hvac ductwork

[astidham]

After getting another out of this world estimate for hvac system install, I have more motivation then ever to do my own total install less the final conection and pull down.
my addition has a total of 1696 sqft. I want to use ridgid duct throughout. My ductwork estimate by itself was 4000.00 and system install was around 10000.00 the unit to be installed indoor and outdoor unit cost 3600.00 delivered to my house.
not to offend anyone, but my engineers design aircraft wing assemblies and other components at less cost per hour.
The sheetmetal house quoted the duct fab at under1200 dollars. Who makes 9,000.00 on a 2 day job? Just venting....
anyone here do there own ridgid duct?
I am not going into this blind, I do have the hvac manual d, and have been doing manual j research, and studying up on energy loss to design. Most local hvac guys do no audits, and none have thought that more than a filtered return at the unit was necessary,  and these are the pros.

[rick91351]

Manual J cost us around a grand - or a little more that I thought was ridiculous.  Then they had to have a guy show up before we finaled and had a weird expensive machine that measured each vent and compared to the paper work and they adjusted the gate vales.  That was included in the HVAC bid but to me this whole energy efficient thing looks like some ones brother in law had his hand out in this mess.   

[Onkeludo2]

I have done some limited rigid duct work.  If you designed or had designed a balanced system, the actual install is not that difficult.  The only hard part is the final plenum and mating to the air handler.

A nibbler or drill-type electric shear makes thing easier.

You should not need an actual break for anything except potentially the turn into or out of the plenum.

Try to use a  flex damper between the air handler and duct work.

Duct sealant is harder to work with but lasts forever...AL tape is easy to work with but more likely to peel due to poor installation and dirty surfaces.

All this assumes you are not wanting to use insulated duct board for any of the system.

[MushCreek]

Have you considered a mini-split? Our top-of-the-line Mitsubishis were only $1600 each, no ductwork, and easy to install. I did my calcs with a $50 program, and the minis and the house are easily exceeding design specs. They are virtually silent inside and out, and make heat down to -13 F.

[astidham]

-13 wow, that is amazing! I have considered mini split units, my only issue is conditioning smaller spaces like bathrooms.
how did you handle these area's?
thanks,
Todd

[Onkeludo2]

Most folks just leave the door open when not in use.  Are you more concerned about heating or cooling these small spaces?

If heating is the concern, electric in floor heating (along with electric water heating) are the only places that I find electricity to be effective and efficient for heating....all you need is a little added comfort.

If cooling is the issue a transom vent and an inch taller than normal door bottom gap can solve most concerns...along with leaving the door open when not in use.

I am a HUGE fan of min-splits for DIY as the pre-charged kits require literally no skills you do not already have, they only require a 3" hole, and you have useful effective zoning.  That being said, they suck in climates that would require heating that exceeds the normal operating range of a heat pump.

[astidham]

Hey Onkeludo,
both really.
I currently have a window heatpump in the cabin right now, it keeps the bathroom tolerable in the summer, but heat transfer is not so great. Im still looking at ducting the addition, and getting a efficient system, but I am now looking at the minisplit option.

[MushCreek]

We keep the doors open throughout the house unless a room such as a bathroom is actually in use. Our house is ICF, 1400 square feet, and one mini keeps the entire house very comfortable throughout. We have a single 12K unit in the open back of the house, which is a kitchen/DR/LR that are open to each other. The adjoining master suite stays within about 1 degree F. with the doors open. At night, we shut the 12K off, and run the 9K in the master suite. So far, we've only been down to 17 F. this winter. According to the Manual J program I used, I'll have to run both units if it gets down to single digits. I'm sure the insulation and thermal mass of the walls helps a lot with the even temperature throughout.

The also make ducted minis, where one interior head can be ducted to multiple rooms, as long as the duct runs are short. For some reason, they don't have the same efficiency and low temperature performance, though. They also make units that can run multiple head units off of one compressor, but they cost more, of course.

[flyingvan]

I did our HVAC in the big house (3k sq')...Installation wasn't difficult but you really have to be mindful of two things:

1) Calculations.  The system has to be the correct size and the ducting has to be designed for the correct airflow over the heat exchanger.  Too much flow and the air won't warm enough.  Not enough and you burn up the unit or worse---crack the exchanger and CO poison everybody.  Do not oversize or undersize the unit.

2) Airflows.  In every single room you have to consider how the conditioned air is getting in, and how it's getting out.

   If you're good with tables and graphs you can design it all yourself.  You might find a professional to design it all for you then you take it from there.

   Be really careful working around sheet metal!

  The 96% AFUE systems are great.  You vent them through plastic pipe out the nearest wall.  These units produce lots of slightly acidic water you have to drain somewhere (our manzanitas love it)
   
    When you buy the unit get a few spare ignitors.  They last a few years and tend to fail on the coldest night of the year when the in-laws are visiting.  Easy to replace, hard to find locally.

[Onkeludo2]

    When you buy the unit get a few spare ignitors.  They last a few years and tend to fail on the coldest night of the year when the in-laws are visiting.  Easy to replace, hard to find locally.

Depending on the unit, add the draft "sail" sensor to the list.  If you have 225#'s of dogs like i do, even with high quality filters, hair can clog them up...much faster to swap the spare and clean the original than sit int he freezing cold figuring out how to clean the relatively delicate sensor.

[db4570]

Do it yourself! It's very straightforward, once you figure out your calculations. I spent many evenings studying everything I could about HVAC ducting, and overthought it a lot.

We replaced our entire heat pump unit including th eindoor heat exchanger (essentially the same as a central air unit, as far as installation) when the outdoor unit went kaput. I bought the entire setup from Ingram's (http://ingramswaterandair.com/) and had it delivered for under $1800. Found a guy to install it for $600.

I did all the ductwork. This involved tearing out the existing duct in a large family room addition that worked terribly, and was very loud. I redesigned the furnace plenum and connected the house ducting to it. For ducts I used insulated flex duct. Originally I was going to use hard metal duct, thinking it would be better somehow than the flex. But the flex was so cheap, and so easy to install, that I went with it and couldn't be happier.

I did have to buy some rigid duct for the furnace branches and odds and ends in the rest of the house where I also made some improvements. This stuff looked inexpensive at first, but adds up fast. I probably spent $300 or so on rigid duct and fittings. I didn't have any sheet metal fabricated except for the plenum, which I think cost about $150.

The system works far better than I imagined it would. It is so quiet in most rooms we can't hear it running. It warms the family room addition beautifully. Our heating bill has gone way down.

I spent many hours learning about this stuff, which became a bit of an obsession. It turns out I made it seem more complicated than it needed to be. I am happy to help you in any way I can, and enjoy the planning part of it. Let me know if I can help.

David

[astidham]

David, that you for your kindness and encouraging words. I am in the process of drawing out a duct plan. I will post it when I am done.
Thanks,
Todd

[Onkeludo2]

Last word of advice..if possible, oversize the ducts (no more than one size) and add accessible dampers.

[astidham]

Thanks Onkeludo2

[db4570]

These might get you started:

http://www.johnrwhite.net/three%20steps%20to%20duct%20sizing.htm

http://www.johnrwhite.net/ductwork%20myths.htm

https://www.rses.org/assets/serviceapplicationmanual/630-148.pdf

http://efficientcomfort.net/ToChart4.html

David