Stack multiple high temp zones on DHW priority using zone controller end switch?

PhillipJones

In a system with radiant slab zones as well as an indirect water heater and an air handler, is it a bad idea to use a zone pump controller for the air handler and indirect water heater, then land the end switch on the boiler DHW terminals? In effect, the boiler outdoor reset could be used for the radiant zones, and the high temp zones would be 180 by using the DHW priority function. this would eliminate mixing down the radiant zones, and I'm fine with the radiant being shut off while the priority zones fire. I know there's no free lunch, but this seems close…. maybe you guys have been doing this for years, and I've just now thought of it; shoot it down if it's a bad idea. Thanks!

Teemok

Good/ Bad? It's in the details of the components involved. Knowing your systems logic details is the only way to identify when there might be problems. Yes it's done regularly, both acceptably and problematically. Indefinite radiant lock outs or incomplete lock outs can happen. Undesired cross temperature mixing can be a problem.

EdTheHeaterMan

It works properly when the ambient temperature is mild. The problems may show up the closer you get to design temperature. That is when the high temperature zone may lock out the low temperature zone for longer time periods. Then the low temperature zones need ot play catch-up when playing catch-up is the most demanding. Oversizing the high temperature zone so it can satisfy quickly is the only way to mitigate this effect. Oversizing may not be the most comfortable answer, but it is the solution to the problem in your proposal.

There are better solutions using mixing valves and other systems to utilize one boiler for 2 or more temperatures. Most folks want to eliminate the use of other temperature reducing controls because the boiler already has the feature built into the internal control system. If that is your idea, then eliminate the need for the high temperature zone. Heat that zone from another source.