I have a gable mounted, thermostatically controlled attic exhaust
fan which I use to hold down attic temps. I know some people think
those are not energy efficient, but I've made my decision and it is
not the subject here.

It occurs to me that there are times when the fan is running needlessly,
because even though the attic is hot enough to trigger the thermostat,
the attic temp and outside temp are almost the same, so the fan is
simply replacing hot air with air that is almost as hot.

I don't want to just increase the thermostat setting to be above the
maximum outdoor temperature (about 105 here in Austin, Texas this year),
because then there will be many times when the fan could run to
advantage but would not.

Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?

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CJT wrote:
I have a ...thermostatically controlled attic exhaust fan ...
Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?
....

Don't know of an inexpensive differential controller offhand, but the
simplest way to approximate one would be a second thermostat (for
outdoor air temp, of course) in line w/ the present controller. Set it
to be on when the air temp is at or below your desired external
temperature for bringing in exterior air and the interior one at
whatever level above that you want it to come on at. The fan will then
only run when both conditions are met -- when the exterior temp gets to
whatever level you want it will open and cut the fan off.

CJT wrote:
I have a gable mounted, thermostatically controlled attic exhaust
fan which I use to hold down attic temps. I know some people think
those are not energy efficient, but I've made my decision and it is
not the subject here.

It occurs to me that there are times when the fan is running needlessly,
because even though the attic is hot enough to trigger the thermostat,
the attic temp and outside temp are almost the same, so the fan is
simply replacing hot air with air that is almost as hot.

I don't want to just increase the thermostat setting to be above the
maximum outdoor temperature (about 105 here in Austin, Texas this year),
because then there will be many times when the fan could run to
advantage but would not.

Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?

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You can do this with a microcontroller. Have it turn on when the
difference between the outside air and inside air has reached your
pre-programmed setting. Have it turn off when the difference is a few
degrees apart. Maybe there is something that exists on the market, but
I have no idea. I would build my own.

Pj

Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?


Sure, what you want is a differential controller. They are commonly used in
greenhouses, so search for "greenhouse temperature controller". Here are some,
about halfway down, look for the Dayton controllers:

http://www.littlegreenhouse.com/acce...controls.shtml

--
Dennis

In article , CJT wrote:
I have a gable mounted, thermostatically controlled attic exhaust
fan which I use to hold down attic temps. I know some people think
those are not energy efficient, but I've made my decision and it is
not the subject here.

It occurs to me that there are times when the fan is running needlessly,
because even though the attic is hot enough to trigger the thermostat,
the attic temp and outside temp are almost the same, so the fan is
simply replacing hot air with air that is almost as hot.

I don't want to just increase the thermostat setting to be above the
maximum outdoor temperature (about 105 here in Austin, Texas this year),
because then there will be many times when the fan could run to
advantage but would not.

Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?

Search for a "differential thermostat". I think most are
made for solar systems and greenhouses but you'll likely
find one that would do the job. This is a low volume item
compared with most consumer products so I would expect it
to be a relatively expensive device compared to a regular
simple thermostat.

--
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~|
| Malcolm Hoar "The more I practice, the luckier I get". |
| Gary Player. |
| http://www.malch.com/ Shpx gur PQN. |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~

On Sun, 27 Aug 2006 18:24:58 GMT, CJT wrote:

I have a gable mounted, thermostatically controlled attic exhaust
fan which I use to hold down attic temps. I know some people think
those are not energy efficient, but I've made my decision and it is
not the subject here.

It occurs to me that there are times when the fan is running needlessly,
because even though the attic is hot enough to trigger the thermostat,
the attic temp and outside temp are almost the same, so the fan is
simply replacing hot air with air that is almost as hot.

I don't want to just increase the thermostat setting to be above the
maximum outdoor temperature (about 105 here in Austin, Texas this year),
because then there will be many times when the fan could run to
advantage but would not.

Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?

Use two thermostats?

CJT wrote:

I have a gable mounted, thermostatically controlled attic exhaust
fan which I use to hold down attic temps. I know some people think
those are not energy efficient, but I've made my decision and it is
not the subject here.

It occurs to me that there are times when the fan is running needlessly,
because even though the attic is hot enough to trigger the thermostat,
the attic temp and outside temp are almost the same, so the fan is
simply replacing hot air with air that is almost as hot.

I don't want to just increase the thermostat setting to be above the
maximum outdoor temperature (about 105 here in Austin, Texas this year),
because then there will be many times when the fan could run to
advantage but would not.

Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?

Wow ... thank you all for the excellent ideas and information. And so
quickly!

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DT wrote:

Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?



Sure, what you want is a differential controller. They are commonly used in
greenhouses, so search for "greenhouse temperature controller". Here are some,
about halfway down, look for the Dayton controllers:

http://www.littlegreenhouse.com/acce...controls.shtml

--
Dennis

Thanks for the info. When I look at that particular page, it seems
the differential they're talking about is what I would call hysteresis
(i.e. the difference between the "on" and "off" temperatures), rather
than control based on the difference between two sensors (e.g. attic
and outdoor). But I do think greenhouse controls are likely to be a
fruitful (no pun intended :-) )area for my search.

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Malcolm Hoar wrote:

snip

Search for a "differential thermostat". I think most are
made for solar systems and greenhouses but you'll likely
find one that would do the job. This is a low volume item
compared with most consumer products so I would expect it
to be a relatively expensive device compared to a regular
simple thermostat.

I did just that, and you are right about the price. They seem to
cost $100 and up, judging by what I found on the Web. I do not
believe that level of expenditure is justified by the likely savings
in energy cost (which, IMHO, is highly unlikely to exceed $15/year).

I may pursue the homebrew option. If I can build something serviceable
for under $25, it might be worthwhile.

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minimize spam. Our true address is of the form .

On Sun, 27 Aug 2006 18:24:58 GMT, CJT wrote:

I have a gable mounted, thermostatically controlled attic exhaust
fan which I use to hold down attic temps. I know some people think
those are not energy efficient, but I've made my decision and it is
not the subject here.

It occurs to me that there are times when the fan is running needlessly,
because even though the attic is hot enough to trigger the thermostat,
the attic temp and outside temp are almost the same, so the fan is
simply replacing hot air with air that is almost as hot.

When is it you think this is true? I have an attic fan in the roof
and I can't think of any time when it is hot enough to turn the fan on
that it it isn't quite a bit hotter inside the roof than outside.

I don't want to just increase the thermostat setting to be above the
maximum outdoor temperature (about 105 here in Austin, Texas this year),
because then there will be many times when the fan could run to
advantage but would not.

Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?

mm wrote:

On Sun, 27 Aug 2006 18:24:58 GMT, CJT wrote:


I have a gable mounted, thermostatically controlled attic exhaust
fan which I use to hold down attic temps. I know some people think
those are not energy efficient, but I've made my decision and it is
not the subject here.

It occurs to me that there are times when the fan is running needlessly,
because even though the attic is hot enough to trigger the thermostat,
the attic temp and outside temp are almost the same, so the fan is
simply replacing hot air with air that is almost as hot.


When is it you think this is true? I have an attic fan in the roof
and I can't think of any time when it is hot enough to turn the fan on
that it it isn't quite a bit hotter inside the roof than outside.

For instance, on very hot nights the attic temp can eventually
approximate the outdoor temp while both are still quite hot.

I don't want to just increase the thermostat setting to be above the
maximum outdoor temperature (about 105 here in Austin, Texas this year),
because then there will be many times when the fan could run to
advantage but would not.

Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?




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On Tue, 29 Aug 2006 22:18:31 GMT, CJT wrote:

mm wrote:

On Sun, 27 Aug 2006 18:24:58 GMT, CJT wrote:


I have a gable mounted, thermostatically controlled attic exhaust
fan which I use to hold down attic temps. I know some people think
those are not energy efficient, but I've made my decision and it is
not the subject here.

It occurs to me that there are times when the fan is running needlessly,
because even though the attic is hot enough to trigger the thermostat,
the attic temp and outside temp are almost the same, so the fan is
simply replacing hot air with air that is almost as hot.


When is it you think this is true? I have an attic fan in the roof
and I can't think of any time when it is hot enough to turn the fan on
that it it isn't quite a bit hotter inside the roof than outside.

For instance, on very hot nights the attic temp can eventually
approximate the outdoor temp while both are still quite hot.

I"ve found that the fan turns off between 7PM and 10PM, rarely going
as late as 10, depending on how sunny and hot the day had been.

The fact that it turns off implies to me that for some time before
then, it was cooling off with cooler outside air.

And my experience when I walk outside after sundown is that it cools
off more quickly than even does the second floor of my house. (I
rarely use AC)

I have doubts that it is ever as hot outside as it is inside the attic
after sundown.

If I ever again see an indoor/outdoor thermometer at a yard sale,
maybe I'll get it, so I can know "for sure".

mm wrote:

On Tue, 29 Aug 2006 22:18:31 GMT, CJT wrote:


mm wrote:


On Sun, 27 Aug 2006 18:24:58 GMT, CJT wrote:



I have a gable mounted, thermostatically controlled attic exhaust
fan which I use to hold down attic temps. I know some people think
those are not energy efficient, but I've made my decision and it is
not the subject here.

It occurs to me that there are times when the fan is running needlessly,
because even though the attic is hot enough to trigger the thermostat,
the attic temp and outside temp are almost the same, so the fan is
simply replacing hot air with air that is almost as hot.


When is it you think this is true? I have an attic fan in the roof
and I can't think of any time when it is hot enough to turn the fan on
that it it isn't quite a bit hotter inside the roof than outside.


For instance, on very hot nights the attic temp can eventually
approximate the outdoor temp while both are still quite hot.


I"ve found that the fan turns off between 7PM and 10PM, rarely going
as late as 10, depending on how sunny and hot the day had been.

The fact that it turns off implies to me that for some time before
then, it was cooling off with cooler outside air.

And my experience when I walk outside after sundown is that it cools
off more quickly than even does the second floor of my house. (I
rarely use AC)

I have doubts that it is ever as hot outside as it is inside the attic
after sundown.


Once they're within about 10 degrees F, it's not worth running the
fan any more, because the fan uses more energy than it removes as heat.

If I ever again see an indoor/outdoor thermometer at a yard sale,
maybe I'll get it, so I can know "for sure".




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