Using Attic Heat -- Recorded Attic Temperatures

There is an ongoing interest in taking advantage of high attic temperatures for things like:

- Preheating domestic water


- Space heating


- A source of pre-heated air for the clothes dryer


- Pool heating


I've been logging my attic temps for a while just to try to get an idea what the potential is for a cold climate location.  The plots below show the results.


The logger was placed about 6 ft up the roof line, and near the roof sheathing.

The ceiling that is the floor of the attic is well insulated (about R50).  The attic space is ventilated by ridge and soffit vents.

The roof has a 45 degree pitch, and the ridge line runs roughly east west. (ie it has big roughly south facing face).

The attic is where the "Potential Roof Location" note in the picture just below.


We are located in Bozeman, MT (southwestern Montana).
Bozeman gets a long, cold, 8100 Heating Degree Day winter, so areas with milder winters may well do better than we would extracting heat from the attic in mid winter.  But, in either kind of winter, there are shoulder seasons where both types of winters would benefit.





Recorded Attic Temperatures


NEW: Another set of recorded attic temperatures for a home in SE VA...

NEW: Added temperatures and RH for Fall -- Winter -- Spring 2011 -- 2012


Winter into start of Spring  --  Mid November through end of March





If possible, I'll keep the logger set up through the summer and fall to get a full year.


Potential Uses of Attic Heat

Some rough conclusions so far for our SW Montana Climate:

Space Heating

- In the mid winter, there are only a few days when temperatures get up into the 80's, which is pretty much the threshold for useable heating.
On most mid-winter days, the attic daily high temperatures are 70F or less.


- In the mid-winter, on the days that do get above 80F for the attic high, you typically have 3 to 4 hours in the mid-afternoon when temps are above 80F.  This is a time period you could presumably blow attic air down into the house and get some free heating.

In the time period from Nov 15 through March 1, I count 22 days on which you could get some limited heat from the attic -- about 20% of the days.


- Starting around March 1, we got many more days with attic high temps above 80F, and quite a few days up into the 100F area (see plot below).  This appears to offer more potential for space heating.  In our climate, March is very much a space heating month, so, it seems like there is some potential here.

My guess is that this will also be true for early to late fall.


Domestic Water Preheating

- One thing to notice right away is that attic temps at night can go below freezing any night during this late fall to early spring period.  Even in the spring, the attic night lows were often down in the 20'sF.  So, some sort of protection from freezing would have to be part of any attic preheat scheme in our climate.


- As far as the potential for useful preheating of water, its probably similar to the space heating story -- limited potential in mid-winter with much more starting in the spring.  In the summer, I would expect nearly every day would offer good water preheating, but you would still want the water being preheated to be insulated from the nighttime attic temperatures.


- In this kind of climate, you would not just want to put a preheat tank or a big coil of pipe up in the attic and hope for the best.  You would need to insulate the preheat tank or coil heavily, and then run the attic hot air from the attic peak through the insulated box only when the attic peak temperature exceeds the water tank temperature.  If you don't do this, you might well end up cooling the water rather than heating it -- not to mention the potential for freezing problems.  This probably holds true even in the summer.

Dryer Air

- One idea is to figure out a way to draw intake air for the clothes dryer from the attic area when attic temps are high.  This way the dryer does not have to do as much heating of the air.  This seems like a potentially good idea to me if one can figure out a workable ducting arrangement.  


If you use the conventional dryer arrangement, the dryer heats room temperature air, and then wastes the heat by venting the heated air outside.  In addition, the room temp air the dryer uses gets replaced by infiltration of cold air from the outside of the house.  A double energy waste.

If you could do dryer loads using attic air (when the attic is hot), and then vent the air outside, you save the dryer some heating and you avoid pulling cold air in from outside to replace the dryer heated air.  If you live in a dry climate, and its an electric dryer, you could go a step further and vent the dryer air inside -- this effectively uses the same attic heat first for drying clothes and then for heating and humidifying the house.

The dryer energy use (and potential saving) are pretty large...


In the summer, drawing the dryer air from the attic would still be a benefit as it keeps the dryer from having to heat the air from room temperature up to dryer temperature.  This is of the order of 4 KWH per dryer load.  You would want to vent the dry exhaust air to the outside in the summer.


Pool Heating

There is at least one commercial pool heating solution that uses an air to water heat exchanger in the attic to provide pool heating.  Since this is a summer demand, it seems like there is some potential here.

The car radiators that have built in electric fans can be very effective air to water heat exchangers, and might be worthwhile trying as pool heaters.


At any rate, I think that people tend to go up in the attic and find it very hot, and then come to the conclusion that its that way all the time.  This may be true in some climates, but clearly not so in Montana.  That said, it does appear that there is some potential for getting some free heat even in cold climates.


One thing that would probably help in harvesting heat from the attic would be to put the attic venting on some type of control that only provided attic venting in the winter when it was actually needed.  I know some of you are thinking this guy is crazy even talking about messing with attic venting, but I would guess that the level of attic venting is chosen for worst case combinations of temperature and humidity, and most of the time its likely well in excess of what is needed.  


More Plots of the Same Data

These are some additional views of the same attic temperatures as are plotted above.


Same time period with Relative Humidity and Temperature.

I was a bit surprised how much the relative humidity comes up when it gets very cold -- even in our very dry climate.



Blow up of Spring Time Period

Quite a few days with useful attic heat.



A couple of the rare days in mid-winter with attic temps above 80F.

Fall -- Winter -- Spring of 2011 - 2012

More temperatures for the winter and early spring of 2011 - 2012.

The winter of 2011 -- 2012 was the mildest winter we have had in the 11 years we have been here by quite a margin.  Just to my eyeball, the milder weather did not seem to make for much warmer attic temperatures than the ones about for 2010.


Attic temperature and RH



Attic temperatures


Sensor location in attic:

Logger height off the floor: 79 inches

Ridge height from floor: 171 inches

Roof slope: 45 degrees

Logger is  41 inches east of the north wall of the attic

Total length of this attic section is 152 inches (the rest of attic is living space)


Logger in middle of rafter truss bay.
Logger case is right against sheathing.
Onset computer temp/humidity logger
View of the attic space.  The logger is
out of sight to the right.



Gary Updated July 10, 2011