$2K Solar Space + Water: Plumbing the System

This page covers all the plumbing associated with the combined solar space and water heating system.

There are three areas that need to be plumbed for the system:

  1. The Collector Loop:  This loop circulates water from the tank to the collector and back to the tank when the sun is out.
  2. The Domestic Water Preheat:  This is the plumbing that re-routes the cold water intake to your existing hot water heater to pass through the heat exchanger in the solar tank for preheating.
  3. Radiant Floor Loop: This loop circulates water from the solar heat storage tank through loops of PEX pipe that are attached to the floor of the room(s) to be heated.

Back to Table of Contents...


On this page:

Plumbing the collector loop...

Plumbing the domestic hot water preheat...

Plumbing the radiant floor loop...

Plumbing the backup heater...

Plumbing the Collector Loop

The collector loop circulates water from the tank, through the collector, and back to the tank when the sun is shining.

This is a drain back system, so the plumbing is arranged so that when the pump turns off, all of the water in the collector and plumbing to and from the collector drains back into the tank.  This is what provides the freeze protection for the system.

For proper drain back,

  1. All of the plumbing in the collector loop needs to slope down toward the tank. 
  2. The collector absorber needs to be rotated a little bit such that the manifolds slope downward toward the lower corner where the supply pipe comes in.
  3. The supply and return plumbing should the 3/4 inch diameter or larger.
  4. The return pipe must end above the water line in the tank -- that is, there must be a air gap between the end of the return pipe and the tank water to allow air to go up the return pipe during the drain back.
  5. The bottom of the collector must be far enough above the water line in the tank to allow the supply and return pipes to drain when the pump stops.
  6. Any components mounted in the supply or return line (eg the pump) must allow fluid to drain through them.



Click on diagram for full size

Tank to collector plumbing -- Tank End

Fluid is drawn from the bottom of the storage tank, up over the tank wall via the U-Tube, and then down to the pump mounted as low as possible.  The pump outlet pipe makes its way to the lower right corner of the collector (see section below).

The pump mounts with flanges so that it can be easily removed.  The valve to the left of the keeps water from being syphoned out of the tank when the pump is removed.  It would be good to have a 2nd inline valve to the right of the pump like the one to the left as is shown on the plumbing diagram.

The valve  mounted off the T just to the right of the pump has a garden hose thread, and allows the pump and U-Tube to be primed and purged of air.  To do this, hook up the garden hose to this valve, open the valve, turn on the garden hose and let the water run long enough to completely purge the lines of air.  Once purged, the pump should be very quiet -- if the pump is noisy, repeat the purge.



This picture shows the U-Tube that extends from the bottom of the tank, up over the side of the tank, and down to the pump intake.  The U-Tube allows the pump to draw water from the bottom of the tank and to retain its prime when its shut off without having to put holes in the tank lining. 

The pump is a Grundfos 15-58 3 speed, cast iron body pump.  The pump comes with a check valve installed -- this CHECK VALVE MUST BE REMOVED TO ALLOW DRAIN BACK.

For the longest service life, this pump should have a bronze or stainless steel case, but I have had good life with the much less expensive cast iron case pumps.

With this system, new water is not being introduced on a regular basis as with some open systems, and this may be one reason that iron pump does OK.










I installed a flow meter in the supply line to be able to monitor flow rate.  The flow meter made a LOT of noise.  When I checked with the manufacturer, I'm told this can happen if the flow rate is just right to match the natural frequency of the flow meter.  I've taken it out for now, but will try another with a different flow range.

If the flow meter is mounted at just the right level, it can also serve as a sight gage for the tank water level -- very handy.

I've temporarily replaced the flow meter with a short section of galvanized pipe.


Making the U-Tube in the shop.

Place for flow meter

Tank to collector plumbing -- Collector End

The supply line is routed from the pump outlet (above) to one of the lower corners of the collector.  In our case it goes to the lower right corner of the collector as shown in the picture.  All of this line must slope down toward the storage tank.  In addition, the collector absorber grid must slope down to this corner for proper drain back.









This is a closeup of the supply pipe connection to the supply manifold of the collector absorber grid.  I included a union just in case I wanted to remove the absorber plate at some future time.

I was lucky in that the solar storage tank is just behind the concrete wall that the picture shows the pipe going through.  But, the supply plumbing can be long as long as all of it slopes down toward the tank, and the pressure drop of the longer pipe is accounted for in sizing the pump.

The hole in the concrete wall was done with a rented drill -- not as much work as I expected.




The exposed pipe was encased with "Great Stuff" polyurethane foam in a can.  The hole through the concrete was also filled with Great Stuff.


Collector to Tank  -- Collector End

The plumbing line that carries the water back to the tank after it has circulated through the collector must leave the collector at the opposite corner of the collector absorber plate.  Since our supply plumbing comes into the the lower right corner of the collector, the return must leave from the upper left corner.  This opposite corner connection scheme is important to insure that all of the absorber risers get the same flow.

This shows the return pipe leaving the upper left corner of the absorber plate.  A union is used to allow the absorber to be removed.

The absorber fin was cut away a little to allow clearance for the return line and union.






This picture shows the return line before the absorber plate is installed.  The return line runs behind the absorber plate to a point where it can go through the crawl space rim joist and into the crawl space.  This avoids running the return line outside the collector where it would be visible.

All of this return plumbing must slope down toward the storage tank.









Collector to Tank  -- Tank End

This picture shows how the return line comes into the tank.  It drops through a small fixed portion of the lid of the tank.  The return line terminates just below the EPDM liner.  There must be an air gap between the end of the return line and the water to allow air to enter the return line when the tank is draining back -- this is very important, the collector will not drain without this air gap.

The return line could just as well have entered the tank through the tank edge board, which would have allowed the full lid to be removable.  The reason for not doing it this way is more historic that logical.

The picture also shows the supply line where it goes out to the collector through a hole drilled in the concrete wall.






Plumbing the Domestic Hot Water Preheat

To hook the solar water up to your existing house hot water system, you need to arrange things so that the cold water line going into your hot water heater first flows through the PEX coil heat exchanger to pick up solar heat. 

The diagram below shows the arrangement. 

A section is cut out of the the cold water line into the hot water heater.  The cut out section is replaced by two T's and and a ball valve.

The open legs of the two T's are then connected to the ends of the PEX coil via two new lines each with a ball valve installed in it.

The three valves allow you to control whether you are using solar heated water or not. 

For solar hot water, close top valve in the diagram below, and open the two lower valves.  In this way cold water from the street flows through the PEX coil and is preheated by the solar heated water in the tank.

To bypass the solar heater, open the top valve, and close the two lower valves.



Tempering Valve

I don't show a tempering valve on the system, but it is a good idea to include one to prevent scalding.  The water in the solar storage tank could  get up to 170F or even a bit more, so there is the potential for scalding if you don't manually mix cold water in quickly enough.   This becomes more of an issue if there are kids in the house.  Some localities will require a tempering valve. 


The tempering valve should be installed at the outlet of the backup hot water heater.  That is, in the hot water outlet from your regular hot water tank.  The valves are readily available, not too expensive, and easy to install.



Connecting Into the House Cold Water Pipe

The pictures show cold house cold water line going into the heat storage tank to be preheated.


The 2nd picture shows the now preheated water leaving the heat storage tank on its way to the house backup hot water tank.

The two valves shown in these pictures should be open for solar water heating, and the third valve (not pictured) in the cold water line should be closed for solar hot water.




See also the last picture in the connecting the heat
exchanger section (just above)


Plumbing the Radiant Floor Heating Loop

This section covers the plumbing that connects the radiant floor loop to the solar storage tank, and the installation of the pump that circulates water from the tank, through the floor loop, and back to the tank.  Installing the staple up floor loop is covered here...

This is very simple in that no heat exchanger is used.  The tank water is simply picked up from the top of the tank and circulated through the floor loop, and returned to the bottom of the tank.

in our system there is only one radiant floor loop heating the master bedroom and bathroom floors.  The rest of the main floor is heated by the Solar Shed system.  If you are heating a larger area, you will need multiple floor loops, and some type of manifold to distribute the flow to each loop.  For an example of this, see our Solar Shed project...


Installing the Circulation Pump

The pump is mounted just outside the tank, and as low as possible.  This type of pump requires some water head at its inlet, and the low mounting maximizes this head.

The pipes are 3/4 inch copper.

The two valves in the inlet line allow the pump and floor loop to the primed and flushed free of air.  A garden hose is hooked up to the lower valve.  The upper valve is closed, and the lower valve is turned on so that the garden hose fills the pump and and entire floor loop.  Then the blue valve is opened to fill the inlet pipe.  Then the lower valve is closed to shut off the supply of water from the garden hose.

If you get all the air out, the pump should be very quiet.  If the pump is making noises, its probably because of trapped air -- try repeating the flush process.





The inlet for the pump goes over the side of the tank through a slot cut in the tank edge boards.  It then turns downward and ends a few inches under the water line.  The inlet should be placed in the upper part of the tank to get the warmest water.








The pump is a Grundfos 15-58 3 speed, cast iron body pump.  The pump includes a check valve.  If you get a pump without a check valve, I would install a separate check valve to both keep the loop full and prevent thermosyphoning.

For the longest service life, this pump should have a bronze or stainless steel case, but I have had good life with the much less expensive cast iron case pumps.  In this system, new water is not introduced on a regular basis, as it is on some open systems, and this may help with the longer life for iron pumps.

Note that in our system, the same pump model is used for both the collector loop and the radiant floor loop. 


The pump inlet goes over the side and down a few  inches.

Pump mounted with flanges.

Installing the return from the floor loop

The return line from the floor loop is brought back into the solar storage tank via a slot in the tank edged board. 

The picture shows the slot being cut with a handsaw (be careful not to damage the EPDM).   The copper pipe in the picture is the return from the floor loop, which will be secured in the slot in the edge board.

The return line goes down toward the bottom of the tank to return the water that has been cooled by going through the floor loop to the colder part of the tank.


Plumbing the Backup Heater

A backup heater was added to this system in 2013 so that the solar system could take over the full task of heating the house. For the full explantion on the backup heater and how its designed, installed and plumbed go here...


Gary Feburary 9, 2011, December 21, 2013