Solar Heat Storage Tanks: The 2P3T Piping Configuration
By Bristol Stickney, Chief Technical Officer,
SolarLogic LLC, Santa Fe, NM
For over 15 years now, I have been using the primary/secondary loop exclusively as my standard “skeleton” piping configuration when designing solar combisystems. I have mentioned it many times my columns. And, anyone familiar with primary loop installations knows that each of the secondary loops is connected to the primary loop with two closely spaced tees. This seems to be common knowledge these days.
I would like to introduce a variation on that theme and describe an application that works nicely with three closely spaced tees. This piping detail is a little unconventional and, in fact, is a bit of a brainteaser at first glance. I have been told more than once by experienced heating professionals that I must have made a mistake. I have come to refer to it as the “2-Pump, 3-Tee” (2P3T) reversible secondary loop connection. I first started installing it on my solar primary loops around 2005 and I have duplicated it many times over the years (well over a dozen times in existing heating systems). I can now say, with some confidence, that it certainly is not a mistake, and has performed as expected with remarkable reliability over the test of time.
Three steps beyond Combi 101
Perhaps you are asking yourself, “Why would I ever need a 2P3T primary loop connection?” To answer that question, we need to take about three steps beyond Solar Combi 101.
To review, the most common and least complicated solar combisystem, which I call Combi 101, includes four heating components:
1. Solar heat collectors (in a single group, pumped by one circulator)
2. A hot water boiler
3. A domestic hot water (DHW) “indirect tank” (with internal heat exchanger coil)
4. Hydronic radiant floors (HRF) of masonry construction (A.K.A. warm mass floors)
In a Combi 101 system, we can design the size and tilt of the collectors to match the heat required by the HRF and the DHW, and often eliminate the need for extra solar heat storage tanks. This is done by taking the heat storage capacity of both the masonry floors and the water in the DHW tank and controlling it to take advantage of the thermal flywheel effect of all that existing thermal mass.
Figure 55-1 shows all four of the Combi 101 components mentioned above, plus a couple more. They are arranged around the primary loop, in the correct temperature-order so that the solar can preheat or bypass the boiler, the DHW is the first heating load and receives the hottest temperature from the primary loop, and the HRF mass-floors on Pump B (with the lowest temperature requirement) are last around the loop. With that understood, now we can discuss going beyond Combi 101.
Step one: Hot water baseboards or non-mass heating zones. Many solar heating systems include some heating zones that do not have any heat storage capacity. The most common example are upstairs zones that use hot water baseboards or warm board HRF systems where the heat tubing is not embedded in masonry and has little or no heat storage capacity. If the non-mass zones are small in comparison to the rest of the heating loads, we can sometimes forgo the extra expense and complexity of a heat storage tank and simply run the boiler to deliver heat to this small heat load. Many systems are designed like this, and since they are one step beyond Combi 101, I suppose that makes them Combi 102. The baseboard zones are shown on Figure 55-1 in the correct location, fed by Pump A. When the baseboards and non-mass floors add up to a significant part of the daily solar heating load, heat storage tanks must then be called upon.
Step two: A single small heat storage tank. In many cases, a single small heat storage tank can be added to the primary loop to act as a relatively simple heat accumulator. They are often chosen to be larger than a DHW tank, but still only one tank, typically tall and mounted vertically. In my standard designs, I always use sealed/insulated pressure tanks and usually fill them with boiler fluid, the same low-pressure water that fills the primary loop and circulates throughout the indoor heating system. The main job this tank performs is to save solar heat overnight at a high enough temperature so that a few hours of baseboard heating can be accomplished without firing the boiler. Also, the heat in the storage tank may be transferred to the DHW tank in the morning to provide more hot showers without running the boiler. Since a single tank has limited heat storage capacity, it is commonly charged up with heat and completely discharged from one sunny day to the next. So I have found that a single circulator and two closely spaced tees will work with the typical single heat storage tank. The circulator draws from the upstream tee, into the bottom of the tank, out of the top of the tank and back into the primary loop through the downstream tee. This type of system can be called Combi 103, and matches the configuration in Figure 55-1 except that the storage tank detail would include only one tank, one pump and one pair of tees (not shown).
Step three: Multiple heat storage tanks, a.k.a. The Tank Farm. In heating systems where there is little or no heat storage mass to work with in the building, most or all of the solar heat may have to be stored in large water tanks. In this situation, temperature stratification becomes an important design issue. In vertical tanks, the hottest water tends to rise to the top and the colder water drops to the bottom. If the water in a large tank is mixed, perhaps by pumping, and the cold and hot are stirred together, the resulting tank temperature may not be hot enough to be useful even though there are hundreds of thousands of BTUs embodied in the water.
So, in large tank systems, the temperature stratification must be maintained to keep the hottest temperature separated from the colder temperatures, for as long as possible. This is true when both pumping heat into and out of the tanks. As you can see in Figure 55-1, I prefer using pressure tanks filled with boiler fluid, even for larger heat storage systems. In the diagram I have included, there are two pressure tanks plumbed in series. In even larger systems, I have strung as many as four such tanks in series and run them in parallel with other strings of four to preserve both vertical and horizontal stratification. To maintain temperature stratification in all these tank systems, I pump heat into the series of tanks in one direction and reverse the flow, pumping in the other direction when pulling heat out of the tanks. To do this, I use the 2P3T secondary connection that is shown on Figure 55-1, feeding the storage tanks.
The 2P3T secondary: How it works
When using two closely spaced tees, the secondary loop is supposed to draw fluid from the upstream tee and discharge back into the primary loop through the downstream tee. The dilemma when reversing the flow through a 2-tee secondary is that when the flow reverses, fluid is drawn from the downstream tee and is discharged upstream. This causes the discharge fluid to re-enter the downstream tee, feeding the secondary with the wrong temperature from its own discharge pipe. There is more than one way to solve this problem. For example, the use of two pumps and four tees could work. The proper placement of a single pump with a motorized 3-port diverter valve and four tees could also work. A reversible pump would be helpful here, but common centrifugal circulators don’t work that way.
This is where the six principles of solar design began to drive the design process. I was looking for an elegant solution that used few components, a single flow path through the tanks and a minimum of plumbing connections. I wanted simple and reliable control. And wherever possible, I like to use the critical components for more than one purpose. After drawing up numerous schematic piping diagrams for a number of days and puzzling over them, the 2P3T detail shown in Figure 55-1 seemed to meet all these criteria.
I have always preferred the simplicity of controlling flow by simply turning pumps on and off. And so I have avoided the use of motorized diverter valves in my primary loop designs. Instead, you will notice that two passive spring-check valves have been strategically placed in the 2P3T configuration. The spring check valves assure that the flow direction and the temperature-order in the primary loop is properly maintained whenever either one of the two circulators is on. Of course, the control system provides that only one pump may run at a time, never both together. Keep in mind that integral check valves are not allowed inside the pump bodies of these two pumps.
Perhaps by now you have noticed that the flow path is a bit unorthodox because the active circulator actually pumps backwards through the inactive circulator. This is the part that looks like it might be a mistake, but it isn’t. A centrifugal circulator is a “wide spot” in the plumbing that contains a wheel with fins on it. Can you pump through those fins when the wheel, the impeller, is not spinning? Yes, quite easily, as it turns out.
My columns refer to residential and small commercial buildings that are smaller than 10,000 square feet. Brand names, organizations, suppliers and manufacturers are mentioned only to provide examples for illustration and discussion. My mention of these entities do not constitute any recommendation or endorsement for any particular installation. Back issues of this column can be found in the archives at the TMB Publishing and SolarLogic LLC websites.
Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.