heat from roof area... 2 years later

[elt]

After reading a few posts two years ago, I decided to pull some heat from the attic area of my shop. Here's one of the threads:

http://www.fieldlines.com/story/2006/1/22/7044/09848

And here's a picture of my air duct running up into the "attic" in my shop:



A duct fan (from Home Depot) pulls air from up there down... It's worked nicely. I put a flapper on it to control back draft but that's pretty much it. It's not a lot of heat but over the course of a few hours in can raise the shop temp by 15 degrees or more.

Up to now, the control has been manual. When I've been in the shop, I turn on the fan and see how hot the air is. (The picture shows an indoor-outdoor thermometer with the remote sensor in the duct.) If the attic is warmer than the shop then I leave it on. The only automation I've had was an AC timer; if the weather is nice I could set the timer to run the fan until a little after sunset.

I've only used it for supplemental heat, to reduce the run-time of the furnace when I've been working. I hope to use it this winter as a primary source. This year, instead of "idling" the shop at 55F when I'm not there I'm going to let the temperature drop down to 42F. With the lower set point, there's an opportunity to get more heat from the attic space, particularly when I'm not there and colder is okay. So I built a differential thermostat to automatically turn on the duct fan.

[Here's a description of the thermostat. Because this is specifically for heating, I've posted to in the "heat" section rather than "controls"; hope that's okay...]

I did find circuits on the Internet. All were basically non-inverting comparators driving a transistor to activate a relay. I had an 2 amp AC switch that I sampled from ST, an ACS120, that I wanted to use and it takes sinking 20ma at at least 2 volts to turn it on so I needed a negatively biased inverting comparator to turn on the switch.

It's time for me to point out that I am not an Electrical Engineer™ but ScottsAI did give me a long lesson in op-amps in a dump controller thread (Thank you!) so I decided to give this one a try without calling for help at the get-go... This is the circuit that I came up. (Parts choices are based on what I had laying around and not really knowing any better.

The opamp is a TS921. The power comes from a ripped open wall-wart and the thermistor set came from a dead indoor-outdoor thermometer. I put the regulator and AC switch on one board and the thermostat on another.

I can't say that I completely understand the hysteresis. I get about .1v which at room temperature is about 5F degrees. I think that I offset the zero a bit with the pot so that on-state hysteresis is all on the warm-side and won't keep the fan running when the attic is (just a little bit) colder than the shop. I honestly don't know how linear the thermistor is; that .1 volt may only be part of degree at cold temps... I think that if I were clever that a thermistor in the feedback position could take care of that.

Here's a picture of the box built:



It turns out that, in use, the transformer is warmer than I thought so for now I've had to pull the op-amp board out of the box to get the room sensor away from the heat; real soon now I'll turn the room sensor into a remote so that I can put the op amp board back in.

Okay, I'm not happy that the circuit is not isolated from the mains but I've tried to insulate and isolate as much as I could. I don't feel so bad in there there are neutrals running through every cord in the shop but it bugs me a little to have one running up the air duct in that I don't think someone walking in would expect it to be there... An isolated solid state relay (SSR) might be a better choice than the AC switch if I build this again but for now my budget was zero dollars for this project and I was able to keep to it.

Thanks all,

- Ed.

[DamonHD]

Very interesting: thanks for sharing.

If you reduce the 1M feedback resistor a little then you'll increase the hysteresis I believe.

I might also be inclined to add a couple of smallish (nF to uF) caps at or near the op-amp inputs to minimise pickup of stray noise and prevent high-frequency oscillation, but then I'm a coward.

Rgds

Damon

[tecker]

This is a good idea .Filter the air though  Lots of fiberglass gets kicked up and can be filtered with a polester filter as the poly is at oposite charge went the air is warm and dry .

[tecker]

Somebody put a Y in there

[elt]

Thanks Damon,

I'm going to have to look at hysteresis more closely. I need to keep the ACS120 from turning on and off too quickly; the data sheet says that if it switches slower than once a second that a heat sink shouldn't be necessary. (I did put a small copper fin on it anyway.) I don't expect rapid temperature swings but I want to figure out what the delta for the thermistor is around 40F or so then do the math for a resistor value that'll give me a couple of degrees there.

It was late when I hit the "post" button and I sure did mean to mention that the unit is subject to noise and I do see it switch when it's in the hysteresis region and I turn some things on. There is a .1uF cap (forgotten in the schematic) across power to the opamp but I don't know how inputs are filtered; are you saying caps directly on plus and minus pins to their respective rails? I can do that...

Thank you,

- Ed.

[elt]

For sure that's a good idea and that has been a concern of mine when thinking about doing the same for the house.

Turns out that the ceiling of the shop is insulated entirely with foam sheet so all I have to worry about is all the dust and stuff that gets sucked there all summer from the powered ventilator... in the winter I turn off the breaker on that and seal the opening to try to keep more heat in the attic.

I do think I could use a bigger duct and some more CFM. I'll incorporate a polyester filter (Thank you!) when I do that. For now, I'd hate to diminish the air flow that I got. I do care what I breath! (It's a woodworking shop, I do run an air cleaner constantly when I'm working and have dust collection on all the machines.)

Thanks again,

- Ed.

[DamonHD]

I'd certainly consider a few nF at least from the + and - to GND.

(I didn't look up the op-amp data sheet: if it's CMOS you have to be careful to avoid causing SCR latchup in some circumstances, eg if the supply rails dip quickly leaving the capacitors to discharge though the op-amp input protection diodes.  You could counter that in various ways but I think that unless you get to many uF you'll probably be fine in practice.)

Rgds

Damon

[DamonHD]

To be clear: from the + and - op-amp inputs to ground...

Rgds

Damon

[framistan]

I am really impressed with your accomplishment!  I am not an engineer either, but

have worked in electronics for 30 years now.  I notice the OUTPUT of your voltage regulator is tied to the mains common ground.  Hopefully that is just a mistake

in your diagram and the circuit is not actually built that way.  If the wallwart was ever plugged in to the wall the "other way" that would place mains-HOT onto your regulator output. SMOKE would result.  The wire is completely un-necessary

you might consider removing it and see.  Your system is truly inspirational and one of the best posts I have read on this website. thanks for sharing with everyone!

.

.

[elt]

Hi framistan,

I'm really glad that you enjoyed the post but I want to say that I don't take credit for anything other than posting what I did.

The "heat from the attic" is other folks' idea and I don't even do it the best way which IMO is described here -

http://builditsolar.com/Projects/SpaceHeating/BlackRoof/BlackRoofCol.htm

The comparator circuit is basic; A National Semiconductor ap note helped me out some on inverting and non-inverting hysteresis.

The data sheet on the AC switch told me how to connect it... the circuit is built as shown with the main neutral connected to VCC. I did not use the wall wart case; I removed the power supply and connected in the middle of three wire, grounded extension cord so it would be hard to plug it in backwards.

I will try without the neutral connection (Thank you!) though the ACS120 data sheet does show the control's VCC tied to the mains neutral... I'll be much happier if it works your way! I'll post a follow up in the next few days after playing with it some more.

Thanks again,

- Ed.

[scottsAI]

Ed,

Here helping like everybody else!

Normally I would not put caps on Opamp +/- inputs, usually creates a nice oscillator.

Since the opamp is being used as a comparator its OK and recommended. Might consider connecting it to JP1 p1 and ground. Cap will short electrical spikes to ground. Make both caps JP1 p1 and opamp - input cap the same value. + input to gnd may not be needed with above two caps. (try first?).

The noise is from the long wire on the remote temp sensor.

Using lower resistance will increase the amount of hysteresis.

Will stay out of neutral question for now. (need more details)

From the picture the room looks like the Attic and work space are one volume? No ceiling?

If true, the small 6 inch pipe will not do much to warm it up. Heat will quickly rise back up. Fan should circulate the air volume in under a hour, then it can warm it up. No pipe needed, just blow heat down.

If the furnace has a blower maybe use it...

Have fun,

Scott.

[Ungrounded Lightning Rod]

I don't like the mains neutral being on the sensor wiring either.

How about using an optical isolator between the op amp circuit and the AC switch trigger?  (I bet there's an application note for the AC switch giving a recommended and cheap way to do it.)

Worst case you'd have two power supplies, one referenced to neutral, one to protective ground, and a little extra electronics.

[elt]

framistan wrote:

> ... you might consider removing [the connection from mains neutral to the logic' Vcc]

I tried it but I couldn't get it to work that way... In hindsight, I think that I'm relieved that the secondary really is that isolated from the primary (in most applications.) This switch is targeted for embedded use in appliances or machines so I don't think it matters much that it needs the neutral connected to the logic supply.

Thanks again!

 - Ed.

[elt]

Hi Scott,

I put 10nF to ground on each side of the remote thermistor and that cured the noise problem... There is a string of 6 CFL's on the same outlet. Switching them on or off used to toggle the opamp but not any more. I haven't put a cap on the + input; since it seems to be working, I'll leave it as is until it isn't.

I tested the thermistor and it's linear, -.7k per 1F... I swagged the sign backwards in text above so instead of wanting more hysteresis, the 1 megohm resistor is about right. The termistor is about 93K at 40F degrees and the 1Meg resistor gives about 3F degrees of hysteresis right around 40F degrees. (And about 2 degrees at 70F.)

If anyone else wants to build this, the hysteresis might change based on thermistors and the pot used but I don't think any of that is critical. Just set it up by making sure that the thermistors have equalized to the same temperature then turn the pot until the circuit turns on and then back the other way until it turn off. Done.

Sometime before Summer I'll have to invert it so I can use it for cooling too.

> From the picture [...] No ceiling?

It sorta does look that way... there is an insulated ceiling and an uninsulated crawl space.

Thanks again,

  Ed.

[elt]

I like the idea of the optoisolator and I see that Digikey has a little transformer with two-secondaries that would fit the bill for only $3.63USD. I'll save that idea, though, for version two or similar applications. For now it's out of the way and diddling with it hasn't tripped the GFI yet; I think it's okay.

Thanks again,

 - Ed.

[stevo566]

Very interesting.  I'm just curious, why not use a 120 volt plug in thermostat set at 80 or 90 to turn on the fan? Did you close off the attic gable vents or leave them open?  With the backflow device you put in, on a really cold day with no sun, do you feel any cold coming from the attic out of the ductwork? I'm already thinking of where I could run ductwork in my house, great,simple and cheap idea. Thanks Steve

[elt]

Hi Stevo,

A regular thermostat only has half the equation: say that it's 50 degrees in the shop but only 40 degrees in the attic, then I don't want the fan to run. But if it's 50 degrees in the shop and 60 degrees in the attic, then I do.

I realize that I made a half-hearted attempt at a duct damper but it's working good enough that I've never taken the time to do better.

The only venting in the attic is a power vent on the east end. The fan exhausts through shop in the summer to there's no soffit vents or other attic openings to worry about in the winter... but please read the article I linked to; if you do it "right" then none of the other attic openings are a concern.

Hope that helps,

- Ed.