I recently moved into a house (30 years old) and the previous owner had an addition (sun room) on the back of the house. It looks nice, but not well heated (not sure about cooling yet, but assume as bad). There are two registers in the room, but it is a long run from the main HVAC shaft to the register (20 feet - other registers in the house are 10 feet from the shaft). There is a single 6 inch flex duct which has a 'T' in it for the second register (at about 9 feet from the closest register). When the furnace is on there is warmth at the register, but it isn't being 'forced' into the room. More like a trickle. Needless to say I am considering one of these duct booster fans to increase the air flow into the room.

The actual wiring of the duct blower isn't a big deal. My question is about getting a control signal from the furnace to control the duct blower.

The furnace is a Goodman Manufacturing Corp. GUPI100-4 gas fired forced hot air system. I think it was installed in 1993. The blower control part number is B18099-03

The blower is multispeed with heat running at one speed and cool running at a different speed.

I was considering the following to control the duct fan.
1) Install a transformer (120v primary/24v secondary) in series with the blower motor (on the common side) to generate 24 volts when the blower is running.
2) Run thermostat cable from this transformer to a 24v relay near the duct fan.
3) Install a 24v relay to control 120v for the duct fan (along with a cut off switch).

Does this sound reasonable? I wasn't able to find a suitable connection point on the board that would give me 24v (or other voltage) when the blower was on.

I'm also wondering if this room needs an additional return to help air flow. I think I have found a way to get a return run through the attic with out too much work (biggest part being cutting through the 2x4 footer of a wall upstairs to get access to the basement ceiling near the return ducting). Could this also help the problem? Is it worth the effort?

Thanks
Dave Sugar

Why not just come off the circuit board LO speed terminal or which ever speed is use one heat, run this wire plus a common and a bare wire ground to the duct motor. This way the duct motor would come on at the same time that the main blower does. Then you could figure out how to do the same thing for your AC to operate the duct motor the same way. If nothing else you could add a Normally open/ Normally closed relay in the furnace to switch from heat to AC to the duct motor.

I really didn't want to run 120 the whole way from the furnace to where the duct fan needs to be installed. It is quite a distance. I'm planning to run the 24v control wire along the basement wall, then outside of the house (behind the siding) and under the addition, There is an existing phone wire I can follow for a good stretch.

If I were to run 120 I would have to run it from the basement, up into the attic, across the attic, then down a space around the chimney and under the floor of the addition.

The reason to install the transformer in series with the blower moter is to only have to deal with one rather than two (one for low and one for high speed). I wasn't sure if this was safe because of the additional load (even though it would be small) in serias with the blower moter.

Thanks

First, you would want to wire the additional transformer in parallel, not series with the blower motor. If you wire it to the low speed lead for heat, then it won't run in cooling mode.

Perhaps forget the duct fan and start with the return. The return will make a big difference if you keep the door to the room closed. Perhaps the easiest thing to do would be a larger duct from the main shaft to where the 2 registers tee off. In along run, the duct size is very important.

I want the duct blower to work in both heating and cooling mode, that it the reason to run in series with the blower motor. I guess based on the resistance of the coil of the motor and the coil of the transformer would define how much voltage that each one gets? If in parallel they get the same amount of voltage is that how it would work?
Series add, parallel divide? Or is that only for resistance?

The room is pretty open. The doorway doesn't have a door to close in it and the door way is the twice the size of a standard door. It was probably where a sliding glass door was before. There is also a small opening were there used to be a window.

Replacing the duct is not possible because it is run in the ceiling of a finished basement. I'm not sure how they got the duct in, I guess they had to remove part of the ceiling in the basement to attach to the main feed.

So I'm really stuck with the 6 inch feed. I can add a return like I said which might make a difference. I have someone coming to give me some help with that (and insulate the attic at the same time). So I will see how that goes and go from there.

To have it run with both heating and cooling, you would have to connect relays to both the high and low speed motor leads. Tie one wire from the relay coils to the motor leads, and the other ones to the neutral from the motor. You could then connect both load terminals of the relays to a wire to the coil of the relay for the duct blower.

You want both the blower and the relay coils to have the full 120 volts.

So, I went ahead with the project over the weekend and got it all hooked up electrically and the duct blower in place.

Electrically I put a transformer in parallel with the blower motor. Ran 24 volts out to the relay. And then use the relay to control the power to the duct fan. All functions perfectly.

But, now I'm not happy with the results. Which surprised me because I have read other posts here and reviews of people who have done similar projects and had great results.

I don't feel that much difference in the air flow before and after. And based on the digital thermometer in the room there is no difference temperature wise. The room is still about 4 degrees cooler than the rest of the house. (When we keep the house at 68 this room being 64 is pretty cold).

Anyway, I'm wondering if the duct is part of the problem.

I'm pretty sure the duct is attached to the main shaft in the center of the house. Then there is about 11 feet of duct work (I assume all insulated flex duct) to a point where there is a tee. The straight through goes for another 9 feet to a register. The 90 degree part of the tee goes about 10 feet where it turns 90 degrees and goes another 9 fee to the register. Both registers have low airflow and I installed the duct fan on the incoming side of the tee.

Now, in addition where the tee is in the duct is below the floor joists for the room so there is a turn down in the flex duct, then the tee, then it goes back up.

I'm wondering if all this turning is negatively effecting the air flow. Would a rigid duct elbow better turn the air than the flex duct? I'm also now thinking that if I rotate the tee so one of the branches points down so most of the air flow is straight through. Then put a 90 degree turn to connect to the flex duct going the 10 feet across the room would be better air flow for the one register.

Anyway, just reporting results.
Thanks all

Looking inside the ducts is a problem, but I wonder if something is stuck somewhere blocking it. I know for liquids the rule is a 90 degree elbow adds the same restriction as 5 diameters of pipe, an elbow in a 6'' duct is like the duct being 30'' longer. My guess is that the flex has a longer radius bend and less restriction. However, the flex isn't as smooth increasing the restriction.

Quote:

---

Originally Posted by dsugar100

So, I went ahead with the project over the weekend and got it all hooked up electrically and the duct blower in place.

Electrically I put a transformer in parallel with the blower motor. Ran 24 volts out to the relay. And then use the relay to control the power to the duct fan. All functions perfectly.

But, now I'm not happy with the results. Which surprised me because I have read other posts here and reviews of people who have done similar projects and had great results.

I don't feel that much difference in the air flow before and after. And based on the digital thermometer in the room there is no difference temperature wise. The room is still about 4 degrees cooler than the rest of the house. (When we keep the house at 68 this room being 64 is pretty cold).

Anyway, I'm wondering if the duct is part of the problem.

I'm pretty sure the duct is attached to the main shaft in the center of the house. Then there is about 11 feet of duct work (I assume all insulated flex duct) to a point where there is a tee. The straight through goes for another 9 feet to a register. The 90 degree part of the tee goes about 10 feet where it turns 90 degrees and goes another 9 fee to the register. Both registers have low airflow and I installed the duct fan on the incoming side of the tee.

Now, in addition where the tee is in the duct is below the floor joists for the room so there is a turn down in the flex duct, then the tee, then it goes back up.

I'm wondering if all this turning is negatively effecting the air flow. Would a rigid duct elbow better turn the air than the flex duct? I'm also now thinking that if I rotate the tee so one of the branches points down so most of the air flow is straight through. Then put a 90 degree turn to connect to the flex duct going the 10 feet across the room would be better air flow for the one register.

Anyway, just reporting results.
Thanks all

---

Typically I don't suggest duct blowers, they go bad to quick, they can be noisy and it is placing one more obstacle in an already poor airflow pipe. I prefer to increase the run or ad a return as Labman said.