Green Home Details – Hot Water

In a previous post were summarized the various green elements of our home in Tiburon. Now let’s dive into the details. This post will deal with how we make hot water including system design, estimated savings and early system performance data.

As mentioned earlier. the overall house is built to have 2 separate living quarters which we call the home and the apartment. When we first purchased it, the home had 2 people living in it and the apartment had one person living in it. Both the home and the apartment consumed natural gas for home heating and for making hot water for showers and sinks. Overall, with careful use these places together used approximately 120 therms of natural gas on average per month – more in the winter and less in the summer when only the hot water was called upon.

The gas furnaces were typical forced hot air systems and the hot water was made in 2 separate tanks – one of 50 gallons and one of 40 gallons.

One way to look at this is that the house was using about 40 therms per month per person. Today we have 4 people living in the house – 1 in the apartment and 3 in the home.

One goal we had for the overall house was to cut our carbon emissions as much as practical. So we believed it would be possible for us to make substantial amounts of electricity from photovoltaic panels and so we converted the gas home heating system to heat pumps. This eliminated about 50% of the overall gas load but shifted it to the electrical side of the equation.

Next we decided that if we were to use solar energy to make much of the hot water we required, that we would only install one solar assisted hot water system to serve both the home and the apartment. After further investigation, we learned that tankless hot water heaters have numerous advantages over the older style tank based systems.

Some of the benefits of a tankless system are –

  • no use of gas during periods when there is no demand. For example, if you go on vacation, a tankless system will use no natural gas, but a tank system (unless you turn it off) will keep a tank full of hot water ready for use even when you are not there.
  • tankless systems work well with solar assist because they make water even easier if it is preheated like naturally happens when the sun shines.
  • tankless systems can be designed to use multiple heating units in series and this has the effect of increasing the capacity of the system; however, if such a system has a light load the extra unit will remain off and use little or no energy. We opted for this approach as there are times when the house has as little as 2 people but there are also times when there are 8 or more people staying there. So the load on the hot water system is highly variable but we never run out even with everyone taking showers at the same time. Cool. But still highly efficient.

Domestic hot water typically accounts for 15-20% of your home utility bills (1), (2), (3).

We also have a gas fired cook top or stove which has 5 burners, 3 normal and 2 higher heat for Woks or large cooking. We estimate that our gas usage splits about 75% for hot water and 25% for cooking. (Most of the energy we need for cooling comes from electricity through the microwave oven, toaster, steamer, coffee machine, etc.)

So if our average monthly usage used to be 120 therms when we had the gas furnaces and the tank style hot water heaters and no solar help. Our goal was to cut our natural gas usage by 90%.

Here is the system block diagram or schematic for our hot water system for the home.

Block Diagram

Block Diagram

The way this system operates, we have 2 hot water panels on the roof and they have plumbing that puts them in series one after the other.  Over a typical sunny day, the heat energy derived from the panels is used to warm water in a 120 gallon storage tank.  As we use hot water, it is pulled from the storage tank and passes through a pair (also in series) of tankless hot water heaters that are natural gas fired.  If the water in the tank is not warm enough for our general use (showers and such) then the tankless heaters warm it further before it is distributed by the home plumbing.  Typically the water is warm enough so that the sun is doing most of the work.  But during a prolonged cloudy period the tankless water heaters can do all of the work by themselves.

With the system described, we make about 75% of our hot water from the sun and the additional 25% of the hot water from natural gas.  Because we also took out our gas furnaces in the home and replaced them with heat pumps (air heat exchangers and not ground source) our overall natural gas usage had decreased from an average of 100 therms per month (annual basis/12) to around 10 therms per month.

So now let’s take a look at how this appears.  In the early morning following a day where the sun was shining but where some showers were taken the previous night and where we did some laundry or dish washing, the system looks like the following –

Hot water system before sunrise

Hot water system before sunrise

You may notice that the collector output temperature is actually warmer than the storage tank, this snapshot of the system was taken just a few minutes after sunrise.  The panels are warming up but they do not have sufficient heat to start up the pumps and begin the heat transfer.  The next snap shot, taken 10 minutes later crosses this threshold –

Hot water system as sun starts heating

Hot water system as sun starts heating

In our system the pumps are turned on and heat begins to transfer when the panels are 18 degrees F warmer than the tank.  And this transfer stops when the panels are 2 degrees warmer than the tank.  Because of the way the tank begins heating you see an immediate small temperature rise in the water temperature.  Next let’s look 10 minutes later –

Hot water system 10 minutes after start up

Hot water system 10 minutes after start up

Again, you can see that the panels are now steadily supplying energy in the form of heat and that the temperature of the water in the 120 gallon tank has risen by about 2.5 degrees over that time period.  After 1 hour, the system now reports –

Hot water system 60 minutes after sun warming begins

Hot water system 60 minutes after sun warming begins

Now the tank has risen about 8 degrees in a period of 1 hour.  therefore we might expect a temperature rise of about 24 degrees in 3 hours –

Hot water system 3 hours after sun warming begins

Hot water system 3 hours after sun warming begins

We get actually 23 degrees which shows that the system behaves relatively linearly while the tank temperature is not too hot.  After 4 hours the tank temperature rose another 8 degrees to around 102 F.

On a fall day – this data was taken on October 11, 2008 – the sun rose about 7:30 am and reached a peak input to the system around noon.  By 3 pm in the afternoon, the angle of these hot water panels begins to be inefficient and little heat gain occurs after that time.

Another point that will come up again and again in this series of essays is that I have found this ability to visually see teh performance and operation of the system most helpful.  Early on, the system had some plumbing problems and lost the working fluid that brings heat down from the panels to the contol system and eventually into the tank.  These might not have been spotted without monitoring. Later we had problems with the control electronics and a circuit board had to be replaced.  Again, without monitoring, I do not believe we could have seen the problem and fixed it quickly.

The system elements in our hot water components are a standard, well insulated 120 gallon tank, the hot water panels, control and monitoring electronics and heat exchanger are all made by Heliodyne, and the tankless water heaters were made by Takagi and all of the internal hot water pipes had pipe insulation to retain the heat as it moves around.

To close this essay, here is the overall performance of the system (but including gas we use for cooking as well) –

Gas usage from August to mid-October

Gas usage from August to mid-October

As you can clearly see our natural gas usage has gone visibly down since the system was installed and we began monitoring in early August, 2008.  More interesting is the fact that the peaks that are visible in early September and early October were times with the solar hot water system was broken.  Now it has been running stable for a couple of weeks and we believe it will be reliable in the future.

This is one of the graphs you will soon be able to see in real time on the SWG website under the Tiburon Home Tab.

Stay tuned!

Posted in Essays, Green Perspectives, Personal Stories

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