2nd T-Stat in Parallel for AC

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Old 06-04-13, 07:57 PM
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2nd T-Stat in Parallel for AC

Issue - too hot at night in bedroom on 2nd floor.

Question - can we wire a 2nd thermometer in parallel so that the the new one in our bedroom controls the AC at night?

Our vacation home has a single zone FHA HVAC system including central air. AC, that was installed last Fall. Cools the downstairs fine, but at night the thermostat needs to be set lower in order to keep the bedroom comfortable. We want to install a second thermostat in the bedroom so we can control the temp in the room. We understand that the downstairs will be cooler.

This past weekend we had very warm temps - in order to be comfortable we set the downstairs thermometer to 69 degrees (it is off by a couple of degrees) and shut the dampers on 6 of the 8 vents on the first floor. Worked well, but woke up around 1 AM and had to lower the temp to 68.

We are using BayWeb thermostat/controlers that have the ability to open/close a damper when calling for heat/cooling. Our thought is too wire one in our bedroom and have it close a damper in the main duct that would shut off 6 of the vents behind it, leaving 2 vents on the 1st floor and 4 on the second floor providing cooling. Cooling is the only issue - during the winter we prefer the bedroom being cooler.

The way we see it, during the day when the downstairs thermostat set at 72 degrees is calling for cooling it will work as it does today - cool air going to both floors. At night we would leave it set for 72 degrees, but turn on the bedroom thermostat to 72 degrees, which would shut the damper sending most of the cool air to the second floor. Sort of a poor mans zoning.

We are getting a quote to have another duct/vent run to the bedroom to get more air in there, but even then the issue will remain at night as the upstairs holds heat a lot more than the downstairs.

Another option we thought of is to simply buy a portable room air conditioner and run that at night in the bedroom leaving the downstairs set to 72 and manually closing a damper.
 
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Old 06-04-13, 08:24 PM
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Have you looked into the Honeywell TH8321R1001 VisionPRO 8000 Programmable RedLINK ThermostatPro?

It can work with the Honeywell REM5000R1001 Portable Comfort Control

It is also available as a Honeywell YTHX9321R5061 Prestige® 2.0 HD Comfort System Kit

The HD stat in the kit is impressive.
 
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Old 06-04-13, 08:42 PM
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Connecting controls in series or parallel arrangements usually introduces more problems than it cures. It can be done with some switching circuits that could also be incorporated into a time clock circuit, but Houston has given you a better option.
 
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Old 06-05-13, 04:11 AM
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I agree with Houston, I would look into the redlink control. I have used it very successfully for my customers in that very situation.
 
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Old 06-05-13, 04:28 AM
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Thanks for the quick replies!

We already own and use the Bayweb devices which have inputs for 3 hardwired sensors (using 2 - one for outdoor temp - one for basement temp), and motion sensors with alerting so it's also a quasi alarm system so no desire to change out to something else.

Furd - besides the obvious "one t-stat on heat and the other on cool" what problems are you referring too? Can you be more specific on the switching circuit you mentioned and what the benefits are. By using a second Bayweb device for the bedroom we will be able to close the dampers for the 1st floor when that t-stat is calling for cooling which will help in not over cooling the 1st floor.

Thanks
 
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Old 06-05-13, 06:22 AM
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You could ruin your ac by starving it for air if you shut off all the downstairs registers. Starving the unit for air can cause compressor liquid slugging.
 
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Old 06-05-13, 06:56 AM
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The damper will only close the supply registers (6 of the 8 on the first floor), not the returns (which are all located on the first floor) I also plan to put in a bypass damper between the supply and return plenums to be sure that the system does not have any excess pressure.

If this works, we may eventually add in a real zone control panel but with 4 supplies going to the 2nd floor I guess I will either need a larger transformer or have it run through a relay to open dampers for the 2nd floor.
 
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Old 06-05-13, 02:01 PM
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I'll start out by stating that I am against "zoning" of residential HVAC systems. The potential problems are often just not worth it. Problems include both high and low temperature differentials across heat exchangers in the heating mode, frozen coils or poor dehumidification during the cooling mode, stuck balancing dampers, general system inefficiencies and increased maintenance. Really, in a multi-level house the best is to have separate systems for the various levels.

I do make one exception and that is where you have a single room that you want to keep cooler during the heating season or warmer during the cooling season. For this I recommend an electric damper in the single duct serving this room with a room thermostat controlling the damper. The room thermostat has no control over the system, just the airflow into the single room so it only allows conditioned air (heat or cool) when the main system is actually operating. By limiting it to a single room the airflow change to the main system is minimal.

The problem with having two thermostats in parallel is that each thermostat will want to control the system. Depending on how badly designed and installed your ductwork you could easily have a worse problem than you do now. If you wire both heating and cooling to both thermostats you will have troubles all year long. If your system performs acceptably in the heating mode then you might be able to accomplish your goal with a second thermostat, but as you already know, the downstairs will definitely go colder than desired. This also means s decrease in overall system efficiency and higher power bills.

You will need to run a four conductor cable from the furnace/air handler to the location of the second thermostat. This cable will connect to both the furnace/air handler and a DPDT selector switch. It is easiest to connect the switch at the furnace but by using a six conductor cable you could mount it anywhere you can run a cable. (some systems will require six conductors, others may work with just three conductors, it depends upon how the blower is controlled in cooling mode.) The selector switch will select which thermostat has control so it should be mounted in place that is not too inconvenient. Using a time clock that has DPDT switching (or a DPDT relay with a time clock that has only SPST switching) would automate the changeover but it only works IF you live by a set schedule.

The secondary thermostat would be wired using the red, yellow and green wires only unless the particular thermostat also requires a common lead. The red (and common) would come from the same designations on the furnace control board and the yellow and green would go to the selector switch on one end. the yellow and green from the primary thermostat would be removed from the furnace control board and connect to the other end of the selector switch with the center terminals of the selector going back to the furnace control board. Keep like colors on the same side of the selector switch.

This arrangement will work on only the cooling cycle and switch control of the cooling from the primary to the secondary thermostat. You DO need to turn the system switch of the secondary thermostat to the OFF position when not actively wanting to use cooling.
 
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Old 06-05-13, 06:18 PM
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Thanks for clarifying the use of the DPDT switch.
 
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