Thermal Performance Test -- Low Thermal Mass Sunspace for Space Heating -- Day 2

This page describes a 2nd day of test of a low thermal mass sunspace that is optimized for providing space heating to the home its attached to.

The test covers heat gain, efficiency, thermal images, and a characterization of the living environment inside the sunspace while its providing space heating to the house.

sunspace for house heating 

On this day, the sunspace started with the same vertical heat absorbing curtain as day 1, but this was taken down about noon, so that the curtain vs no curtain could be compared.

Bottom line is that the sunspace provides as much heat to the house as a good active solar collector of the same size would, while also providing an area that can be used for lounging, clothes drying, and many other uses.

The additional finding on this test is that adding the vertical curtain in the middle of the sunspace does not appear to significantly change performance.

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Page Index

Sunspace Configuration  -- January 5, 2013 Test

In this test, the sunspace has been optimized for space heating the attached house via the in the following ways.

Its the same basic configuration as day 1.

More details on the construction, objectives,...

This test starts with the same vertical black weed fabric suspended vertically down from a support pipe that runs about 2.5 ft below the ridge line.   At about noon, the vertical curtain is removed to see if it results in any change in heat output.

sunspace for space heating

Vertical Screen

Test starts with the same vertical screen as the day 1 screen.

A vertical screen was added that starts about 36 inches directly below the peak and hangs vertically right below the ridgeline.  It extends the full east-west dimension of the space.  When the screen hits the floor, it is carried southward on the floor to the south wall.  The screen is Scotts black weed fabric from Home Depot. 

The idea of the screen was to provide surface where solar radiation could be absorbed and converted to heat.  The screen is located directly under the peak, so heated air can rise directly up both the front and back surfaces of the screen directly to the collection area at the peak.

The radiation that gets through the screen encounters the back (north) wall and is absorbed there.  Air heated by the back wall and roof rise up the wall and roof slope into the collection area in the peak. 

At about noon, the vertical screen is dropped and just laid out on the floor as in this picture.

So, the solar radiation that was absorbed by the screen now hits the north wall and floor and is absorbed the dark paint.










Inlets and Outlets

Same as fro day 1 inlets and outlets

Heating Performance

This was another very clear day.  Its also only a couple weeks after the Winter Solstice, so the days are short and the sun is low in the sky (about 23 degrees at noon).  The outside air temperature was in the 20'sF.  Wind was generally light.


sunspace performance plot

Blue solid line -- sun intensity (w/sm)

Red Lines -- Fan outlet temperatures for East and West fans (F)  -- this is the temperature of the heated air going to the house for space heating.

Green dash-dot -- Ambient temperature (F)

Purple dash line - Temperature at about the 5 ft level in the sunspace (F)

Yellow-Green solid line -  Temperature 1 inch from glazing in shade (F)

The sunspace was run in the same configuration as day 1 until 1:10 pm, at which time the vertical curtain was dropped to the floor.  The black curtain was spread out on the floor so that it covered the whole floor.  The floor surface without the screen would have been the reflective film on the 1.5 inch Styrofoam that was on the floor.

Total flow rate for the outlet ducts was 670 cfm (based on 614 fpm at outlet ducts).  Same as day 1.

Start Up 


Right at the start of the day (8:55 am), the sun comes on the sunspace, and all of the air temperatures rise very rapidly.  After about 10 minutes (9:05 am), it gets hot enough for the controller to turn on the fans, which stops the rapid rise in temperature by exporting the heated air outside.  It gives an idea how fast the temperatures rise when the sun hits the sunspace, and how effective the fan system is in controlling the temperatures and exporting what would otherwise be excessive heat.

The three peaks in the fan outlet temperature (red at 9:10 to 9:20 am) is the fan short cycling.  There is also some fan short cycling at the other end of the day around 4 pm.

There is a strange bump in the temperature sensor mounted at 5 ft about the floor north of the screen for half an hour at noon.  This also appears on the day 1 plot.  Not sure, but I think that during the time period, the sensor is getting direct sun from a gap between the two fabric panels in front of it.

Dropping the Curtain

At 1:10 pm, the vertical curtain is dropped to the floor (marked by twin triangles on the plot). 

There is a rapid temperature drop of about 10F in the fan outlet temperature, followed by a gradual rise.  During the period, the sun intensity is steadily dropping.

At first glance, it looks like dropping the curtain does result in a significant drop in temperature and heat out, but looking more carefully, I don't think so.  The rapid initial drop may be due to the curtain dropping process and transients caused by that.  If you go a few more minutes down the plot, it looks like the fan outlet temperature curves just about regain the temperature they would have had (with the dropping sun levels) if the curtain had been left up.  I'm inclined to think that the efficiency difference between the two is small if any.

A couple things that do definitely change:


Temperature logger about 1 inch from the glazing.  It is shaded by the
yellow paper.

So, in the end, it does not seem to make much difference in performance if the curtain is up or down.  It does make a difference in the comfort level.  I think its nice to be able to have some form of movable curtain, probably closer to the glazing, that can be deployed to create a more shaded spot during the brightest part of the day, and then slid over to the side at other times.  It looks like you can do this with little to no performance drop.


Cool Down at Sunset

sunspace cool down

This is a zoom in on the full day plot above to the time near sunset.

At about 4pm, the sun intensity is down enough so that the fans start to short cycle.

From the time the sun intensity drop to less than 100 w/sm until the sunspace is within a couple degrees of outdoor ambient is about 2 hours.  So, with the low thermal mass, it warms up fast and it cools off fast.

Heat Production and Efficiency

Heat production and efficiency were similar to the day 1 test


Temperature Environment in Sunspace

The thermal images of the inside of the sunspace were very similar to the day 1 test

Air and Surface Temperatures

The two temperature surveys before are with and without the vertical curtain in place.

11:47 am with curtain up

sunspace temperature
Temperatures at 11:47 am -- vertical curtain in place.


Temperatures at 1:33 pm -- no vertical curtain.

In these pictures, the grey filled circles are surface temperatures (taken with an IR temperature gun), and the teal filled circles are air temperatures taken with a couple indoor/outdoor thermometers.

As expected, the back wall and the lower north roof temperatures went up because they now have the sun directly on them. 

The air temperatures on the 1:33 pm survey are a bit high for comfort indicating that somewhat more fan capacity might be good to remove the  heat a bit faster, or there may be other ways to encourage the heated air to get to the peak area without overheating the lower area?  As a practical matter, it did not seem that uncomfortable, and all of these sensors were in the sun and probably should have been shaded to get true air temperatures.


Got comments, Questions, Suggestions -- go here...




January 11, 2013