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Honeywell T8400B Thermostat Display Intermittently Blank

Two of the 4 thermostats in our house (for water circulating radiant heat) have on occasion had blank screens for several hours, then spontaneously came back on. Turning them off and on or changing the temperature had no effect. Is this a known problem> Should they be replaced?

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I have the same problem. It's because your domestic hot water (dhw) is on the same system as your radiant heat. In these systems, dhw is given priority over floor heating. That is, when your dhw needs heat, power to your Tstats are shut off until your dhw is satisfied. (Makes sense: cold showers no fun) Your T8400B's have no battery, so they are dead unitl power to them is restored by your zone control. It's really stupid that they use these Tstats, because you can't even SEE your room temp or settings let alone change them. I will be replacing my Tstats with battery powered soon; hopefully that will allow me to see and adjust during dhw cycles.

Posted on Nov 07, 2009

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Most likely a bad tstat i would replace I recommend honeywell focus pro 6000

Posted on Aug 29, 2009

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