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Solar Thermal 101: Calculating the Expansion Tank Size

This is the seventh in a series of posts written by Free Hot Water’s co-founder and senior mechanical engineer, Gal Moyal. We’ll be posting this series every Wednesday, so please make it a date. Some of the information may be very technical, but if you have any questions at all, please feel free to contact us. We sincerely want to help. If you would like to have a more hands-on experience, explore our certified Free Hot Water training courses.  –Solar Fred.

Calculating the Expansion Tank Size [Vet]

Through out the course of its life span, a closed loop solar hot water system will get exposed to wide temperature changes. To prevent the system from exceeding the pressure range allowed by the designer, there’s a need to “park” the fluid volume expansion.

To calculate the size of the expansion tank you will have to make the following calculations:

Click to Enlarge

Click to Enlarge

System potential expansion volume:

Va = 1.1(Vc+Vp)ά+Vc

Vά – Collector Expansion to be “parked”

Vc – Collector total volume

Vp – Collector Volume other than the collector

ά – Expansion coefficient (Water- .045 Glycol – 0.07)

Static Pressure at the relief valve location:

Pi = H(0.45)+7

Pi- Pressure at the relief valve location (PSI)

H – Height of collector above relief valve

To calculate minimum expansion tank needed:

Vet = Va [{Pf+14.7}/{Pf-Pi}]

Vet – Min expansion tank volume required

Va – System potential expansion volume

Pf –  Relief valve max allowable pressure

Pi –  Static pressure at relief valve location

If you’re an installer who wants more more information or a home or business owner interested in solar hot water, please email us at info@freehotwter.com, and one of our experts will walk you through the process.

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