can't I put a relay in series, in between thermostat and furnace?

ckurtis

I'm not a hvac person, but I have a decent understanding of electronics. I have a forced air furnace. I recently added a 2nd thermostat (in what we'll call the cold wing) that I've wired in parallel to the original thermostat. I've also installed a "normally on" damper to the "hot wing" of the house. Both thermostats can turn the heat on, air on, fan on etc.

What I'd like to do, is run a relay that would switch on the damper when the cold wing is the only thermostat wanting heat. Wiring the relay in series, results in the relay working fine, but I'm guessing there's not enough juice (too much resistance, induction?) getting to the furnace for it to turn on via that thermostat. I'm using a spdt relay (http://tinyurl.com/m6ublqz) that is 24v ac.

Is there a better type of relay that won't get in the way of the furnace starting, or some sort of transistor that works with alternating current I can use? I thought that this would be simple enough to not get a zone controller into the mix.

Thanks,
CK in Chicago

wmtandson

Seems logical,but without more information no help is available.
system type?
zones etc?

ProblemSolver

Think about what you're saying. A zone controller has a number of relays on it because that is what it takes to accomplish what you are wanting to do. If one relay would work, then there would be no need for a zone controler. Plus, those black relay boxes are noisy when they close, and a zone controler uses solid state relays which are quiet.

ProblemSolver

But more important! A furnace is engineered to have a desiged amount of air flow through it (CFM's - cubic foot per minute, of air). By zoning, you end up reducing the amount of air that is available to flow through the furnase. This will slowly kill the bower motor and heat exchanger; basically the whole furnace. It's like eating only fast food; it won't kill you after one meal, but over a number of years you'll get sick and eventually die. Your furnace was not sized for zoning, it was sized to feed every register in your home, not half the registers!

ProblemSolver

But there is another solution. Forced air is not about defeating the undesirable temperature in the home, it's about removing the undesirable temperature (through the return air grills), and replacing that with the newly conditioned air from the supply air registers; thus, washing the whole room with newly conditioned air. This is how forced air works efficiently. Back to the 12K classroom when we learned that high pressure always goes to low pressure; therefore, the return air grills that are removing air out of a room are creating a low pressure, and thus the supply air (high pressure) has no choice but to follow that low pressure. This means the rooms that are lacking comfort are also lacking return air. The supply air cannot wash a room with newly conditioned air if the return air grills are not removing air from the room. As a quick fix, and to prove this point; go to a home improvement store and purchase some magnetic sheets (found in the register aisle). Cover most (but not all) of the return air grills in the rooms that are comfortable (these rooms are also the one's closes to the furnace). This will force the blower to reach out and pull air from the rooms that are uncomfortable. To get the supply air balanced we must first balance the return air.

ProblemSolver

If you're still set on zoning your ducts, try using a transformer with a 70va rating. Most furnaces have a 40va transformer which sounds as though it doesn't have enough amps to operate everything. The 70va transformer should handle it.

Mark N

How much voltage are you dopping across the coils of the new relay? Does the thermostat control a relay on the furnace the powers the gas valve? If it is a relay what is the voltage across its coils? What is the pickup voltage of these relays?

pecmsg

As others have stated the furnace is designed to move X amount of air with a certain temperature rise across the heat exchanger. By closing off the ducts you are slowing the CFM and may exceed the design temperature rise of the heat exchanger.
Get a contractor in to calculate why this room is cold and correct the issue.

ProblemSolver

He hasn't used a relay yet. The zone damper is not powered up because it interferes with the operation of the furnace.

Mark N

Reread his post, he wired the relay in series, it picked the new relay but the furnace didn't start. The new relay is in series with what turns on the furnace, Need to know the voltage drop across the coil of the new relay.

ProblemSolver

Well if the furnace didn't srart, then it could easily be assumed that he disconnected it so he could use his furnace.

Zman

As far as the flow rate across the coil and damaging the furnace. Yes, you should verify that you will have adequate flow.

This whole wiring loads in series thing is a really bad plan.They furnace was not designed for voltage drop. Don't do it.

I would lean towards a solution using a DPDT relay so that you can wire the 2 loads in parallel.

ckurtis

You guys are definitely a funny bunch. I resolved the issue without relays, controllers, or even magnetic pads. Lol

I used the green wire (fan) coming from the 2nd thermostat that could be configured to switch on along with the white wire when the cold wing needed more heat. I used the power transformer that pushed the humidifier when it was on, to power on the damper, and let the common and green activate it. I figured I'd never need to turn the fan on in that wing without heat or cool.

As far as pushing too much air into too small of a space, I didn't set the damper full close, just enough to send more that way...

Frankly I'd rather replace the heater 2-3 years sooner than to be cold. That's what heaters are for, I suppose it would last forever if I didn't use it.

Thanks for the advice

icesailor

Some install a "Bypass Damper" on a WA system with zone dampers to avoid having a too low air flow and damaging the HX. Some manufacturers suggest them. Or so I was told.

ProblemSolver

I gave you, ckurtis, some really good engineering knowledge on how a FA system is suppose to work and do it efficiently. Efficiently means you spend less each month to be comfortable. But as it goes with the American mind-set; taking the easy route is better than working on the best solution. It is going to cost you a lot more money down the road when you throw a bandade on a problem insread of dealing with the cause. Yes your idea will help make your home more comfortable, but at a much higher expence.