Economics of a Solar Space Heating Retrofit

Expanding an Existing Solar Hot Water System

Inside this Article

Solar heating system
This solar heating system uses both flat-plate and evacuated-tube collectors to supply radiant heat for the barn, as well as a house and office. The system’s size, seasonal resource, and application all play an important part in its thermal and economic viability.
Drainback solar water heating collectors
Drainback and antifreeze solar water heating systems are easily adapted as an add-on home heating system.
Solar hot air collectors
Simple and inexpensive solar hot air collectors can often augment space heating systems more economically than hot water collectors.
Solar heating system
Drainback solar water heating collectors
Solar hot air collectors

When it comes to using active solar heating systems with a conventional home heating system, adding on to a solar domestic water heating system can be a viable solution—but the costs and benefits should be carefully considered.

Drainback and antifreeze solar water heating systems are easily adapted as an add-on home heating system—an additional one or two solar collectors serve the home’s space-heating load. Balance-of-system components needed include a larger (or second) storage tank, pump, control, and heat exchanger. (Locations in which simpler direct forced-circulation SWH systems or passive water heaters usually have little or no need for space heating.)

To understand the economics of each system, the local winter solar resource and length of the winter heating season need to be examined. A common SWH system has an 80-gallon storage tank with two 4- by 8-foot collectors. Such a system might have an installed cost of $8,000.

Adding a third collector for space heating and replacing the tank with a 120-gallon tank brings the cost to $11,000. The first two–collector system cost $4,000 per collector, and the third collector adds $3,000. Let’s assume that the heating season of the installation is six months, thus, the third collector is only used only half of the year.

However, because we’re installing this system in a location with a lower winter solar resource (where half of the yearly average resource is available), the value of the third collector doesn’t stack up to the first two. The math shows that the third collector gets only one-quarter of the useful production of the first two (0.5 for half the resource × 0.5 for half the year = 0.25). To have the same benefit-to-cost ratio of the original system, the add-on in this simple scenario would need to cost $1,000­—not $3,000.

But the economics of the system change with location. Let’s look at the add-on system scenarios in four different regions of the United States (see the “Add-On Heating System Economics” table). The Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors ( provides the solar resource and winter length information needed to evaluate active solar heating systems.

The heating season length is estimated using the heating degree days (HDD) per month in the solar radiation data. In the cities selected, the heating season varies from seven to nine months if we discount any month with fewer than 100 HDDs. (This is reasonable, since a month that measures fewer than 100 HDDs translates to about 3 HDD per day, for which even a small amount of passive solar gain will meet this need. If not, that’s what sweaters are made for.)

The average solar resource is listed as “kWh per square meter” (aka peak sun-hours) per day for five different tilt angles. The data is given with a monthly and cumulative average. To compute the solar resource for the heating season only, the heating season months’ resources are added and sum is divided by the number of months.

We used the “latitude plus 15° tilt” row for this evaluation, since, in most of the United States, a higher tilt angle gives better winter performance. However, in a few areas, such as the Pacific Northwest, heating season average peak sun-hours per month is slightly better at a tilt angle equal to the location’s latitude. This is due to the extremely cloudy winters.

In all cases, during the coldest three months of the year the system benefits from having a greater tilt angle. The latitude plus 15° angle also provides a hedge against the possibility of summer overheating. Note that the “Add-On Heating System Economics” table contains the data gleaned from the U.S. Department of Energy’s Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors (aka the “Redbook”) and has a multiplier to represent the relative benefit-to-cost ratio of additional collectors required for space heating only.

Comments (4)

Fred Golden's picture


Having given your barn heating some more thought, how about hooking up a SHWH to the roof, then storing the heat in a non-insulated tank? Then the tank itself will give off heat all the time. I know it is not as controlled as a insulated tank, or perhaps you could build a walled area within the barn, then insulate those walls with 3.5" fiberglass, and open the door when you want to warm the barn, close it when you do not need additional heat in the barn. With a water tank 80" high and 60" diameter (15 feet of surface area for each foot tall it is) that is a pretty large surface to dissipate heat into the room it is located in.

Collectors would depend on the area you live in. If you get snow, then evacuated tube will work fine. If you are near a gulf state, then flat panels work well and are less overall cost. One evacuated tube system in New Jersey was mounted along the south wall of the house to prevent summer overheating and also provide the ideal winter angle for the panels. The owner could clear it of snow with a broom fast.

Good Luck,


Fred Golden's picture


I did not look at the links that Ben provided, but would also suggest a simple south facing solar system might work out great for the barn, with some thermostatically controlled dampers to open in the day when barn temp is less than solar collector temp, then close at night. Also they would open to outside air all day and night when the barn temperature is over say 75F in the summer.

The solar collector can be as simple as installing 2X4 on the entire south wall of the barn, then paint it all black. Have an opening at the bottom (say left) then chanel the air to the right side, with a 2X4 say 10" wide about 2' above the bottom of the collector. Then the air will all have to go to the right side, then up. Another 10' long 2X4 will direct all the air to the left, then let it go up another 2-3 feet to another 2X4. At the top, then it will cross one more time, and then reach a damper either going into the barn or blowing the air outside, so it will not overheat the barn in the summer. Cover all these 2X4's with clear plastic sheeting 1/8 - 1/4" thick (for it's insulating value - something like plexiglass) to keep the wind from blowing away all the heat that you collected. Seal all the joints to avoid air infiltration. (and dust coming in).

You will not have heat storage, but it will warm the barn a bit during the day, passively.

For a active system, you might consider a 4X8 collector that will warm large volumes of water from say 60F to 90F. Evacuated tube systems are great when warming water above 90, say as warm as 120F. One system used a pair of 4X8 flat plate panels then a 30 tube evacuated tube panel to warm the water first to about 80, then 100, then 130F before returning to the solar collector tank. In your area, a drainback system or one with antifreeze protected to -10 will work fine. The problems with drainback is that if the pipes are not tilted enough toward the tank it might not all drain, and that you need a higher "Head" pressure pump. The antifreeze system can stay in the panels at -10F, and will be totally full of water, so a head pressure pump of about 10 - 15 feet at 3 GPM will work fine. This is a very low wattage pump. The problem with antifreeze systems is if they get over about 180F then the antifreeze is likely to break down.

AS for tank size. 500 gallons of water X8 pounds = 2,000 pounds. Heat it 10F and you have stored about 20,000 Btu's. About 1/5 therm of fuel saved. Normally you might be able to warm the water from 90F to say 130F on a good day, or about 40F. That would be about 80,000 Btu's. If the tank weight is 500 pounds, then it will also store about 30 Btu's per 1F warmed or about 20,000 Btu's stored in the tank mass. (it can only store the heat it can collect, so if the panel Btu per day rating is only 5,800 Btu's per day, then you will need to increase panel number to meet your heating needs, and increase a little more to cover cloudy days.) Flat panel 4X8 can collect more heat on a 50 - 100F day than a 30 tube evacuated tube panel will collect, despite them each covering about the same amount of your roof. On a really cold day, then the evacuated tube collector will excel a LOT over the flat plate collectors. Flat panels are also typically much lower cost too.

I am considering a 800 gallon steel tank, with a copper tube heat exchanger for fresh water wrapped around it. This will store 6400 pounds of water, or at 40F temp rise about 256000 Btu's total. This would also require a solar system capable of providing that much heat, it might actually take more than 3 days to warm it up with 3 panels, 6 panels will store enough heat for a couple of sun free days. I am in Portland Oregon and would only use evacuated tube panels because I want to store 120 - 140F water. This would not be practical with flat plate panels, and I might lose as much heat as I could collect on a cooler day if I pumped 90F water from the bottom of my storage tank to a flat panel then to a couple of 30 tube evacuated panels that have a output temp around 150F. Air surrounding the flat plate panel with wind blowing at say 15 MPH can take away a lot of heat from those panels, but will barely take away any heat from a evacuated tube array.

It also depends on how much tubing is under your floors and what the minimum hot water temperature can be used while still heating the home. Say it is 15F outside, you will need more gallons of water per minute circulating or warmer water to have enough Btu's of heat left in the floor to keep the house at say 65F. If your boiler will not properly warm the house while set at 105F, then solar heating might not work well there. However if there is adequate tubing in the floor, then 105F water will heat the home nicely. Even if the boiler is running 120F now, don't give up. Spring and fall you probably can heat the home with 105F water, and 15F days probably would require the boiler anyway.

It would take detailed heat load calculations to determine for sure, but a system with 4 each 4X8 panels and in series with a pair of 30 tube evacuated tube solar panels might warm a 500 - 800 gallon tank to 130F in one day. That size tank should store enough heat to keep the house boiler from running 1-2 cloudy days in a row at 35F outside temps, more than a week when the outside temps are in the upper 40's overnight and low 60's in the day. At least it would save a LOT of propane. And you could meet most of your domestic hot water needs all year long. Yet all of this depends on your boiler size and how well the home is insulated and air sealed. You can easily lose 100,000 Btu's overnight on a windy 45F night if the drapes are moving around with each breeze. If the home is built to modern standards, say built after 1990, it should not have a huge BTU loss each night, and a smaller system will heat it overnight with ease.

Wayne Gale's picture

Curious about heating my 32X48 pole barn with regular concrete floor. A hydronic system was not installed and currently no insulation. I'm thinking spray foam in the future but heating this beast even with foam installed could be costly. My idea is 2-3 4'X8' SHW panels and a 500 gal insulated concrete tank buried in the ground. I've heard that concrete tanks tend to disintegrate in this application so maybe a fiberglas or poly tank would work. Anyway, plumb the tank to ceiling mounted radiators (thinking automotive) with fans and distribute the heat. Maybe even run the fans and pumps off a PV system. Location is in SE Idaho where we get a decent amount of sun. I'd prefer to use hydronics but that would mean tearing up the existing concrete.

My house is hydronic and uses a natural gas fed boiler. Duplicating the same basic SHW system is definitely an option. Both the home and shop have excellent southern exposure. So my concern is having a big enough storage tank for each application and transferring the SHW to a shop radiator system. Just don't know if the cost to install would be worth it. NG is pretty cheap right now but that could change at any time.

Ben Root's picture

Hi Wayne,

I'm just wondering if, as a work space only, you may not need round-the-clock heat. Check out these article on solar hot air collection, for heating the space directly and way more simply than hydronically. You can dump solar heat into the space during the day, then just not worry so much if it cools off at night. See: and it's DIY companion article in HP145, pg 122.
Also, (probably the best one) check out the article in HP109 pg 30, which I can't seem to find on the HP site. Or see it here on the author's web site (scroll down a little for the HP article) There are many other cool active and passive hot-air space heating articles on this site too.

There's a lot of good info in back articles of HP.
I hope this helps.

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