S-plan vs Y-plan
Supports revision for: C&G 6035 Unit 208. Coverage references only; no awarding-body affiliation or qualification is conferred.
A conventional S-plan uses separate two-port motorised valves for heating and cylinder circuits. A conventional Y-plan uses one three-port mid-position valve to direct flow towards heating, the cylinder, or both. The names describe a control arrangement, not a cylinder pressure rating.
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| Control idea | S-plan | Y-plan |
|---|---|---|
| Valve arrangement | Separate two-port valves on the branches | One three-port mid-position valve at the split |
| Separate demands | Each branch valve responds to its demand | The valve position responds to the combination of demands |
| Both demands together | Both branch valves can be open | The valve can supply both branches in its mid position |
A common mix-up
A similar-looking three-port valve is not necessarily a mid-position valve. These concepts do not identify terminals or prove an installation is safe. Electrical control work needs competent assessment; for gas work in Great Britain, use a Gas Safe registered engineer.
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Key numbers
- 2-port
- S-plan: a separate two-port valve for heating and for the cylinder.
- 3-port
- Y-plan: one three-port mid-position valve at the split.
Common mistakes
- Assuming any three-port valve is a mid-position valve.
- Treating S-plan and Y-plan as cylinder types. They're control layouts.
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