It&#x27;s a pressure valve. Here&#x27;s the patent:<p><a href="https:&#x2F;&#x2F;heatinghelp.com&#x2F;assets&#x2F;documents&#x2F;141.pdf" rel="nofollow">https:&#x2F;&#x2F;heatinghelp.com&#x2F;assets&#x2F;documents&#x2F;141.pdf</a><p>I don&#x27;t think I&#x27;ve a clear enough understanding of how it works to try to explain it.