Replies

1. 
   CloudHidden | Sep 04, 2003 05:19pm | #1

   What would be your rules for overriding?

   If you want fancy rules, or can't find another way to do it, I've successfully used a PLC...programmable logic controller. The tstats become input to the plc, and the A/C is the output. Through your computer you program the logic. I use it both as zone control and to limit water heater operation to the least expensive $/kwh periods of the day. Magic Module from Elk Products $250-350 or so.

1. 
   TRIGGER | Sep 05, 2003 08:15am | #4

   That sounds like a really nice system C.H.. Probibly not too hard to program either. I guess there must be instructions on wireing all the different components?  Thanks!

1. 
   CloudHidden | Sep 05, 2003 08:28am | #5

   Programming was in something like Basic, which was easier than the ladder-logic of the typical PLC, but still quirky. The wiring was easy for the guy who did it. It's a nicely designed piece of electronics. I have a few quibbles with it, but far less than any other system I researched.

2. 
   BillHartmann | Sep 04, 2003 07:37pm | #2

   What are you controlling? Does the AC have multiple states? Variable speed fan? Zoning dampner?

   If so you should already have some kind of controller.

   But if it is just a basic unit with upstairs and downstair vents then you can connect two thermostats to it by paralleling them.

   If EITHER one calls for cooling them the AC turns on. So you will always be overcooling one area.

1. 
   TRIGGER | Sep 05, 2003 08:41am | #6

   I found one manual damper right out of the plenum that feeds the upstairs. Typicaly, will there be anouther damper for downstairs, 'cause I can't tell yet without more demo. I assume I can replace or add dampers compatible with the comp. program from CH.  Two new stats, lots o wire, dampers, maby the variable speed fan motor(this one is a one speed, far as I can tell),  the PC program and a clear head is all I 'll need?  Hope It's in my budg. cause it sounds real nice.  Thanks!

1. 
   DaveRicheson | Sep 05, 2003 01:46pm | #7

   Normal dampers in residential HVAC systems are manual dampers. What you need to install are VAVs (variable air volume) boxes sized to your duct runs. They have actuators that control the damper position from a maximum to a minimum setting.  Without the minimum damper settings, either manual stops or electronic, when one zone calls for cooling and the other doesn't, you could over pressurize the demand side duct work. Properly sized duct work is designed for the correct cfm of air  to each rom at a specific static duct pressure. A variable speed AHU fan generally selects the speed from the static pressure  settings.

   Lots of variables in a properly designed zoned HVAC. a lot more than just two t-stats.

   Dave

1. 
   TRIGGER | Sep 08, 2003 10:22am | #8

   Does this mean, my duct and manual dampers are probably sized and adjusted for the proper cfm per zone and the fans output is close to the total cfm for the two zones? And by using the VAV's to restrict air flow to a predetermined minimum at one zone and the maximum at the other zone, I will need a variable speed fan to maintain the same proper cfm to the non restricted zone? Or does  having the restricted zone's VAV set to the calculated minimum setting prevent over pressurizing the demand zone alone? Or is it even  possible to answer w/o all the specifics of my system?  BTW, this stuff is excellent input. Thanks.   Jack

1. 
   DaveRicheson | Sep 08, 2003 01:37pm | #9

   You said that you think you have a single speed motor. That would indicate, with only one T-stat. that you have a single zone system. The manual damper that you found, if on a main distribution trunk, may be the only one in the system. It was probably installed and set when the house was built. Most of the time it would have been set with a best guess adjustment by the installer. The equipment to air balance a system properly is expensive and rarely used in residential constructio.

   Before you go to the expense and work to install two t-stats., PLC, and a multispeed fan, try adjusting the damper that you found. A few days of incremental adjustments may solve you heat/cool balance problem. You should also find indiviual damper on each duct run to the room diffusers. Adjusting them may also help your distribution problem.

   Dave

3. 
   MrPita2 | Sep 04, 2003 07:44pm | #3

   Just a guess about you want, but I'll say sure, it can be done.  I think you might be looking for a zoned A/C system.  I'd think you can get it done with two T-stats, and two or more controlled dampers (depending on duct layout; assume one main duct branch for each floor, & one damper each)

   I assume these are the scenarios you want:

   1st floor hot, 2nd floor hot: A/C on, both dampers open
   1st floor hot, 2nd floor cold: A/C on, 1st floor damper open, 2nd floor damper closed.
   1st floor cold, 2nd floor hot: A/C on, 1st damper closed, 2nd damper open
   1st floor cold, 2nd floor cold: A/C off, damper positions kinda irrelevant.

   I'd think this simple enough that any HVAC contractor could do it easily.  You can likely do it as well, but you need to look into the proper controllers (sorry, can't help you there).  The dampers I suggest only because it will help direct airflow properly to the areas that need it most.  It could be done w/out dampers as well, but you'd be wasting A/C on areas that don't need it, and may make some areas uncomfortably cold, while hot areas are just tolerably cool.  Of course, if ducting is not easily accessed to install dampers, costs go up considerably.

   If everything seems to be going well, you've obviously overlooked something.

   
   

   ---

   Edited 9/4/2003 12:45:55 PM ET by Mr. Pita

4. 
   ClayS | Sep 25, 2003 12:19am | #10

   So I'm taking a hour for lunch and just surfing around and I read all the posts. Heres what I think about what your looking for. Kinda like a miss wired set of travelers on a light circuit.You turn one on the light goes on, you turn the other one off the light goes out, you go to the other switch to turn it on and the light doesn't work. So say it either doesn't have 3way wire or the light sw was replace with a non traveler/3 way.

    So lets look at what you can do with the2 thermos setup with nothing more than 1 extra thermo. Your up stairs and want it colder, you set it lower and the air comes on to the temp set say 70- temp gets to 70 and system shuts down, so far that's just like normal but if you go downstairs and say oh to cold, but unless you have turned down the upstairs it's still going to run to the lower setting. Well if you consider the cost You may be able to just take a little extra effort when needed to turn the thermo down or up depending on the need. after a couple of times up and down the steps to reset the thermo you'll remember to do it. also the minor adjustment of the registers to try to balance the system will help, not perfect but a possibility considering the expence of the other options.

    Clay