Tarms piping recomendation (PT1)- see attachment- calls for a Termovar model 6440AF. This is not the standard termovar used on the boiler return. The 6440AF that is used in this piping arrangement is a diverting valve that allows both the zone and the storage system hot water at the same time thus preventing boiler idle. The valve goes for $190.00. Does anyone know of a cheaper alternative?