Replies

1. 
   Tim_Rice | Feb 14, 2002 03:29pm | #1

   *
   Brian,

   You might want to try some of the Engineering trade publications and organization websites.

   HPAC(Heating/Piping/Air Conditioning) Engineering (www.hpac.com)

   Engineered Systems (http://www.esmagazine.com/) or

   ASHRAE, the American Society of Heating, Refrigeration and Air Conditioning Engineers, Inc., (http://www.ashrae.org/).

   Lots of good info, though you will not find any canned "rules of thumb". Those are for contractors and estimators, not designers.

1. 
   Mike_Smith | Feb 14, 2002 08:43pm | #2

   *:-) mike

1. 
   Luka_ | Feb 15, 2002 04:01am | #3

   *:-)Luka

1. 
   Bill_Hartmann | Feb 15, 2002 04:48am | #4

   *:-)Bill

1. 
   calvin_ | Feb 15, 2002 02:20pm | #5

   *ha ha ha.

2. 
   Tim_Rice | Feb 15, 2002 03:36pm | #6

   *Inside joke?

3. 
   calvin_ | Feb 16, 2002 02:23am | #7

   *Nope.

4. 
   Brian_Smith | Feb 16, 2002 08:28am | #8

   *;-)brianps. they are just a means of bookmarking threads where the contributions are right on and valuable to me. this way i can run a search that includes my name and search term and the result includes only the threads i know to be definitive, instead of the entire archive.

5. 
   Brian_Smith | Feb 16, 2002 08:29am | #9

   *pss. love you guys... brian

6. 
   calvin_ | Feb 17, 2002 04:11pm | #10

   *Memory isn't what it used to be eh brian?

7. 
   Art_B. | Feb 18, 2002 09:10pm | #11

   *My favorite "rule of thumb" for DIY ducting - put in as big of ducts as will fit, as big of blower with most speed selections as you can, use dampers to adjust for system balance and lowest noise by trial and error.Till the mid 1960s, the trial and error method was even how aircraft avionic rack cooling airfow was adjusted.

8. 
   Brian_Smith | Feb 21, 2002 07:43am | #12

   *as a matter of fact, no. brian|:-)>|

9. 
   Larry_Schuette | Feb 23, 2002 02:29pm | #13

   *Go over to alt.hvac and ask. Do it in the winter and the flame throwers the pros will use on you will keep you warm for days. In the summer, the icy daggers they hurl will keep you comfortably cool... As near as I can tell, HVAC is the last of the guilds that protects its secrets (sic). Buy a copy of Althouse's "Modern Refrigeration..." It's the book the trade uses to teach the subject from. Pretty simple stuff. If you can use an Excel spreadsheet and apply some common sense you can calculate the correct duct sizes and overall system size for your house. Note that the epa won't allow you to install the A/C unit, but that's the easy part of the job. YMMV

10. 
    Tim_Rice | Feb 23, 2002 07:54pm | #14

    *If you want a text book, the Air Conditioning Manual by Trane has been in publication sine 1938 and is excellent, also the ASHRAE Fundamentals is an excellent reference. The entire ASHRAE series is a must if you design HVAC systems for a living, free with membership.

11. 
    Brian_Smith | Feb 24, 2002 02:19pm | #15

    *found this free demo for anyone else interested...http://www.wwwebworks.com/hvac/newfeatures.htmbrian

12. 
    bckelly1 | Feb 28, 2002 04:14am | #16

    *Brian,I'm with Larry and Tim. We have a GSHP that the manufacturer wouldn't allow us to DIY. Our HVAC sub proposed ducting that was clearly inconsistent w/ GSHP unit literature. So, went to the library and dug up the ASHRAE Fundamentals book for a little education. Paired this with our own duct plan and a large EXCEL spreadsheet and now have a nice duct system. I'd warn you that it doesn't go very fast though if you've never done it before.BCK

13. 
    Brian_Smith | Feb 28, 2002 05:44am | #17

    *thanks everyone. ashrae fundamentals handbook $70 at eBay. probably more than worth it, but not in my budget at this time. looks like a library trip is in order.BCK... are you willing to share the spreadsheet?brian

14. 
    bckelly1 | Feb 28, 2002 02:47pm | #18

    *Brian,You're welcome to our spreadsheet work, but it'll only be of marginal use since it's specific to our duct plan. We used two; one for supply and one for return. Never done this, but I'll try to attach one.

15. 
    Tim_Rice | Feb 28, 2002 03:52pm | #19

    *Brain,Ever hear of a Ductulator? Its basically, a piece of cardboard with a wheel and scales on it. Talk to someone like your local Trane factory rep. They'll probably give you one for free. If you are sizing ductwork, its indispensable. For any size duct, rectangular or round, this thing will tell you, for any flow rate, velocity and pressure drop (per 100'). Similar devices exist for heating piping. All you need then is the criteria you plan to use.

16. 
    bckelly1 | Mar 01, 2002 01:17am | #20

    *Tim,I bought a Leslie Locke brand air duct calculator at HD for about $5. I'll point out right off that this is not my business, so I'm not an expert. But, it looked like this was a useful tool but couldn't give the result I was looking for. My GSHP unit is rated for a maximum external static pressure of 0.3 in WG. I've looked at a few of these "ductalators" and it didn't appear possible to actually determine static pressures soley with them. It had some nice info. printed on it, some nice conversions like round to square equivalence, but just didn't seem adequate. This is why I ended up playing with an EXCEL spreadsheet; so I could size my supply and return in a manner that limited external static pressure to 0.3 in. If this was not necessary, I'd be glad to know why?

17. 
    Brian_Smith | Mar 01, 2002 06:30am | #21

    *tim,that was the gizmo that i was looking for...thank you, bck1. the built in calculations are valuable. thank you for providing the groundwork.brian

18. 
    Tim_Rice | Mar 01, 2002 04:02pm | #22

    *bck,System design, duct sizing and layout is my job. Typically, starting from scratch, I size ducts using the ductulator at 0.05"/100', return registers at 500 feet/minute and supplies at 700 feet/minute or to get the throw or noise level required. I did not look at you spread sheet, but I have created some just to help add up losses and see the affect system changes quickly. Other than maybe reading it of a chart or graph, I know of no other way to determine friction in ducts than a ductulator. (There are softwares out there that will do all of this automatically) Filters, coils, economizers, curbs, dampers, registers and diffusers all have tabulated data for losses.To determine the total external static in any system you need all this data and a detailed layout. There are tables that show equivalent lenghts for turns, takeoffs and boots.In your case, a dirty filter and the coil will take up 90% of that 0.3".

19. 
    bckelly1 | Mar 02, 2002 05:00am | #23

    *Tim,Thanks for giving us the perspective of someone who does this type of thing full time. Looking at my ductalator I guess I see what you mean. I suppose the primary purpose of my spreadsheet is simply accounting. Once we laid out our runs, this made it simple to tweak duct sizes to get where we wanted to go. I can see now that this would also be fairly simple with the ductalator for short runs, but I'm afraid I'd start to get lost fast as things add up. ASHRAE Fundamentals Handbook has charts and graphs for friction like you suggest. In addition, it has closed form solutions for friction loss that aren't too bad to dump into a spreadsheet.What exactly are you referring to as a "coil". Layout and sizing of our system was NOT a small task. I'm sure you're much more efficient than we were, but I'd think it's still difficult to get a decent fee for this work, right? Or not?

20. 
    Tim_Rice | Mar 03, 2002 08:19am | #24

    *Your heat pump is a refrigeration cycle that is reversible. In the types of systems that are common in homes, mechanical refrigeration, i.e. air conditioning, is done by whats called direct expansion. Outside of the house is the condenser, which is a compressor and a fan and a coil, similar to the radiator in your car. Hot, pressurized refrigerant gas goes through it and rejects heat the the outside air. Inside the house, usually downstream from the blower is another coil, the evaporator. This coil takes heat from the air stream and "rejects" it to the refrigerant in the tubes. This is the coil that I was referring to. Lots of tubes and fins and when it is in the cooling mode, it is usually wet. Add a little dust and you have a pretty significant pressure drop at design flow. The ESP, or external static pressure of your HP probably accounted for the coil. A furnace, on the other hand, since a coil is added depending on if you want AC or not, does not. This is a detail that you need to know.Layout and sizing of any system, large or small, is not a simple task by any means. We are usually paid as a percentage of the estimated installed cost and yes, it is difficult to get a decent fee for this work, especially on smaller jobs. Experience and efficiency, like in most professions, is required to compete and survive.