HVAC Duct system?

[gitmo234]

So, I have a ranch style house, and the duct work makes a sort of a cross (the main supply) like this

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With small ducts running off the trunks into the registers. In that figure North is UP.

My propane furnace is in the center. My Harmon PF-100 is on the extreme north end.

Last weekend I did a complete inspection of my duct work because my basement was about 90 degrees. I found that the north side ducts were all flex duct except one. One of them was chewed entirely through, so it was spilling heat into a crawl space. The non-flex duct wasnt even connected. There was about a 2 inch gap between the trunk and the 7" pipe and the pipe went to nowhere. The register was under the floor and had been smashed to shut it. No hole in the floor.

Going south my ducts are all metal and uninsulated (even in the unconditioned space), and there were a few similar gaps and wholes in the piping.

I found a few other disconnected pipes off the main trunk.

So after a run to home depot, I bought several caps, HVAC duct tape (shiny foil), some flex duct connectors, and sheet metal screws. I fixed or capped all the holes, re-attached some of the ducts, and capped off a few I dont use.

The goal was to increase air pressure in the vents because some on the extremes (east and west) had almost no airflow. This seemed to work.

Now, I have half a mind to convert all flex duct to metal (insulated) and sort of re-work the ducting. I have no HVAC training.

Also, I'd like to replace my propane furnace with the harmon entirely. And install the central air coils above it.


Does anyone have any experience or guidance they'd like to share on this potential project? Any help or resources on load calculations? My home is 2500 square feet. I've re-insulated the attic throwing R-30 on top of the blown in, which went up and down from R-30 to R-19 in depth.

I'm actually considering enrolling in the local community college to take an HVAC course or two since it would be paid for by the national guard. I get obsessive over these things and want to jump in head first.

Side note, my harmon thermostat is in the master bedroom, which has four registers... Since my capping, my basement temp has dropped drastically (almost chilly with ducts closed), and my bedroom is now so hot that the rest of the house stays around 68 degrees when the bedroom is at 73 or 74. I may move one of those registers into the master bath which is typically colder. It woudl be about a two foot move.

 

[semipro]

I'm not expert on this but I'll dive in to get the discussion started.

- Consult an HVAC forum. There are some good ones out there. You've probably already done this.
- Look up "Manual J". It may be a good starting point for you. You use it, or software based on it, to calculate heating and cooling loads
- Consider putting your Harmon in series with your gas unit in a single HVAC system. That way you'll have the flexibility of using either.
- Placement of your return(s) is critical especially as related to potential circulation of combustion byproducts.
- Move ductwork into conditioned space if possible. If you can't make sure ducts in unconditioned space are well sealed and insulated.
- The centrifugal blowers used on air handlers are designed to work efficiently within a range of flow rates. If you get outside that range by not matching your ductcwork to the blower, you may have problems.
- There are some pretty cool aftermarket systems that use inflatable bladders installed within your ducts (and maybe a variable speed blower) to regulate flow to individual zones/rooms. These may be useful for your situation. They use a central control unit with temp sensors in each zone/room.
- While working with HVAC duct is not that hard, designing a system that is effective, efficient, and safe is not easy. Many HVAC contractors seem to have a problem doing it. You may want to get professional help, at least for the design.

Edit: Here's a link to one company that uses the duct bladders and central processor for control of temps in different zones/rooms. http://www.retrozone.com/?gclid=CM2U9JjKtq0CFScRNAodzlcxlQ