T7300 tstat with furnace/split system

[olc]

I want a furnace split system to be controlled by a Honeywell T7300 thermostat. There will be an economizer and remote temperature sensor/override. The problem is that I do not think the furnace control transformer can handle the load of the control. How do you isolate the control power for the thermostat from the furnace control?
I have no clue. At the furnace, thermostat, economizer, none of the above?
I have spent half a day looking at furnace and honeywell installation lit with little to show for it. Most of Honeywell's lit kind of assumes a separate control transformer.

[service guy]

If I'm not mistaking You can purchase a subase that has a aux set of contacts however I forget what the max amp draw is. If what your installing exceeds the max amps you can simply wire the device using a relay of your choice mounted in the furnace. I also think there is a subase that has a set of contacts for outdoor air control you may want to contact your local supply house.

[ac/dc]

You think the unit Xfmer is to small for the econo. cntrl.
& actuator? or the t7300 ? you cant isolate a load without
an additional xfmer and relay. Have you done the math to
determine what va is required? (considering some loads wont
run at the same time) t7300 and econo cntrl together dont
use much power but econo actuator may. can you simply up-
size unit xfmer?

[maxster]

here is the TR page:they are calling for another 6VA for the heat side thru that TR on the RH i can put up the subbase with letterings and listing of all the terminals and the econo hook-up page if you need it..let me know.http://www.imagewiz.net/usr/hvac/959_H_TR3.JPG

[maxster]

http://www.imagewiz.net/usr/hvac/960_Htrlist.JPG

[maxster]

http://www.imagewiz.net/usr/hvac/961_Hsubbase.JPG

[olc]

The furnace instructions (Rheem in this case) say to not use "current robbing" thermostats and if required to use an isolation relay. The T7300 certainly is a "current robbing" and if it were not the economizer is.
It looks like the best way may be to use relays for the heat, cool, and fan connections on the furnace and size a power supply for everything else. Basically the bottom diagram in the picture above.

[DeltaT]

Originally posted by olc
The furnace instructions (Rheem in this case) say to not use "current robbing" thermostats and if required to use an isolation relay. The T7300 certainly is a "current robbing" and if it were not the economizer is.
It looks like the best way may be to use relays for the heat, cool, and fan connections on the furnace and size a power supply for everything else. Basically the bottom diagram in the picture above.
A T7300 is not a current robbing thermostat. Neither is an economizer. Are you in the trade?

[olc]

OK - what does "current robbing" mean?

The T7300 may not require too much power but the economizer dampers require power.

[service guy]

All it means is the Stat power stealing (uses R and C) terminals on the stat to power the display and control the equipment and run a program on or off. However some stats just use AA Batteries to power the stat these are non-power stealing and will control on and off and heat, cool,and fan operations. Hope this helps OlC.

[maxster]

if you are running only 2 or 3 mod motors you can get them with a built in TR and supply 115V to them.then control the R B W thru the mixed air outdoor air or enthaphy setups for free cooling and mixed air setpoints in the winter

[ac/dc]

olc, What service guy is saying is if the the stat requires
a common (like T7300) it is not a power stealing stat.
power stealing stats dont require a common...they steal it
thru the coil of the non operating mode component...so
when its in cooling mode it steals its common thru the heat
relay coil or gas valve coil. Econo controls are never
power stealing or anything else that Ive seen besides a stat. So...its likley your unit xfmer is big enough.
You can be sure by simply adding all the xfmer loads
together to determine what VA is required. hope this helps
let us know

[skarpes]

I agree w/ Mr. ac/dc. (t7300 is not a power-stealing stat)It sounds like it'll likely work. But, i would personally use the new 8000 series of Honeywell Touchscreen tstat because they are MUCH cheaper and work alot better. (plus have 5 year warranty) Good luck.

[jasond1011]

take out the oem 40va transformer and put in a 75 va fused transformer good for up to 3.1amps.

[olc]

Thanks for the info.
I just looked at my particular application and the load looks like it is right around 40VA. There are a few other dampers in addition to the economizer dampers. It would be easy to put the dampers on a separate power supply. If you change the transformer in the furnace, things will get screwed up when they replace the furnace.
The T8000 does not have aux. contacts.

[skarpes]

hmmm. aux-contacts? am i missing something here?

[olc]

The occupied/unoccupied auxiliary contacts.

[skarpes]

there are no 'aux-contacts' on a t7300, it's a programmable tstat with occupied/unnoccupied settings...same as 8000 series and many other programmable tstats on the market.

[stopro1]

do in fact have aux occupied spst n.o. dry contacts a1 and a2. 2000 series 7300's even have spdt a1,a2 and a3. alot of rtu oem's use the n.o. contact to disable min pos oa during unocc periods- wired to n and p terminals on econo module.

[service guy]

Skarpes we are talking about the sub-base which you can change for multiple applications I just wanted to make sure we are clear on what we are talking about.

[needlevalve]

Is the economizer matched to split system or in-house design? You can add as many dry contacts and transformer you'ld like; however, less is more. It is likely the transformer ship with the system is 40va and should provide plenty of amps for your requirement. You can always remove the existing transformer and replace with larger capacity one.

[ac/dc]

huh...that sounds familar

[seaboard]

Lots of scattered advice here. A "power stealing thermostat" will not have a common terminal. It bleeds voltage through a coil in order to operate. Similar to a small solid state timer, there will be a voltage drop across the thermostat terminals when in a given mode. There are several model's of gas heating equipment that are equipped with solid state boards which are looking for a 24 volt input to "call for heat" but there really is no appreciable current draw. A power stealing thermostat will not work properly on these units. A simple pilot relay can do the trick to get around this. A Lightstat is a very popular model "power stealing thermostat".
I agree that the Honeywell touchscreen has an abundance of uses and should be good for your application. The T7300 should work fine as well. It never hurts to bump up the transformer VA and should when in doubt.
Many thermostats with batteries are simply using them to back up the program in the event of power loss. There are a few models that are actually battery powered but again these are not "power stealing".

[olc]

Thanks for all the info. Very helpfull. In our case there is a ventilation heat exchanger which will have two dampers (outside air and exhaust air). Also there are a couple relays to disable the heat exchanger when the the economizer is running. Like I said, I do not want to change the furnace transformer because when the furnaces is replaced it will be forgotten. Very easy to add a transformer for the dampers.