Tip Archive
August 17, 24, 31
September 7, 14, 21, 28
October 5, 12, 19
November 9, 16
December 7, 14, 21
January 11, 18
February 8, 15, 21, 29
March 7, 21, 28How do solar heating systems compare to solar electric (part 2)?
Last week I wrote about the difference in cost between the two styles of systems and a reader challenged me to do better with my comparison so here it goes. As discussed last week a solar electric system costs between $7 - $10/ watt installed. Using the numbers from last week a solar energy heating system is typically installed for $1.20 - $2.20 /watt installed. When you look at the system having an expected life of 25 years that means you are selling clean renewable solar energy for $0.06 - $0.10 /kwh. Not a bad deal. And you get to own your power generation rather than leasing it from the power company.
How do solar heating systems compare to solar electric?
Currently, there is tremendous interest in solar energy in the United States with apparently a lot more interest in solar electricity than in solar heating. How do the two technologies compare? Typically, installers will talk about BTUs when they talk about solar heating systems. That is unfortunate. The general public doesn’t understand a BTU nor do they care to. They are billed in kilowatt hours or therms or gallons for their energy so why do they want to go back to high school chemistry to learn a new item that has no bearing on their daily life? While the two types of solar energy generate their energy in different ways there is a very simple way to compare the two systems. Both solar heating and solar electric systems can be talked about in terms of their peak energy produced. Solar electric systems are marketed by their peak energy production or the amount of energy produced under ideal circumstances. When a solar electric system is installed it is sized, installed, and quoted based on this peak number. A typical residential installation might run from 2 to 5 KW. In residential applications a solar electric system might run $7 - $10 per watt installed. So, a 4 KW solar electric system would cost $28,000 - $40,000 installed (before any incentives). How does solar heating compare? A solar heating system can (and should) be talked about in the same way in terms of peak energy. A standard system with 2 4’x8’ panels a peak output of 4.1 KW. This same system would typically install for between $5000 - $9,000. A huge difference. The next time you are at a trade show or talking to a potential customer let them know that you can install their 4 KW solar energy system for under $10,000 and let’s see if we can get some of the solar electric people to sit up and take notice.
I was told that I need to mount the collectors due south at the same tilt as my latitude to get the most efficient system. Is that right?
While I have heard this statement a number of times it misses the mark. Frequently a homeowner will have a roof that is not ideal for collecting solar energy. The roof might be east or west of south and have a pitch that doesn’t match the latitude. Rather than discouraging the homeowner from pursuing solar we should be educating them so they know how much energy they can produce. For example, a 4 person home in Raleigh, North Carolina with 2 of our platinum collectors and an 80 gallon tank would receive approximately 73% of their hot water from the sun. If you take the same demand and mount the collectors flush on a 5/12 (23 degrees) roof the system would produce 72% of the families hot water needs. The same is true if the panels are mounted on a 12/12 pitch roof. So the angle of the collectors is fairly insensitive to the angle the collectors are mounted for a year round hot water systems overall performance. What about the angle of the collectors? If you take the same two panel system and point it south-east or south-west you would only reduce the annual output to 70% of the families hot water needs. If you went so far as to point the collectors due east or west you would only reduce your solar fraction to 62% of the families hot water needs. As you can see solar water heating systems are fairly robust when it comes to placement and performance. There is rarely a case where the performance of a system would dictate mounting the collectors at a funny angle. So the next time you are asked about installing collectors on somebody’s roof that doesn’t match the perfect profile. Don’t sweat it.
How do I balance the flow in a multi-bank array (part 3)?
Over the last two weeks we covered the two most basic ways to balance flow through multi-bank arrays (outlet temperature measurement and reverse return piping). These methods of balancing flow are simple and robust but each have limitations. If you have a multi-bank array that doesn’t lend itself to either of the two previously mentioned methods then you are left to use balancing valves to achieve proper flow.
The basic concept behind a balancing valve is that any valve will generate a given pressure drop at various flow rates. A balancing valve works by having ports for measuring the pressure both before and after the valve. The difference between these measurements is cross referenced to a graph that shows the flow rate –vs- pressure drop. Once you have measured the pressure drop you are then able to find the flow rate on the graph. A good quality balancing valve will have the ability for fine tune adjustment. The process of setting the balancing valves in large systems with many banks can be time consuming but it does provide a proven means to guarantee consistent flow.
How do I balance the flow in multi-bank arrays (part 2)?
Last week I discussed flow temperature sensing as a way to balance the flow in multi-bank arrays. Another concept that has been used to great effect is parallel reverse return piping.
You need to balance the flow in multi-bank arrays because water will choose the path of least resistance. What that means is if you have two banks of collectors supplied by a single pump the water will tend to flow through the piping path that provides the least resistance. In multi-bank arrays this means that water will flow through the shortest path. To insure that each bank has similar flow path you should use parallel-reverse return piping. Parallel-reverse return piping insures that each bank has the same overall flow length (and therefore piping resistance). Among the different principles for balancing flow this method generally involves the most cost (in copper and additional installation) but it does provide a robust solution that is not subject to a system owner changing the balance by adjusting ball valves or balancing valves.
How do I balance the flow in a multi-bank array? (part 1)
When you install larger collector arrays or sometimes when you have to deal with limited roof space you end up having more than one bank of collectors. It is important when you have multiple banks to insure that you have relatively equal flow through each bank. If you don’t you may significantly compromise the performance of your overall system. There are a number of ways to accomplish this including; balancing valves, parallel reverse return piping, ball valves/pressure gauge combinations, and flow temperature measurements. Each of these concepts has advantages and disadvantages. For today I will only focus on using flow temperature measurement for two bank arrays. When you use the Steca 0301 differential control (part #: S-DC-12S) you have the ability to use three temperature sensors.
Two of these sensors are used as part of the differential control logic and come with the control. One of these sensors connects to the tank bottom and one to the collector bank exit. If you purchase and install the third sensor (part #: S-S-1KPS) on the outlet of the second collector bank you now have the ability to measure the outgoing fluid temperature from each collector bank. You would also need to install ball valves on the outlet end of each collector bank (before the return lines of each bank meet). With the ability to monitor outlet temperature of each bank you can now restrict the flow (by partially closing the ball valve on the exit of the collector bank) through the bank that has the lowest temperature. You would do this until the outlet temperature of each bank is the same. This system of balancing flows is probably the least expensive when it comes to equipment costs but you need to have a sunny day and a little time to dial in the flows.
Can I use a Temperature and Pressure relief valve (T&P valve) on my collector loop?
No but … While many installers try to use these valves on their systems because; a) the are cheap, b) they are readily available, and c) they frequently come with your tanks (meaning you probably have one lying around), using them on your collector loop will cause problems. A standard hot water heater T&P valve (the most common type) is set for 150 psi and 210 degrees Fahrenheit. If you use one of these in your system the high collector temperatures will cause it to periodically release collector fluid on both drainback and glycol systems. Released fluid means service calls to recharge the system, which we want to avoid. Some installers advocate cutting the temperature probe off flush with the casting to eliminate the temperature sensing portion of the valve. While this will eliminate the temperature portion of the relief valve, you will break the protective seal on the thermocouple. This will then start to oxidize and cause the valve to fail prematurely. This premature failure can take a year or 20 years. Avoid the warranty call-back and spend the few extra dollars to install the right pressure relief valve (Part #: P-V-3PR76).
How do I find out how much heat a given solar system will produce? This is an important question since you want to insure that your customer has reasonable expectations about how much hot water their system will produce. There are a number of tools that installers use to determine how to size a system. I would recommend using Retscreen.
Retscreen is a software tool available for free download from www.retscreen.net that was developed by the Canadian department of natural resources. The software uses NASA weather data for its modeling. As such, there is regionally specific weather data available for every area of the county. The tool incorporates factors like slope of the collectors, direction of the collectors, # of people in the home, size of the storage tank, heat exchanger efficiency, occupancy rate, desired water temperature, incoming water temperature, and a host of other factors. It is an extremely powerful tool that can model everything from a standard 4 person home application through a 300 room hotel.You should use the tool every so often to make sure that you are calibrating your customers expectations about what they will get from their system. Make sure your customers know what to expect from their system to insure a satisfied customer.
I have heard that the expansion tank needs to be before the pump but I have seen some European product where the expansion tank is after the pump which is right?
The first step in answering this question is to understand the role of the expansion tank and then to understand how a pump generates it’s flow and the relationship between the two.
Once an expansion tank has started to accept fluid it acts as a point of constant pressure in the system. What I mean is as the pressure tries to increase the expansion tank will accept more fluid to maintain overall system pressure. When the system pressure tries to decrease the expansion tank will expel fluid into the system again maintaining overall system pressure.
At its most basic level a pump generates flow by creating a pressure differential, lower pressure before the pump and higher pressure after the pump. At a given flow rate each pump will generate a given pressure differential. When you combine the features of these two elements you can see what happens. If the expansion tank is on the outlet side of the pump (holding the outlet pressure constant) the pump will reduce the pressure on the inlet side of the pump. If the expansion tank is before the pump the pump will increase the pressure on the outlet side of the pump. See the diagram below.
These drawings show what the pressure is throughout the solar loop. By installing the expansion tank on the outlet side of the pump you risk damaging the pump with cavitation. In order to prevent cavitation you want to maintain the inlet pressure on the pump to at least 15 p.s.i.. You can do this by either jacking the system pressure up really high to make sure you have it or you can design the system properly and install the expansion tank prior to the pump.
I was told that I shouldn’t use the insulation that I picked up at the plumbing supply house. Is that right? There are a number of different types of pipe insulation on the market. The most common type is Polyethylene insulation. This is a semi-rigid foam insulation that is similar to the swimming noodles that kids use in the pool. Frequently, water heaters will come supplied with 2-24” long pieces in the box. Polyethylene insulation is found at most hardware and plumbing supply stores. Polyethylene insulation should not be used on solar heating systems!!! Polyethylene insulation melts at 160°F – 180°F.
A more appropriate choice is Rubetex insulation. Rubetex (or other brands) rubberized insulation can be found at HVAC supply stores. Rubetex can be identified since it is much more flexible than Polyethylene with a higher melting temperature. The high temperature limit of standard rubetex insulation is 230 degrees which is suitable for most solar installations. If your installation has higher temperature needs then Rubetex has a higher temperature version that has a high limit of 300 degrees. Some installers prefer fiberglass insulation, which also works fine at the high temperatures.
Keep your installation looking good by choosing the right insulation for the job.
From time to time I hear stories of glycol systems that have their pressure fluctuate wildly or systems that have the pressure relief valve on the glycol loop discharge from high pressure. Why?
While there can be several culprits that can cause this situation the most common is that the pressure in the expansion tank hasn’t been set properly.
The purpose of an expansion tank in a closed loop glycol system is to allow the volume of the system to expand as the temperature rises without sending the pressure through the roof (remember the Ideal Gas Law: PV=nRT). The expansion tank needs to have enough volume to accommodate any potential expansion of the fluid without reaching the trip point of the Pressure Relief Valve. Prior to charging the system the desired system pressure should be calculated by using the following formula;
Cold System Pressure = 15 p.s.i. + [(1 / 2.31) x (vertical height of the system)]
Example: If the top of the collectors are 23 feet higher than the storage tank the appropriate cold system pressure should be: 15 + [( 1 / 2.31) x 23)] = 15 + 10 = 25 p.s.i.
Now that we know the cold system pressure we charge the expansion tank to 5 p.s.i. more than the cold system pressure; 30 p.s.i. in this case. By charging the expansion tank to slightly greater than the cold system pressure the expansion tank won’t accept any fluid until the system starts to heat up. If you charge the tank to a pressure less than this you run the risk of reducing the capacity of the expansion tank and not providing enough expansion room for the fluid as it heats up. This undercharging of the expansion tank can lead to the system discharging from excessive pressure. Another common problem is having too much pressure on the expansion tank. This problem is less catastrophic but it can lead to wild pressure swings between the cold system pressure and the expansion tank pressure. Effectively the expansion tank isn’t doing anything until it’s charge pressure is matched with system pressure. Only then does it begin to expand and accept fluid.
The final caution in establishing the “right” pressure in a system is to understand that no tanks come pre-set to the right pressure. Non-lined expansion tanks come preset to 12 psi (lower than it should be for any glycol system) where lined expansion tanks come preset to 40 psi (higher pressure than is required for almost all glycol systems).
Next time you charge a glycol solar system use these guidelines to make sure you don’t get that call back from the homeowner.
How do I connect the Solvelox to a two port hot water tank? For years installers have been using the drain port of tanks as a “cold” supply to feed external heat exchangers. While this can work it has a number of drawbacks. One of which is that any sediment in the water coming into the home through the hot water tank will accumulate in the bottom of the hot water tank. This sediment will accumulate first around the bottom edge of the tank. When you use the drain port as a supply you are pulling the dirtiest water in the system which can lead to premature clogging of filter screens (which you better have) and unnecessary wear on the your pumps. But, there is a better way…
On tanks equipped with two top ports (hot and cold) and a top T&P valve you can reconfigure the T&P port to accommodate both the T&P valve as well as the hot out. This frees up the port marked for hot out to be used as the solar hot water return (in conjunction with a diffusing dip tube (part # P-DT-955)). Now, you place a tee on the cold water inlet and use that to supply cold to the solvelox. By pulling the cold water up the cold water dip tube you are avoiding any of the sludge that can plug up your system.
By using this technique you are able to have the advantages of a four or five port tank without the cost and aggravation of finding that tank. (see picture) In using this technique be sure that the T&P probe protrudes into (but not past) the top 6” of the tank. If needed ask for a T&P with an 8” long probe (part # P-V-3PT76XL).
What are some of the safety concerns I need to keep in mind when installing my solar system? I will address this in multiple parts by taking the easy out first. Go to this website and watch this segment of Myth Busters about a water heater rocket to understand the force that heated water can exert. http://www.spikedhumor.com/articles/132341/Mythbusters_Water_Heater_Rocket.html
The pump on my drainback system doesn’t consistently pump high enough to get flow through the collectors. Or I hear a ticking noise in the collector side pump of my drainback system. Both of these issues can be caused by cavitation in the pump. Cavitation is caused when the fluid traveling through the pump vaporizes (boils) because of the pressure drop going into the eye of the impeller. As cavitation takes place you hear little popping sounds as the air bubbles are created and then collapse within the different pressure zones of the impeller. The more air bubbles created the greater the impediment on the pumps ability to generate flow. Cavitation can also lead to pre-mature wear on the pump components. We want to avoid it at all costs.
Some of the factors effecting cavitation in pumps are; static head on the suction side of the pump, surface pressure, fluid, fluid temperature, and resistive losses on the supply side of the pump. Since your system design dictate your fluid, temperatures, and resistive losses many installers shoot to eliminate pump cavitation by increasing the static head on the supply side of the system by requiring the drainback reservoir (and thus the static head) to be 1 – 3 feet higher than the pump. While this is always recommended combining this solution with an increase in surface pressure is recommended as well. Wilo and Armstrong recommend a minimum inlet pressure of 14.5 psi for their pumps when pumping hot fluids.
So, the next time you install a drainback system bring your compressor and charge the system up to 14.5 psi to reduce the noise and reduce the chance for a call back from the pump not working.To learn more see: http://www.mcnallyinstitute.com/11-html/11-12.html
How do I find out about government contracts for solar installations? With the boom in interest in implementing green technologies, governments at all levels have started aggressively pursuing and implementing green project. With local, state and federal projects focused on implementing green projects how can you “get a piece of the action?”
While each state operates their procurement system according to different rules it is generally true that most projects are open bid. In order to publicize the open bids states publish the bids on interactive web sites that can be searched for relevant criteria.
So, if you are interested in possibly landing one of these government jobs do a little searching on google to find the interactive state purchasing website for your state. Be sure to check back frequently since bids are regularly being added and withdrawn.
A few examples are;
http://www.mmo.sc.gov/cgi-bin/htsearch
http://www.ips.state.nc.us/ips/pubmain.asp
https://www.gssa.state.co.us/BdSols.nsf/OByCats?OpenView&Start=1&Count=500&Collapse=9#9How do I get the word out there about my solar installation business? One of the challenges with any business is getting the word out about what you do. There are countless books out there about marketing your business. With the many books available I won’t presume to be an expert although marketing a solar business might be both easier and harder than other businesses. Many marketing professionals will speak about the need for print advertising, radio, billboards, television the yellow pages and the like. While I have seen and heard of many installation businesses that have invested their resources in these different sources of advertising I have yet to hear of an installation company that got value from their investment.
On the other hand there are lower cost and more effective marketing options that are available. First, I would encourage everybody that has a solar business (installation or distribution) to take advantage of the free referral services that are available on-line. The most popular are http://www.sourceguides.com/ and http://www.findsolar.com/. There are many other sites that will provide referrals for you but these two can be very productive.
Other low cost options include participation in local/regional shows. These shows can be particularly effective if they have a green or renewable focus. We have customers that receive most if not all of there leads through these events.
The next time the guy from the Yellow Pages calls and tries to get you to spend $1,000 for a year listing remember there are other options that have served solar installers better.What is the proper pipe size for my solar installation? I get this question all the time and it depends on a number of factors. A few factors to consider include length of run, desired flow rate through the collectors, size of pump, cost of pipe and type of system (open loop, drainback or glycol).
From a cost and efficiency perspective you want to use the smallest diameter possible to accomplish the job. Because of the special requirements of a drainback system the minimum pipe size was discussed a few weeks back so I won’t rehash that. On glycol systems you have the chance to go with a smaller line set. Two of the requirements that you are working with for pipe sizing are; 1) keeping the velocity high enough that you keep air entrained in the heat transfer fluid to reduce the chance of the system getting air locked and, 2) keeping the flow low enough that you don’t suffer from pipe erosion by having too high a flow rate. To accomplish both of these requirements many sources recommend that you have a flow velocity between 2 and 4 feet per second. The following chart gives you the minimum and maximum flow rates that you should use for Type L copper of various diameters;
Tube size (type L copper)
Flow rate at 2 ft/sec (gpm)
Flow rate at 4 ft/sec (gpm)
Maximum collector area supported
3/8"
0.9
1.8
66
1/2"
1.5
2.9
106
5/8"
2.2
4.3
158
3/4"
3.0
6.0
220
1"
5.2
10.3
375
The column at the right gives you the maximum collector area that a given pipe diameter can support (from a flow rate perspective). This chart represents the maximum amount of flow for a given pipe diameter. Since higher flow rates lead to better heat exchanger and collector performance you may want to consider stepping up to the next larger pipe size where practical.
What is the right gas for my torch? For the solar installer doing a small amount of work with a torch is part of the territory. Whether it is using copper pipes for a glycol installation or tying into the existing lines many jobs require that you have to solder to finish the job. There are a couple of different gases out there available for soldering but which one is right for the job?
In general the choice is between propane and MAPP (Acetylene requires mixing so therefore is not a viable choice for on-site work). MAPP gas burns hotter than propane enabling you to solder faster. Faster soldering means less time on the job. This higher heat is also a necessity (or nearly so) on pipe sizes greater than ¾” in diameter. This means any time you are soldering on the collector headers you should be using MAPP gas. Propane can be used effectively on the smaller diameter lines particularly with ½” diameter and smaller.
If you are currently using propane I would encourage you on your next visit to pick up a canister to try MAPP. You will be glad you did.
Do I want a higher flow rate through my system or the lowest flow rate possible?
I have seen this question in various forms from many different sources. Generally speaking, installers will assume that the lower the flow-rate the better. Their logic is “I want to see a significant temperature drop across the heat exchanger otherwise the heat exchanger isn’t doing it’s job.” Another justification for the low flow rate is frequently to match the “design” or tested flow rate. Almost without exception this approach is wrong.
For uniformity of testing ASHRAE established a design fluid flow rate for solar collectors so they could be tested with a uniform set of criteria. Since the flow rate has an impact on the performance of the solar collector they needed a standard flow rate to provide consistency in their testing. This flow rate was not designed to represent the optimal flow for any particular collector arrangement.
The assumption that you keep the flow rate down to achieve better heat exchange is based on a limited understanding of heat transfer. All heat exchangers work by passing a hot liquid (or gas) along one side of a heat transfer surface and a colder liquid along the other side of the surface. How much heat passes from the hot side to the cold side depends on; the temperature difference between the sides, the flow rate on each side, and the heat transfer surface. Assuming no change to the heat transfer surface, if you increase either the temperature differential or the flow rates you increase the heat exchange rate. For example; if you take a plate heat exchanger with the following parameters; Hot inlet temp – 120, Cold inlet temp – 100, collector flow rate 5.5 gpm, storage flow rate 3.5 gpm you would transfer 24,113 btu/hr for a given size heat exchanger. Using the same heat exchanger, temperature settings, and collector flow rate but increasing the storage flow rate to 5.5 gpm you would increase your heat transfer rate to 29,906/hour. That’s a 24% increase in heat exchanger output. You argue, “but I will pay more to run my pumps to get that increased heat exchanger performance.” Here is the rub; the incremental cost to run the pump at the higher flow rate for that hour is about 30whr. The incremental heat available is 1.7KWH. In other words you will capture 57x the amount of energy expended.
Is it fair to claim that you are saving 57x the energy by running the higher flow pump if the solar collector can’t produce that much additional energy? Yes. If your collector array can’t produce all of the extra heat to keep up with the higher heat exchange your system will shut down. Using the numbers above if your solar panels are only producing 24,000 btus per hour then your higher flow rate system would only run 24/29.9 % of the hour. Either way this ratio holds.
Although the heat exchanger and collector will increase their performance by running at a higher flow rate there are a few reasons you don’t want to go overboard;
- Your flow rate should be constrained by the size of the pipe that you use.
- Too high a flow rate on the tank side can destroy stratification if used improperly.
- Higher initial cost to go with the larger pump.
Next time somebody tells you they run a lower flow to get better heat exchange just smile.
I’ve hooked up solar to my water heater and now the water heater doesn’t work.
This is the time of the year when the calls start rolling in. “I have a single tank solar water heating system and the water heater has stopped working.” This is not a service call you should need to run out on.Using a standard electric water heater for both your solar storage as well as providing your back-up heating has many advantages including; reduced floor space, ability to use standard tanks, less standby losses from the tank, better heat recovery, and less cost. But now we come to the rub. Electric water heaters have a thermostatic breaker built into the top element. When the thermostat senses temperatures over 170° F it automatically trips this safety breaker. This breaker is designed as extra protection in case you have a run-away element in your tank to keep the system from getting too hot.
Frequently a solar water heating system will supply 100% of your hot water needs during May through September. If at any time during that period the top of your tank exceeded 170° F the tanks thermostatic breaker will trip. As the cooler and cloudier weather sets in the back up element is no longer able to kick in and you receive a service call. Fortunately, the fix for this problem is simple and within the reach of even the least proficient home-owner.
Simply remove the cover plate over the upper element, depress the red button with the eraser of a pencil and voila you are back and running again. (see picture)
So next time you get this call don’t fret and think about the little red dot.
How do I prevent the pressure relief valve from releasing on my solar storage tank?
We have seen a number of installers call in with the pressure relief valve on their storage tank venting. They assume it is either a defective pressure release valve or that the temperature in the top of the tank is a lot hotter than the temperature in the bottom. They might see a temperature of 135 degrees on the bottom sensor and still have the release valve releasing. Is the temperature at the top of the tank 80 degrees hotter than the bottom? No.What these installers are most likely experiencing is the pressure relief valve releasing due to too much pressure. In many areas of the country it is written into the code that a backflow prevention device must be installed in the line. By not having any place for the heated (and therefore expanded) water to go, the system pressure may spike and cause a release through the P&T valve. But, there is a way to prevent this.
An expansion tank should be used on the potable side of any system where there is backflow prevention (or a check valve) installed in the house line. You can assume that any home built within the last 5 years would have one of these. By installing a properly sized expansion tank in the line you will eliminate the call back for the pressure relief valve blowing.
The exact size of the expansion tank required depends on incoming water temperature, tank high limit temperature, and incoming water pressure. As a rule of thumb; you should use an ST-12 Therm-x-trol (4.4 gallon) on the potable side for all of your 80 gallon solar storage applications. We have these in stock.
If you would like to size the expansion tank yourself use the following link; http://www.amtrol.com/pdf/MC4090%2006_07%20TXT%20Brochure.pdf Standard city water pressure runs around 40 p.s.i…
Note: Only use expansion tanks with polypropylene liners for potable water applications.
What size line can I use for a drainback system?
Traditionally, Drainback installations have focused on using 3/4” line for both the supply and return lines on a system. During July 2007 at the ASES show in Cleveland I had lunch with Chuck Kutscher from the National Renewable Energy Laboratory and he had some information on “a better way.” Several years ago they did testing where they used 3/4” line on the supply of drainback systems and 1/2” line on the return. By using the smaller line on the return side they were able to generate suction from the falling water thereby reducing the effective head requirements on the pump. By reducing the head requirements on the supply pump they were able to get more flow with the same energy use. More flow through the collector and through the heat exchanger leads to higher overall system performance. In addition, a given thickness of insulation around the pipes has a better effective R-value the smaller the diameter of the pipe. In conclusion, be reducing the diameter on the return section of the pipe you improve the overall efficiency of the system at the same time you reduce your material costs. A win for both you and your customer.
August 31, 2007
Can I use PEX instead of copper for an installation? According to the SRCC a Drainback system may receive OG-300 certification using PEX instead of copper if the system meets the following criteria;
- The system must be a Drainback system
- The PEX shall not be exposed to sunlight
- The system will use water ONLY as a heat transfer fluid
- Drainback systems employing PEX shall be non-pressurized (capped at atmospheric on the day of installation) or the system shall be vented to the atmosphere.
- At the outlet of the collector(s), a length of uninsulated copper tube of no less than 3/4” nominal diameter shall be used for a distance of no less than three feet before the conversion to PEX for the return line is made.
- The system shall have a pressure-only relief valve installed at the drainback tank location or the tank shall be vented to the atmosphere. The pressure rating for this valve shall be no less than 25 psi and no more than 50 psi.
What angle do I need to incline my solar collectors to insure that they drain properly in a drainback system?
Different installers have advocated different answers to this question for years. In Tom Lane’s book he advocates a pitch of 1/16” per foot. While this can work I would advocate a more nuanced approach. A collector can drain if it is mounted with the headers parallel to the ground although I would never advocate this approach. The general principle that we are trying to accomplish is having just enough angle to enable the water to run out of the collectors. The steeper the roof pitch the more confidence we have that the headers will drain even with a slight tilt back towards the inlet pipe. When you are working on a shallow pitch roof (4/12 or less) more pitch is recommended. I would recommend doubling the pitch of the collectors when dealing with shallow pitch roofs of 3/12 or 4/12. I would not recommend using a drainback system with a pitch of 2/12 or less. Another concern to watch out for is on lower pitch roofs pay special attention to any waviness in the roof. It can be catastrophic as in the case of this picture where the collectors were ruined from freeze damage because they didn’t drain.
Does it matter which sensor I place on the collector?
Yes, it matters a great deal. The sensors supplied with controls in the U.S. historically have come in two different formats; lug and probe. Traditionally, the probe sensor has been designed for a thermocouple well in a storage tank leaving the lug sensor to be mounted on the collector. This is no longer the case. With the huge success that Steca has had penetrating the market with their 0301 control there is now a change that installers need to be aware of. The Steca 0301 control comes equipped with two sensors; a probe sensor for mounting on the collector and a lug sensor for mounting on the tank. That’s right, the probe sensor is designed to be mounted on the collector. Steca’s probe sensor is designed to handle temperatures up to 356 degrees F while the lug sensor is only designed for temperatures up to 221 degrees F. If you use the lug sensor on the collector you are asking for a service call back because of a premature sensor failure.
Probe sensor and lug sensor for Steca 0301 control. The probe sensor is designed to take the higher temperatures on the collector the lug sensor is not.
