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Who knows thermostats? Need help with my house setup!
This thread has 11 replies. Displaying all posts.
Post 1 made on Wednesday May 4, 2016 at 10:50
Loyal Member
July 2009
My house has no central air, but radiant baseboard heat provided by a boiler (Weil McClain) the boiler was installed around 2005 and uses a traditional thermostat. I'm having central air installed and the installer (old guy) told me it's possible to just use one thermostat but wants to just put a separate one in for air.

I'm providing the C4 aprilaire thermostat (will do remote temperature sensor) so I rather suspect this is why he doesn't want to try and tie in the two systems.

How can this be done? It has to be possible! He's hung up on both systems having a transformer and doesn't want to tie them both in (understandable) can I just have the Tstat powered by the boiler transformer? Isn't a thermostat just a contact closure essentially when heat, air or fan is called for?


Current thermostat only has a wire connected to RH and W
The Bitterness of Poor Quality is Remembered Long after the Sweetness of Price is Forgotten! - Benjamin Franklin
Post 2 made on Wednesday May 4, 2016 at 10:56
RC Moderator
October 2002
Any time there's forced air and Radiant(Or geothermal), we've seen and used separate thermostats.

Just an observation. But Ive run into this several times over the years.

If the cost of 2 thermostats is a concern, look at getting 2 Aprilaire 8810's THey are a bit less expensive than the C4 stat.
I'm no engineer, but I did stay at a Motel 6 last night!
Post 3 made on Wednesday May 4, 2016 at 11:02
Super Member
May 2003
I haven't used that particular AprilAire. Look in the documentation for instructions about tying the two systems together. Just about any T-Stat allows this. I can't imagine why you'd need to do this, but in the worst case you'd need to add a relay to isolate the two systems. Most heating and cooling systems expect a simple contact closure for control. On the air conditioning side there may be a separate Fan circuit for a total of three contact closures. Heat pump systems may have additional circuits for the emergency electric heat.

The worst part of this adventure is figuring out the color codes. It's best if you first forget any color codes that you presently know. (and keep your fingers crossed that the original installer used the correct codes.)
Post 4 made on Wednesday May 4, 2016 at 11:25
Loyal Member
March 2005
if it is regular relay control, you should have no trouble coming up with something that would work. get the specs for the thermostat he wants to use and see what it wants to be happy and go from there. hit me up if you want. my brother is in the hvac bus and his father in law has owned a big hvac company for 35 years
I love supporting product that supports me!
Post 5 made on Wednesday May 4, 2016 at 13:00
Active Member
December 2011
Call Aprilaire. They have good tech support.
Post 6 made on Wednesday May 4, 2016 at 13:10
Founding Member
December 2001
I have baseboard hot water plus central air run off of 1 thermostat per zone with no problems so it can be done.
Post 7 made on Wednesday May 4, 2016 at 13:17
Elite Member
June 2007
On May 4, 2016 at 13:00, oprahthehutt. said...
Call Aprilaire. They have good tech support.

Civil War reenactment is LARPing for people with no imagination.

Post 8 made on Wednesday May 4, 2016 at 13:59
Loyal Member
November 2006
When you mix multiple unit, be it forced air, radiant, solar, geothermal, etc. you'll most likely end up with HVAC controller.

I, for example have few BMPlus7000 from , but have used or worked on many different ones from too many MFGs to recall.

There is some logic to the controller for things like: 2nd stage temperature or time deltas, emergency heat calls, changeover delays, short cycle prevention, damper control, etc.

As long as you understand that all t-stat does is close contacts (relays) then you can figure out how to make any system work with any device.

There are some exceptions, like digital or proprietary communication systems with things like variable fan speed controls, but even those can be beat into submission.

You've got my number if you need to chat further.
Post 9 made on Wednesday May 4, 2016 at 15:21
Long Time Member
September 2010
I did this on Aprilaire 8870 in a house with radiant baseboard and forced air conditioning. As I recall it was as simple as a couple of settings in the menu and leaving off the jumper between R and RH
Post 10 made on Thursday May 5, 2016 at 19:25
Long Time Member
May 2011
Separate r and rh as mentioned above and use a relay to
Keep the two systems separated.
Post 11 made on Thursday May 5, 2016 at 22:32
Audible Solutions
Super Member
March 2004
You need a thermostat with A. isolated relays, has primary and secondary heat relays ( W1 and W2 ). Because both units have different power supplies you will need either a zone controller or a separate relay. There is absolutely no reason to use more than 1 TSTAT.

If you are only going to use Y1 for cool and W1 for heat you may not need to do anything other than connect the unit. If you are adding central Air to your radient heat you can may only need to increase the wires at the TSTAT.

For example, you need to power the TSTAT. I prefer to power the unit separately from the HVAC system. You contractor will protest, but it is done. It will cost you about 40 dollars for the transformer.

Then you will bring power from the AC system in to RC and power from the heating system into RH. Wire W1 and Y1 to the heating and AC systems and you are done.

If you have W1 and W2 you will need either a zone controller or a relay to isolate the the different power supplies. You only have a single RH so W2 needs to be isolated from the secondary heating system.

April Air does make a TSTAT that can meet these requirement. But there are a number of TSTATs that have these features. Interestingly, I've seen lots of Crestron TSTATs installed by HVAC contractors that will never be hooked up or controlled by a Crestron system. Why? It can control any digital system, from FCU, forced air, or heat pump without needing a zone controller. It also has Y1, Y2, W1, W2, B,O, Humidity relay, and temp sensor inputs. This means a single TSTAT can be installed by that contractor without needing expensive zone controllers ( 300 ).

"This is a Christian Country,Charlie,founded on Christian values...when you can't put a nativiy scene in front fire house at Christmas time in Nacogdoches Township, something's gone terribly wrong"
Post 12 made on Friday May 6, 2016 at 03:05
Long Time Member
February 2005
On May 4, 2016 at 15:21, jrainey said...
I did this on Aprilaire 8870 in a house with radiant baseboard and forced air conditioning. As I recall it was as simple as a couple of settings in the menu and leaving off the jumper between R and RH

I've done the same with radiant in floor and an 8870.

If you end up using separate t-stats, Houselogix has a driver that will combine them into a single t-stat on the UI's.

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