Combi Heating System Design 9.13.2010avwFm87mDz
SUNMAXX SOLAR HOT WATER SOLUTIONS
COMBI HEATING SYSTEM DESIGN
Well I’d like to welcome everyone to Monday’s solar webinar series. Good morning everyone or good afternoon where ever you are in the country. Today I’d like to talk about Combi System designs. The Combi Systems are becoming more and more popular these days. Especially when the quality of the collector increases such that you can really harness a considerable amount of energy in the winter time.
So again I’d like to welcome everybody and remind you if you have any questions please feel free to type them in the chat box there. I’ll try my best to get to them. I actually have a lot to talk about today. Any of you that know me realize there’s always a lot to talk about in solar. So I’ll try to keep it to thirty minutes. This is really just an ongoing conversation and this thirty minutes is devoted to Combi System design.
Without further adieu…Over the years these are a few things that we’ve realized are most important to consider. In terms of the solar fraction often a higher solar fraction of the Combi Systems and the end result is a much longer payback. So thirty to sixty percent…And sixty is a really high solar fraction for Combi System. Generally the solar fractions that are working the best are in the neighborhood of thirty to forty percent are usually the greatest return. As you may know the reason for that is you produce a considerable amount of Btu’s in the summer time that you can not use. So now that thirty to sixty percent is pretty typical. You can get a higher solar fraction. But there really needs to be a way for you to utilize rather than just dump. But utilize that extra energy that you produce in the summer time. So right off the bat you should always realize and consider that a Combi System is a supplement. It’s just a solar supplementation to your existing fuel source. It supplements it usually in the neighborhood at thirty and at the very high end sixty percent.
Another very important consideration is that there has to be a heat dump or a way to get rid of the excess energy. And I know that many of you know this already. Yet there’s still some systems that are over heating. Although you’ve installed a heat dump has it been designed properly? Is it capable of dissipating the appropriate amount of heat? Fluctuating production levels in some cases I know for New York State for example. We might be producing four times or five times the amount of energy in June then we do in December. So it’s very important that we look at the production not the average production or year round production but the production during the summer months.
Also roof size is usually the limiting factor for heating systems. So when you’re talking to clients about designing a supplementation solar system. You really have to understand how much solar potential they have on the roof because often for residential systems the roof size is the limiting factor. When you design these systems or choose from some existing designs the ones that are giving us the highest utilization, greatest solar gain are the ones that eliminate as many heat exchanges as possible. I’m going to go through several different design strategies in just a few minutes. And you’ll realize that every time we have a heat exchanger we suffer a performance penalty.
Storage volume is probably one of the most important things that you need to properly design. Although your storage is just going to store Btu’s that you can dump into your house. It’s not as straight forward as that because in many different temperature. Ambient temperatures require different quality Btu’s in order to heat your house. So what we’re really after is a balance between creating that quality Btu that you need for your heat dissipaters. But also being able to maximize collector efficiency.
So traditionally what happens is the higher the quality the lower the collector efficiencies. So we want to try to identify exactly what that quality Btu is. When I say quality I’m referring to the temperature of the water. Also heat exchangers it can be….You try to design a very simple Combi System using one of the SunMaxx tanks or another type of tank. And you think well I can just add a three way valve and dump it into the house. But the fact is that many heat exchangers inside the storage tanks aren’t necessarily large enough to maintain a temperature in your house. Because the Btu load in your house is greater than the heat exchange capacity of that coil. So you have to understand what the heat exchange capacity is and also the flow rate.
I just realized I’ve been talking for five minutes and haven’t had any confirmation. Would somebody please just confirm that they can hear me talk?
Now another consideration is the angle…Okay good. Thanks Jamie. Now when you install the collectors you can fluctuate the performance directly by changing the angle. But unlike PV we typically set the angle once at a fixed angle. So you have to realize what it is that you’re trying to do. If you’re trying to create heat in the winter time then you really should set your angle at latitude plus fifteen degrees. The difference in performance although it’s relatively small every little bit helps. So for example if I have a angle my roof at forty five degrees and I want a nice flush mount for aesthetic appeal versus a fifty seven degree pitch. I can expect to lose less than ten percent performance. Probably even closer to five percent loss. But five percent loss might mean several hundred dollars a year. So you have to consider the aesthetic appeal but also the losses in efficiency. And your sales rep can help you determine what those losses will be. And exactly how many Btu’s are you sacrificing for the aesthetic appeal.
Lastly and probably the thing that’s going to thread this entire next twenty three minutes together is that low temperature systems have the highest solar yield. Hands down. So there’s a few things that you can do to lower that temperature requirement beyond replacing your entire heating system. But whenever you have the situation that requires low temperature you can get in the neighborhood of forty to fifty percent more energy out of you collectors over the course of the winter with such a system.
Okay just to do a quick little review the types of heating systems. You have radiant, the forced hot air and radiant base board. Radiant in floor heat is the lowest temperature of those three. So we see a higher utilization from our solar collectors when we have in floor radiant or sub floor radiant. I’ll talk about that in a few minutes too. Sub floor and in floor are both low temperature. In some cases with a concrete floor we can do ninety to ninety five degrees perhaps even less in the floor to maintain temperature. Where as radiant baseboard often needs one hundred forty and one hundred sixty degree temperature and even higher. So radiant baseboard although very popular in the US and relatively efficient becomes one of the least heating systems for solar tie in.
Forced hot air works well although it’s a very inefficient way to heat your house because of the expansion and contraction of the air inside the house. It does have a quick recovery rate so the furnace turns on and the house can heat up in five minutes. Well what happens there is the house will expand. Then as it cools off it’s going to contract and naturally contraction is going to create a low pressure and it’s going to pull cold air in from the environment. Which causes the furnace to kick back on. One good thing about forced hot air with solar is that if you can adjust the fan speed it gives us a much larger degree of temperature range that we can tie into. We can blow a hundred degree air on the house as long as it’s not going to high above the speed. So that gives a lower temperature which means higher utilization on the solar.
Just real quick as far as storage capacity. Okay just to throw it out there some of you maybe familiar already. But we typically size up our storage volume anywhere from one to three gallons per foot squared. But in order to determine what ratio we’re shooting for we have to know what the temperature requirement is. So storage capacity is completely dependent on first the temperature requirement. Secondly it’s dependent on the size of the collector array. So for high temperature also means high quality Btu we tend to be on the low end of that ratio. And lower temperature on the higher end.
Now with baseboards the efficiency of the baseboard makes all the difference in the world. You can get some standard thin copper but what happens with this is it works very well at high temperatures but it doesn’t work well at all at low temperature. In fact if I send a hundred and twenty degrees through my thin copper. I’m only going to be able to dump thirty percent of the Btu’s into my house then I would if I was sending two hundred degree Fahrenheit water through the thin copper. This is a standard single thin copper run. There’s several different styles of baseboard and obviously the style that works the best with the solar is the one that operates at a lower temperature. You can see the Btu output of per foot with a sixty five degree air temperature. As the temperature increases we get exponential increase in our Btu output.
One thing that you can do with regard to an existing non-modulating boiler is you can add…For example Techmar makes an outdoor ambient reset boiler control 256. What this is going to do is compare the outdoor ambient temperature relative to the necessary temperature indoors. And it’s going to re-adjust through some logarithms the necessary temperature of the fluid in the heat dissipaters. So as temperature decreases outdoors the solar utilization increases. And conversely as temperature decreases outdoor the solar utilization decreases.
So what this will do is look and see outdoors if the temperature is say forty degrees well if a boiler is normally set at a hundred and seventy degrees. Then the boiler’s going to kick on at a hundred and seventy regardless of the outdoor temperature. An outdoor ambient reset is going to allow you to utilize your solar field for a much longer period of time. Especially during those shoulder seasons at the end of fall and the beginning of spring. Because without a modulating boiler or without an outdoor ambient reset the solar will not work unless it meets the set point of the boiler. What this does is it automatically adjusts the set point of the boiler relative to outdoor air temperature.
So this is one of those holistic approaches to reducing and conserving your energy load. This is one of those components that can really save a homeowner the payback on one of these the installation might be one winter season. So it not only does it by itself independently reduce their fuel bill. But it also in conjunction with the solar field it increases utilization. So you get a positive feedback mechanism with an outdoor ambient reset. There’s a lot more I can go into it but I’m really just trying to shoot for a half hour discussion.
Just to remind you if you have any questions I’m happy to help you out afterwards or point you in the right direction for some more resources.
Heat emitters most radiant heat emitters operate at low temperature. Relatively low temperatures especially the newer ones. The ones we’re bringing over from Germany and Austria. They’ve been operating at lower temperatures for quite some time. They tend to be a bit more expensive. In floor heated slab with a low resistant covering or no covering at all is going to give you the lowest temperature. So you get the same amount of heat delivered into the house at a lower temperature. So the low quality Btu is less expensive. A thin slab on a floor is the next second best thing.
I’m getting a message that the video is frozen. I can’t really….I’m going to have to continue because I don’t know if that is the case for everybody. But if for some of you that have technical difficulties this is being recorded so you can always access it again as it is being presented now.
One thing we discovered is when it comes to in floor radiant systems. For example concrete the concrete is your thermal mass. And it’s your battery in a sense. So a six inch slab of concrete compared to water it takes about seven square feet. Seven square feet of six inch concrete to store the same amount of energy as six gallons of water. So if we’re designing a heating system and I look at the storage volume. Say I have a storage volume of four hundred gallons. Well four hundred gallons what that really means is that same amount of energy can be stored in seven hundred square feet of concrete slab. Which is a pretty small space. In that same line of thinking we realize that with concrete radiant floor the storage tank really only serves as a buffer. It doesn’t do much for storing energy because all the energy we produce can be stored directly in the floor. So often in knowing that you can get a higher utilization from your collectors with low temperature heat emitters. And some cases it may make sense to replace or supplement your existing heat emitters with low temperature heat emitters. And when done in conjunction with a solar array there’s other tax breaks that you maybe eligible for as well by replacing your heat emitters.
Let’s take a look at a couple of examples of Combi System design. This first one is forced hot air. Most people maybe not you folks that are listening now. But most people don’t realize that we can take solar energy in liquid form off of the roof and dump it in through the air with a forced hot air furnace. Now one important note is that on the return plenum or the cold air return comes into the furnace is open and in this case we put the hydronic coil on the supply plenum. You can also put the hydronic coil on the return plenum. Some people have their cooling coil or their AC coil that’s taking up the space. So if you need to this hydronic coil can also go on the return. The only drawback is you’re going to be running hot air across your fan. There’s some speculation as to the longevity of your fan as it relates to having hot air rather than cold air move across it.
Just looking at this real quick what we have is two heat exchangers on that solar tank. One heat exchanger on the bottom solar loop then the top coil is tied into the hydronic coil. That circulator between the tank and the hydronic coil will only activate when the set point. The set point of our…This thermostat right here if you can see this top thermistor on the solar tank. When that registers a temperature above the set point of the blower. Then this circulator will kick on if there’s a load. So all we have to do is re-wire the contacts in the furnace. So that when the thermostat calls for heat this takes priority. The solar tank takes priority and the circulator kicks on. When this circulator is on then the fan is one. So the fan actually receives a current from two relays. One relay is in parallel with this pump so that when the pump is one then the fan is on. The second relay is the same relay that fires the furnace. So when the solar tank is not up to temperature the circulator is off which causes the burner to kick on. Which will in turn cause the blower to kick on.
Notice where you’re also using the storage tank as a hot and cold water. So the cold water comes into the bottom and as it passes through here to the diverting valve. This is an on demand modulating water heater. As the water passes into this diverting valve if the water is up to temperature already then the water continues on its normal path up to the load. As it reaches the diverting valve and it’s less than desired temperature then the hot water is diverted up into this on demand modulating boiler. Which picks up the slack and then dumps it out to the load. So from a single storage tank we’re tying in a furnace and a modulating boiler.
I do realize that I’m going to go through these diagrams pretty quickly. Please remember to always access these they’re available to you and we can help you afterwards.
Here’s another example where we have some space heating circuits that are zoned off. There’s a manifold valve actuators on each of the zones. In this case we brought the exchanger outside…External heat exchangers do have a higher efficiency of heat transfer then internal heat exchangers. The drawback is that they do require a second pump.
Now there is occasionally some mineralization in on demand and other types of heat sources. So if you have a particularly hard water one of the first things if you haven’t done already is you want to try to treat that water. Many people for example Peter uses a primary secondary piping to prevent mineralization of the on demand. Now that’s a whole other topic all together. Perhaps I can do a webinar on secondary primary piping to avoid mineralization.
I want to get through this real quick. Now this external heat exchanger requires that second pump. This is pump is going to be tied in parallel with this pump. So when this pump is on P1 then P3 is going to be on pulling heat out of the top of the tank through the heat exchanger. Just like the diverting valve in the previous diagram. As the return passes through this heat exchanger to the diverting valve if it’s up to temperature then its gong to send the flow back out to the zones. Okay which is going to be delivered by this pressure regulating circulator and these actuators will open to their corresponding zones. If the temperature at this point after the heat exchanger is not up to temp it’s going to run up into the boiler. Where the boiler will pick up the slack and then dump it out. Notice the close spaced tees. These basically promotes a hydraulic separation as it appears this pump one and pump two appear to be in series. But they don’t work against each other nor do they work with each other. Because of these closely spaced tees separate hydraulically these two pumps.
So one of the common things that you’re going to see in all of these are a three way diverting valve. Three way diverting valves are easy to use. They run off a simple relay in most of the controllers. Certainly the controllers that SunMaxx offers.
Now this next diagram is similar to the one I just showed you. The only difference is if we go back real quickly. In this case we’re using a heat exchanger between our solar tank and our heat zones. In this picture in this diagram we’re using a heat exchanger between our domestic hot water and a solar tank. Now this is very effective way to tie your domestic hot water into a storage tank. This storage tank contains the fluid that’s being delivered to the heat zones. Okay in this case we’re using low temperature panel radiators with thermostatic mixing valves in each room. So the difference just to repeat myself is that in the previous slide we have a heat exchanger externally that separates the heat load with the solar tank. In this diagram we’re using the entire volume of fluid in the tank being delivered to the heat load.
Basically you suffer less performance penalty because the differential between the heat load and the solar tank is much less then the differential between the cold water supply and the solar tank. So in a sense you get a better heat transfer through this heat exchanger on the domestic side then you do in this previous picture using the brace plate to separate the solar and the heat.
Now this diagram is probably the single best way because we see one heat exchanger here and that’s for your domestic supply. This is a drain back system. Yes, in the previous slide Don just to go back real quick. This heat exchanger is used for the domestic supply. One thing I didn’t mention is there needs to be a flow switch if you’re going to use a heat exchanger on the domestic side. Because this flow switch will activate the circulator that’s going to pull this circulator here…Pulls out of the storage tank under two conditions. One if there’s a load and the tank is up to temperature. The second condition is when there’s flow on this cold water side as indicated by the flow switch.
Now back to the drain back system. If you notice the fluid that we’re sending through the collectors in this schematic is being delivered to the house. So the energy that we make is being swept away through the heat transfer fluid. Being stored temporarily in a buffer tank. Then being delivered to the zones. This situation is very similar to what I showed you before. We’re pulling off the top of the tank using a three way diverter valve. If the temperature meets the requirement then the flow continues out to the zones and is delivered by this pressure regulator circulator. If the temperature is not up to the set point then the three way valve will send the return up into the boiler. The boiler being modulating will pick up what it needs to and dump ten, twelve, twenty degrees back into the loop. Then it will send if off into the zones.
Well I just had a comment regarding air pads. So regarding this particular set up here perhaps you could elaborate the…I’m not familiar with an air pad. But what we do have here is the drainage of this because of this air pressure control valve. So when these circulators shut off the air is going to rise up the small narrow diameter tube into the collectors. Which is going to cause all the fluid to drain into this pressurized storage tank. Now the coil inside the storage tank is transferring the heat into the domestic supply. So just based on flow no need for a flow switch. As the fluid enters the coils it’s being delivered right out of the tanks. So the only exchange that we have in this setup is the coil.
Yeah, I just did have a note regarding air pads. So if anybody would like to comment on an air pad. More than happy to take a moment. Otherwise we’re going to keep on moving forward here.
Now heat dumps you definitely need to have some type of a heat dump. If you have a pool you’re in good shape. Good so this air will not leak out of the drain back system. This is a closed loop drain back. There’s no where for air to escape. This bubble of air right here is essentially going to be nitrogen. If it is nitrogen then we know we have a closed loop drain back. The reason its nitrogen is because any of the oxygen that’s been in the system will be oxidized and the remainder of that air will be nitrogen. So with a closed loop system this air bubble is trapped by the force of those pumps right there. Those pumps are trapping the air in here. Once those pumps shut off then the air can escape back up into the collectors. But there’s no where for the air to go. This air is going to be in the tank or it’s going to be in the collectors.
PSI is entirely dependent on the head that you have in the system. It’s dependent on the flow. It’s dependent on the pressure drop. PSI is not to exceed ninety. In a situation like this you’re going to need at least one atmosphere. So anywhere to fifteen to ninety PSI.
In the event that leaks occur somewhere and you have some problem. Then hopefully this little air release will allow for any of the extra air that was…Eventually when the air comes into the system it’s going to work its way into this bubble. This bubble will displace any of the fluid that’s left up in the collectors as long as they’ve been pitched properly. But that’s a whole other discussion. In fact I think we’re going to do a drain back webinar soon. So I’m definitely going to make note of that and discuss the air elimination strategy.
Now sizing heat dump real quick. You have to understand what the maximum production is going to be. The magic number for maximum production per square foot of standard thermal collectors both flat plates and evacuated tubes is 300 Btu’s per hour. So knowing that I have three hundred Btu’s per hour maximum production in June. You size up a heat dump relative to the square footage of your collector. For example if I have a hundred and twenty square feet times three hundred Btu’s an hour I need a heat dump that’s capable of the thirty six thousand Btu’s an hour. Now that’s not a very big heat dump. Most heat dumps in standard hydronic coils like this are going to be good for. Even a small one is forty to fifty thousand Btu’s an hour.
I want to show you the picture maybe you’re familiar with a butler max over temperature unit. These are not recommended for Combi Systems as they aren’t as much a heat dump as they are a heat dissipater. Now let me just clarify the difference. A heat dump is something that’s going to actively dump excessive amount of energy out of the loop. These are a very effective insurance in any event that your pump shuts off and it protects your glycol from burning and turning acidic. But it’s not an operational component you should rely on to dump excessive amounts of energy out of your system.
Now here’s another example of a heat dump the swimming pool. As you notice on this diagram we’re using again a three way diverting valve. Now this three way diverting valve is going to be drawn off of a relay from the controller. And it essentially heat exchanger is in parallel with your solar storage tank. So when your solar storage tank is up to temperature as indicated by the differential controller. The three way valve will automatically open. Now many of your controllers have thermostatic functions as well as differential functions. So in this case you’re going to use your differential controller’s thermostatic function. That is once this sensor at the top of the tank reaches one hundred and seventy five degrees. Whether the solar loop is running or not it’s automatically going to send voltage to this valve. Which is going to open it towards the shallow tube heat exchanger side. What that’s going to do if your solar circulator is still running. Your supply side of the collector is going to pass through the shallow tube and you’re going to bypass the storage tank. Now pools take a tremendous amount of Btu’s. So rarely is a pool unable to handle the Btu load produced. But to play it safe you should still understand the maximum production.
Another way to size up a pool in terms of a heat dump is you want a one square foot of pool for every square foot of collector. If you’re less than that then you’re good to go. If you’re more than that then you should consider an extra heat dump. So one square foot of surface area to one square foot of area for the pool collector to pool 1:1 you should be good or less.
Now in terms of storage tank. One thing I want you to notice here is that the solar fraction begins to drop off as you increase your storage tank size. So the recommendations that we make in terms of storage tank size relative to your collector field. Is completely dependent on your temperature requirement. As I mentioned before the lower the temperature requirement the larger the storage tank volume. But you reach a point where the storage tank volume becomes a hindrance to your total system efficiency because of all the losses. The thermal losses through conduction and radiation out of your storage tank become a hindrance to your total system efficiency.
Although there’s a lot of different ideas and schools of thought in terms of storage tank. One thing’s for sure as you approach four to five gallons of storage per square foot of collector. The system efficiency will begin to drop. Solar fraction will also drop. It won’t continue to increase like this and you do see the relationship. So what you need to do is try to find and pinpoint where that intercept is. And that intercept is dependent on your temperature requirement.
Another consideration is that your storage capacity can fluctuate. So we try to design systems that have fluctuating storage capacities whether we use two tanks or we maximize the stratification of one tank. And play with the stratifications so that in a sense we can have two storage tanks inside one storage. Because your production changes all the time as does your consumption. It’s not as straight forward relationship. So most heat exchangers are going to operate at about fifty percent for external heat exchangers and internal heat exchangers often times or even less than that.
So here’s an example where you can design a system with two storage tanks to fluctuate the volume. These are in parallel off of the same solar loop. With a Combi System controller you have the ability to not only energize your solar loop and your re-circulation loop and have a high temperature shut off. You can have freeze protection and a Btu meter and variable speed drive. But you also want to be able to control your diverting valves. Those diverting valves not only tie into heating systems effectively but they can also be tied to multiple tanks. So whether you do multiple tanks in series or in parallel as shown here. So we have tank prioritization going on where the domestic hot water is taken care of for. The tank to your left or anatomical right is the domestic hot water tank. Once that tank is up to temperature the diverting valve will automatically send the solar fluid into the bottom coil of the second tank. Any surplus energy is then used to tie into heating system using the conventional method that I showed you before.
The controllers that are used for Combi Systems are slightly more complex then your standard domestic hot water controller. Well, Don this would be an example where you’re not using glycol. Some systems are just using water and they’re circulating from the storage tank up to the roof at night when the temperature drops below thirty five. Its sacrifices estimated of ten to fifteen percent of your annual production. But some places some clients don’t want to use glycol at all. That gives you an ability to do that.
The IntellaMaxx Combi Plus has twelve sensors. It allows you to tie into nine different. I has nine different functionalities. So it has pump logic and also valve logic. So we can control several different heating system, domestic hot water system, pumps and dump loops.
So here’s an example I know you can’t see this. I took it right out of the manual for the IntellaMaxx Combi Plus. Just to point out though the important take away here is there’s several different arrays that you choose from. When you’re designing your Combi System try to stick with what we know works. So you might download our Combi Plus manual just to get an idea of what types of strategies you can use for a heating system. So I think that’s an important point and I’m going to repeat it. If you go to sunmaxxsolar.com you can find this manual. Download the Combi Plus manual just to see what the possibilities are so you have a framework that you’re working within. And try to design systems based on what works and what already works. I know many of you like to think out of the box which is a great thing. If you do come up with a great idea that you want to try out. Please consult one of your SunMaxx representatives or our engineering design team. Just to get a feel for how well we think it’s going to work and if there’s any drawbacks to it.
So basically for Combi System you want to pick one of these arrays that seems to represent what you’re trying to do. It can be tweaked and modified a little bit. But you want to stick with… These systems have been proven and tested for many years in Europe. That’s how the controllers have been designed around systems that were working properly. So the functionality of the Combi Plus represents tried and true methods of Combi System layouts in Europe. I want to leave you with this website if you download the pdf it’s hyperlinked. The website is solarage. It’s www/solarage.com. What solarage is it’s basically case studies of European Combi Systems and there are hundreds of large scale, small scale, residential and commercial Combi System designs. Solar fractions detailed at post installation analysis and a lot of software was used to describe the performance of these. Your clients might want to see it as well. So I strongly encourage you to check out solarge if you are inclined for some case studies, analysis of these systems having been installed and maintained in Europe for a long time.
I’ve gone over a little bit. But I would like to thank you for your attention. I want to remind you to visit solarwebinars.com for any archives. I did go a little bit fast. This Combi System is really a day or two or three days of discussion and training. The solar webinar series is designed to really just help you out and answer some questions that you have. To stimulate thought and really get you thinking. Really bring everyone together on the same page and be a resource for everyone. I’m always open for future suggestions for future webinars.
I’m doing one today on drain back systems. So thanks Pete for those recommendations. I’m interested to learn more from you about the side arm heat analysis. So thanks again everyone I hope you have a great solar day. The weather is turning a little bit maybe that’s the best for some of you. But here in upstate New York it typically means low solar production and cold weather. So I’m trying to make the most of it. Have a great solar day everyone. Take care.