I was recently sent a set of plans for a solar hot water system at a military base. The design featured solar preheating a hot water heating system for a barracks — basically two separate systems. Two 100-gallon hot water heaters re-circulated through an additional 400-gallon tank, for a total of 600 gallons of 140° water — all simply waiting for someone to turn on the tap. The solar had its storage, too. Unfortunately, the solar couldn’t heat the 600 gallons when water wasn’t being used.
Using tank-type water heaters and recirculating through additional storage for those occasional peak demands — such as a bunch of bodies in the shower — is not always the most efficient design. Do you cover the load at all costs? And, at what cost? There is a better approach, thanks to improved technology.
Simple principles tell us that the higher the temperature and larger the tank size, the higher standby losses will be and the more energy will be wasted. With highly efficient modulating and even cascade water heating technology, matching the ever-changing hot water demands can be achieved with less storage.
So why, exactly, do we need the tanks? A water heater or boiler has an efficiency rating that is measured when the unit is operating under normal conditions. Anything that is started repeatedly uses more energy just to start, i.e. water heater, electric pump — even a light bulb. We need a properly sized buffer tank so that the water heater won’t fire anytime someone washes their hands. The storage is used to bridge the gap of occasional hand washing.
Now, let’s turn on the shower. To heat 1 gpm (60 gph) from 50° to 120° requires 34,860 Btus per hour. If we have 10 shower heads and invite 10 dirty Army guys, now we need 348,600 Btu. If only five of the 10 soldiers shower, 174,300 Btu is needed.
So, conditions range from an occasional hand wash to full-on towel-flipping shower time. That’s swinging from 34,860 to 348,600 Btus. If there is a 120-gallon tank and the sensor is located 1/3rd from the bottom, the burner will not fire until several hand washes and showers have been started — that’s 40 gallons. Then the water heater will engage and operate at peak efficiency, modulating according to the changing demands. As the tank reaches set point, the firing rate slows. If the showers load up again, the water heater may not need to waste energy to re-fire.
Adding solar: Pre-heat or not to pre-heat? Stratification or mechanical control
When solar gain is achieved at times that hot water is infrequently consumed, it is necessary to use a design that allows solar to heat all of the stored water (including the water heater portion). The design must prevent the backup water heater from heating the solar storage portion of the tank(s). This can be achieved through Mother Nature (stratification) or mechanical control.
Example of stratification
Stratification: An upright tank with a VWH (volume water heater) piped in the top ½ or even 1/3rd of the tank will stratify the injected heated water. When heating only the top part of the tank, that introduced heat will not sponge to the bottom. The solar is always introduced into the coldest water available in the system — the bottom. This, too, increases solar efficiency.
Once the cooler part of the tank is solar heated, it will overrun the set point of the water heater. Storage tank temperatures can be set to as high as 180°. Remember, it is code mandatory to use anti-scalding devices that allow safe storage at higher temperatures. Setting the solar control to a higher temperature allows the heat to be retained instead of wasted and lost. When studying historical data from a properly designed solar hot water heating system, you’ll find the water heater only comes into play when the hot water load exceeds what the solar can provide or has stored.
Often during hot seasons, the water heater will stay offline for weeks at a time — even if this is a military facility and the solar fraction required is only 30% annual average. Summer solar can often cover 100% of needs. When this happens, the designer or installer looks good.
Example of mechanical control
Mechanical control: In this example with two tanks (at left), the tank on the left is partly heated with the VWH. The volume that water heater heats is the amount in the buffer tank; 100% of the stored volume of both tanks is the solar calculated volume. With this design, the water heater and the solar storage volumes overlap. It does not employ a tank to tank recirculation pump. A control modulates a tank-to-tank transfer pump via Delta T from temperature sensors mounted on the top of the right tank and bottom of the left. When solar gain builds in the right tank, its top sensor will tell the controller to start the pump moving the heat to the left tank stacking it with solar energy. The transfer pump can only start if the top of the right tank is hotter than the bottom of the left tank. Email me for control tips.
Example of preheating
Pre-heat: In this example with two tanks, the tank on the left is heated with the VWH. The tank on the right is solar only. The volume that the water heater heats is the amount in the buffer tank. The volume that the solar heats is the calculated solar volume. With this design, more storage is needed, and cross-tank solar storage cannot be achieved. When water is not being used, solar gains cannot prevent the water heater from firing.
One last thing, I want to stress that anti-scalding devises must always be installed; and they must be solar rated. Many off-the-shelf anti-scalding valves will not work on solar applications.