Solar Shed -- Controls and Plumbing

The controls for the Solar Shed space heating system are very simple.

Basically, a standard off the shelf Goldline GL30 is used to control the pump that circulates fluid through the collectors.

When the pump is off, the collectors drain back to the tank for freeze protection.

The pump that circulates hot tank water through the floor loops is controlled by two thermostats. The first senses whether the house needs heat, and the 2nd senses whether the tank has heat. If both of these things are true, the pump is turned on to circulate hot water through the floor.

Basically the plumbing in the collector loop is all one inch copper and is all sloped toward the tank for drain back.

The underground pipe to the house is CPVC insulated with extruded polystyrene rigid foam board.

More detail and parts below.

Go back to the main Solar Shed page ...

This controls diagram is from the Mother Earth News article on the system.

The full diagram and labeling are available here ...

The full Mother Earth News article on the system is available here ...

This is a more detailed wiring diagram for the pump and controls...

Referring to the controls diagram above,

For the collector plumbing circuit:

The Goldline GL30 differential controller reads the temperature of the tank from sensor 16, and reads the temperature of the collector using sensor 4.

If the collector temperature is greater than the tank temperature by an amount that you set on the differential controller, the controller turns on the pump (3).

The pump circulates water through the collectors (1) as long as the collector temperature remains at least 4F greater than the tank temperature.

If the collector temperature drops to less than 4F above the tank temperature, the controller (15) shuts off the pump (3). The water drains from the collectors back to the tank -- this keeps water from freezing in the collector and damaging it.

For the circulation through the floor loops,

For heating, hot water flows from the tank (5), through the underground pipe (7), through the thermal mixing valve (8), through the circulation pump (9), and into the supply manifold 10.

From the supply manifold, water flows out to each of the 6 floor loops of PEX pipe, and then back to the return manifold (11).

From the return manifold, water flows back through the underground pipe (6) to the tank.

The function of the thermal mixing valve (8) is to mix some of the cooler returning water with the incoming hot water if the temperature of the incoming water is too hot to circulate through the floor.

The valve is adjustable over a wide range of temperatures, and prevents the floor from being damaged by circulating excessively hot water.

Water only circulates through the floor if 1) the house needs heat, and 2) the tank has water hot enough to provide heat.

The two thermostats (13 & 14) turn the pump on only under these conditions.

Thermostat (13) senses the house temperature, and only turns on when the house temperature falls below the value you set.

Thermostat (14) senses the tank water temperature, and only turns on when the tank is above the temperature you set.

These two thermostats are Johnson Controls A419s. They will directly control the 120VAC circuit that powers the pump -- so no low voltage control wiring is required.

Both of the two thermostats are located together next to the supply and return manifolds. Thermostat (14) senses the tank temperature via a 150 ft long set of wires to the temperature sensor located in the tank.

See pictures below.

Plumbing components

15    Goldline GL30 Differential controller

2      One inch copper pipe between the collector and tank -- sloped toward tank

3        Taco 008 120 VAC bronze circulator pump

16      Temperature sensor -- senses tank temperature for GL30 -- a 10K thermistor

4        Temperature sensor -- senses collector temperature for GL30 -- a 10K thermistor

5        Tank is an insulated plywood shell lined with EPDM rubber sheet.

6, 7    Insulated CPVC pipe that runs from the tank to the house.

8        Taco thermal mixing valve

9         Grundfos UPS 15 120VAC circulator pump (probably to be replaced with Taco 008 bronze pump)

10, 11 The supply and return manifolds. These have connections for up to 6 floor loop circuits in my case

12        Floor loops of PEX pipe heat the floors when water is circulated through them

19       Air bleed valve

13, 14 Johnson Controls A419 thermostats to control the pump (9)

18      Temperature sensor that reads the tank temperature for thermostat 14

17      Temperature sensor that reads room temperature for thermostat 13

The manifolds also have valves and connections for filling the system, and have shutoff valves to shut off flow between the tank and the manifolds.

The manifolds also have valves and flow meters that allow the flow in each floor loop to be controlled and monitored.

I plan to replace the iron Grundfos pump with a bronze of stainless steel pump -- I used it because it was on hand. Systems like this that are vented to the atmosphere should not use iron parts because the air in the system can rust the iron.

The tank collectors and floor loops all share the same plain water -- no antifreeze.

There are no heat exchangers in the system.

A heat exchanger may be required in the floor loop system depending on the elevation of the floor compared to the elevation of the tank.

Pictures of the Plumbing

Floor Loops:

The supply and return manifolds with the orange PEX pipe running to the 6 floor loops are on the left.

The two Johnson control A419 thermostats that control the system are to the upper right.

The Grundfos pump and Taco thermal mixing valve (just above the pump) are in the center.

The manifolds were bought as a package, and include a number of helpful features: fill valves, air bleed valve, supply and return shut offs, supply and return temperature gages, and individual flow valves and flow gages for each floor loop -- this makes the system much easier to fill and troubleshoot.

The fused switch to the right turns power off to the whole system. The switch right next to it turns the pump alone off (for troubleshooting mostly).

Picture showing floor loops being installed.

Plywood spacers are used to make a groove for the PEX pipe. Homemade aluminum sheet heat spreader plates are used to improve the heat transfer to the room

This shows the Taco 008 bronze circulator pump that circulates water to the collectors mounted to the side of the tank.

The connection through the tank wall is just below the pump body.

This connection is made through a bulkhead fitting.

This is the only plumbing line that penetrates the tank wall -- the rest of the connections go over the top edge.

The tank sits low enough that the inside of the pump is always flooded -- the pump is not self priming.

The insulation arrangement is set up to allow the pump to pick up enough heat from the tank to prevent freezing, but not so much as to overheat the pump.

The pump is rated for continuous operation at 200+F, so the insulation arrangement is probably not too critical.

This shows the new arrangement of all of the plumbing connections to the tank except the pump (see above).

All of the connections come in over the top of the tank through grooves in the boards that surround the top edge of the tank.

This allows a lid with no connections through it, so it can be removed easily.

A little messy, but functional.

The connections from left to right are:

- Return water from collectors -- must terminate above the tank waterline

- Tank vent line -- just runs over a bit and up a bit -- allows air to enter or leave the air space above the water.

- The tank temperature sensor lives inside the angled copper pipe -- its a 10K thermistor.

- CPVC line to the house. The T on the end of this line is set up about 3 inches under the tank water surface -- it conveys hot water tot he house.

- CPVC line coming in from the right corner is the water return line from the house -- it continues over the the lower far end of the tank

The white gunk is silicone caulk to keep water vapor from escaping the tank.

There is a stainless screen filter cartridge in the CPVC supply line to the house just beyond where it exits.

At the end of the first season there was nothing in the filter.

The lid for the tank is lined with EPDM on the bottom, then a couple layers of rigid foam insulation, topped off by a layer of OSB.

The lid is clamped down to the tank with long deck screws at about 1 ft intervals. This connection has proved to be vapor tight.

Gary Dec 1, 2007