Solar Panel for Radiant Floor Heat

Solar Panel for Radiant Floor Heat


When my brother and I originally planned the shop, we were aiming for floor heat as we had experienced that at some other guy’s shop, and really liked it.  Back in 2001/2002, we went to a lot of work putting in the foam, the rebar, the heat tubing, and even the manifolds.  Then we hit a stage where we needed a boiler for the system.  We originally tried a hot water heater, but unfortunately we tried it in the dead of winter, and the entire system was super cold.  So all it did was make the electric meter spin, and no heat to show for it.  We realized we probably needed a boiler for it, and then spent time researching boilers.  The problem we ran into is that boilers are expensive!  Not only that, but once you have the boiler, you still have to pay the electric company or buy propane… neither one are exactly cheap either.  So we procrastinated on buying a boiler, and so now over 10 years later, still no boiler.  However, at this point, I’ve run more numbers and realized I don’t want to pay for heat in the shop from now until forever.  There’s got to be a way to harvest the sun’s energy and heat the shop for (almost) free.  During that time, I did some research, and stumbled upon an air-to-air style solar panels on the builditsolar webpage.  Based on that research, I built my own large air-to-air solar panel.  That’s been operational since 2010, and has been able to keep the shop at or above freezing temperature most of the winter.  Not bad considering winter temps can easily be well below 0 F, and often times as low as -20 or -30 F.

A year later I moved to China (Feb 2011) and here there a millions of roof tops that have these glass tube style solar panels.  They look like a water heater tank tipped on its side with these glass tubes coming out of the side to form a panel area.

hot water solar panel

People in China use this as their sole source of hot water for showering, dishes, etc.  The upfront cost is actually fairly reasonable, and the long-term cost is virtually zero!!  Hmmm, seems like that could be tied to a radiant floor heat system and harness the power of the sun!

After many van rides around in the city, I noticed that some of the roof top ones had a manifold instead of the tank, and a whole array of panels, which I assume tie to some sort of heating system.

Once I saw that, I did some more research, and found that those systems are available for purchase in the USA.  Unfortunately, they are somewhat expensive, ranging $900 to $1500 per panel.  While on some training in the US, I stumbled upon a company almost next door to the training location that was specialized in these.  I went and visited them, and it turns out they source all of their panels from China, and then specialize in selling and installing the systems across the US.  These panels are called Evacuated Glass Vacuum Tube Solar Panels.  I did get a quote from this company, and it wasn’t cheap!

Before just jumping into buying panels, I clearly needed to learn more about them.

Here’s what I have discovered:

A single large panel can easily provide the daily hot water needed by a family of 2-4.  The tubes are generally 58mm in diameter (2.28″) and usually 1.5m long (59″) up to 1.8m long (70.9″).  They range from manifolds that have 10 up to manifolds that have 30 tubes.  The manifolds can be strung together in series.  Because the inside of the tube is a vacuum, it makes it a super insulator.  That allows the heat that’s captured to then be transferred up an internal heat tube and transferred into the water in the manifold.  A more detailed explanation can be found here.

However, its not just the panel that you need, there’s also the insulated piping to get the water to the collector and then back down to the floor.  You also need a pump, and then a controller with temperature sensors, as you don’t want it running if its not producing heat.  From there, you should also have a heat exchanger to make the radiant floor heat system independent from the solar panel system.  You also need an expansion tank on each system to compensate for the pressure fluctuations in the system.  To make it work right, 2 panels is the minimum I need, and 4 is really recommended.  They also recommend a large storage tank to store excess energy for non-sunny days. Whew, that’s a lot of stuff!

On the Radiant floor heat side, I already have the tubing in the concrete floor, manifolds for the multiple zones, pumps, expansion tank, and water heater tank as a storage tank.  The only thing missing is the heat source!

Since a heat exchanger is recommended, I’m going to try to spiral PEX tubing inside of the water heater tank, remove the heating elements, and with some unique fittings, bring the PEX tubing out those ports.  It can then connect to the solar panel system with pump, expansion tank, and controller.  More on the heat exchanger later….

Since the whole panel system seems a bit expensive to just jump deep into, I decided it might be nice to try to find a small panel, and do a bit of testing with it before spending a lot of money.  A little searching on Tao-Bao (a China version of eBay) and I found some….

small solar water heater

Hmm, 598 rmb is only $96!!!!  That seems like a cheap way to do some testing…

Since Tao Bao is only in Chinese, I found a company in Shanghai that can be the go-between for foreigners to buy stuff on Tao Bao, called Tao How!  I contacted them, and they said that vendor was out of stock on that one.  Same story with all of the similar small ones.  I finally found a slightly larger one, and it was in stock…..

75L solar heat tube

838 rmb for the panel, plus 200 rmb for shipping, and a 30 rmb agent fee, 1068 rmb total is still only $172!!!

So today (5-28-2014) I ordered one!  I also ordered some battery powered digital thermometers to do some testing with.

digital thermometer Hard to beat the price at ~$3 each.

So once those show up, then I can start to hook it up and do some testing here in China.  If it works the way I think it will, my plan is to bring it to the US in suitcase(s), and then install it at the shop and tie it into a just a baseboard radiant heater for now.  That will give me the time to work on the heat exchanger in the water heater tank, and then monitor the heat output over the winter.  Then maybe it will be time to jump in and go with the full-on system.


The Solar water heater arrived!!!! And….. it was a wee bit bigger than I thought it would be… Okay, a lot bigger!!!


Total of 3 boxes:

  • Tank Box: 52″ long x 17″ wide x 19.5″ tall, 25 lbs
  • Glass tubes box: 62.5″ long x 13″ wide x 8″ tall, 52 lbs!!!!!
  • Frame box: 59″ long x 9.5″ wide x 4″ tall, 18 lbs

Total weight: 95 lbs!!!

I had initially hoped that I could bring this back as airline luggage from China to the US, but these exceeded the limits for regular luggage, and with the excess size fees, it would be more than the items are worth…

I then contacted the account rep at Tao-How to see if I could return the solar water heater… turns out that return shipping was going to be ~$80… almost half of the value of it.  For that much, I’m keeping it for research.

So, here’s detailed pictures of the tank, and the tubes:

IMG_6916 IMG_6918 IMG_6920 IMG_6926 IMG_6922 IMG_6923 IMG_6924

After getting this system, and looking at all of the parts, I realized that the water heater setup’s that are used in China are different than the other evacuated vacuum tube systems that I’ve looked at online.  The tubes are of doublewall construction, with the inside layer coated black.  Water from the tank flows all the way down into the tubes, which heat the water, and by natural convection form a flow of cold water down and hot water up.

Here’s a good schematic from

u-tube solar hot water

These are also referred to as U-tubes or open tubes.

This was something I had not encountered before.  Another company I’ve been researching sells a panel that the call the U-tube version, but no explanation behind it.  This type of tube is less efficient, but much cheaper.  The reason?  well, read on and find out!!

Since this system was going to be too big to bring back as luggage, I was back to researching.  This time I was on eBay, as I’ve looked at multiple systems there before.  I easily found a small one, one that I had looked at before, but had skipped it before because shipping was more than the selling price!!  It was a small one, 10 tubes that were only 19″ long, and the price was $193, but shipping was $200!!!  Reason? It was coming from China!  But now I had a good thought, I could just have it shipped to my place in China, and bring it back to the US myself!  Ha, so I contacted the ebay seller, and after some good discussion back and forth, the shipping would only be $32!  I also found a solar controller for $69, and a little pump for $11 that I’m going to try running directly with a PV solar panel.

Then I had an issue with eBay, as the seller did not have China as a viable shipping location.  After a call into eBay help and a few more emails with the seller, I was able to complete the purchase on Friday, 6-6-2014.  Lo and behold, Saturday morning (6-7-2014) I had a knock on the door, and there it was!  1 day shipping!  That’s one thing that China does well!

Here’s the items I ordered:


And here’s what it looked like when it arrived:

IMG_6939 IMG_6943 IMG_6941 IMG_6944 IMG_6945 IMG_6947


  • Long box: manifold, copper tubes and brackets: 37″ x 7.5″ x 6.5″ and 19lbs
  • Main box: glass solar tubes: 24″ x 16″ x 8″ and 18lbs
  • Package: controller and pump: 16″ x 10″ x 3″ and 4lbs

Now that’s some boxes that will fit in suitcases and stay below the limits of 62 linear inches and 50lbs.

After getting the pieces out, it was now more apparent the differences between this solar panel and the solar water heater.  These glass tubes have some extra pieces that I’ve read about before, but not fully understood.  There are two main pieces to the system: the double wall glass tube, just like the other system, but then this sealed copper tube system slides into a metal fin that is inside of the glass tube.  How it works is that there’s a fluid inside of the copper tube that boils very easily when its heated by the fin that’s been heated by the black film inside the glass tube.  The vaporized fluid flows quickly up to the bulb top of the copper unit, where it is cooled and transfers it’s heat to the water running through the manifold.  Condensing, it then flows back down where it’s heated again.  This happens very quickly and efficiently, thereby keeping the temperature inside the glass tube low so that there’s minimal losses, and transferring the heat into the manifold where its transferred into the rest of the system, either heating water or going to a hydronic radiator.

Here’s a good explanation from


The copper tube, aluminum wing and the manifold add cost to the system, so that’s why these systems are more expensive than the U-tube water tank heaters.  That right there was exactly the reason for going through this exercise.  The next step is to take the small system back to the US to the shop, buy a hydronic radiator at a big box store (an 8′ long one sells for $66) and hook up a simple solar heating system for testing.  After that, it will be time to scale up and get the BIG solar panels for the floor heat.

Additionally, the engineer in me wanted to see how well the simple U-tube system worked.  Rather than setting up the whole tank and tubes and brackets, I set up a test that was much simpler:  as single tube filled with water, a bracket using a sawhorse and a cooking thermometer:

IMG_6962 IMG_6963


At 9:16am, the water started out at 24° C (75.2° F) and ambient temperature was barely more than that.  It was sort of sunny, but more smog than I would have liked, which made it more like a cloudy day… typical for Shanghai, China.

I monitored it most of the morning and watched temperature slowly rise.  We had lunch plans, so my data logging took a break until we got back around 3:30pm, and the temperature was up to 52°C.  After another hour it hadn’t gone up any, and the sun was basically done for the day.  Sidenote: China is all one time zone, so during summer the sun comes up around 4:30am, and is going down by 7pm.  So by 4:30, there isn’t much sun left, and it’s further hampered by the smog.  The clear blue sunny skies of South Dakota rarely happen in China.

So then I dumped the water from the tube into a measuring container and mixed it make sure there was some lower temperature layers, and ended up with 1300 mL or 48 oz that was at 50°C.  So that’s a 26°C rise in temperature, seems like I could calculate the energy content.

After a bit of internet searching to find the right equation (engineering school gives you the knowledge to find and calculate what you need, but its impossible to remember every equation and method, hence the internet search)

So here’s the equation, thanks to

Q = m cp dt


Q = energy transferred (kJ or Btu)
m = mass of substance (Kg or Lbs)
cp = specific heat of the substance (kJ/Kg°C, kJ/Kg°K, Btu/Lb°F)
dt = temperature difference in substance (°C, K, °F)

Energy calculation from engineeringtoolbox

So here’s the actual calculation:

1300 mL of water = 1.3 liters = 1.3Kg of water

Q = (1.3Kg)(4.19 kJ/Kg°C)(26°C)

Q = 141.6 kJ or 134 Btu

Not too bad for a smoggy cloudy day using the less efficient solar tube.




Stay tuned for upcoming progress as I get the solar panel to the US and hook up the radiator system, try to make a heat exchanger, and do some more testing in the shop.

More updates to come, so stay tuned!