Heating and cooling issues in finished room over garage: mini split?

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Jasonir129

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Hi all, back to get advice on another part of my house with HVAC issues. Thanks for all the previous help -- it all worked out great so far (installed mini split in sunroom)!


We have a large finished room over our garage. Details on this space:

- 33' long x12' wide
- sloped ceiling from 4.5'-8' with the flat part up top roughly 60" wide
- windows on the east and west facing walls
- R19 insulation in walls and R39 in ceiling
- insulation in the floor of unknown R value
- climate zone 4 in MD

The house was built in 1998 and the builder had some hack do the ductwork which is a total mess. The duct run from a heat pump in the attic to this space is roughly 60' long and was previous served by 6" flex ducting. I hired a company to come and replace the run with larger diameter (either 7 or 8”) smooth walled solid ducting and add a return (previously there was none in this space). They assured me it would improve air flow... it didn't. You can feel a small amount of air coming from the supplies, the return hardly seems to do anything (can't hold a piece of toilet paper to the grill). This didn’t really improve the situation – on really hot or cold days this room can be +/-15 degrees from the rest of the second floor (and feels very humid in the summer). The company that ran the ductwork is now saying they could add dampers to our mess of ducting to divert more airflow to this room but said the best solution is to add a mini split instead. Not sure if we should waste more money experimenting or just move on and install a mini split. I’m leaning toward the latter.
 

Jasonir129

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Here are some images of the space. Note how there isn't enough space over the window for a wall unit and the sloped walls also prevent using a wall unit. The main frontrunner is to install a low wall console 1 ton unit like the Fujitsu 12RLFFH either to the left of the window or on the other wall (see image with boxes labeled 1 and 2, both are relatively easy for running line-set and drain etc.). Does this seem like a reasonable plan that would fix our HVAC issues for this area? Would the entire room volume be treated including the other end (not pictured) where I have a desk setup for working (we don't want outside unit on this side of the house for aesthetics plus other side is easier to run power to)?
 

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Breplum

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Dampers definitely are appropriate in that it can help balance a whole house, room by room. Of course, rooms demand can vary between heating and cooling, so it is never perfect, just an averaging contribution.
For a 33' long room, you'll never get real comfort at the far end of the room because the 'throw' can't get there. An additional free-standing fan will help
 

WorthFlorida

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Look at the air handler installation or user manual. Some will allow fan motor speed settings. Most new ones with an ECM motor are variable, older units may be three speed. Increasing fan speed may help if the unit is sized correctly. Modern HE seer units work best with large duct work, 18"x18" main trunk. And branch off with smaller ducts.

If this is one unit for the entire home, where is the air handler? You want it in the attic or second floor. At lot of heat from the first floor raises to the second.

From everything else that has been going on, a mini split maybe the best option. They are quite, very efficient, cost wise usually reasonable. Depending on size it can operate on 120 volt. You do not need to run it all the time as the rest of the home. The only down side might be the lines running up the side of the home. Advantage is you are taking heat load away from you main unit and should run less. Winter can provide extra heat during deep freeze warnings. Pushing more air through a furnace plenum is a big restriction So changing fan speed may do nothing. Breplum made a good point that 33' long room be tough to get air that far down.

My sons house just down the street from me here near Orlando, Fl. His two story house had some issues. The second floor unit did not cool down the furthest bedroom. The fix was a new larger duct to the room via attic space. His first floor office, the furthest from the first floor air handler could not increase duct work, added a mini split on 120 volt.
 

Jasonir129

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@Breplum and @WorthFlorida , thanks for the feedback so far. I don't disagree with any of your points. I can imagine the dampers helping, but is it going to make the improvement needed. I'm skeptical. I asked about adding an inline duct fan and the pro said it's likely to be ineffective. I do wish they did larger diameter ducting to the room but there were some tight space constraints that the ducting had to pass through.

On the mini split pushing to the furthest parts of the room -- that's why I was leaning toward position 2 in the image above. It's a little closer to the middle. Only issue is the direction it's aimed. I have 2 ceiling fans (still need to install second one) that should hopefully help with the mixing and reaching the ends?
 

John Gayewski

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You first need to see if your system is even capable of heating and cooling the house with this space, this would require a room by room heat loss calc.

There really should be no guessing on what a particular system can heat or cool its actually just math using known (or measured) variables based on your building and the system installed. After the calculations you can then make an informed decision on what to do.

For instance there's a possibility that diverting enough air from your current system to this room will leave a shortage on other rooms in the house, but there's no way to know without measurements. If your hvac contractor doesn't know how to do this I would find a new one asap. Competent professionals from the start on this one might have saved some money and annoyance.
 

Jasonir129

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You first need to see if your system is even capable of heating and cooling the house with this space, this would require a room by room heat loss calc.

There really should be no guessing on what a particular system can heat or cool its actually just math using known (or measured) variables based on your building and the system installed. After the calculations you can then make an informed decision on what to do.

For instance there's a possibility that diverting enough air from your current system to this room will leave a shortage on other rooms in the house, but there's no way to know without measurements. If your hvac contractor doesn't know how to do this I would find a new one asap. Competent professionals from the start on this one might have saved some money and annoyance.
I'm familiar with the manual J and went through the process for an addition we did in another part of the house. I think it's unrealistic to do a fine grained calculation at this point and not really sure what I would gain. Probably more beneficial could be something like the HERS test to get a baseline on available airflow etc., but not sure anyone does that around here (my frustration with my HVAC company is they're generally shooting in the dark on the airflow problem, but I think it's more an issue of no return flow and I'm guessing there's nothing we can do about that?). The room has been connected to the system for the life of the house... that doesn't mean the system or duct work was properly sized back then, but it is what it is. The rest of the second floor is well served, just this one room is insufficient which is why I'm leaning toward going the mini split route unless anyone sees any other realistic paths to avoiding the mini split that doesn't involve replacing the heat pump and duct work.
 

John Gayewski

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I'm familiar with the manual J and went through the process for an addition we did in another part of the house. I think it's unrealistic to do a fine grained calculation at this point and not really sure what I would gain. Probably more beneficial could be something like the HERS test to get a baseline on available airflow etc., but not sure anyone does that around here (my frustration with my HVAC company is they're generally shooting in the dark on the airflow problem, but I think it's more an issue of no return flow and I'm guessing there's nothing we can do about that?). The room has been connected to the system for the life of the house... that doesn't mean the system or duct work was properly sized back then, but it is what it is. The rest of the second floor is well served, just this one room is insufficient which is why I'm leaning toward going the mini split route unless anyone sees any other realistic paths to avoiding the mini split that doesn't involve replacing the heat pump and duct work.
So you know the unit can heat and cool this space ALONG WITH your house sufficiently? But it's not? If that's the case then balancing is gonna be cheaper and easier. Not sure why you'd add more to a system that you know is big enough.

A system can only deliver btu's (or wring them out) as much as air can be delivered (with a hot/cold air system). So if you're system can handle what's been measured then why add to it?

Either way guessing isn't the way to go.
 

Jasonir129

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So you know the unit can heat and cool this space ALONG WITH your house sufficiently? But it's not? If that's the case then balancing is gonna be cheaper and easier. Not sure why you'd add more to a system that you know is big enough.

A system can only deliver btu's (or wring them out) as much as air can be delivered (with a hot/cold air system). So if you're system can handle what's been measured then why add to it?

Either way guessing isn't the way to go.
In theory it was supposed to heat and cool the space with my house sufficiently but it doesn't mean it was accomplished (or properly designed in the first place). Adding in dampers and balancing the airflow is roughly half the cost of adding a mini split and I was not given a guarantee that it will solve the issue. The current system is likely insufficient. Plus balancing out the airflow with dampers will likely only improve the supply of conditioned air, there isn't a clear way to improve the return in that space (there's a lot of humidity that sits in the room). I've spoken with several companies around here about the issue and they all insist that a mini split is the solution. I don't see a path to getting a pro who is going to get better performance out of my current setup. It's likely not cost effective. I'm an engineer myself -- I understand that guessing is a crappy way to approach problems that can measured and modeled, but that process isn't cheap and I have yet to talk to an HVAC pro that wants to actually deal with the issue in a systematic way.
 

John Gayewski

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In theory it was supposed to heat and cool the space with my house sufficiently but it doesn't mean it was accomplished (or properly designed in the first place). Adding in dampers and balancing the airflow is roughly half the cost of adding a mini split and I was not given a guarantee that it will solve the issue. The current system is likely insufficient. Plus balancing out the airflow with dampers will likely only improve the supply of conditioned air, there isn't a clear way to improve the return in that space (there's a lot of humidity that sits in the room). I've spoken with several companies around here about the issue and they all insist that a mini split is the solution. I don't see a path to getting a pro who is going to get better performance out of my current setup. It's likely not cost effective. I'm an engineer myself -- I understand that guessing is a crappy way to approach problems that can measured and modeled, but that process isn't cheap and I have yet to talk to an HVAC pro that wants to actually deal with the issue in a systematic way.
Then it sounds like you've made up your mind.
 

Jasonir129

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Then it sounds like you've made up your mind.
That's not helpful. I didn't make up my mind -- I just don't have this option near me. No HVAC companies come out to do this sort of work unless your system is down etc. Is there a different type of HVAC professional I'm unaware of? This isn't my decision; I'm all about working with what I have and not adding more equipment unless necessary but don't see another feasible path.
 

Jasonir129

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Anyone have experience with inline duct fans or duct booster fans? It seems like it could be helpful for overcoming all of the losses from the long run of ductwork to the supplies and return. Potentially a combination of dampers and inline fans on supply and return? I have relatively easy access to the first 1/3-1/2 of the overall supply and return ductwork. The HVAC pro I asked about this discouraged this but didn't really have a great answer for why.
 

Jasonir129

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Spoke with a fantech tech and they recommended this fan (FG8EC 49901) for the supply and return lines and use the potentiometer to dial in the speeds. Up to 400cfm so it could bring a good amount of air movement and displacement. Any thoughts on this route?
 

Fitter30

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Mini with two ceiling fans blowing down can adjust the modulating damper fixed position to blow straight out have the mini wall unit high.
Duct fans might work with one run but not anymore. Damper system the whole system would have to be design for it. Two or story and a half really needs its own system to make it right.
 
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WorthFlorida

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Spoke with a fantech tech and they recommended this fan (FG8EC 49901) for the supply and return lines and use the potentiometer to dial in the speeds. Up to 400cfm so it could bring a good amount of air movement and displacement. Any thoughts on this route?
It might be worth a try if not too expensive. If it is after another register, it can pull air from the other room.

The big question here is can you're cooling plant able to meet the heat load. For cooling, slower airflow is wanted to across the coils to extract moisture by dropping the air temperature below the dew point. Most of the energy is used doing just that, removing latent heat. In a perfect world, the maximum temperature drop across the coils is about 18 degrees.

I know you stated you're an engineer but it can get too easy looking for simple solution. If you're in a jam needing to use this space, for $200 get a 6k window rattler to get you through the season if there are any left.
 
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