I have heard that regulations require that a motorized valve is fitted on the FLOW (hot side) connection of the solar water heating coil in the hot water cylinder.  I guess this is powered open by the solar controller when in turns on the solar circuit pump and is wired through a high temperature thermostat.  Then if the system over heats the cylinder the stat switch will open, taking power off the zone valve, closing it and preventing any further heating of the hot water cylinder.  I am qualified to install un-vented cylinders and can see this has been taken from this type of installation. This is to protect the un-vented cylinder from a boiler gone out of control.

As the solar systems gets very hot it vaporises the fluid as part of their normal operation.  I thought it best to fit all the components in the RETURN (cold side). This includes the circulation pump, expansion vessel, pressure relief valve and flow meter etc. Then place the NRV (non return valve) between them and the solar collector to protect them from the fluid particularly when it vaporises. So you cannot really rearrange the solar water heating circuit to put the NRV after the zone valve.

So if the motorised valve closes at high temperatures and subsequently the NRV closes as the fluid vaporises this will give a section of the circuit that is completely closed and isolated from the expansion vessel or pressure relief valve.  Bearing in mind zone valves, unlike mid position valves, are meant to give 100% closure.  I would say that this will cause a dangerous build up of pressure in the FLOW side including the solar collector manifold’s internals.  I guess the zone valve will fail first, if it has not already succumbed to the high operating temps of the solar water heating circuit thus rendering this safety feature useless.  If not a joint may blow allowing the escape of very high temp vapour which would be particularly unpleasant if one of the high energy transfer fluids have been used.  An oil burn is much worse than a water burn.

I thought again about rearranging the circuit but could not because of the arguments in paragraph two. Even if regulations were written, say with two expansion vessels, I think it would only be a matter of time before someone got confused.  Basically putting two stop cocks on a circuit that could be heated and subsequently become pressurised is just bad engineering unless it had vent points in every section!

As a 2^nd safety back up the to solar controllers systems a high thermostat on the hot water cylinder could be fitted to break the power to the solar water heating systems circulation pump.  This would also protect against installation errors.

This entry was posted on Friday, April 17th, 2009 at 12:39 pm and is filed under Green Energy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.