Solar Hot Water Systems

Dan Lepinski, P.E.

On 4/30/22 8:26 PM, Jim Duncan wrote:

but the fluid eventually broke down like they tend to do over time
when they overheat.

Since solar hot water systems aren't a commonly discussed topic, particularly among the NTREG Conversation Group members .. some explanation will help those who aren't familiar with solar hot water system construction or operation.

Solar hot water systems can be constructed in several different configurations. In all cases, the aspect of the hot water system that heats the water using sunlight is called a "solar collector". Solar collectors are analogous to solar electric panels. However, instead of generating electricity, solar collectors heat a fluid (typically water or water combined with anti-freeze). The fluid is referred to as a "heat transfer fluid", or "HTF".

One solar hot water system configuration that's very suitable for north Texas is called a "drainback" system. When the system isn't operating, such as at night, the pumps are off, and the fluid "drains back" into a small storage tank. In such cases, the solar collectors are empty of any fluid. This makes a drainback system ideal for our area because it's freeze-proof. When the sun is high enough in the sky to heat the collectors, pumps turn on and circulate the fluid through the collectors. A "heat exchanger" then transfers the heat in the fluid into the water we use for washing and other purposes. Because drainback systems are freeze-proof, no anti-freeze is needed. More on this in a moment...

In other configurations, the fluid is always present in the solar collectors, and is circulated through the solar collectors only when it's hot enough to be of use. During freezing weather, the anti-freeze in the fluid prevents the water from turning to ice and destroying the collector plumbing.

In all cases, the heated water is stored in a tank such as an ordinary water heater or other tank designed specifically for solar hot water systems. When the water in the tank reaches a predetermined maximum temperature, the pumps are turned off to prevent overheating. It's all electronic .. and automatic.

In solar hot water systems, anti-freeze is often mixed with tap water in the heat transfer fluid to prevent degradation of the plumbing due to corrosion when different metals are present in the system, such as copper and galvanized pipe. Anti-freeze also degrades with time and temperature, and must be replaced with fresh anti-freeze every few years. If this isn't done, the anti-freeze breaks down and is no longer effective. (Same reason we get the cooling systems serviced in our vehicles.) Systems don't need to

The need for anti-freeze in drainback systems can be completely eliminated by using distilled water as the heat transfer fluid. Distilled water doesn't conduct electricity the way tap water does. Distilled water also doesn't have any minerals in it. Subsequently, for both of these reasons, corrosion doesn't occur. Our home solar hot water system uses distilled water for the heat transfer fluid. It's been there for more than 15 years, and is still as clear as the day I filled the system. No leaks. No corrosion. No anti-freeze to "wear out".

Solar hot water systems don't need to "overheat" to degrade the anti-freeze (when present). The simple actions of heat and time break it down. In a properly operating system, this takes several years. Use exclusively distilled water, and this problem doesn't happen.


On 4/30/22 8:26 PM, Jim Duncan wrote:
Philip, You may recall that 4-tracker install near Glen Rose back in 2006 you worked on with me. The owner put 2 solar DHW panels on the greenhouse adjacent to the inverter room for a radiant heat floor of the green house side. There was plenty of heat but being south-facing the system was hardly run at all. It was a drainback system but the fluid eventually broke down like they tend to do over time when they overheat. Otherwise it  did keep the room warm and steamy.....Jim

Jim Duncan
Solar Acres Farm

On 4/30/2022 10:29 AM, Philip Timmons via wrote:
Has the general consensus become that Thermal (Solar) Collectors are not worth the time / effort? 

Asking for both Water and Concrete Floor Heat.


On Saturday, April 30, 2022, 09:59:15 AM CDT, Mark Witte <witte.m@...> wrote:


To help progress the discussion about heat pump water heaters, I'm forwarding an email from the Advanced Water Heating Initiative.  Their email includes links to great information, including the numerous models that are available, or soon will be, rebates that may be available, etc. 


----- Forwarded Message -----
*From:* Advanced Water Heating Initiative <info@...>
*To:* "witte.m@..." <witte.m@...>
*Sent:* Thursday, April 21, 2022, 05:44:34 PM CDT
*Subject:* Thank You For Your Interest


Hello Mark,

Thank you for getting in touch and for your interest in Heat Pump Water Heaters. Together we can move to more efficient water heating, save energy and the environment.


Below, you'll find information on heat pump water heaters.



As AWHI learns of new products coming to market, we will keep you updated via occasional emails. More details about existing HPWH products are available online:   

* Hot Water Solutions – Buyer's Guide <>
* NEEA <>
* EnergyStar <>  
* Redwood Energy <> - specifically, the section on domestic hot water on p. 43  

Thanks again for getting in touch and good luck on your heat pump water heater projects!



AWHI is a member-funded collaborative of building owners, utilities, federal agencies, state and local governments, manufacturers, engineers, installers, advocates, researchers, and building industry professionals from across the U.S.   Our mission is to make HPWHs universal in every home and business.  

Facebook <>

Twitter <>

Website <>

Advanced Water Heating Initiative, 151 SW 1st Ave, Suite 300, Portland, OR

Unsubscribe <> Manage preferences <>

Join to automatically receive all group messages.