Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Zone priority: only heating one zone at a time?

Hi, I am a home owner and have a question about HVAC zone priorities:

A few years ago we installed a brand new Lochinvar Knight condensing high efficiency gas boiler (WHN110). Heat is distributed to 2 zones (upstairs and downstairs) using forced hot air. It also has a superstor water heater. The whole thing works great. But it uses a "Taco" priority relay to heat only one zone a time. When it was installed, the HVAC guy asked me which zone I wanted to prioritize. I didn't think much of it, and went with his suggestion of prioritizing the upstairs (where bedrooms are located). But now if both zones are cold, it will only heat the upstairs ... the downstairs will blow cold air (actually luke warm air). That is pretty annoying to blow cold air in the winter!

During yearly maintenance, I asked the technician about this, and they said I could turn OFF the taco relay with the flip of a switch. OK, problem solved, I guess. So now it heats both zones at the same time. But my questions are:

1. Why would you even install a priority relay?
2. Is there some benefit to leaving the relay active, and only heating one zone at time? E.g. Am I ruining the efficiency or doing something bad by disabling the relay they installed? Is it maybe overloading the system?


  • GGross
    GGross Member Posts: 1,072
    You use priority to cut a heating demand when you get a call for DHW. This allows the DHW to take priority over space heating calls, which is generally desirable as someone may be in the shower.

    Personally I would never set space heating zones to a priority, these boilers modulate to match the load and should be capable of heating the entire space at the same time. That said there may be particular instances where this could be desirable. But I can't think of any standard residential application, with properly sized equipment, that I would put one space heating zone on priority over any other, just DHW priority

    In short, the installer should have never suggested you put a heating zone on prio, only the DHW
    Alan (California Radiant) Forbesbburd
  • Alan (California Radiant) Forbes
    Zone priority is meant for your indirect water heater, not a heating zone.

    When you're using hot water and draining the tank, zone priority turns off your space heating and dedicates the boiler output to making domestic hot water.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab