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Hot Water

The domestic hot water system is designed to meet several goals:

  • Provide hot water at a constant temperature regardless of the situation
  • Take advantage of all available sources of heat
  • When heating with wood, provide usable hot water for as long a period of time as possible
  • When heating with oil, minimize the number of burner firings

Original (normal) installation

The domestic hot water tank is a 40 gallon tank with an internal water-to-water heat exchanger coil. It's configured as an extra zone on our oil-fired hot water baseboard system. It has an aquastat mounted about halfway up the tank which is set to call for heat when the water temperature drops to about 105°F at the tank center. At that point, the tank top is generally around 115°F. The aquastat is connected to a zone valve, and the zone valve is in turn connected to the oil burner. Cold water enters the bottom of the tank, and hot water is drawn from the top. Thermal stratification occurs as water is withdrawn, with cold water remaining in the bottom of the tank while hot water is drawn off the top. During heating, vigorous convection mixes the water in the tank pretty thoroughly.

This approach is effective, but way too simple. First of all, it seems wrong to introduce ice cold water into the bottom of our hot water tank.


Preheated makeup water (Solar)

It's much more desirable to warm it by first passing it through a coil in our outdoor heat storage tank, which is conveniently heated by our solar panel. Now we're using oil less often, since we're heating warm water instead of cold water.

This works well during the summer, but during the winter the outdoor tank can be as high as 175°F. This is dangerously hot. If we used enough hot water, the hot water tank temperature could reach unsafe levels.


Mixing Valve

What we need is a mixing valve to ensure that we are providing hot water at a safe temperature. This simply mixes cold water with the water coming from the top of the tank to provide hot water at the desired temperature. Not all mixing valves are created equal. Some require a large temperature difference between the hot inlet and the outlet. We used a Honeywell AM101-1-C mixing valve designed for domestic hot water use. It requires only a 3°F difference between the hot inlet and the outlet.

Superheated Tank

Since we have a mixing valve that will control outlet temperature, we can heat the hot ware tank itself tank to higher temperatures, providing more useful hot water per heating cycle. A temperature sensor was added alongside the aquastat. This allows the TS7260 controller to monitor the temperature of the domestic hot water tank and increase the temperature beyond the aquastat setpoint. The hot water tank is heated to 160°F during wood boiler operation, and it is heated above the aquastat setpoint during oil boiler operation via oil boiler heat scavenging.

This is better, but still not complicated enough. We are mixing ice-cold water with the precious hot water from our tank.


Final configuration - two mixing valves

We can use two mixing valves to achieve even better results. The first mixing valve attempts to provide water at about 115°F - just a few degrees cooler than our desired temperature. Depending on the outside tank temperature, it may not be able to reach 115°F. In some cases, it's output may be as low as 70°F.

The output of the first mixing valve goes to the second mixing valve, where it is combined with water from the domestic hot water tank to provide hot water at 118°F. If the outside tank is hot, very little water from the inside tank will be needed. Also, a minimum of cold water is introduced, and the inside tank is replenished with preheated water from outside.

This higher temperature combined with the mixing valves and preheated 'makeup' water provides us with three days of normal use from a single tank of hot water. This compares quite favorably to our experience with the original installation with 'as-designed' oil heat. We used to average slightly more than three oil burner heat cycles per day to maintain hot water.


Putting it all together

In this graph, there are several things happening. The heavy light green line is the temperature at the top of the outside heat storage tank. The dark green line is the temperature about halfway up the hot water tank. The heavy black line is the temperature at the outlet of the wood boiler. The two thin black lines show the state of the aquastat and the hot water tank zone valve.

It's been three days since the last fire in the wood boiler. The outside storage tank is down to 120°F, and the domestic hot water tank is down below 110°F (the temperature at the top of the hot water tank is higher, but unmeasured).

At about 7:08, the morning series of showers starts. The hot water tank temperature starts to drop as the thermocline passes by (colder water is coming in the bottom of the tank). A few minutes later, the temperature drops below 105°F, which is the aquastat setpoint, and the aquastat trips. The oil boiler is disabled by the controller, so the oil boiler does not come on at this point.

At about 7:30, showers are done. We don't know the exact temperature at the hot water tank outlet, but it was still hot enough - probably around 110°F. No complaints from the user community.

At about 9:30, a fire is started in the wood boiler. The heavy black line shows a spike upwards, and at about 10:00 it's hot enough so that the wood boiler circulator pump starts. Since the hot water tank is cold enough to represent a risk of irate residents, the controller goes into a 'hot water crisis' mode, where it prioritizes hot water above all else. The hot water zone valve is opened at this point, and we see a slight increase in hot water tank temperature.

After a very short time, the boiler temperature drops too far and the circulator shuts off and the hot water zone valve shuts off. The boiler temperature climbs again until it's hot enough for the circulator and zone valve to come on again. This cycle repeats a second time.

By about 10:40, the hot water tank is hot enough so that the aquastat is satisfied. The hot water crisis is over. The controller shuts off the hot water zone valve, allowing the wood boiler get hot enough to provide effective heat for the baseboards.

As it turns out, the boiler temperature rises very quickly, and just a few minutes later it's above 168°F. As long as the boiler is that hot, the controller opens the hot water zone valve and begins 'superheating' the domestic hot water tank. This process continues until the hot water tank reaches about 158°F, which happens at around 12:25.