Attic fan control

[electricmanscott]

I have a customer that has a standard attic vent fan with a t stat control that activates when the temp rises and deactivates when the temp goes below the set point.

They want to have it run in the winter too to draw in cold air to help prevent roof snow melt and ultimately ice dam prevention.

I can't think of a good way to accomplish this. Any ideas?

 

[electricguy61]

How about a manual switch in the living area of the home? I usually wire a bypass of the thermostat so the customer has control when they wish.

 

[Jraef]

I have a customer that has a standard attic vent fan with a t stat control that activates when the temp rises and deactivates when the temp goes below the set point.

They want to have it run in the winter too to draw in cold air to help prevent roof snow melt and ultimately ice dam prevention.

I can't think of a good way to accomplish this. Any ideas?

Aside from sounding like a bad idea (sucking in moisture?) you would need to have a reversible motor. Most cheap fan motors are NOT reversible (without disassembling the motor and creating new lead connections) so you would need to change out the motor to one that is. Then you get into having to match the motor mounting, weight etc. Might be easier to see if you can find a new attic vent fan that is reversible. Then you would have to wire in a reverse switch somewhere that they can access it easily.

If they want it to remain fully automatic, you would need something a lot more intelligent than a t-stat now, because it has to "decide" when to suck and when to blow and under what conditions, taking into account more than just temperature. For example, do you want it sucking in air and snow during a blizzard? You might need one of those "smart relays" that can be programmed for a number of sensor inputs such as temperature and humidity. But how would you detect snow? I don't know actually.

You also have to make sure they don't have automatic shutters, which can be problematic in wind storms. The whole thing sounds more and more like a bad idea to me, but then I don't live in an area that snows, so I have little experience with why they think they need it.

 

[Jraef]

Oh wait, duh!

No need to reverse the fan, what was I thinking? It still draws in cold air from somewhere!

The other issue of the decision process is still probably valid, but in thinking about it, there may be an easier solution.

If THEY can decide when they want to and not want to use it during cold dry conditions, then you can go to an appliance parts store and get what's called a "freeze stat". This is what they use in refrigeration to turn on the compressor. It changes state on temperature RISE above a freezing set point, say 30 degrees. So you would need one with NC contacts, then wire those in series with a switch they have access to. How it would work is, they decide they need it, they turn on the switch. If the temperature is above 30, the contacts are open and nothing happens, but when it falls below 30, the contacts close again and the fan comes on.

I hope they have good insulation though...

 

[LarryFine]

How about a manual switch in the living area of the home? I usually wire a bypass of the thermostat so the customer has control when they wish.

I did that when I installed our gable-mounted attic fan. I ran 14-3 NM to it, with the red connected to the junction between the T-stat and the fan motor, and installed two switches by the attic stairs.

One is for on-off control, and the other is a manual bypass, so I can keep the fan off or keep it on, as well as allow automatic operation. I also set attic fan T-stats lower than they usually recommend.

 

[don_resqcapt19]

You need to be careful about bypassing the internal control. Most attic fans have a high temperature shut down function as a part of that control that will shut the fan down in the event of a fire.

 

[K8MHZ]

I have a customer that has a standard attic vent fan with a t stat control that activates when the temp rises and deactivates when the temp goes below the set point.

They want to have it run in the winter too to draw in cold air to help prevent roof snow melt and ultimately ice dam prevention.

I can't think of a good way to accomplish this. Any ideas?

If this is an unheated garage, how would this help?

I know an antique dealer that has a detached garage and has the vent come on in the winter when the humidity gets above a certain level. I have never seen a bit of improvement as far as snow accumulation goes. Other garages with no ventilation have the same amount of snow.

Now, if it's a heated garage with an attic, ventilation would help, but I don't see where anything more than a good vent design would be needed.

 

[electricmanscott]

If this is an unheated garage, how would this help?

I know an antique dealer that has a detached garage and has the vent come on in the winter when the humidity gets above a certain level. I have never seen a bit of improvement as far as snow accumulation goes. Other garages with no ventilation have the same amount of snow.

Now, if it's a heated garage with an attic, ventilation would help, but I don't see where anything more than a good vent design would be needed.

Garage?????


I was thinking along the lines of what Larry posted. Thanks for the replies.

 

[K8MHZ]

Garage?????


I was thinking along the lines of what Larry posted. Thanks for the replies.

Duh...

I don't know how I read garage into that. Nevermind....

:roll:

 

[iwire]

Just add any line voltage close on fall T-stat in the right temperature range in parallel with the existing cooling stat.

Here is my own set up ...

I have an indoor and outdoor temp sensor so the fan will not try to cool the attic during cool weather. I also included a time clock to look it out on hot nights and the three way provides a manual override without a back feed into the other controls.

I could program the indoor one to close on rise and close on fall with a huge differential between them.

 

[M4gery]

I did that when I installed our gable-mounted attic fan. I ran 14-3 NM to it, with the red connected to the junction between the T-stat and the fan motor, and installed two switches by the attic stairs.

One is for on-off control, and the other is a manual bypass, so I can keep the fan off or keep it on, as well as allow automatic operation. I also set attic fan T-stats lower than they usually recommend.

When you are in bypass mode, you are essentially backfeeding the T-stat. Is that bad?

I want to do something similar with a photocell to bypass it, but doing so will backfeed the photocell, I was wondering if that would hurt it in any way?

 

[LarryFine]

When you are in bypass mode, you are essentially backfeeding the T-stat. Is that bad?

No. This T-stat is a simple mechanical bi-metallic switch. It's simply two SPST switches in parallel.

I want to do something similar with a photocell to bypass it, but doing so will backfeed the photocell, I was wondering if that would hurt it in any way?

I really don't know. That's something the manufacturer could tell you. It could even depend on whether the line is hot when the load is back-fed.

You could use a SPDT switch (a 3-way switch, for example) with the common feeding the load, one traveler fed by the photocell's load connection, and the other traveler fed from the always-hot line connection.

This way, when in bypass, the load terminal is left floating, which is no more harmful than a missing or burned-out bulb.

 

[mtfallsmikey]

I did that when I installed our gable-mounted attic fan. I ran 14-3 NM to it, with the red connected to the junction between the T-stat and the fan motor, and installed two switches by the attic stairs.

One is for on-off control, and the other is a manual bypass, so I can keep the fan off or keep it on, as well as allow automatic operation. I also set attic fan T-stats lower than they usually recommend.

If anyone has ever had problems with ice dams, running the fan in winter manually will keep cold air moving thru the attic, warmth from the living spaces causes the ice dams...

Very creative, Larry, I like it!

 

[LarryFine]

Very creative, Larry, I like it!

Thanx! You may also like the idea of placing a diode across a switch to make a hi-lo switch (incandescent) out of an on-off switch.

When my son started using the bathroom himself, this was a great night-time solution. It's bright enough for still-half-asleep eyes.

 

[mtfallsmikey]

Thanx! You may also like the idea of placing a diode across a switch to make a hi-lo switch (incandescent) out of an on-off switch.

When my son started using the bathroom himself, this was a great night-time solution. It's bright enough for still-half-asleep eyes.

Well, in Mt. Falls, placing a diode across the switch, and making it a hi-lo would make my house LEED Gold certified, wouldn't it?

When whole house fans came out, the mfr's used horse hair in the humidity sensors (no joke!)