HVAC: Aftermarket Cold Air Return

[ZomblibulaX]

Bear with me, tough to explain:

Casa de Z has a converted garage/mother-in-law suite, with a separate entrance (~200 sf). Floor is stick framed across the stem walls above the garage floor, so no room to get under it anywhere. When it was built, previous owner punched a hole in the stem wall and piped in heat ducts, but there's no cold air return in the unit, which means the heat isn't very effective (it's ~10 degrees colder than the main house all winter). Furnace is in the crawlspace on the other side of the house, ~20 ft away. Question is, what's the best way to put a cold air return in the unit? The hole through the foundation for the ducting is only big enough for the ducting, and is near a bearing element so I'm reluctant to make it any bigger. I'm wondering if there's a compact way to install a cold air return that will increase air circulation without forcing me to make giant holes in the floor. Can I just install a standard floor register and pipe it in to the main cold air return? I think I can squeeze in another 4" duct without blowing open the hole in the wall any further.

[SnowMachine]

You just need to make sure the room isn't pressurized. If you open the door back to the house is it better? If not, then you probably have further issues with that room: Quantity of heat, quality of insulation, etc.

There are calcs to look at CFM/SQ.FT. In commercial application we always sized appropriately and balanced. Residential rarely gets balanced or even has balance dampers.

Generally speaking, rooms in homes don't have returns. Doors are undercut enough to allow return. Space heater is probably your best bet.

[nickwm21]

If the spacing of the return to other supplies isn’t right you could “short circuit” the room.

Is the cost of running a space heater less than the “cost” of spending a weekend on your belly in the crawl space?


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[I Skied Bandini Mountain]

No access up top?

[ZomblibulaX]

Guess I should clarify, the unit has a separate exterior entrance, no connection to the main house other than a shared wall. So yes, it is effectively pressurized.

There is a damper in the junction box that services the unit, and it's wide open. There's air going in, but without any air going out it isn't very effective. Space heater is functional, but you'd be surprised what it's worth to me to not have to listen to my MIL bitch about how cold it is when she comes to take care of the kids.

No attic space; roof is a 1/12 pitch with vaulted ceilings. The place was built in the '50's, so not our finest hour of carpentry as a country.

The only reason I don't want to punch a hole in the wall between and install a louver is that its already pretty loud over there, especially with two kids in the house. I was hoping to find a way to increase air flow through the unit without increasing noise from the main house.

That's why I'm wondering if I can't just put in another standard floor grille, jury rig some filter material in it, and run it back to the main cold air return for the house.

[El Chupacabra]

Is this space not occupied regularly? If not (and I know this isn't what you're asking for here), I'd vote for a large radiant oil filled space heater. Reasonably efficient.

[ZomblibulaX]

It's not, just for guests. But when my MIL is in town it's usually for 2+ weeks, and she's constantly griping about it being cold over there. If I crank up the heat in the main house to 75, it'll get up to about 64 over there, and then no one else in my house can sleep at night for it being too hot.

A space heater totally works, it's a small space. I just prefer permanent solutions.

[I Skied Bandini Mountain]

MIL's gripe about shit, tell her to wear a sweater.

[Core Shot]

A separate thermostat and zone valves are needed.
Even if you get a return air.

Return is not the problem.

Easiest solution is portable or permanent space heater.
It’s an old garage. With a concrete slab. Seems like you have sleepers and insulation. But it’s always colder.

If you’re on gas then install a cool fireplace feature. Or an ugly wall furnace.
If electric. Then resistance or ductless split

[Rideski]

Radiant heat under flooring?

[nickwm21]

Can you easily get 240v there? A baseboard heater would do it nice and clean.


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[dmcd]

If it's truly just that room is so well sealed it can't heat because it's pressurized what if you just put a through wall exhaust fan or ERV/HRV to pull the room air out. Added benefit that then you can use at as ERV/HRV for that separated space in other seasons. Does the converted garage share a wall with another converted space that does have a cold air return?

[Danno]

I'm no HVAC guy, and in fact I'm not handy at all. But you mention the room shares a wall with your house. Would a simple vent in that wall make the room not pressurized, and create a passive cold air return?

[dmcd]

That's what I was getting at above. If that wall between garage and house is just a normal framed wall just put a return air pathway through it to balance the room pressure. They're $100 on Amazon.

[I Skied Bandini Mountain]

He's worried about sound transmission which is a valid point.

I think the electric baseboard is the way to go.

[ZomblibulaX]

240 is an option, the circuit breaker is right on the other side of the exterior wall.

In floor is not an option, there's eight inches between the slab and the floor joists.

Through vent = noise. My kids are loud as fuck and I want to keep the option open of renting the unit if all hell breaks loose in my life.

The unit is three small rooms: entry/kitchen, bedroom, 3/4 bath, 225 sq ft. There's a grille under the window in the front room, and one under the window in the bedroom. Previous owner built it out for his daughter while she was in college, I'm just trying to make it more livable.

[Core Shot]

There’s your answer. Either pay for zone dampers or pay every year for electric heat.

At least they put in sleepers to get you up off the slab.

[skinipenem]

Mini split (a/c option) or space heater seems like a good option with the capability of controlling the new 'zone' separately.

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[Hopeless Sinner]

I'd have to go w the supplemental heat. It's the simplest way to insure the ability to control the temperature w the downside being the operational cost. The last curveball of renting it out calls for a separate electric meter. Focus on the main problem and avoid complexity preparing for something that's likely not going to happen.

[Core Shot]

It’s already got one inflow. That isn’t quite enough.
Main house at 75 and garage at 64 isn’t working.

Electric. Whether resistance baseboard or mini split won’t cost that much for the extra few degrees you need.
Baseboard would be way cheaper install. And smarter if you don’t need ac.

[SnowMachine]

Having read the replies, a mini split is really the right answer for the application. Independent temp control and you can shut it down when the space isn't being used. AC during the summer.

Temperature balance on a single zone system is always finicky. Sun exposure, exterior temp, etc...

Electric baseboard is the cheapest/quickest bandaid. Least efficient.