HVAC ducts

[ShawnS]

I've been adding on to my house and have some questions on what size I need to account for for hvac ducting. I built a 24 x 40 extension on one end of the house. The down stairs part give me parking and wood storage while the upstairs will be a living room. I will be adding on a new heat pump unit and wandering what size tunnel to frame in for the duct work. I can't put it above the ceiling I don't have enough room so I will be building a tunnel down the center of the room with vents on each side of it. there will be a small room in one corner ( 10x10) to house the air handler and other related stuff. I will be running the round insulated ducting and there will be insulation in the ceiling above it. Anyone here have any suggestions?

 

[Endless Pursuit]

A contractor used 8" round insulated duct to run a 45 foot trunk down the middle an attic that had 6 "drops" 4, 6" and 2, 8" at the far end. Took a couple adjustment in the ceiling mounted registers but it works great.

 

[aplpickr]

Do not undersize the heat pump duct work. Too small ducts will kill your HP compressor. With furnaces blowing 145° air that is ~50° above desired room temperature you can get there with 6" ducts. With HPs blowing 110° air you better plan on 8" ducts. If 103° air blows on wet skin (just out of the shower) you will feel cold even with the room temperature at 90°. A main duct can not properly supply the same size drop as itself along with other drops, 8" yields two 6"s, 10" yields two 8"s, 6" yields two 4"s. Drops should be made with wyes, not tees. 45° bends much better than 90°. Same engineering as with dust collection, moving air is moving air. Proper size of return ducts is just as important as supply air. There should be 2 square inches of NET return grille opening for each cubic inch of air flow (cfm). Grille fin area should be subtracted from grille size. Any less and the system will be noisy! More duct work is expensive, but it is a ONE time expense. Blown compressors will get you about every four years. Higher light bills will get you every month. An unhappy wife will cost you daily!

 

[ShawnS]

Thanks I won't be running the duct work myself I'll leave that to the experts , just want to know about how much room I need to leave him while I'm framing it up. I don't want to give up all that much room but I also don't want to leave the space to small to get the proper ducting in.

 

[aplpickr]

I suggest that you let the duct installer tell you how much room he needs!

 

[JimD]

We had an extra garage added - for my shop - and had the HVAC upgraded for the room over the existing 2 car garage. The total space is about 24x34. The installer ran a 12 inch duct twenty feet into the room where there is a wall for the bedroom. At that point it necks down to 10 inch. The duct is the rigid spiral wound metal and is exposed. That let us have a much higher ceiling. We think it looks good and it works well. The rigid duct costs a little more (maybe $300) but I think it works better and the higher ceiling height was definitely worth it. If that is still an option for you, you might want to look into it.

 

[ShawnS]

Thanks for the suggestions guys, I just talked to the HVAC guy and he will be here later this evening to look things over. I'll then decided what to do.

 

[KenOfCary]

Did you consider a totally self contained unit with heater/cooling and air handler all in one outside unit. It keeps all of the noise outside and is great for maintenance. No inside unit to worry about. And you might be able to get that 10x10 room back. Would make a nice spray booth.

I have such a unit - it used to be what they called a gas pack but is now a heat pump because the cost of LP Gas got so high when I was ready to replace it a year or so ago. It all sits behind the house and a technician can service it without me being there or having the keys to the house. All the blower noise is outside.

 

[ehpoole]

Another option if you are trying to minimize the height of ductwork, but are less concerned about overall width, is to run two or more smaller ducts rather than a single large diameter duct. For example, two 8" ducts will move nearly the same volume of air as a single 12" duct as the two combinations have roughly the same cross-sectional area (within about 11%, or 101 sq.in. versus 113 sq.in.).

You could also use insulated metal ductwork (square or rectangle) for the main run down most of the length of the room, then branch off with smaller diameter ducts for each ceiling register. The square or rectangle shape maximizes the cross-sectional area for a given framing dimension.

 

[ShawnS]

well the HVAC company sent a guy over but that was really waste of time, he had no clue on what I needed or was asking, he is supposed to have the boss call me tom. Hopefully he will be better info than the guy they sent over this evening! That was a waste of 75 bucks.

 

[Charlie]

And they are the "experts"?:gar-La;
I think I would be looking for another contractor.

 

[manfre]

Another option is to get a ductless mini-split system. You'll lose about 3 sq. ft. of wall space and 3 sq. ft. of space outside for the condenser. I had one installed for a 24' x 32' room above my garage. This was cheaper than running a ducted system.

 

[ShawnS]

Manfre

How did that mini split work? does it do a good job of heating a cooling? I have looked around some and those seem like a good option. Is it hard on energy bills? did you do a single inside unit or multiple? How big of a unit did you use? your room is not much smaller than mine.

 

[manfre]

Manfre

How did that mini split work? does it do a good job of heating a cooling? I have looked around some and those seem like a good option. Is it hard on energy bills? did you do a single inside unit or multiple? How big of a unit did you use? your room is not much smaller than mine.

It's been working well and has no problems keeping the room comfortable. The only maintenance so far is cleaning the filters for the wall mounted unit, which is much easier than replacing the filter for the house's heat pump. The room is well insulated with R-38 ceiling, R-15 walls, R-30 floor, and also have foil for the roof and gable ends. The garage has an insulated door and R-15 in the walls. When I leave for work in the morning, my car reports the garage as 15-20 degrees warmer than outside. It is a bit confusing when I see ice on the streets, but my car's outdoor thermostat hasn't yet walked its way down in to the 40s. The unit is sized properly for the room and doesn't have to work that hard because of the proper insulation. Your space might be large enough that it needs a higher BTU unit that needs more than the 15-amp breaker I have for mine. Your HVAC company will know the proper size.

I have a single wall mounted unit installed on the short wall and I can feel the fan on the opposite side of the room. The outside condenser sits on a half sized concrete pad on the ground outside with the necessary piping run up through the framed 2x4 wall. I didn't notice any increase in my electric bills, except for the last one. It was noticeably higher, but I think it might be a result of my wife setting the fan to only blow at the ceiling, instead of oscillating, for most of the month. Her sewing machine is in front of the unit, so I don't blame her for not wanting a fan aimed at her while she tries to get stuff done. The thermostat is currently sitting on the knee wall on the other corner of the room, so the unit had to work overtime trying to get the heat to fill down 5'.

I have this unit: 18,000 BTU Mitsubishi 19.2 SEER model-MUZGE18NA-1 mini split heat pump (pdf)

 

[Cbozz]

I too would suggest a mini split. Daikin even makes one with a split coil and a humidistat to control humidity as well - Quaternity line. Fujitsu and Mitsubishi have some super efficient models as well.

The real benefit to a mini split is that there are fewer ways for the installer to screw up. I'm an architect, and we have tremendous problems finding knowledgeable HVAC designers and installers around here. Most don't know how to properly run a manual J, much less anything about matching equipment or sizing ducts. With mini splits, we mostly just have to get the loads close to correct.

 

[gritz]

The mini-splits often provide more air movement than most folks find comfortable, particularly at the beginning and the end of a cycle when temperature differentials are greatest. Given the size of the space, and not knowing the cubic volume, exposure, insulation, etc. I would likely opt to heat/cool both levels with a small split system.

 

[ShawnS]

The entire space is well isnulated, r 38 under the plywood roofing which has 1 inch foam between it and the tin roof, r 19 walls, r 19 between the 2 floors, all seams caulked up, 4 small windows upstairs all sealed up very well and no windows down stairs only 2 doors. At this time I don't think I'll be putting anything from this system downstairs as it does get some heat for the propane furnace that heats the garage part downstairs its attached to so even on nights with single digit temps that part normally stays in the upper 40's low 50's. which I can warm up to 70's quickly with a small propane heater when I need to. But the upstairs part will eventually be livign space so I would like to keep it in a decently comfortable temp year round.
The more research I do the more i'm leaning toward the mini split system something the 2 1/2 ton range. Anybody have recommendations on either a single inside unit or more than one?

 

[Cbozz]

Not having done the math... I'd be shocked if you needed 30k BTU/hr (2.5 ton) for that space in VA...that's less than 400sf per ton. Oversizing is less of an issue with modern inverter based mini splits (the whole thing will just dial down as needed), but I'd bet you'd be fine with significantly less, rough guess 1 or 1.5 ton as long as you have a good handle on air leaks.

Try going through this to get a rough idea of your heating/cooling loads:

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-1
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-2
http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-cooling-loads

In most of NC, it's the cooling loads that drive equipment selection. I'm pretty sure VA will be the same.

Single unit vs multiple: is it one big living room or are there bedrooms included as well? One big room, one unit is fine. If there are rooms that will have closed doors for extended periods of time, they can vary in temp from the main area due to lack of airflow. You can use additional units knowing they will be oversized, or use fan based transfer systems to move air in and out of the closed off rooms.

Also, if this space is as well sealed as you say, you should think about ventilation. Spaces over a garage will collect air pollutants pretty quickly.