electrical advice for zone control on downdraft system

[sprangdog]

I've adapted a zoning damper for use in a cooktop downdraft system and just need some confirmation on principles of electrical components and any other feedback on this setup.

Kitchen downdraft system with external/outdoor blower unit. I installed a 24v transformer (wired to the 120 primary connections) on the line between the downdraft which controls the variable-speed blower outside. The transformer controls a power-open zone damper in order to keep the cold air from drafting in through the 8" duct between the outside blower and downdraft in the island when not in use. I am not real keen on electrical, and since this is out-of-spec for a zone control, I have two questions:

1) When the downdraft is on low, it supplies 80 volts the the transformer which steps down to 16 volts on the secondary connection to the damper motor. This lower voltage won't harm the transformer or the damper control, will it?

2) If the downdraft is started on low, the 16 volts to the damper only allows it to open about 10-20%. If I start the unit any higher than lowest setting it gives enough voltage to open the damper wide, and then if I turn it down to low the damper motor will hold it open. If I do this is there any chance of damaging equipment by supplying less than 120v to the transformer and less than 24v to the the damper motor?

Thanks for any advice here.

[crab master]

Welcome to the site. Since this is not a DIY site please post some background/qualifications about yourself either here or in your profile.

The only thing I will throw out there for now is many items are only good for +/-10% of rated voltage. You'll likely burn up your actuator prematurely with your current conditions and that's why it wont' open all the way.

[sprangdog]

This is a project in my own home, but perhaps I need some clarification on what constitutes DIY. I did read the forum rules, and I'm not looking for how-to advice on wiring a t-stat, or step-by-step on eliminating the drafting through the downdraft... Maybe I gave too much background or need to rephrase the post? I'm obviously not a HVAC pro, and my only qualifications are from the school of hard knocks, talking to/watching the pros that I have hired as a contractor and homeowner, and self-study. I consider myself a rehabber/homebuilder and skilled at carpentry and woodworking. Without going into detail, I have done fairly extensive remodeling and hvac repair work for myself and others. With the projects I am involved in, I definitely have an interest in HVAC systems and I was quite pleased to read that this forum is not "pro-only."

I have already installed the damper control and so I'm not looking for someone to tell me what to do here, but rather if someone can help me understand what impact, if any, there will be if I continue supplying less than 24vac to the damper motor. My understanding of transformers tells me there's no harm there because unless you overload it the transformer is just going to spit out a voltage proportionate to what is supplied, which I confirmed by putting a meter on it. So, perhaps my question boils down to:

Does anybody know if occasionally going less than 10% of rated voltage is going to damage the actuator on a damper control?

I do have a plan B, involving a relay, but I'm not even going to go there.

@crab_master: Let me know if I'm misunderstanding the forum rules.

[BACnet]

I'm not sure about the DIY ramifications of this thread. But-

I would like to know more about the damper actuator.

Do you pulse it open/closed or drive it open and allow a spring to return it? Knowing the type of actuator is the key to solving your question.

[sprangdog]

BAC - It is a spring close, so it is only open while actuator is energized.

From my limited understanding of motors, lower voltage means it will draw more amps, depending on the mechanical load on the motor? I was thinking about checking the current of the motor at the lowest voltage (16) to see if it is in spec (motor is rated 6VA) so if it reads 6 amps or less while the damper is open, does that mean I'm ok? I figure the load on the motor while it is opening the damper is only a few seconds so I'm thinking if it overheats it will be while it is holding the damper open?

Thanks for your help.

[Cuziwanano]

First of all, Volt-Amps (VA) is Volts x Amps

6 va at 16 volts is something like .39 amps or so.
24 volts and 6 va is around .25 amps

[Cuziwanano]

6 amps across that actuator and transformer, you will need the ventilator to extract the smoke that gets released.

[BACnet]

Go with the relay- don't stress the motor

[fxb80]

BTW, same rules apply to relays, not a good idea to have less than rated voltage going to the coil.

Somebody that works with actuators, transformers, and relays for a living needs to help with your project.

[crab master]

Although in my mind DIY, but since I like your honest answer; I am simply wondering why the transformer is wired downstream of the rheostat?

*Unless the rheostat is also your on off switch....

[sprangdog]

I just assumed a high voltage relay would still work with lower-than-rated voltage. It will burn up too?

For now I just won't run the thing at less than full power until I find somebody who knows a solution. I'm sure there's a way to do it. Trouble is the thing is oversized for the kitchen at 900 cfm using the downdraft in the winter I might as well just turn on the whole house ventilator fan in the the attic... and putting a fire in the hearth and cooking fish won't mix without opening windows.

I should have just left the rolled-up towel there to stop the draft.

[sprangdog]

Although in my mind DIY, but since I like your honest answer; I am simply wondering why the transformer is wired downstream of the rheostat?

*Unless the rheostat is also your on off switch....

sorry if it is diy, not sure what kinda questions you guys get from Joe Homeowner if this is diy... thanks for your help in thinking it through.

It's a fancy built-in unit with the rheostat molded in under the power button, it is actually concealed when the thing retracts down. I'm sure theres a way to splice in between the power and the rheostat, but the transformer would have to mount to the cabinet and the rheo moves up and down with popup. I can't see behind the unit to see how the supply power gets into the part that moves without tearing the thing out...and then there's the issue of flexing wire. But that is a good idea and I may try it yet. Do they make a smaller-profile transformer that I could maybe rig up inside the unit? Now I have a typical bell-type which is way too bulky to get inside.

[Cuziwanano]

Hey Crab, I think you should use a current sensing relay like this one from RIB and put your transformer on a constant power source. Assuming that your fan would draw alt least .25 amps

[crab master]

Do they make a smaller-profile transformer that I could maybe rig up inside the unit? Now I have a typical bell-type which is way too bulky to get inside.

Now I am worrying that you may end up using an undersized transformer.

I just assumed a high voltage relay would still work with lower-than-rated voltage. It will burn up too?

Yes.

These kind of questions is why you need to bring in a professional. This is a relatively simple thing to do; however, the assumptions you have made and your attempts are all leading to likely burning up a transformer, actuator and possibly more. If space limitations are such then one could mount the transformer in an external j-box and pull wire, or better yet get an actuator that is rated at 120VAC and get rid of the transformer. If your fan is so oversized I would look at putting an external switch anyway just to open the damper so you don't have to run the fan just to cook something little. *Just realized downdraft, so no fan then draft will likely go the wrong way, but* Then one could interlock an end switch and.....bring in a professional.

We don't want a conversation such as:
your insurance agent "Well it seems the electrical in your kitchen ventilator caused the house fire and since it wasn't inspected/proof that it was professionally wired your insurance policy is null and void on the damages caused.."
You to your agent "Well I learned how to do this on the internet at HVAC-Talk and CM, ...told me how!"

Good luck to you and you might post your general location, who knows maybe one of us lives near you/knows someone that does.

[sprangdog]

Thanks for talking through this, still not sure exactly what I will do, but I have a few ideas and maybe the current sensing relay is something I'll look into. Don't worry I won't do anything based on assumptions, I do want to respect the no DIY policy here, so the bottom-line is that you guys think the 24v actuator can be damaged by under-powering it. Thanks again for the help.

[Cuziwanano]

Yo Dog, most guys (or gals) on this site would probably love to help engineer a solution for you but the variables are too vast to give responsible advice over a bulletin board and still feel comfortable that you will not light yourself up.
Having said that, the fact that the actuator motor only opens partially tells me that at 16v the motor does not have the torque to overcome the return spring. So I think that would eventually lead to the demise of the actuator motor. Without knowing how far "point A" is from "point B" its hard to say what the best way to do something would be.

My email is in my profile.
Best of luck.

[sprangdog]

Thanks again. I've ordered a low-voltage current-sensing relay (the kind typ used for humidifiers) and will wire the power to the transformer on the primary feed. This will keep the solution on the 24v side. AS a side issue I love the outdoor blower/ventilator setup but it really surprises me that these systems don't integrate a damper... I know that homes can be a little too tight these days but an open 8" round duct from the kitchen to the outside isn't getting it in my (midwest) book.