Solar Hot Water Part VI: Performance

Last summer after the system came up, I turned off the gas hot water heater. The gas hot water heater has a pilot and pilots are just about the most energy wasting thing around. I was seeing temperatures on the solar tank of around 150F-160F in July, and it never rains in summer in California (like the old song says) so I figured we were good without the backup.


The results were mixed. Most of the time, we had plenty of hot water with the temperature being fine. With the shower on the top setting, it was not as hot as with the gas heater on (when you really need to turn the setting down to make it tolerable), but it was adequate for a nice hot shower after a long bike ride. Sometimes, though, the water was just lukewarm. This seemed to happen most often when we didn't use hot water for a number of days (like we showered at the gym) or when we went away for vacation. A couple times we had cloudy weather and the tank temperature dropped to 130F, which is I would have expected, but it didn't affect the domestic hot water temperature unless we didn't use much hot water.

The problem is that the gas-fired tank cools down if the hot water is not used for a couple days. Since there is a lot of water in the gas-fired tank, it takes a while for the hot water in the solar tank to draw through, even though the solar water is really hot. By and large, this wasn't much of a problem; it being summer, an occasional lukewarm shower wasn't an issue. I kept the gas-fired tank off until the end of October then relit it. At that point, the temperature in the solar tank was getting down below 130F.

Below you can see a comparison of our gas usage for July through November in 2008 and 2009:

As you can see, the usage was reduced from July through October, but it shot up in November. That's because we had a cold snap in the beginning of December (the November bill includes a couple weeks into December) this year, and November was generally much colder than last year. One of the difficulties of using year over year comparisons of energy usage is this kind of variability due to changes in the weather, etc. In fact, the increase in November completely overwhelmed the savings for July through October. July through November 2008 was 98 therms while the same period this year was 97 therms, just one therm less, or around 1%. If you look at the period from July through October, not counting the time when space heating was generating most of our gas demand, the reduction in gas usage is around 71%.

From last summer's data, I now know we use about 1 therm a month for cooking. In 2004, I discovered that our forced air furnace had a pilot light (yeech!) and I started turning it off in summer. I can therefore use the summer gas use from 2004 through 2009 prior to installation of the solar hot water system to estimate how much  gas we used for hot water heating. That gas will be completely eliminated in the summer 6 months (essentially April through September) with solar hot water. For the other 6 months, estimating usage is more difficult, because the solar acts as a preheater for the gas heater. So the amount of gas eliminated won't be the same as for the summer; on the other hand, it also won't be zero. To be conservative, we can say that it is, in fact, zero; that is, that the gas tank is responsible for all the hot water heating.

The average gas use in the summer months just for hot water heating is 6.32 therms per month. The average annual monthly use for 2004-2008 is 36.5 therms per month. Figuring 6 months of 6.32 therms reduction  and 6 months of 0 therm reduction, the percent savings comes out to 8.65%. So, say, around 10% reduction since we were on the conservative side about the winter use (yes, I know this is a fudge factor but it is the best I can do with the data I have). Anyway, I think I will need some more data for at least another year before I can really say what the reduction is like. There are also some ways to collect data that require measuring the hot water heater output, which I may decide to collect in the end.

The other interesting issue is the effect of the various insulation treatments I've tried. Below is a graph showing the temperature in the morning and evening in July before I installed the radiant barrier insulation:

I didn't use any sophisticated monitoring device for recording these temperatures, I just wrote them down on paper. That's why there are so many holes in the recording, some days I forgot, others we were on vacation. In general the temperature is running around 160F

Below is a chart of the morning and evening temperatures with the tank completely covered in radiant barrier insulation:

The gap between morning and evening is slightly smaller, and the average temperature of the tank drops over the month. The dates here are getting into fall, when the days are shorter and there is less sun on the collector.

Below is a graph of the morning and evening temperatures after putting on the R-13 fiberglass batt blanket:

The temperatures now are a lot lower because the time period is into December when the sun is at the lowest point, but the gap between the morning and evening temperature has narrowed considerably.

Insulation was highly effective in reducing heat loss from the tank. The temperature drop in degrees F per hour was 0.68 for no insulation - noticeably *above* what Superstor advertises. For radiant barrier only, the drop was 0.54, average of two sets of measurements, one shown above and one taken just before the batt was installed in December. For radiant barrier plus batt, the drop was 0.23. The temperature outside  the tank had little impact, the measurements in September and December for the radiant barrier insulation were close (0.57 in September and 0.52 in December). This might possibly be due to the fact that the tank closet itself has 6" of closed cell foam on the top and around 3-6" on the walls, keeping the tank enclosure toasty.

In fact, the insulation was so effective in cutting heat loss from the tank that I am now a bit worried it might overheat in the summer and cook the glycol. We shall see. I've signed up for a maintenance contract to have them come and check the glycol once a year to make sure it isn't acidic, at least for a couple years until I feel comfortable with the system. And, as a practical matter, the Schueco system is specifically designed to purge if the collector reaches stagnation temperature, to avoid cooking the glycol. So I think the system should be OK, and the extra insulation will sure help in the winter. Why can't Superstor put that extra insulation on the tank in the first place? It can't really be a question of cost. And I wonder, since it has about cut the heat loss in half, whether it would have allowed me to get by with one instead of two Schueco panels? Something for a pleasant evening of calculation in the near future.