[Greenbuilding] Looking for a whole-house fan thermostat

[Nick Pine]

Chris Hutchinson <chris at trebuchetgroup.com> wrote:

>... Here's what I would like my mythical thermostat to do -
> namely, turn on when the outside temp is lower than either the
> set point or the inside temp and turn off when the set point is
> reached OR when the inside is the same as the outside.

That seems a little hard to understand. Reading up through the first
words "set point" above, we might expect the fan to be on in the first
line of the table below, even if it were 30 F outdoors. But reading
further, the fan should be off if "the setpoint is reached." And you
might want the fan to turn off if the outside temp is less than say, 5 F
below the indoor temp, vs equal to the indoor temp, because the fan
won't do much cooling compared to the electrical fan power, with
a smaller difference.

>I could then simply set the thermostat for, say 60 degrees and
>here's what would happen (starting in the morning and cycling
>through to the following morning):
>
> Out        In            Fan
> 55           60           off   [too cold--turn off the fan]
> 58           62           on   [we'd like to keep the room 60 F--turn 
> on the fan]
> 62           62           off   [the fan won't cool in this case, so 
> turn it off]
> 80           64           off   ["]
> 85           66           off   ["]
> 75           68           off   ["]
> 68           70           on   [cooler outdoors--turn on the fan, or 
> maybe]
> 60           65           on   [wait till it becomes 5 F cooler 
> outdoors...]
> 55           63           on   ["]
>
> Does this make sense?

Sure.

>I'm beginning to think that I'll have to get an X10 controller, 
>receiver,
>and 2 temp sensors to do this -

You might go further than that, with a $119 eway TU PC loaded with
ubuntu and a USB one-wire adapter and a USB X10 adapter and 2
$44 Aagelectronica AG 8540A temp/RH sensors to measure indoor
and outdoor temperature and humidity. That way, you could also heat
the house on a spring day when outdoor air is warmer than indoor air
and dry enough that there's no danger of condensation on cold surfaces
indoors (with a dew point calc.) You could try to keep the house on
the cool side in summertime and the warm side in wintertime this way.

> yet it would seem like something that >someone should have made
>for people who want to use outside evening air to cool the house
>instead of an air conditioner!

Sure does. "Economiser controls" do this for large buildings.

>If we can't find one I've got dibs on the patent -

I'm sure it's covered by expired patents.

>>this could even be a plug into the wall unit that could do window fans 
>>or
>>stand fans.

Sure.

Corwyn <corwyn at midcoast.com> wrote:

> If you were to combine a differential thermostat with a regular
> thermostat in series, you will get AND logic

As in "run the fan when outdoor air is at least 5 F cooler
than indoor air AND indoor air is above the setpoint."

Nick