Minisplit and Deadvent design

You control the temp of the air in the exchange source usually the rest of the baement. You also pray that you guesstimated the correct volume of air exchange to keep up with the heat load in the theater. If you wanted you could install a line voltage thermostat to control the fans in the dead vent system. But even if the temp is OK you may want some fresh air.
Edited by BIGmouthinDC - 3/13/13 at 6:18am

Maybe I'm misunderstanding what you're asking, but if you put a ductless minisplit in the room, I don't think you would have to have a deadvent. The minisplit will handle cooling the air in the room. However, as BIG said, you might want to have a deadvent supply and a deadvent return that is ducted to another part of the basement for fresh air exchange.

I think the setup you are getting mixed up here is where deadvents are used the exchange air from other parts of the house to cool the room without a minisplit. In that case, you have to make sure you get sufficient air movement between the cool space and your theater to keep the theater cool when it's full of people and whatever heat generating equipment you have. Not as easy as it sounds. Again, you assume that the rest of the house is a comfortable temperature (or even cool) and using that air sets the temperature in your room. Again, a thermostatically controlled fan can also be used as BIG pointed out if you are fortunate enough to have air cool enough to make the theater uncomfortably cool (I can't remember a thread where that's happened, though).

Have you actually gotten an HVAC installer to give you an estimate for adding zone control or are you basing costs on retail hardware pricing? A quick internet search shows the typical minisplits appears to run around $1000 + installation (not a DIY job). When we had our HVAC systems replaced a few years ago, adding zone control for the theater room was about an $800-1000 upcharge, depending on the quote. Obviously, price may vary with the difficulty of the install and my installer may have absorbed some of the zone cost into the overall system replacement cost.

With our zone setup, only clue that the HVAC is running in the theater is if you happen to feel the air flow or hear the relay in the 'stat click...no noise from the vents themselves. :-) You probably want to check out minisplits in operation first hand before you decide if they're quiet enough for you...a video recording tells you nothing about actual volume levels. Speaking of anecdotal evidence, we rented a mountain cabin a couple of years ago that had minisplits throughout...while quieter than a typical window A/C, they were still considerably louder than even my old Infocus X1 projector...certainly louder than I would find acceptable in my theater room and loud enough that we had to ride the TV's volume control as the A/C cycled on/off. Noise will vary by brand and model, though, so maybe this house just had bad examples of the breed. YMMV and caveat emptor. :-)

-Brent

I am planning a ducted mini-split, with the air handler mounted in the attic outside the theater sound envelope. We don't normally air condition homes here, so the theatre will be the only room in the house with A/C. Thus we will have "wooly worm" ducts to to and from the theatre, with the obligatory 4" x 36" Nailor-type bar diffusors for supply and return. This setup also facilitates fresh air exchange. One of the challenges I am working on is that the mini-splits typically come with integrated remote controls, which is a complication when you want to integrate it with your remote controls (iRule, in my case). Still trying to figure out if I can substitute a conventional automation-friendly thermostat.

I disagree with the above DIY assertion.

A mini-split is quite simple to install. The only part most would probably not be able to perform is connecting the refrigerant line since it has to be evacuated and potentially topped off, but that's roughly a $100-200 service call for an HVAC tech. The model you appear to have had experience with must have been very old or inexpensive. Current models are very quiet.

Agreed that running the linesets shouldn't be much harder than pulling cable. As you mention, the refrigerant charge was the main "installation" I was referring to. However, there's also the electrical connections to be considered...while possible to DIY, I"m sure most would rather let a contractor add the necessary dedicated circuits. So figure another couple $100s for electrical. Don't forget to pull your permits, at least in my jurisdiction, for the electrical and HVAC work, so you're probably looking at minimum $500 on top of the purchase price of the hardware.

One also needs to look at the limitations for lineset lengths for the particular minisplit they're considering. You might find you can't place the outdoor unit in an aesthetically acceptable location.

-Brent

I just had a quote for a Mitsubishi minisplit. Our local power company offers a $750 rebate, but even then, the installed price was over $3700...

Are you planning to add a supply?

You have several options for how to control the fan. You can put in a line voltage thermostat as BIG suggested, the other easy options that I see would use a relay to turn on the fan whenever some other component in your theater turns on such as the AVR or PJ. That way you are circulating air whenever the theater is in use. You might be able to setup a relay to turn on a fan when the minisplit turns on, but I would think that would be getting more complicated.

I may be wrong, but I don't think a ductless minisplit brings in any fresh air. So the return wouldn't have anywhere to pull fresh air in without another deadvent just for that purpose. I don't have any experience with minisplits, so I'm just going on the pictures that I've seen.

It's recommended to have four to six fresh air exchanges per hour. If you decide to meet that, then you can run a slow fan the whole time, or a fast fan for short intervals. It doesn't matter. Just however you want to get there. Keep in mind that you'll want to minimize the noise, and faster fans tend to be louder.

A central air unit doesn't move air from outside to inside. It only cools the air inside the house. The unit on the outside expels the heat removed from the air inside the house.

You have to calculate the volume of your room. Let's say 200 s.f. with 8' walls. That's 1,600 cubic feet (c.f.). You need to move that a minimum of 4 times per hour (4 times/ 60 minutes). That works out to 107 cfm (1600 c.f. * 4 times / 60 minutes). Now, you can either get a small fan connected to a small duct that operates around 110 cfm which will likely be loud, or you can get a larger fan connected to a larger duct that moves the same amount of air at a lower velocity. The Panasonic whisper line fans are pretty popular here because they are quiet. If you turn your fan off for half that time, then it will need to run at 2 times the capacity to make up for the time it's off.

The standard that the four to six exchanges comes from is intended to provide "fresh" air exchanges as you've pointed out. I don't think most homes meet this criteria, but there is a larger volume of air to dilute the "stale" air. Opening and closing exterior doors as well as all the vents in the bathrooms and the kitchen vent provide some air exchanges. if your house is older and/or has fiberglass insulation, then you also get a lot of exchange through leaks in your building envelope. Newer houses, and particularly homes with spray foam insulation have little to no leakage, so they rely solely on the air exchange mechanisms mentioned above unless they have an ERV of some variety.

At any rate, the idea is four to six people in a small room will get stuffy pretty quickly. Compared to that, the air in the rest of your home is "fresh." I don't think it really meets the intent of the standard, but it's an oft quoted value, and it's seemed to provide good results for a lot of people on this forum. As an example, you can checkout The Black Cat Theater where Morph1c used dead vents to keep his room cool. He documented quite well the amount of air he's moving as well as testing his room with several heat sources to see if his approach was viable. It's a good resource for anyone looking for cheap alternatives to HVAC.

I've seen this mentioned before, but I can't think of a thread off hand that has done this. The primary advantage is to limit the noise in the room, and, as you said, eliminate the wall clutter. The down side is you have to add a powered supply and return into your room. A ducted minisplit might be just as economical. I really have no idea.

A quiet minisplit will work in the room. You may not even need the fresh air. It just depends on how many people will be in there and how tightly your room is built.

Well, you could round up all the neighborhood dogs, preferably some big ones (think great danes or mastiffs) and lock yourself in your theater for three hours. If you haven't decided you need the extra ventilation by then, well, you're probably good to go without it

yes just make sure it is quiet, Panasonic makes some "whisper" series units designed for in-ceiling mounting in one room, Not sure how quiet they are. This one looks pretty quiet in the low speed mode.

http://www.iaqsource.com/product.php?p=panasonic-whispercomfort-spot-erv-fv-04ve1_panasonic_fv-04ve1&product=171090&utm_source=google&utm_medium=Product_Search&utm_campaign=google_base&gclid=CNfoycWY_LUCFQKrPAodaiwACg

Not to make things more confusing, but here are my experiences.

I installed a mini-split in my theater room. Installed the electrical, wall unit, outdoor condensing unit and copper lineset myself. I had a buddy come over and pull a vacuum on the lineset and then I released the refrigerant charge. Pretty simple depending on how comfortable you are with that type of work. I spent about $800 when all was said and done, but I had a good source for the split system, I isntalled the unit myself, and had a buddy evacuate the refrigerant lines.

A few comments I have on my system now that it has been operational (and used a few times).
-If it is just me and the wife watching a movie, the split system is not used. The heat does not build up enough with just the two of us.
-When I have 4 or more people in the room, its very nice to have. Put it on low and it cools the space perfectly (and it quiet).
-I do not have supply or return vents installed in my room for fresh air. It does not seem to get stuffy in there at all.
-I installed ductwork in the soffit area in case I wanted to install them at a later date. I do not feel it is necessary.
-My house does not have any true "fresh" air coming into the house, infiltration or an open window are the only ways fresh air is introduced. I personally think the ERV is unnecessary.
-For purely aesthetic reasons, if I were to build my room over again, I would install a ducted mini-split. Mainly because a nice painted linear diffuser would look better than the white wall mounted unit.

When you say the room is hooked up to the furnace with 2 supplies and 1 return... is that in addition to another return on the floor? The answer leads to 2 possibilities:

1) If you essentially have 2 parallel loops - supply & return for the theater, and supply & return for the rest of the floor... you're in pretty good shape. I would be inclined to just see how it goes. Air the furnace pulls in from the room will mix with the air pulled from the other return and you'll get "fresh" air, same as dead vent. Zoning a system like that should be relatively inexpensive. When you check prices, explain that you already have dedicated supply & return ducts run to the room and are just looking to make it a separate zone. I don't see a dead vent really buying you anything. A mini-split would be overkill.

2) If the only return for the floor happens to be in the theater room, then things are more complicated. That would mean the 2 supplies in your theater plus all the other supplies on the floor were intended to return through the theater room. In that case, at a minimum you'll want to put in a 1 way passive dead vent that will allow the other rooms to pass their air into the theater so it can be returned properly. If you don't, and your theater is sealed, you'll starve the furnace of return air. In turn it won't be able to supply as much. When you're cooling and have less than the designed airflow there is actually risk of the condensation removed from the air freezing on the coils. The ice blocks more air further compounding the airflow problem. It creates a vicious cycle, performance plummets, and there is risk of damaging the equipment (or having the unit shut itself down to prevent damage). So put a dead vent in and see how it goes. In this case, though, zoning is not as easy because they would have to run additional ducts (to create 2 parallel loops). Depending on your house layout it could range from more expensive to cost prohibitive. A minisplit could help if you placed it opposite the return, but that conditioned air would still make it back to the furnace return. Not a big deal unless the mini-split is trying to cool the room while furnace is set to heat (it would just be really inefficient).

For my room I considered putting a mini-split in a dead vent. That way I could control the temperature of the air coming into the room and control it locally. I decided against all the complication and just went with a conventional forced air heat pump with 2 loops - 1 for theater, 1 for the rest of the basement. I also have an ERV which I agree isn't necessary, but a nice to have in new houses that are tightly sealed. I'm going to see how it goes. My 2nd floor is zoned with the master suite on it's own zone. If I need to put the theater on its own zone, I will, but the HVAC contractor thought it was overkill.

Edit: not saying those are the only 2 possibilities. For example, you might have 1 furnace for the entire house. But I don't feel like going through every possible permutation, so just assumed the 2 most likely cases.

The theater already has a supply and return in it? I'm really confused now.

If I was in your situation, I would look into DIYing a separate zone for the theater if I didn't want to pay someone to do it for me.