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SUNMAXX SOLAR HOT WATER SOLUTIONS
DRAINBACK SYSTEMS
Date: 10/18/2010
Good morning. Welcome to the next addition of our SunMaxx Solar hot water series webinar. Today I’d like to discuss drainback systems and how to optimize the performance for both flat plates and evacuated tubes. As always I welcome you to type any questions you have in the chat window. I can try to respond to those immediately. For those of you who are having trouble with audio just please refresh your screen. You might be able to eliminate any problems that you have.
I’d like to talk about the basics and we can get into some more complex topics. Although a half an hour is very short period of time to discuss too much. I’ll try my best to cover as much detail as I can in those next thirty minutes.
So essentially drainbacks are a closed loop system. There’s different style drainback. You can do an open loop or direct and indirect. What I’m going to talk about today is a closed loop drainback that is indirect heat source. So we’re going to use a heat exchanger to dump into both heating system and hot water. Essentially drainbacks flood the collectors when the pump is on and when the pump is off for different reasons. When the pump is off the collectors will drain. Draining allows the fluid to leave the collectors preventing any overheating situation and also preventing any freezing situation. So drainbacks are relatively unique to the United States for them most part. In other parts of Europe and Asia most of the systems there are closed loop glycol systems.
Drainbacks have been around for quite some time over thirty years. Particularly down in the Florida and the southern areas. But they’ve slowly made there way up north for heating systems. Particularly for systems that are sized for large heat loads in the wintertime and zero heat loads in the summer time. So drainback systems as you’ll see have several different merits. Obviously the first one is there’s’ no need for glycol. So as this picture depicts when the pump is off the collectors and the piping outside are drained. When the pump is on then the water in the loop displaces any of the air that is trapped in the collectors and in the tubing outside.
Couple of the benefits of drainbacks it really in the function of anti-freeze. So from experience the only good thing about anti-freeze is that it doesn’t freeze. So with a system that doesn’t require anti-freeze. You don’t have all of the other drawbacks that anti-freeze is associated with more viscosity and heat transfer. Cost is another one. Drainback systems additionally have no need for an expansion tank. Presumably the volume of air that’s trapped inside the tank when the pump is on will suffice for any expansion that occurs during the heating up of the system. There’s also it’s an added safety for when the power fails or there’s a brown out or black out. The worst case scenario is the pump will shut off and if that happens then the system is protected if it’s designed properly. As opposed to a closed loop glycol system when the pump shuts off there have to be measures built into the system design that protect from overheating.
Additionally there’s less annual maintenance. If they system is designed and installed properly there should essentially be no maintenance each year. Maintenance that would typically coincide with checking glycol acidity, maintaining system pressure and measuring the concentration of the glycol that can all be eliminated. Also there’s less oxidation of the system because presumably all of the oxygen will be used up during the system’s initial commissioning. In very short period of time the oxygen is consumed and what’s left behind would be the atmospheric nitrogen. So this will occur if the systems are installed properly and are tight. That way we don’t’ have any new oxygen gain or any pressure losses.
A couple of drawbacks and the one that’s most apparent to me is that the collectors need to be mounted at a slope. So when you have a collector run of ten to twenty feet. What would be required is a drop in elevation from the high side of the collector to the low side of the collector of anywhere from four to eight inches. Now when mounted on a house that has straight lines the aesthetic appeal really drops. Here’s a perfect example although this collector system is probably working perfectly well dumping a lot of heat into the house. It is crooked and so as people drive by or notice the system on the roof that is crooked. They don’t’ realize the functionality of that slope so they might pass judgment. So you have to be very considerate of what the collector field will look like. In some cases aesthetic is paramount.
Also if the system is not installed properly it can lead to lots of things that will go wrong for instance freezing pipes. All of the external piping needs to be mounted at a quarter inch to a foot of slope. With the drainback system it typically requires a larger pumps or secondary pump which I’ll show you in just a few moments. That secondary pump can be used to overcome the initial head that’s required from system startup or when the pump initially kicks on. The second pump needs to be used to help overcome gravity. Once you create a siphon in the system gravity no longer plays a role.
Another drawback is that there have been some complaints that drainback piping tends to be noisy. There can be some air entrained in the line that causes mini-explosions when high temperature meets low temperature. There can be some knocking in the pipe. So if you have piping that’s installed in the house. Clients will hear it when they first kick on their system until they get that nice siphoning going.
So in most cases if it’s installed properly and out of sight. Then the first two drawbacks are irrelevant. Now with our newest controllers that offer variable speed function the third drawback can also be irrelevant. Because we can use a large pump to overcome that initial head and then the controller will throttle back the pump power. So that we’re consuming just what’s needed and nothing more. Obviously if you have pipes that are installed outside then the noise is also irrelevant. So there are many important benefits to drainbacks but it becomes more and more important that you consider some of the drawbacks before you make your final decision.
Now let me get into a little bit of fundamental here. One of the benefits to drainbacks is it reduces the need for extra heat exchangers. Every time you go through a heat exchanger you undergo what’s called a performance penalty. Performance penalties can be removed from systems that are well designed. But regardless when you have a heat exchanger very rarely will you get a hundred percent heat exchange. For example this heat exchanger operating at fifty percent effectiveness. In other words the amount of energy that a heat exchanger can transfer across a heat exchanger is typically between fifty to eight percent effectiveness. So this means that if a collector is yielding a ninety four percent….They only yield ninety four percent of what they are expected to yield because of the fifty percent heat exchanger effectiveness. By looking at this graph there’s a diminishing return as you increase the performance penalties. So by eliminating this heat exchanger you essentially increase the system performance by six percent. So you have to be careful not to assume that just because the heat exchanger is operating at seventy five percent effectiveness. Doesn’t mean that you’re losing twenty five percent of the energy because it’ll just take more cycles to work through. However six percent losses can be pretty substantial. And why would you suffer six percent loss if you didn’t need to. So drainbacks eliminate that need to suffer from performance penalty because of this heat exchanger effectiveness versus quicker performance graph.
Okay so here we have two drainbacks. We have an open loop and a closed loop drainback system. The open loop drainbacks which you see on the left are typically used with non-pressurized tanks. Non-pressure tanks are very popular with Combi Systems where you have a large reservoir of energy two, four, five, six hundred gallons storage tanks. These are very simple to design because being that the tank is not pressurized. All we’re going to do is pull fluid from the tank up to the collectors when the collectors are producing energy and dump back down into that non pressure tank. So we’re literally pulling the water from the tank to the collectors. Now it does require a system on the left like an open loop…It does require a separate exchanger because typically heating systems and certainly domestic hot water systems maintain an existing pressure. So we can’t introduce a pressurized system into a non-pressure system without the use of a heat exchanger. So although that system on the left the open loop drainback systems perform quite well and are very easy to tie into heating systems we do need to add a heat exchanger.
The system on the right the closed loop drainback system where we’re maintaining a certain pressure. Now this system can be tied directly into heating systems. For example we can tie that storage tank being pressurized into pressurized floor, into pressurized baseboard or into a pressurized domestic hot water line. So first you decide whether you’re going to do pressurized or non-pressurized. One of the considerations of a non-pressure tank is the availability of large high volume storage tanks. For example if you designed a system that had a five hundred gallon storage tank. It becomes quite costly to have a five hundred gallon pressurized system. So non-pressure systems tend to be more cost effective when you’re talking about large Combi System designs. With closed loop they’re typically reserved for smaller systems they’re much easier to tie into because there’s no need for heat exchanger between the storage tank and the heating system. However there’s a limitation to the size of the pressurized vessel. I have seen three, four, five thousand gallon pressurized tanks but the cost is sometimes two to three fold that of the non-pressure tank of the same size.
Okay here’s an example of a Combi System design that uses a pressurized tank and as you notice the fluid that’s passing through the collectors is the same fluid that’s delivered to the heating system. Whether it’s the heat dissipaters there…Whether the heat dissipaters are radiant floor or tubing or baseboard. It can also be forced hot air. The fluid from the collectors and the tank and the heating system are all the same. So there’s no performance penalty loss there. The only exchanger that you see in the middle of this tank is the domestic hot water exchanger. So when the tank calls for heat the boiler will add heat to the tank if the solar is not producing any energy. So the only time the boiler will dump heat into the storage tank is when the solar is not active.
Now in this case the cold water comes into the bottom of the tank and works it way up and out through that coil. So the only heat exchanger is for domestic hot water. Typically heat exchangers on domestic hot water lines have a higher heat exchanger effectiveness or heat exchange capacity. Because of the larger delta t. The incoming cold water temperature versus tank temperature tends to be higher. Therefore we get a greater heat exchange rate and losses are much less.
So the design that you see here is probably one of the best ones out there for domestic hot water heating system solar collector tie a pressurized drainback closed loop.
You can also have a drainback system like this that does not use a heat exchanger. In this case the storage is the pressurized drainback tank in the case of the one before Don there is no storage for hot water per see. The storage is actually in the drainback tank so presumably we are getting a full recovery of cold water coming into the coil and leaving at the temperature of the tank. Okay. So we’re getting a large delta t from the cold water supply that comes in at fifty and leaves at a hundred and five or whatever you have it set for. At night our storage tank is going to be maintained by the boiler if the solar hasn’t been able to take care of it. So all day long the solar collectors bring the storage tank up to temperature. At the end of day if the storage tank is not up to temperature then the boiler will add heat to that storage tank maintaining that storage tank at a hundred fifteen or a hundred twenty all the time.
Now with this one the difference is the drainback tank that is located outside of the storage tank. This small drainback tank only has to store the volume of fluid required to flood the collectors and the piping outside of the house. So in most cases this is a small ten or twenty drainback tank. By mounting it up towards the roof it eliminates the need for a larger pump or a second pump. Because you only have to overcome the vertical height that’s associated with the distance between the top of the water in the drainback tank and the top of the collectors. So the smaller that height the less pump powered required.
Here’s a picture I have right out of Tom’s Lane lesson learned book of a drainback system that uses a pressurized drainback tank and a coil. Now this system is used mostly for domestic hot water system. The water in the storage tank is used for domestic purposes. Here is a diagram of a larger commercial pressurized drainback system that uses three tanks in parallel. Each of these tanks would ideally be the same temperature. And there is a larger drainback tank that will allow for any of the fluid in the collectors to drain into. Like I said before those separate drainback tanks can be anywhere from ten, twenty, forty and even eighty gallon drainback tanks outside of the normal storage.
Now mounting flat plates to allow them for drainback as I mentioned it usually requires a slope of a quarter inch per foot. Now that slope can be apparent in the installation of the collectors or that slope can be taken care of in simply the piping. You only need stainless steel pumps for lines that are associated with domestic hot water or oxygenated loops. So for any system that’s a closed loop that has a pump cast iron will be sufficient. If the system is open that is with domestic hot water or a non-pressure tank then stainless steel or bronze pump should be used. In fact if it’s domestic hot water then stainless steel or bronze is code.
So as I mentioned the flat plates don’t always have to be mounted crooked. But the piping will need to be mounted crooked. It depends what style. On the top you see the header riser style where we have all four panels are essentially piped in parallel. At the bottom we have serpentine. Now there are two different types of serpentine. This is a horizontal serpentine where the pipes are moving back and forth left and right. Horizontal serpentine collectors with a port on the top and a port on the bottom can in fact be used for drainbacks. However vertical serpentine where the manners go from top to bottom they can not be used in a drainback because it won’t drain. Typically flat plates have a header riser style internal piping. And most of those header riser style that have four ports can be used in drainback when they’re mounted properly.
In terms of the mounting of the heat pipes there’s much less consideration it’s very straight forward. There’s a single pipe running through the manifold. That pipe like any other drain pipe needs to be mounted a quarter inch tilt per foot. Now that tilt can be accomplished in many ways by…For instance mounting it flush on the roof and rotating the entire piping at a quarter foot. Or mounting it on a flat roof like you see in this picture with one side proportionally higher than the other which allows for the drainage from left to right. But again when designing systems you have to consider what the aesthetics will be like. When you have a long run of forty or fifty feet. For example a four foot run will see just about an inch and a half of elevation change. So one flat plate will need to be tilted at about one point five inches. But for four flat plates now we’re up to six inches. So that slope becomes pretty noticeable as your runs become longer and longer.
Now in order to overcome the initial vertical head we use a double pump system or a single pump that has a variable speed function. When you put two pumps in series you essentially double the head. Not double the flow but you double the head. That’s what the issue is with breaking gravity getting the flow from the drainback tank all the way to the collectors is like turning on a fountain pump. So it has to be able to overcome that vertical head. Now once the siphon is created so once we’re able to flow the fluid across the collectors and then fall back down. Then you count on the effect of gravity and that second pump can then go offline. Or a single larger pump would be throttled back.
Now like I mentioned by taking the drainback reservoir and mounting it as high as possible you will loose some of the friction head because there’s a much shorter flow path. So if you have the opportunity to raise your drainback tank up to in an insulated space up to the point where just before it becomes non-insulated then you can dramatically reduce the amount of friction you have to overcome and take out the effected gravity. There are many questions about how to pressurize drainback systems. It’s got to have at least fifteen PSI in the system in order for the siphon to take effect. What’s going to happen if you start off with just fifteen pressure at the pump by the time you move to the top of the system the pressure will decrease. If it drops below atmospheric pressure then the boiling point is going to drop as well. It’s going to begin disassociate the…
Yes, that is correct. So anytime you are introducing air into the system. You have to use stainless steel or bronze pumps.
Now proper sizing for drainbacks in order to maintain a two foot per second of velocity in some cases a larger diameter pipe although it will reduce your friction will prevent siphoning from happening. So you have to be able to maintain a minimum velocity in your system. And velocity is going to be directly related to the volume and your pump speed. The volume is related to obviously to the diameter of the pipe that you choose. So for systems that require for example one point six gallons per minute we can go down to half inch type m copper or type l and that’s going to maintain two foot per second. If we need a gallon and a half a minute and we use three quarter inch copper then our velocity is going to be less than two feet per second. Which means you’re going to have a hard time maintaining that siphon.
So the rule of thumb is you want to try to have the smallest diameter pipe possible while still maintaining a certain velocity. That minimum velocity is two per feet per second and the maximum velocity is five feet per second. So we want to try to maintain at least two feet per second in the lines and not to exceed five feet per second. So again by reducing the velocity you put more stress on the pump to maintain that siphon. So you have a better opportunity to create that siphon if your velocity is above two feet per second. You can use this chart to help guide what volume of piping I should use while maintaining two feet and not to exceed five feet per second.
There’s many different options for drainback storage. There are ten, twenty, forty and eighty gallon tanks often they need to be mounted on top of an existing top. Or they can be mounted as I said in a larger space up in the attic. Some manufacturers have large eighty gallon tanks that eliminate the need for a separate tank all together. So most often is the case that the installers will choose a separate drainback storage tank that may or may not include a coil heat exchanger. Many times when a coiled heat exchanger is included in a drainback tank since it’s such a small volume. They increase the surface area by using thinned copper or dimpled copper to help increase the heat exchange capacity through a smaller coil.
The UniMaxx drain back is ten gallon tank that uses a double pump. So we’re pumping through the collectors and a separate pump is going to re-circulate from the drainback tank into an existing tank with the use of a raised plate as opposed to an internal coil. There are many manufactures of stand alone units. We are just now bringing one to market that includes all of the components built in. Similar to a pump station but as you see on the left here it’s a pump station that stands up on it’s own with a separate drainback reservoir that re-circulates into an existing tank.
A couple very important considerations before I close here. Is that all horizontal piping must be sloped and one of the challenges for many installers is to design a system that is both aesthetically appealing and can drain properly. So by looking at the lay out of the roof and the runs of the pipe. You have to be able to visualize every single horizontal of pipe needs to be sloped. In some cases it just doesn’t fit the aesthetics of the house so installers will choose to go with a closed loop glycol. Another important factor to remember is velocity must be two feet per second. Also often pumps come with check valves built in. I know Takeo and Burnfoss the standard pumps that might come with a pump station. Whether you get a pump station from SunMaxx or whom ever else often they have check valves built in. Now Takeo check valves are very easily removed with a pair of needle nose pliers. Never the less you need to be sure that your system does not have a check valve that’s going to prevent the fluid from draining back effectively.
Many of the controllers now have a high limit shut off. Not only do they shut it off but they will keep it off. Now make sure your controller has this function enabled especially with evacuated tube drainback systems. Because as your pump shuts off because the tank reached a high temperature. If there’s a load on your storage tank causing the storage tank temperature to drop maintaining a delta t between the collectors and the tank. The pump is going to want to turn back on again. If this happens if your collector field is in excess of two hundred and fifty degrees you’re going to get some flash steam. Which isn’t necessarily a problem but it’s something you want to avoid having happen consistently every single day. It’s going to put undue stress on your piping and fitting and valves.
So the controllers are designed now a days for drain back functionality where it will turn off the pump when the tank reaches maximum temperature. It will keep your pump off regardless of your tank temperature if the collectors exceed a temperature it’s typically set at about two hundred and thirty five degrees. Which in the case of evacuated tubes and flat plates as well…. When the pump shuts off in full sun those collector temperatures can climb up to two hundred thirty or two hundred forty degrees in a matter of minutes. So that’s a very important function to make sure is enabled.
Also you need to maintain a minimum pressure inside the system to help enable that siphon. With lower pressure in the system you’re going to increase the siphon requirements. In other words you’re going to lower the vertical height that siphon can still occur at. So the more pressure in your system the higher the total vertical head can be while maintaining that siphon. And you don’t need an air vent. In fact you don’t want an air vent in the system. Obviously drain backs have no need for expanse state.
Hi Pete. Just wrapping it up here. I am actually going to be done. I’ve gone two minutes over. If anyone has any questions I welcome you to bring them to me now. Otherwise I’d like to conclude and I hope to see you again perhaps next week. I’m glad Pete joined us maybe….Now would be a good time to bring it up Pete. To clarify the pressure I’m certain that you have something to say.
But from my understanding the lower the pressure the lower the total vertical head that we can accomplish and still maintain a siphon. So I believe at one atmospheric pressure in the system we’re only going to be able to get about thirty to thirty five feet of head before the boiling point is so low. That we begin to break the siphon as the water moves over the top of the system. So by increasing that pressure it allows us to attain a higher vertical head and maintain that siphon.
Okay. Well if anybody else has any questions I’d be happy to answer them now. And otherwise I wish you a happy solar day and hope to see you again. Take care. Bye.
