HPAC Magazine

Lowering the Limit

How to extract more heat from a storage tank.

April 1, 2012   By John Siegenthaler

The article titled Big Buffers in HPAC’s January 2012 issue (available in the HPAC archives) reviewed the options for large thermal storage tanks used in combination with wood gasification boilers. The piping between the boiler and storage tank is shown in Figure 1.

In this article we will add some hardware to configure the load side of the thermal storage tank and discuss a way to “squeeze” the maximum amount of heat out of that tank before resorting to heat generation by the auxiliary boiler.

Figure 2 shows one way to set up the load side of the system for zoned heat delivery as well as incorporation of a modulating/condensing boiler for auxiliary heating. The distribution system uses a variable speed pressure-regulated circulator to supply three zoned manifold stations for radiant panel heating. The tank circulator (P2), and the auxiliary boiler circulator (P1) are hydraulically separated from the variable speed distribution circulator by a set of closely-spaced tees. A three-way motorized mixing valve operates based on outdoor reset control to deliver the proper supply water temperature to the manifolds under all conditions.

Upon a demand for space heating from any heating zone, an outdoor reset controller is powered on to measure the temperature at the top of the storage tank, and compare it to the “target” supply water temperature. If this controller determines that the tank is sufficiently warm, the storage tank circulator (P2) operates to deliver heated water to the closely-spaced tees and then on to the space heating load. If the storage temperature is too low to supply the load, the auxiliary boiler is turned on along with the circulator (P1) to deliver heat to the closely-spaced tees. In this mode, circulator (P2) is turned off to prevent heat generated by the auxiliary boiler from going into the storage tank.

The graph in Figure 3 shows the logic used by the outdoor reset controller. In this case, that controller has been configured to deliver a “target” supply water temperature of 109F when the corresponding outdoor temperature is zero Fahrenheit. The “on/off” control output of the reset controller requires a differential to prevent short cycling. This differential is represented by the two dashed lines shown in Figure 3. The lower dashed line shows the temperature at which the auxiliary boiler is turned on, assuming there is a heating demand. The upper dashed line shows where the auxiliary boiler is turned off, and thus all heat is delivered from the storage tank. What happens when the tank sensor temperature is between the dashed lines depends on the “direction” of temperature change. If there is a demand for heat and the tank is supplying the load, it will continue to do so until the tank temperature drops to the lower dashed line. If there is a demand for heat and the boiler is supplying the load, it will continue to do so until the tank temperature rises above the upper dashed line.


One limitation of this basic control concept is that there is no operating mode where both the storage tank and auxiliary boiler can supply heat to the load.

Consider the situation where the temperature at the top of the storage tank is slightly lower than the lower limit established by the outdoor reset controller, but is still warmer than the return temperature of the distribution system. Under these conditions the tank can still contribute some heat to the space heating load. The balance of the heat input is supplied by the auxiliary boiler.

The addition of a differential temperature controller, like those used to control solar thermal systems, allows the storage tank to continue supplying heat to a lower temperature. Figure 4 shows where the sensors for the differential temperature controller would be located. Figure 5 shows a ladder diagram containing the hard-wired logic for the control system.

If the outdoor reset controller determines that the tank is too cool to supply the load, its normally open contacts close to power up the differential temperature (T) controller as well as a relay that enables the boiler to operate, and places control of the circulator (P2) solely at the discretion of the differential temperature controller.

The differential temperature controller measures the temperature difference between water at the top of the buffer tank and that returning from the distribution system. If the top of tank is at least 2F above the return temperature, the normally open contacts in the differential temperature controller close to keep the circulator (P2) operating.

The boiler and circulator (P1) are also operating at this time. The boiler is monitoring the temperature downstream of the closely-spaced tees. Ideally, it modulates to add just enough heat to maintain this temperature close to the target temperature required by the distribution system based on the settings of its internal reset controller.

The mixing valve, operating under its own outdoor reset controller, and with the same settings as the boiler reset controller, should be at or very close to its fully open position.

If the temperature difference between the top of the buffer tank and the return side of the distribution system drops 1F or less, there is very little heat being extracted from the tank. Under this condition the differential temperature controller turns off circulator (P2) to prevent heat supplied by the auxiliary boiler from being added to the buffer tank. The auxiliary boiler and circulator (P1) remain on to supply the space heating load as required.

When the buffer tank temperature again rises to where the temperature differential between the top of the tank and the return side of the distribution system reaches 2F or more, and there is still a demand for heat, the circulator (P2) is turned back on to again extract available heat from the tank.

This strategy makes sense when a large storage tank is used in the system. I suggest it only be used when the storage tank contains at least 500 gallons. The ability to lower 500 gallons of water by an additional 10F implies a release of almost 42 000 additional Btus from the tank. Smaller tanks would contribute proportionally smaller amounts of heat, and thus provide less justification for the additional controls.


If you implement this strategy, be sure of two things:

1. That the accuracy of the differential temperature controller and its sensors is capable of consistently reacting to a temperature difference as low as 1F.

2. That the temperature sensors for the differential temperature controller are mounted in identical, or nearly identical manners.

Ideally, both sensors would be mounted in identical sensor wells, immersed in the system water, and with ample coatings of thermal grease. If mounted to the surface of copper tubing, be sure the sensor surface makes good contact, is well secured, and is fully wrapped with insulation. Remember, you are asking the differential temperature controller to detect differences in temperature as low as 1F. That is about one third of the temperature difference you can detect with your own fingers.

If you cannot ensure the accuracy of the controller or the identical sensor mounting, it is better to use a wider differential temperature setting on the differential temperature controller. For example, it could be set to close its contacts at 4F and open them at 2F.

Finally, keep in mind that this technique can also be used with storage tanks heated by other sources, such as solar collectors or heat pumps. <>

John Siegenthaler, P.E. is the author of Modern Hydronic Heating (the third edition of this book is now available). For reference information and software to assist in hydronic system design visit www.hydronicpros.com.