$2K Solar Space + Water:  Overall Design and Sizing

This section covers the overall design of the DIY combined solar space and water heating system.
 It gives an overview of how the system works, and goes over the advantages and disadvantages of the system.

It also gives some design ground rules and methods that will help you size the system to your specific needs, including a method that will allow you to roughly estimate the solar fraction a given size system will produce for your house and climate.

Cost saving and payback information is also provided.



Back to Table of Contents...

Overview of the system

This is a quick overview of how the system works:

- Collection: Water from the tank is heated by  circulating it through the collector when the sun is out. Water drains back to tank for freeze protection.  No antifreeze, no heat exchanger, no expansion tank.

- Domestic Hot Water: Cold water from the street is preheated via a single pass through a simple, efficient, and cheap heat exchanger on its way to the regular hot water tank.  That's all there is to it.

- Space Heating: Hot water is taken directly from the solar tank, circulated through the radiant floor loops, and back to the tank.  No antifreeze, no heat exchanger.

Really simple!


Objectives of the Design

The design emphasis on this system is to:

- Keep it very simple

- Keep it easy to construct with ordinary tools, ordinary DIY skills, and materials available from local stores.

- Strive for a long life with little maintenance.

- Achieve an aesthetically pleasing look from the street.

- Keep it cheap

Design Description

The system consists of three main parts:

Collector Loop:
The collector loop consists of the solar collectors, and of the pump and plumbing that circulate water from the tank to the collectors.

The collector loop is very simple.  Plain water from the solar storage tank is circulated by the pump up through the collectors and back to the tank.  When the sun goes down, the controller senses that the collector is cooler than the tank and shuts the pump down.  All water in the collectors and the supply and return plumbing drains back to the tank.  The fact that the water drains back provides the freeze protection for the system -- no water in the collectors and plumbing means nothing to freeze.

Drain back systems like this are well proven and very common.  It is important on the original installation to follow some simple rules about maintaining drain back slopes in the plumbing, but once the initial build is done correctly, these systems require essentially no maintenance -- this is in contrast to closed loop systems that use antifreeze for freeze protection, and that must be checked and replaced on a regular basis.

In contrast to many drain back systems that have a small, non-pressurized drain back tank for the collectors to drain back to, and then a larger pressurized solar heated water storage tank, this system just has only one larger, non-pressurized tank that both stores heat and acts as the drain back tank.  This eliminates the cost of one tank and plumbing connections associated with an extra tank.  It also means that no heat exchanger is required at all in the collector loop -- this is a big plus because it eliminates both the cost and inefficiency associated with a having a heat exchanger. 

The collectors are homemade.  The design has gone through several iterations improving with each one.  I believe that these collectors will have a very long life with just a bit of maintenance to the exterior case over the years. 
The performance tests indicate performance equivalent to commercial collectors.

The cost of these collectors range from about $6 per sqft up to $8 per sqft depending mostly on the glazing used.  This compares to commercial collectors at over $30 per sqft plus expensive truck shipping.

The control for the collector loop is an off the shelf commercial differential controller, and the circulation pump is an off the shelf  HVAC circulation pump.  Both of these are the same units as are used on many commercial solar water heating systems, and are readily available.

Domestic Water Preheat:
The domestic water preheat system uses a single pass of the incoming cold water through a heat exchanger in immersed in the solar tank to preheat water before it gets to your regular water heater.   The preheat is accomplished by running the cold water from the mains through a very large coil of PEX pipe that is immersed in the solar heat storage tank. 
On our system the coil is 300 feet long and 1 inch in diameter.    While PEX is not a great conductor of heat, this coil has so much surface area that it does quite well as a heat exchanger.  One advantage of this approach is that the coil itself holds about 10 gallons of water, and since this water will have been sitting in the coil and warmed up to full tank temperature, the heat exchanger is 100% efficient for the first 10 gallons of any hot water draw.

This approach 1) allows the main storage tank to be non-pressurized, which allows it to be large without getting expensive, 2) makes the plumbing very simple, and 3) avoids the use of a circulation pump that is used in many system.

The three valves shown in the diagram allow the solar water heating coil to be bypassed if desired.

Radiant Floor Heat Distribution:
The final element is the distribution of the heat from the heat storage tank to the house for space heating.  In our system, we use a simple, staple up, radiant floor heating loop.   For us, this is a single 200ft loop that heats only one area of the house.  It is certainly possible (if you have room) to add more collector area, and additional heating loops to heat more of your house.

The system we use is very simple.  The radiant floor circulation pump picks up water from near the top of the heat storage tank, and circulates it up through the half inch PEX radiant floor loop.  After completing its circuit around the floor loop and giving up some heat to the floor, the cooler water is returned to the bottom of the tank.  A check valve built into the pump discourages any tendency the water might have to drain out of the heating loop, and insures that the loop stays full and the pump keeps its prime.

No heat exchanger is used -- basically all there is to the system is a standard HVAC circulation pump and some plumbing.

The controls for the heating loop consist of a regular thermostat (as would be used with a furnace) that senses when the room needs heat, and a thermostat that senses the heat storage tank temperature to see if the tank has heat available.  These to are hooked in series so that when 1) the room needs heat AND 2) the tank has heat, the pump is turned on to circulate the heated water through the floor.

The tank is basically a plywood box that is framed (carefully) with 2 by lumber. The inside of the tank is insulated with high temperature rigid foam board insulation, and then lined with a single piece EPDM rubber membrane.   There is a tight fitting and well insulated lid to complete the tank.

This type of  tank design has been used since at least back into the 70's, and has a good track record.  I have heard from people with tanks of this type that date back to the 80's who are just now replacing the original lining.

The advantages of this kind of tank are 1) it can provide large storage volumes at a low cost, 2) it takes only simple tools and regular DIY skills to build, 3) materials to build the tank are locally available, 4) it has a good long life, 5) when the liner does wear out, it can be replaced at a low cost, 6) it can be insulated to whatever level you like, 7) its dimensions can be customized to the space you have available, and 8) for spaces with limited access (eg crawl spaces) it can be passed in through the access in parts and assembled in place.

Full construction detail is provided in the construction sections for all pars of the system -- see the Table of Contents...

So, this system can't all be good news -- what are the negatives?

- While constructing this system is not difficult, it is not a small project, and it will take more than a few hours of your time.  I don't recommend it as a first DIY project, but if you have done a few projects already, you should be fine for this one.

- The system as constructed does not qualify for the 30% federal tax rebate.  The federal program is not friendly to DIY builders and requires that the collectors be certified by the SRCC.  This is not really a problem, and there are a couple of good approaches that are explained here...

- There are some situations in which a drain back system cannot be installed, and in these cases, it is necessary to go to a closed loop system.  This is certainly not the end of the world, but it does make the system a bit more complex and a bit more expensive.  The circumstances that will make it difficult or impossible to do a drain back system are explained here...

- Compared to commercial collectors, which are largely maintenance free, the wood cased collectors used in this project will require a repaint once in a while -- this basically just like wood trim on your house.  It is also workable to build metal cased collectors, and this is covered in the alternatives section on collectors.

- Some of the components of the system (e.g. the tank) don't have the shiny stainless steel look -- if this bothers you, all I can say is "get over it" -- its performance and reliability that matter :)  There are actually some people who have done the $1K system, have done very nice looking tanks.
On the other hand, I think the collector looks better than commercial collectors -- I may be a bit prejudiced on this :)

Sizing the System for Water and Space Heating

This section provides some guidelines and methods for sizing the collectors and heat storage for the system -- first for domestic water heating and then adding solar space heating.

For Domestic Water Heating

For solar domestic water heating only (no space heating), the guidelines that are used for most commercial systems are along these lines:

- Collector area of 15 to 20 sqft per person for the first 2 people, then add 8 sqft for additional persons in warm climates, and add 14 sqft per additional persons in colder climates.
This works out to:

About 30 to 40 sqft for a family of 2

About 50 sqft for a family of 4 -- maybe 60 sqft in a colder climate.

- Storage size that is about 2.0 gallons per sqft of collector in warm climates to about 1.5 gallons per sqft of collector in colder climates.

About 50 to 70 gallons for a family of 2

About 80 to 120 gallons for a family of 4

- Tilt angle equal to the local latitude -- that if you live in Bozeman, MT at 46 degrees north latitude, you would tilt the collector at 46 degrees up from the ground.   This is the tilt angle that gives maximum year round energy harvest in most locations. 

I believe that a system sized to these ground rules  will likely provide nearly all your hot water during the summer, and about 50% in the winter (probably more in warmer climates), for an overall of about 80% of your hot water needs over the year.

Part of the logic for not going to a larger system that would supply a greater fraction of your needs is that the systems are expensive, and you reach a point of diminishing returns.

For DIY systems I think that a solar water heating system designed to the ground rules stated above will work fine, but I advise people to consider making these changes:

- Increase the collector area so that the collector has enough area  that on a sunny day it can make hot water for more than one days use.  Maybe around 25 sqft per person.

- Increase the storage area so that you have enough storage for 2 or 3 days worth of hot water -- this way a sunny day can put away hot water for that day plus a couple following cloudy days.  I don't think that 50 to 80 gallons of storage per person is at all excessive as long as you have the extra collector area to take advantage of it.

- Increase the tilt of the collectors to favor winter collection.  More tilt increases winter collection because the sun is lower in the sky and is shining more directly on the high tilt collectors.   Higher tilt also favors winter space heating (see below).

The aim of all these recommendations is to improve the year long solar fraction of the system by 1) being able to weather more cloudy days without running out of solar hot water, and 2) to increase winter production, which is the time when systems installed to the conventional guidelines tend to fall short. 

The changes listed above are a package of changes that tend to work together.  For example, the increased collector area could tend to cause overheating in the summer when there is lots of sun, but more storage and higher tilt angle reduce any tendency for summer overheat.  And, having more storage volume to store water for a cloudy day or two only makes sense if the collector area is large enough to heat up more than one days worth of water on a sunny day.

You may ask, why do these bigger system recommendations make sense for a DIY system and not for a commercial system?  Its because with a DIY system the extra cost is quite small.  At $6 a sqft for collector area, adding another 20 sqft of collector is only $120.  Adding some extra capacity to the tank with the kind of tank we use is very inexpensive -- a few dollars.  Commercial systems cost of the order of $8000 when  sized to the conventional ground rules --this is already borderline on ever paying back, so adding more capacity is pretty painful.  A DIY domestic water heating system can be done for $1000 even  to the more generous ground rules...

Adding Space Heating

The system is intended to supply both domestic water heating and space heating, so how does adding space heating effect the system?

 - Collector area should be increased significantly.  I give some methods below for making a fairly precise estimate of how much adding a given amount of collector area will decrease fuel bills and for estimating what kind of solar fraction a given collector area will provide for your house.  BUT, the simple answer is that for most homes you can add as much collector as you can find room for and as your budget will stand and not worry about overheating your house. 

If you happen to live in a house that is very well insulated, or you live in a mild climate, or you have a thick wallet, then you will want to go through the more precise methods below to make sure you don't overdue it, but for most of us, Bigger Is Better.

- Storage tank size wants to go up in proportion to the collector area increase.  A common rule of thumb that works well for most homes is that you want to add 1.5 to 2 gallons of storage per sqft of added collector.  This basically works out so that the tank will store the heat that the collectors can generate over one sunny day.   That is, warming up 1.7 gallons of water by about 50 to 60F takes about as much heat as one sqft of collector will generate on a sunny winter day.
For most US homes, all of that stored heat will be used overnight, so the storage will be back down to its depleted temperature the next morning and ready to take another day of sun.

If you have a home that is very well insulated and sealed, or you have a whole lot of collector area, or you live in a mild climate, then you may want to consider adding more than 2 gallons per sqft of collector storage so that you can store heat for multiple days.

Again, the more detailed methods give below will allow you to make a better estimate.

- Tilt angle for space heating systems should be steeper.  I would say local latitude + 15 degrees is a minimum, and that you can go all the way to vertical.  We did our system with vertical collectors, which have some advantages:

- With the low winter sun, vertical collectors get the suns rays at near perpendicular incidence.

- If you have a snow field in front of the collector, vertical collectors get a boost from the sun reflected off the snow.

- They fit against walls nicely (but you can still build a steeply tilted collector against a wall and have it fit in well...)
A vertical collector on the wall can also use the wall for support and for its back, saving some material.

For wall mounted collectors, its important evaluate the roof overhang and make sure that not too much of the collector will be shaded when the sun is high in the summer.  These overhang shadow tools will help you do this...

More Precise Sizing Methods for Space Heating Solar Systems

For those who want a more precise way to figure out how much a given size collector array will do in solar fraction and in fuel saving, these tools will help you figure this out.

Figuring out how much or your heating a solar system will provide is an interplay among several factors:

- Your home heat loss -- basically how well its insulated and sealed.

- Your climate -- how sunny and how cold it is.

- The size of your solar collector array -- bigger collectors mean more heat.

The tool listed below takes into account all of the factors above, and should provide a reasonable estimate of the fraction or your heating a given size set of solar collectors will provide for your particular house, climate, and collector size.

Tool to estimate solar fraction...

The method is fairly lengthy to go through, but not difficult.  It will give you a much better understanding of how your home heat loss relates to your systems solar gains. 

Example output from the Solar Fraction Tool

If you run into trouble using the method, drop me an email...

Minimum Size for Combined Space and Water Heating

If you look at the collector area for a solar domestic water heating system only (see above), and you are trying to decide if you want to add the space heating feature, but you have only limited room to add more collector, what is the minimum that its worthwhile to add? 

I would say that its not worth incurring the cost of adding the extra components for the heat distribution system unless you can add at least 50 sqft of extra collector over the size needed for domestic water heating, and more is better.  Adding the heat distribution system for space heating will likely add several hundred dollars to the cost and some extra labor -- you want to be adding enough collector area to make this extra cost and effort worthwhile. 

An exception might be if you have a nice simple way to distribute the heat  -- some options for really simple/cheap heat distribution:

-  A single hydronic baseboard unit just large enough to dump the solar heat into the house.
One might even work out a way to to let water from the tank thermosyphon through a hydronic radiator and avoid a circulation pump?

- A PEX radiator....

- Just having a way to remove insulation from the tank and letting the tank act as a radiator might also work for a very simple heat distribution system (assuming the tank is in a conditioned area).

If you can do one of these very simple heat distribution systems, than potentially it would be worth adding as little as 25 sqft of collector for space heating -- in very rough terms, an extra 25 sqft of collector area will produce about a $50 fuel saving per year -- you can compare that to the cost of the heat distribution system and see how many years it takes to pay off.

Note that our system is right at the lower end of the range as we did not have space for any larger a collector on the south wall.  We have just a bit over 50 sqft more collector area than we would have done for just domestic water heating.

Balance Between Space and Water Heating

Since the space and water heating come out of the same tank, there is some interplay between the two.

There are a couple of control strategies you can use to set the balance between space heating and water heating:

Scheme 1: Set the space heating control so that the minimum tank temperature is about 90F. 
That is, if the house needs heat, and the tank is at or above 90F, the floor loop will pull heat out of the tank.
With this arrangement, and depending on your collector area, home heat loss, and climate, the tank is likely to spend a good deal of time through the winter near the 90F mark.  That is, the space heating demand will likely pull the tank down to 90F fairly soon after the solar gain raises it above 90F. 
This means that the domestic water preheat will only get your water up to about 90F.   Depending on where you have your hot water tank thermostat set and your ground water temperature, this will provide about half of your hot water heating through the winter.
In the summer, the tank temperature will go up as the space heating need goes down, and the system will provide nearly all of your hot water demand.

Scheme 2: Set the space heating control so that the minimum tank temperature is about the same as your hot water tank temperature setting. 
For us, this would be about 110F.   With this arrangement, the solar heater will provide most of your hot water demand through the winter, but will provide less space heating. 

There will still be times when the solar heater does not provide all of your hot water -- for example, you get a sunny day, and the tank temperature will go well above 110F, but the space heating demand will fairly quickly lower it to 110F.  Then you get a cloudy day or two.  Under this scenario, the tank temperature will continue to drop due to domestic hot water demands over the cloudy days, so you will only get a partial solar preheat.  If space heating had been completely turned off for this scenario, the tank would have gone well above 110F on the sunny day, and would drift down toward 110F over the cloudy days while continuing to supply 100% of domestic water heating.

Either of these schemes will work.  With the first scheme the tank is on average at a lower temperature, and this will make the collectors more efficient.  For example, with 35 F outside temperature and full sun with a tank temperature of 120F, the collector efficiency is about 44%, but if you lower the tank temperature to 90F, the collector efficiency goes up to about 54% -- this is actually a 23 % increase in heat output at the lower tank temperature. 

So, from the point of view of getting the most "free" heat out of your solar system, first scheme is better.  But, if there is some reason you want to favor domestic water heating, you can just set the minimum tank temperature higher and take a hit in total output.  One reason you might want to do this is if you use a fuel for water heating that is more expensive or more polluting than what you use for space heating --  e.g. you might be using electricity for water heating and natural gas or wood for space heating.

One design challenge you might want to take on is some way to stratify or partition the tank in such a way that the the domestic water preheat coil can be in a warmer part of the tank.  I'd like to hear if you come up with a good scheme.

A word on cost and Payback:

This system is derived from the $1000 Solar Water heating system.  It is basically the same system with 1) an increase in collector area to allow for some space heating, 2) and increase in tank size to go with the larger collector, and 3) a system to distribute heat from the tank to the house for space heating.   These extra features raise the cost of the system to a bit over $2000 for a system with a modest amount of space heating.  The cost will, of course, go up as more space heating capacity is added, but even adding an additional 100 sqft of collector and corresponding tank size would not push the total cost a lot above $3000.

The cost of a commercially installed solar domestic water heating system (no space heating) typically runs about $8000.  So, the cost saving is pretty spectacular.  Many people have built the $1K solar water heating systems with good results.  Costs have generally run a bit above $1000, but not a lot.  Performance has been comparable to commercial water heating systems.

I've not attempted to determine the cost of commercially installed systems that do both water and space heating, and this would of course depend a lot on how much space heating.  But, I'd venture a guess that the prices would  be North of $15,000. 

So, I believe, you can save a ton of money building one of these systems.  The feedback I get from people who have built the $1K systems supports this belief.  Why the savings are so large compared to other DIY projects is puzzling.  Perhaps it has to do with the relatively low volume of commercial installations.  In addition, we have really worked to make the system simple -- simple is always better and cheaper whether you are designing airplanes or solar water heaters.  The tank design in particular is different than the typical commercial approach and much less expensive.   Whatever the reasons, the large cost saving is nice :)

I do want to make the point that the low cost is not due to cutting corners, the materials used and figured in the cost are new and high in quality -- in many cases they are exactly the same components used in commercial systems.

The payback for the $1K solar water heating system is discussed in detail here.  It runs about 2.5 to 3.5 years depending on the fuel you use to heat water. 

I have not tried (yet) to estimate the payback for the additions to the system for space heating.  I will do this, and add the numbers later, but my guess would be that the payback for the space heating part might be a bit longer than the water heating part.

Make or Buy and Rebates

See the Cost Page -- rebates section for the ins and outs of buying your collectors instead of building them to collect the federal rebate.

Basically you have to buy SRCC certified collectors in order for the system to qualify for the federal rebate.    Because commercial collectors cost about $30 per square foot plus expensive shipping and even high quality DIY collectors are only about $8 per sqft with no shipping costs, the system with commercial collectors will cost quite a bit more even with the rebate.  But, you do save the labor of building the collector and you get a high quality product -- so, its something to consider.

Question, Comments, Suggestions?...



I'm not a solar professional, not a plumber, not an electrician.   I do not take any responsibility whatever for the correctness of the design or construction advice give in on this system.   I do not take any responsibility whatever for any damage or pain or inconvenience that errors in this section may cause you.  You have to take on all the risk involved in building this system yourself -- Do Your Own Homework!


Full description of the $1K Solar Water Heating system that the new system is based on...

For those interested in the $1K Solar Water heating system (which is the basis for this system), all of the pages covering design, component testing, performance testing and logging, cost, design issue, and example systems are here...

The Solar Water Heating page...  (many alternative approaches to solar water heating)

The Solar Space Heating page... (many alternative approaches to solar space heating)

Gary February 17, 2011