ERV and ducts in our new place. They installed the AC earlier in the year so that is in and working and can be seen now.

#17

Brilliant overview Stuart (and great site!)

I'd put together a chunk of what I think I need to know on HVAC via books/research and in particular reading posts by Gregwor, yourself and John Steel's posts and blogs - but this puts it all the design options together in one place.

I'm in the process of designing a two room studio down here in Oz and have been trying to get a reasonable line on HVAC needs before building - as a priority, silencer box sizing/weight. (FYI - I have based my isolation plan around the advice you provided on the Dog and Bear build as my isolation needs and environment are pretty aligned ). Mine is a room-in-a-room structure with separate and isolated live and control rooms both circa 4.2 x 6.1 x 3.0m h. I posted the first iteration of the design on John Sayers site a while back but am going through a design revision and will post the revised proposal here as soon as I can close the loop on a couple of engineering questions.

One question I'm really grappling with is whether to go with a mini split in each room with a supporting ventilation system (8 silencers - the inner ones most likely in the style of the dual split style I've seen here) OR an AHU in the roof space with 8 silencers. To some degree the answer may depend on silencer box size but I am favouring the AHU approach at the moment as
- I have available accessible roof space
- I like the idea of complete removal of AC noise from the rooms; and
- I'm keen for as few as possible penetrations of the space - especially ones that have to be done by someone else

To my question:
I've seen John Steel's post where he sets out the calculations for silencer box sizing for a multi-split system.
What I'm wondering is whether the calculation differs at all for a ducted mini split setup?

Many thanks
Andrew

#18

Happy Friday all!

I have a question regarding HVAC design, and am curious if there's any consensus on utilizing the ceiling joist/rafter bay (2- 2x12"s & subfloor/wood flooring above) as part of the outer leaf baffle/silencer, or would it be measurably better to build all four individual B/S's, and mount them individually instead?

Thanks in advance, I hope that you're all doing well.

#19

So long as the joists or rafters belong to the leaf that the silencer belongs to then you could use them as one face of the silencer box. If you look at the photos of my silencers in post 56 of my build topic, here, you will see that not one silencer was a fully enclosed box. I used the walls and ceilings rather than joists and rafters but the principle is the same.

I am not sure I properly understand your question regarding building all four silencers separately. I presume you intend to combine the incoming air and outgoing air into a single twin-channel silencer, one on the inner leaf and one on the outer. I can't say I recall ever having seen it done. I would not want to have done that simply for the practicality of getting them into position as they are so heavy and a twin silencer would be twice the weight. I am not sure whether there is an acoustic reason why you shouldn't do it; maybe someone else does.

#20

Thanks for the quick reply, Starlight, I have in fact scoured your thread and used it as a reference for a couple things already.

Without hijacking this amazing thread, I'll keep it short and say, that was exactly what I was looking for. I guess I didn't look close enough at your baffles to realize those few little details, but I do now.

Thanks again, I'll be sure to bring any other questions to my thread (a continuation from the other forum), I'll put it together one of these days.

Enjoy your weekend!

-Chris

#21

gearjunk1e wrote:Brilliant overview Stuart (and great site!)

Thanks Andrew!

I only just saw your post now (I'm still playing "catch-up"), but hopefully the info is still useful for you.

(FYI - I have based my isolation plan around the advice you provided on the Dog and Bear build as my isolation needs and environment are pretty aligned ).

Great! So you already have a pretty good basis for your place then.

4.2 x 6.1 x 3.0m h.

That's a reasonable size, and the 3m height is great. Very useful.

am going through a design revision and will post the revised proposal here as soon as I can close the loop on a couple of engineering questions.

One question I'm really grappling with is whether to go with a mini split in each room with a supporting ventilation system (8 silencers - the inner ones most likely in the style of the dual split style I've seen here) OR an AHU in the roof space with 8 silencers.

If you go with the AHU, it might be possible to use fewer silencers. In some cases, it is possible to have just one single "intake" silencer on the outer leaf, rather than one for each room, and ditto for the "return" path: just one that combines the return from all of the rooms. Not always possible, but could be an option.

I am favouring the AHU approach at the moment as
- I have available accessible roof space
- I like the idea of complete removal of AC noise from the rooms; and
- I'm keen for as few as possible penetrations of the space - especially ones that have to be done by someone else

Sounds like a good plan! Without seeing the actual layout and details you have in mind, I can't really comment more, but in general your reasoning makes perfect sense.

I've seen John Steel's post where he sets out the calculations for silencer box sizing for a multi-split system.
What I'm wondering is whether the calculation differs at all for a ducted mini split setup?

If you use mini-splits in each room, then your ducts and silencers can be smaller, since all they are doing is supplying fresh air and removing stale air: they are not recirculating air through the rooms, which is the case for an AHU based system.

It gets a bit confusing, but there's two issues involved: One is circulating enough air through the room to keep all of it cooled evenly, and the other is supplying enough fresh air to keep the people inside alive. That's rather important! With a mini-split unit, it is not supplying any fresh air: all it is doing is recirculating air through the room at the correct rate. The ventilation duct is supplying the fresh air, but is not recirculating anything, and the exhaust duct is removing stale air. However, with an AHU, all of the air is going through the duct system: both fresh air AND ALSO recirculated cooling air. The AHU is a complete circuit, that moves all of the air through the system. At some point in that circuit, a distance before it arrives back at the AHU after going through the rooms, you bleed off some of the returned air and dump it overboard. A bit closer to the AHU, you have the duct that is bringing in fresh air, and that mixes with the rest of the recirculated air just prior to going through the AHU, where it is conditioned, then goes into the circuit again. So, since the ducts to and from the AHU carry ALL of the air, they need to be bigger than the ducts for a mini-split based system.

Hopefully I'm managing to explain that clearly! I just read it over, and confused myself, I think!

But that's the general gist. However, even though the ducts are larger, I'm still inclined in favor of AHU, for the reasons you mentioned.

- Stuart -

#22

Hi Stuart

No probs - I had just assumed you were busy - all good

The weird thing was that I hadn't seen your reply til now :oops:

Needless to say your reply is extremely useful.

BTW - my design post is at https://digistar.cl/Forum/viewtopic.php?t=915 and construction at https://digistar.cl/Forum/viewtopic.php?f=6&t=918. Don't worry - none of the construction yet is anything more than building the outer shell!

What your comments have done is make me realise that the silencer plan in my recent post will only work for ductless mini-splits so I'll have to rethink this - perfect timing

Andrew

#23

OK! I just replied over on your design thread, in more detail...

- Stuart -

#24

Hi. I was hoping I could clarify a few things.

In your initial example a ductless system provides 200CFM and 80CFM of fresh air is required. Does that mean if the same system were ducted, you would still need 200CFM but 80CFM from that would be injected into the system from outside?

If so, would that mean the air handler specs only need to be least 120CFM & therefore run at a lower speed than it would otherwise? Or does the fresh air just feed into the air handler which still pumps out 200CFM? I'm trying to get my head around two sources of air, perhaps competing with each other.

I've also seen it suggested that an ERV or other fresh air delivery system could just dump air right over the inlet of the ductless Air Handler. Would this work in a ducted system (assuming there is an adjacent equipment room for mixing to take place)? If not OK, is a closed plenum necessary and does that require additional calculation for size & avoiding static pressure issues?

Finally, on baffle boxes. Do I understand correctly that you would, in a sense, set a speed limit for your entire supply system but allow that speed to fluctuate? For example, the duct leading into the outer box could be 300fpm, then change to 200 inside the box (due to larger CSA), then, say, back to 300 in the next duct, 200 in the next box & finally 150 at the supply register? Hopefully that makes sense. I've seen it suggested that the CSA must gradually get larger as you get closer to the supply register to avoid wind noise issues. I would think either situation generates insertion loss but my example keeps the ductwork smaller!

#25

Hi there Pequod, and Welcome to the forum!

Glad you found us.

In your initial example a ductless system provides 200CFM and 80CFM of fresh air is required. Does that mean if the same system were ducted, you would still need 200CFM but 80CFM from that would be injected into the system from outside?

Exactly. The 200 CFM figure is what needs to circulate through the room, regardless of where it comes from and where it goes. The 80 CFM figure is how much fresh air you need in order to keep the people alive and healthy and happy, because that's how much stale air (CO2 and other nasty stuff) you need to remove, and replace with the same amount of fresh air (O2). These are two different concepts, but interconnected.

If so, would that mean the air handler specs only need to be least 120CFM & therefore run at a lower speed than it would otherwise? Or does the fresh air just feed into the air handler which still pumps out 200CFM?

The AHU still needs to move 200 CFM in this hypothetical room, regardless of whether it is ductless or ducted. That's how much air needs to be circulated through the room, in order to provide the cooling, dehumidification, and general air circulation. That's what you need to keep the conditions evenly spread out through the room, and moving around nicely. How much fresh air is in that, isn't really relevant to the amount of air that must be moved through the room. (It is only relevant to keeping people alive / healthy / happy.) In some cases (open loop systems), all of the incoming air is fresh, and all of the outgoing air is stale: there is no return path in such systems. That's the ideal, but very expensive to run in extreme climates. In that case, too, the AHU would still need to move 200 CFM, because that's how much flow the room needs to keep things moving smoothly.

I've also seen it suggested that an ERV or other fresh air delivery system could just dump air right over the inlet of the ductless Air Handler.

Right! That is, indeed, an acceptable option.

Would this work in a ducted system (assuming there is an adjacent equipment room for mixing to take place)?

Some people do suggest doing that, but I'm not very keen on the idea, for several reasons, even assuming it is legal. In some places it might not be permitted by code, for example if the "other room" is classified as habitable space: Code usually prohibits supplying air from one room into another,l or exhausting air from one room into another, if they are "living spaces" or "habitable spaces", even if there is nobody in there. For code, practical issues like that don't matter. Only the classification matters.

Would this work in a ducted system (assuming there is an adjacent equipment room for mixing to take place)? If not OK, is a closed plenum necessary and does that require additional calculation for size & avoiding static pressure issues?

A ducted system is generally a closed "loop", with an exit point for stale air, and an entry point for fresh air. The AHU itself is part of that, and has the return duct arriving on one side, with the supply duct leaving on the other side, everything sealed up air-tight. The return duct feeds the recirculated air coming back form the room(s) into the AHU, where it is cooled and dehumidified. That return duct generally is a plenum, and several feet upstream of the AHU is the point where the stale air is taken out thorugh the "exhaust" duct, to the outside world, then just before the AHU the inlet duct brings in the fresh air stream. You generally want about 6 feet between those two points, to prevent backflow in the wrong direction, and suchlike. So the stale air is sucked off first, then there several feet of duct or plenum, then the fresh air is pumped in, then maybe a couple more feet and it all goes into the AHU.

Finally, on baffle boxes. Do I understand correctly that you would, in a sense, set a speed limit for your entire supply system but allow that speed to fluctuate?

The only velocity that really matters, is the velocity through the registers: at the point where the air comes into the room, and at the point where it leaves the room. At those two points, the velocity must be low, and the flow smooth, for at least several times the duct diameter. But beyond that, inside the silencers and other ducting, the speed can be higher. Ideally, the highest speed is in the distant ducting, on the far side of the silencers from the room, and the lowest speeds towards the "room" side of the silencers. If you have seen my silencer box design, you'll not that the incoming airflow from the duct is immediately split into two parallel paths, which drops the air flow velocity by 50%, and increases the static pressure similarly.... but for each individual arm! The TOTAL airflow velocity is still the same, and therefore the total pressure is too. There are then additional changes in velocity/pressure along the rest of the path through the silencer, but once again, they are in parallel, not series, so the overall effect is minimal. Normally, I have a larger plenum-like region at the two ends of the box, where the CSA is similar to that of the actual "sleeve" that reaches through the wall or ceiling to the register, so the flow is slow and smooth for as long as possible before it hits the register.

I've seen it suggested that the CSA must gradually get larger as you get closer to the supply register to avoid wind noise issues.

Not gradually! There must be sudden, abrupt, drastic changes in CSA, to get the additional benefit of impedance mismatch: each time you have an abrupt change in CSA, you also have an abrupt acoustic impedance mismatch, which can gain you an extra couple of dB.

Hope that helps!

- Stuart -

#26

That does help, thanks!

One other point on the exhaust. I'm looking at your diagram from the first page with the whole system laid out. Is there any trick to that Return plenum? I see now that most of the air goes right back to the air handler but some is siphoned off to go outside. The CSA of the plenum inlet is pretty big (as is the air handler) while the exhaust looks pretty small. Just wondering if there is some devil in the details there or, maybe not.

And, if one was using an ERV or HRV, do you have a visual of how that would fit into your "big picture" diagram with the silencers, plenums and air handler?

#27

The fresh-air intake and the stale air exhaust are the same size: they have to be, because they are moving the same amount of air. You need to bring in a certain quantity of fresh air, which goes into the closed-loop circulation system, and because it is a closed loop, you must also dump overboard the exact same amount of air that came in through the intake.

Also, the air return duct for the "closed loop" is the same size as the air supply duct: They are both much larger than the intake and exhaust ducts, yes, but they are still the same size as each other. In other words, Fresh air intake CSA = stale air exhaust CSA. That's on one side of the system. And on the other side: Supply duct CSA = return duct CSA. But the CSA of the two systems is NOT the same, since the fresh/stale air system only moves a smaller amount than the Supply/Return system. (That being said, the CSA of the return duct/plenum is often a bit larger than that of the supply duct/plenum, for other reasons).

Also, that diagram is not really about studios at all! As I mentioned in the write-up, that is actually for a house furnace system. Thus, it does not completely represent how this is implemented in a studio. A house is ca lose analog, but not exactly the same as a studio. Houses are not sealed totally air-tight, like studios are, for example. And other differences, too.

I really should do a better diagram myself, to fully illustrate the concepts! (If only I had more hours in the day.....

).

- Stuart -

#28

as a note, generally intake and exhaust are the same size, however, in the overall balancing of the system, there is usually some cross-duct control valves to allow for pressure changes (e.g. a multi zone system with fan speed changes and VAV operation). so there may be rare cases where intake and exhaust are not only sized differently, but sometimes there are even multiple openings to change volume.

#29

I know supply registers need to be of good quality to get the correct throw and avoid turbulence noise but what about the exhaust registers? Is there anything special about those or can they be of average big box store quality?

#30

as a general rule, you should plan on your exhaust being the same as the supply. you can adjust the return duct via values to balance the pressure once out of the room. but you want to avoid allow the exhaust to become the noise source (wind, vibration, motors, etc) just as much as you would the supply. i prefer symmetrical supply and returns in the CR taking into account "stale" pockets in the room. and in booths or live room, asymmetry is ok, and ensure the air flow works across the room.

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