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We are, hopefully, closing on our house out in the country this week. The house is unique in that it is a duplex* that has had the wall between the living rooms removed making one big room. The rest of the house was renovated keeping the large master suite, a utility room, and a mud room on one side of the living room while the kitchen, bath, and a couple of bedrooms are on the other side of the living room. Same square footage on either side.

The house has two identical HVAC units on either side in the original furnance closets. The thermostats are located at opposite ends of the living room. One duct from each unit feeds the living room from opposite ends.

I see advantages and disadvantages with this dual system.

One advantage is that the house can be zoned to some degree.

The big disadvantage I see is the difficulty of getting both thermostats to energize at the same reasonable time causing one unit to try to heat/cool the living room more that the other unit and overworking.

I am considering tying both units into one thermostat but I'm concerned that this may overload the low-voltage transformer.

I've also thought about moving one thermostat to the master suite but this gets back to one unit trying to cool more than its design.

Another consideration that may or may not muddle things is the living room is on the east side of the house (in line with kitchen, utility and mud rooms) and has a covered porch running the length of the living room. The bedrooms are one the west side of house with no shading. This means the bedrooms will tend to heat up more in the afternoon sun during the summer. So it may be better to move the thermostat(s) to the west side of the house to better keep up with the cooling. I don't see as big of an issue with the heating during the winter.

Does anyone have any suggestions on how to wire two HVAC units to one thermostat (replacing the thermostat with an upgrade is very acceptable)?

Any other suggestions?

I may be making a mountain out of a mole hill but I'm trying to "simplify".


*What's kind of cool is this house was originally a Reese AFB officer's housing duplex. My understanding is the were constructed with no interior load-bearing walls so the Air Force could repurpose them as needed. Original oak hardwood floors and solid construction. Utilities were updated when the house was moved to its present location, just a few miles from the AFB. Some of the interior walls were moved to convert to a one family house. The previous owner bricked the outside and added the covered front porch. If you didn't know better you'd just think it was a typical pier and beam ranch style house.

I miss Reese AFB and the jets overhead.

yes you can put them on one Thermostat, gonna have to get relays and another transformer, wire the relays to take care of the A/C and heat inside each unit then use the 3rd transformer to energize the relays from the stat, if your not familiar call a competant tech to do it for you, however I wouldn't tie them into one stat, I would make sure the stats are properly located and set up an actual residential zone system that can be installed by a licensed and insured company, this will save money, time and headaches.

I'd leave them separate. They will work the same as when they were separated by a wall. When I build a 2 story the 2nd floor is usually on a separate unit and thermostat. If you are still on 2 meters you have a code violation...

The best solution IMHO would be to leave them seperate and relocate 1 thermostat. Then, take the duct from the outlet off of the grille and pull another duct from the unit with the thermostat remaining in the living room to that grille. You could utilize the duct that was removed by simply switching with another grille from the other unit.

This all makes sense to me but after I read it, it may take someone a couple of times in reading to understand what I am trying to say. If you PM me your email address I will send you a drawing of what I am talking about.

JP171 wrote:yes you can put them on one Thermostat, gonna have to get relays and another transformer, wire the relays to take care of the A/C and heat inside each unit then use the 3rd transformer to energize the relays from the stat, if your not familiar call a competant tech to do it for you, however I wouldn't tie them into one stat, I would make sure the stats are properly located and set up an actual residential zone system that can be installed by a licensed and insured company, this will save money, time and headaches.

As noted, the thermostats are unlikely to coordinate even under the best of circumstances. If you are not going to use the two systems as separate systems, the best way to coordinate them is to use a single thermostat. Since a single thermostat usually cannot control two separate systems, using it to control those two systems would involve a relay to energize them that is controlled by the one thermostat.

Of course the other scenario would be to use them as zones and relocate the thermostats to where they would be most effective.

Would there be an advantage to converting to electronic thermostats that can read/interpret remote sensors? Do remote sensors on thermostats even work that way?

I only ask because my thermo has the capability to read a remote, but I've never studied the details, and don't know if the remote overrides/replaces the built-in temp sensor...

if it works "in the loop" with the built in, you could place a remote near the other-unit's main thermo and average things somewhat...maybe....

Just an idea. On re-reading my own thoughts, maybe I should stick to my usual meddling and peddling LOL

RPBrown wrote:The best solution IMHO would be to leave them seperate and relocate 1 thermostat. Then, take the duct from the outlet off of the grille and pull another duct from the unit with the thermostat remaining in the living room to that grille. You could utilize the duct that was removed by simply switching with another grille from the other unit.

This all makes sense to me but after I read it, it may take someone a couple of times in reading to understand what I am trying to say. If you PM me your email address I will send you a drawing of what I am talking about.

I understand your concept. The problem would then involve the capacity of the HVAC unit with the added half of the living room. I don't even know what the capacities of the units are at the moment. Seems this has the best potential of doing what I want.

To further complicate things, within the next 5 years my son will be moving out. When that happens we can shut down that side of the house for day to day use. Aside from the kitchen, that unit could be relocated behind a door to the hallway and set at more economical temps. Then all we would have to do is heat the master suite, living room, kitchen, and utility/mud room (and they could be closed off from general circulation most of the time).

I think I will need to move the thermostats, one to the master suite and one to the "north wing" hall, then do some creative duct relocating to keep things balanced. I don't guess I have to get in a hurry and living in the house a while will let me figure out how it breathes.

Thanks for the help, it got me out of a mental rut and opened up some new options.