By Audrey HayesUpdated November 2026
Choosing parts for a solar thermal boiler system comes down to matching components to your loop: circulation pumps, expansion tanks, heat exchangers, controllers and collector kits all have to work together at the temperatures and pressures your system actually sees. This guide compares seven widely sold options. Our selection weighed manufacturer specifications, brand reputation for hydronic and solar work, and patterns in owner feedback, with a focus on how well each part fits a closed-loop or drain-back setup. We did not install or test these products ourselves; the details below come from listing specifications and published manufacturer information.
Our top picks at a glance
- 1
TOP CHOICEBest overallTOPWAY 3/4" NPT 110V Hot Water 3-Speed Circulation Recirculating PumpView on Amazon - 2
TOP CHOICEBest for higher flow loopsTOPWAY 115V 17 GPM Flanged 3-Speed Cast Iron Hot Water Circulation Recirculating PumpView on Amazon - 3
TOP CHOICEBest for boiler-side pressure controlHydronic Heating Expansion Tank 102-1 Fits Amtrol Extrol Residential Boiler Systems. OEMView on Amazon - 4
Best for small potable loops1 Gallon Potable Thermal Expansion Tank, 3/4" MNPT Connection, CompactView on Amazon - 5
Best for pool-style collector arraysIndustrial Grade Solar Pool Heater DIY Kit – Superior High-Wind StraplView on Amazon - 6
Best for separating loopsUL Certified Sealed Plate Heat Exchanger, 4 x 12 Inch, 60 Plates, 316L SS, Copper BrazedView on Amazon - 7
Best for pressurized solar controlsMISOL 110V Controller of Solar Water Heater, Used for Separated pressuView on Amazon
Our Top Picks
Quick Comparison
| Product | Best for | Key feature |
|---|---|---|
| TOPWAY 3/4″ NPT Recirculating Pump | General hot water loops | Three speed settings |
| TOPWAY 115V 17 GPM Cast Iron Pump | Higher flow circulation | Flanged cast iron body |
| Hydronic Expansion Tank 102-1 | Boiler-side expansion | Amtrol Extrol-compatible fit |
| 1 Gallon Potable Expansion Tank | Compact potable loops | 3/4″ MNPT connection |
| Solar Pool Heater DIY Kit | Collector arrays | Strapless high-wind mounting |
| 316L Plate Heat Exchanger | Loop separation | UL certified, copper brazed |
| MISOL Solar Water Heater Controller | Pressurized solar systems | 110V controller |
TOPWAY 3/4″ NPT 110V Hot Water 3-Speed Circulation Recirculating Pump
TOPWAY 3/4" NPT 110V Hot Water 3-Speed Circulation Recirculating Pump Circulator Pump for Water Heater System
Best for: General hot water loop circulation at moderate flow.
This suits homeowners building a basic closed loop between a collector array and a storage tank who want a pump sized for residential-scale runs.
Key specs
- 110V operation
- 3/4″ NPT connection
- Three selectable speeds
Pros
- Speed selection lets you tune flow to pipe length
- NPT threads fit common residential fittings
- Widely used format for hot water recirculation
Cons
- Thread connections need PTFE tape or sealant and careful torque
- Speed and head ratings should be checked against your specific loop
Bottom line: A sensible first pump for a small residential loop where you want adjustable flow.
TOPWAY 115V 17 GPM Flanged 3-Speed Cast Iron Hot Water Circulation Recirculating Pump
TOPWAY 115V 17 GPM Flanged 3-Speed Cast Iron Hot Water Circulation Recirculating Pump Circulator Pump for Water Heater System
Best for: Higher flow circulation in larger loops.
Suited to systems with longer runs or multiple collectors where a small pump would struggle to move enough fluid.
Key specs
- 115V operation
- 17 GPM rated flow (per listing)
- Flanged cast iron body
- Three speeds
Pros
- Flanged connection suits larger piping
- Cast iron body is a common choice for heavier duty circulation
- Higher rated flow for bigger arrays
Cons
- Heavier and larger footprint than compact pumps
- Flanged installation requires gaskets and proper alignment
Bottom line: The better match when the loop is long or the array is large.
Hydronic Heating Expansion Tank 102-1 Fits Amtrol Extrol Residential Boiler Systems. OEM
Hydronic Heating Expansion Tank 102-1 Fits Amtrol Extrol Residential Boiler Systems. OEM
Best for: Boiler-side pressure control in closed hydronic loops.
Useful when the solar loop shares a boiler or hydronic circuit and needs an expansion tank matched to that equipment.
Key specs
- Designed as a replacement fit for Amtrol Extrol residential boiler systems
- OEM-style part number
Pros
- Matches an established residential boiler expansion design
- Straightforward replacement format
- Appropriate for closed hydronic loops
Cons
- Fit is only as good as your model match, so verify before buying
- Tank pre-charge must be set for your system pressure
Bottom line: Pick this if your existing boiler is an Amtrol Extrol-compatible unit and you need a matching tank.
1 Gallon Potable Thermal Expansion Tank, 3/4″ MNPT Connection, Compact Design for Water Heater, RV, Marine, Boat, Off-Grid Systems, Residential Plumbing & Solar Thermal
1 Gallon Potable Thermal Expansion Tank, 3/4" MNPT Connection, Compact Design for Water Heater, RV, Marine, Boat, Off-Grid Systems, Residential Plumbing & Solar Thermal
Best for: Compact potable-side expansion in small systems.
Fits a domestic hot water side where thermal expansion needs absorbing and space is limited.
Key specs
- 1 gallon capacity
- 3/4″ MNPT connection
- Potable-rated design (per listing)
Pros
- Compact footprint
- Standard MNPT connection
- Listed for solar thermal and water heater use
Cons
- Small volume limits use on larger systems
- Confirm pressure rating meets your code and relief valve setting
Bottom line: A good fit for small potable loops, not for large boiler-side systems.
Industrial Grade Solar Pool Heater DIY Kit – Superior High-Wind Strapless Mounting System and Maximum Freeze Resistance (2-4’x6.5′ / 52 Square Feet)
Industrial Grade Solar Pool Heater DIY Kit – Superior High-Wind Strapless Mounting System & Maximum Freeze Resistance (2-4'x6.5' / 52 Square Feet)
Best for: Collector arrays sized around 52 square feet.
Works for DIYers building a collector field who want a kit with a defined mounting system and stated freeze resistance.
Key specs
- Covers about 52 square feet (2-4′ x 6.5′ panels per listing)
- Strapless mounting system
- Stated freeze resistance
Pros
- Defined panel count and area
- Strapless mounting suits windy sites
- Designed for outdoor exposure
Cons
- Pool-oriented design may need adaptation for a boiler loop
- Panels need correct orientation and tilt for your latitude
Bottom line: Best for DIYers who want a complete collector array and can adapt it to a closed loop.
UL Certified Sealed Plate Heat Exchanger, 4 x 12 Inch, 60 Plates, 316L SS, Copper Brazed
UL Certified; sealed, compact design for reliable heat transfer in hydronic heating, water heating and snow melting systems. Plate Heat Exchanger, 4 x 12 Inch, 60 Plates, 316L SS, Copper Brazed
Best for: Separating the solar loop from potable or boiler water.
A heat exchanger lets a glycol solar loop transfer heat to domestic or hydronic water without mixing fluids, a common design choice for closed-loop systems.
Key specs
- 60 plates
- 316L stainless steel plates
- Copper brazed construction
- UL certified (per listing)
Pros
- Keeps glycol and potable water separated
- 316L stainless resists corrosion well
- Compact sealed design
Cons
- Sizing must match your flow and temperature demands
- Requires correct piping and flow direction for efficiency
Bottom line: Recommended for closed-loop designs that need fluid separation.
MISOL 110V Controller of Solar Water Heater, Used for Separated Pressurized Solar Hot Water System
MISOL 110V Controller of Solar Water Heater, Used for Separated pressurized Solar hot Water System
Best for: Controlling pumps in pressurized separated solar systems.
Suited to builders who want a dedicated controller to run the circulation pump based on collector and tank temperatures.
Key specs
- 110V operation
- Designed for separated pressurized solar hot water systems
Pros
- Purpose-built for solar hot water control
- Simplifies pump operation
- Fits a pressurized separated design
Cons
- Sensor placement and wiring require care
- Check compatibility with your pump and sensors before wiring
Bottom line: A useful control choice for a pressurized separated system with a dedicated pump.
Buying Guide
Circulation Flow and Pump Sizing
Flow determines how quickly heat moves from collectors to storage. Match the pump to the loop’s length, pipe diameter and head loss. Undersized pumps starve collectors; oversized ones waste power and can cause noise.
Good picks for this: TOPWAY 3/4″ NPT Recirculating Pump, TOPWAY 115V 17 GPM Cast Iron Pump
Expansion Tank Volume and Pre-Charge
Expansion tanks absorb fluid growth as the loop heats. Size volume to total system fluid and set pre-charge to match your fill pressure. An incorrect pre-charge is a common cause of pressure problems.
Good picks for this: Hydronic Expansion Tank 102-1, 1 Gallon Potable Expansion Tank
Heat Exchanger Material and Separation
Choose stainless or copper-brazed exchangers rated for your fluids. Separating glycol from potable water is usually the goal in closed-loop designs.
Good picks for this: UL Certified 316L Plate Heat Exchanger
Controls and Sensors
A controller that reads collector and tank temperatures prevents pumping when the array is cooler than storage. Check sensor compatibility and wiring before purchase.
Good picks for this: MISOL 110V Controller of Solar Water Heater
Collector Array Size and Mounting
Plan collector area against daily hot water demand and local solar resource. Mounting must resist wind loads and be tilted and oriented correctly for your latitude.
Good picks for this: Industrial Grade Solar Pool Heater DIY Kit
Freeze Protection and Climate
In freezing climates, choose a glycol loop or drain-back design and insulate exposed piping. Freeze resistance claims should be verified against your local minimum temperatures.
Good picks for this: UL Certified 316L Plate Heat Exchanger, Industrial Grade Solar Pool Heater DIY Kit
Materials and Long-Term Durability
Look for corrosion-resistant metals, proper fittings and certified components. Durability depends heavily on water chemistry, temperature limits and correct installation.
Good picks for this: TOPWAY 115V 17 GPM Cast Iron Pump, UL Certified 316L Plate Heat Exchanger
FAQ
What is a solar thermal boiler system?
A solar thermal boiler system uses solar collectors to heat a fluid, which then transfers heat to water or a boiler loop through a pump, controls and often a heat exchanger.
Do I need a heat exchanger?
In closed-loop designs that use antifreeze, a heat exchanger keeps the solar fluid separate from potable water. Direct systems with potable water in the collectors may not need one, but local code often governs this.
How big should the expansion tank be?
Size it to the total fluid volume and expected temperature rise, then set the pre-charge to match your fill pressure. Follow the manufacturer’s sizing charts.
Can I use a pool heater kit for a boiler system?
Pool collector kits can work as a starting point, but they are designed for low-pressure pool loops. Adapt them carefully and confirm pressure and temperature ratings.
How do I control the pump?
A differential controller starts the pump when the collector is sufficiently hotter than the storage tank and stops it when the difference falls off.
Is a DIY solar thermal boiler system safe?
Pressure, temperature and electrical work carry real risk. Consult a licensed plumber, electrician or solar installer, and confirm your build meets local plumbing and electrical codes.
Conclusion
For most home solar thermal boiler system builds, start with a correctly sized circulation pump and expansion tank, then add a heat exchanger if you use antifreeze. Choose a higher-flow pump for larger arrays, a boiler-matched tank for hydronic circuits, and a dedicated controller for reliable pump operation. Verify every fit, pressure rating and code requirement before installation.
Sources
- Federal guidance on solar water heating design, operation and maintenance (energy.gov)
- Research on solar thermal system performance and modeling (nrel.gov)
- Standards and technical resources for building heating and water systems (ashrae.org)
Related reading
- Top 5 Solar Thermal Storage Tanks for Home Hot Water Systems
- Solar Thermal Energy Collection: How It Works and How to Build a Working System
- Solar Thermal Air Conditioning: How It Works and How to Build a System
- Installing Solar Thermal Systems: 5 Best Guides and References (2026)
- 7 Best Solar Thermal Heating System Options Compared (2026)
- 7 Best Solar Thermal Cooling System Options for Homes (2026)
About Audrey Hayes
Audrey Hayes is part of our editorial team and reviews and edits our solar water heaters guides, checking product details against manufacturer specifications and owner feedback.
