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Solar Thermal Hot WaterThermomax solar thermal hot water

A solar thermal system uses the sun's energy to create hot water than can be used for space heating, domestic hot water, pool heating, ice melting, industrial process heat and more.  A solar hot water system can save you money on fuel, your boiler will not need to run May - October, and you will feel good about cutting your environmental impact.  Hot water systems are a cost effective choice especially for families.  For small households an on-demand tankless hot water system, or air source heat pump tank is often more cost effective.

What works in much of the US does not work in our harsh climate.  In Florida a simple black tank, holding the actual domestic water, sitting in the sun may be enough.  Here in Maine we need something a bit more effective. 

In general solar thermal systems come in two styles.  A flat plate collector or an evacuated tube system.  Both have advantages and disadvantages.  They both need a sunny location.  South facing is ideal, but East or West can work also especially with tube systems.  Collectors are mounted on the roof or on a ground rack.  A plumbing line circulates the hot fluid to a heat exchanger which warms the water in your hot water tank.  The collector plumbing loop is filled with a glycol water mix to prevent freezing in the winter.  This is a closed loop system and the fluid never comes in contact with your domestic water.  In addition we use a non-toxic glycol.  A controller makes sure the pumps only run when the sun's energy is making heat. 

Solar thermal systems are safe, work with all existing mechanical systems, and need almost no maintenance.  Rain will wash them clean.  Every few years you should test the glycol PH with a test strip.  After 10 years or more it may need to be changed.

Solar energy is limitless and free.  But before considering a solar energy system is it important to insure the building and systems are efficient as possible.  The quickest return on your investment will be to properly air seal, insulate and then upgrade older heating equipment.  To help with this process we offer energy audits, all types weatherization services and install high efficiency heating equipment. 

Flat plates vs Evacuated tubes

If you have ever seen a solar electric photovoltaic module a flat plate thermal collector will look much the same.  It is basically an aluminum frame rectangle with copper pipes inside covered by a copper sheet and then glass.  Within the frame the copper pipes are protected by insulation.  The glycol mix flows through the pipes and is heated and circulated to warm your tank. 

Apricus evacuated tubeAn evacuated tube is a glass tube where all the matter has been removed.  Temperature requires matter to move.  So a vacuum makes a perfect insulator since heat and cold cannot move through it.  Usually an evacuated tube system will consist of 8-30 tubes connected to a manifold.  Within the solar thermal evacuated glass tube is the copper heat pipe that actually creates and moves heat.  This narrow pipe is sealed, under pressure, and has a bit of fluid inside.  When the sun strikes the absorber surface on the evacuated tube the energy is transfered to the heat pipe and the liquid inside flashes to steam.  This makes the bulb at the tip of the heat pipe very hot.  This tip is plugged into the manifold which contains the glycol fluid.  The heat is transferred to the circsingle wall evacuated tube ulating fluid and the tip cools causing the steam to turn back to a liquid then the cycle repeats.   The hot glycol fluid is circulated to a heat exchanger and warms the water in your tank.

Both systems are very durable and even normal hail will not damage the collectors.  With flat plates part of the heat created is radiating back out so snow quickly melts.  With evacuated tubes no heat is radiated because it is contained by the vacuum.   With tubes the snow tends to blow off or fall between due to the round shape.  In both cases snow normally clears within 2 days. 


 Advantages of flat plates

  • Lower cost
  • In areas of high snow, such as roof valleys, the snow clears sooner since they are always loosing heat
  • More of a skylight look
  • Work best when the pitch is at least 35 degrees, ideally more

Advantages of evacuated tubes

  • Create higher temperatures.  Hotter water in the tank means more usable domestic hot water
  • Great for space heating or industrial process heat
  • If one tube breaks it can be easily replaced.  Until then you only loose a percentage of your capacity
  • Lighter weight

What about rain, snow and cloudy weather?

Our New England winters are much harsher than most of the country.   This means we need to have a slightly larger collector than someone in the south.  In addition some technologies simply do not work well up here.  We have been installing solar in Maine since 2008 and we know how to so it right.  There are three important peices to this.  First is we always need a backup system.  The most cost effective option is to leave in place your current hot water system.   Second, we use a large hot water storage tank.  These tanks contain enough water for about 2 days so your backup system does not have to come on very often even in the winter.   Third, any decent solar collector will do an excellent job in summer.  We use top of the line products that work well in our winters.  Plus we angle the collectors towards the lower sun angle that occurs in winter.  This means a less than ideal angle in summer but since the days are much longer you still get plenty of hot water. 

We will tell you the expected "solar fraction" your system will deliver.  This number is the percentage of hot water that will come from solar vs. your backup system.   We consider average weather patterns in this number. 

Some products are more suited to our weather.  Thermomax makes the top of the line evacuated tube.  The absorber coating does an excellent job at collecting diffuse radiation, what you get when clouds or moisture breaks up the straight paths of energy rays.  Collectors with titanium oxide coatings are generally blue and perform better than other coatings.  To make a good buying decision it is good to get expected performance number for Maine on a month by monthly basis and take a close look at the winter performance.  Unfortunately few manufactures can provide this. 

Features and differences of collectors and components


Apricus tubes have a coating around the entire tube which gives them the advantage of being able to collect the sun no matter which side it come from.  Many tubes have a flat collector inside and we can rotate the tubes so it points towards the sun if your site does not face true south.  There are flat plates with poly-carbonate glazing.  These are a great inexpensive option for camps as they are light weight but their winter performance is low.  At the end of the day all the sales pitches and unique features become irrelevant.  What counts is what output you can expect at your unique site.  Picking the right product is where our experience becomes important.  

The SRCC is the US rating agency that tests and certifies solar thermal collectors.  They are similar to UL ratings for electric appliances.  Every manufacture can shoot holes through their testing procedures and ways of reporting.  But, they do give some way of comparing collector to collector.  Unfortunately their site is complicated if you are not sure of what you are looking for.  When you get a proposal for a system ask the installer to provide a copy of the certificate and to explain how to read it.

Evacuated tube differences

A true evacuated tube is a single glass chamber, such as Thermomax, or Sunda.  In these cases the glass tube has all the matter removed and the heat pipe resides inside the vacuum within the glass.  It takes high quality manufacturing equipment to successfully make this type of tube.  Thermomax produces their tubes in the UK at a state of the art facility.  The advantage is the all important heat pipe resides in and is protected by the vacuum.  Hybrid tubes cost less and they use a double layer of glass with the vacuum between the layers.  The heat pipe resides in the air chamber that is blanketed by the vacuum.  These are slightly less efficient as part of the heat produced is use to keep the air chamber warm.  A key component of these types of tubes is how well the air chamber is sealed and how long that seal will last.  They need to fit into a well insulated manifold.

The vacuum is what allows these tubes to perform so well in the winter.  The heat pipe is protected from the outside temperature and performs just as well when it is 0 as when it is 70 degrees.  Proper manufacturing will insure the vacuum will be maintained for 30 years or more.  Thermomax gives a 20 year warranty, many other manufactures offer 10.  Most of the less expensive tubes use borosillicate glass which can allow the transmission of gasses into the tube.  Over the first five years you can see a drop in production until the vacuum stabilizes with our air.  Thermomax uses a soda lime glass that does not allow helium and hydrogen to pass.  These tubes maintain their ability to produce high temperatures over the years.


Racking is generally made of stainless steel or aluminum.  A few manufactures use power coated steel.  These are heavier at install time and if they get scratched they can rust.  If this happens you could end up with rust stains down your roof.  Overall the vast majority of racking is well made. 

Despite the fact we use the finest caulking and insure they are compatible with your roof, we are firm believers in flashing all roof penetrations using a product such as QuickMountPV or EcoFasten.   We expect our workmanship to last at least as long as your system and than includes doing everything we can to prevent roof leaks.  Plus all roofing trade associations require their use even if the building code doesn't.  In our opinion risking a leak to save a bit of money is simply not worth it. 

If you have a metal roof with standing seams there is a clip called S-5 that allows us to attach the racks without drilling into your roof.  They make a number of different shapes

Ground mount systems are either poles or racks.  Before the price of solar electric, photovoltaic, dropped the idea of sun tracking racks made sense because it increased output 30% or more.  It mattered because PV is less than 20% efficient at converting the sun's energy to electricity.  Solar thermal systems are often about 80% efficient.  It simply is not worth the extra initial cost and the maintenance expense to use a tracker with solar thermal.  Plus engineering the plumbing lines can become a nightmare. 

Controllers and pumpsSolar pump station controller

Controllers can make a huge difference when installing, debugging, or tracking performance data.  Many installers are using an integrated pump and controller system.  They include the pump and controller as well as pressure relief valves, filling ports, flow meters, gauges, and many other parts.  They all have windows that display operating information.  These all-in-one boxes are quicker to install thus saving you money.  Some of these pump stations have the ability to control many functions, such as pool heating, dissipating excess summer heat, east/west applications, or controlling multiple tanks.  Some come with built in SD cards to record operational data.  This is what we use the vast majority of the time.  With the historical data collected we can see the collector temp, top of tank, bottom of tank, we can even program it to record how often the backup boiler comes on.  This greatly reduces the time we spend on service calls and saves you money.  Other systems have web based monitoring.  These usually include some sort of wall mounted box that displays performance information. 

Tank solar controller and Nyle heat pumpHot water storage tanksmarathon hot water tank

In most residentail applications we use a pressurized tank of 80-120 gallons.  Hot water storage tanks are large and thus expensive.  We prefer tanks with stainless steel linings because they last longer than glass line tanks.  When you are spending $2000, or more, on a tank you do not want it to rust out after 10 years.  Thermomax guarantees their tanks for 20 years.  Another favorite of our is Marathon.  These poly-carbonate tanks are light weight, excellent for sites that have corrosive water, and have a heat loss of only 5 degrees per day.  These tanks work well in homes that do not have a boiler for backup.

In most cases our clients are currently using boilers for space heating.  Generally we add a dual coil storage tank, for homes about 80-120 gl.  In the lower coil we route the hot glycol loop from the collectors.  Heat always rises and it will warm the entire tank.  The domestic water is drawn from the top of the tank and cold water comes in on the bottom.  At times, especially in winter, your solar system may not be able to keep the entire tank warm enough.  The top coil in the tank will be tied to the boiler so it can raise the temperature when necessary.  A thermometer tells the boiler when to turn off.  Since the coil is in the top half, above the thermometer the boiler will only warm the top.  Thus you never use fuel to heat the entire over-sized tank.  To prevent scalding a mixing valve will insure your domestic water never exceeds a safe level.  Other backup options include gas backup tanks, an electric element in the tank, or a stand alone Nyle heat pump as shown in the picture above. 

If you are doing space heating or have an industrial application it may be necessary to have 500 gl of storage, or more.   In these cases we often use unpressurized tanks.  A large heat exchange coil inside draws the heat out and send it to your primary tank.  

Excess summer heat

Residential hot water tanks are only rated to 180 degrees.  In the summer it is possible to exceed this.  With a flat plate system the pump will stop circulating and the heat will radiate out.  Evacuated tubes don't loose their heat and need some other way of dissipating it.  A pool is the perfect place, just ask your kids.  Normally we put in a divert valve and the glycol is routed to a "heat dump" until the tank temp drops.  This can simply be a piece of baseboard in the basement, garage, or outside.   It could be a PEX loop that warms your vegetable garden soil.  There are limitless ways to handle this and it is not difficult or expensive. 







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