New t87 with Navien issue

Hydronicz

Ok 1st time running into this issue. I installed a ncb 250- 150H. It has the 3 zone relay built into the unit. Basement zone had a pretty new t87 yoyo, same as top fl zone. 1st fl zone digital type. Only 2 wire R&W for each thermostat. The issue is the basement and top floor with the t87 stats you hear it click but the zone never shuts off. Navien still shows the 2 zones calling for heat until i pull the stat off the base. I tried different dip switch settings and no luck. Replaced both stats with a basic programmable works perfectly. Has anyone ran into this issue? I've probably put in a few dozen of these but dont recall any having a t87 and if so i probably had it into an external switching relay. 

EdTheHeaterMan

The T87F obsolete mercury switch thermostat would never give you that problem. the new T87K that looks like that old one is really a digital thermostat without the digital screen. There may be some power stealing going on in that thing. Let me look at it for some more details.

EDIT:
OK the T87K thermostat has a lithium battery. When you connect only 2 wires, there is a power stealing feature the charges the battery when there is no call for heat. That circuit is designed to have a large enough resistance so the batter charger will not interfere with the standard heating control system. The Navien control may not see it that way. The resistance may be just enough to act as a holding circuit. a holding circuit isn't enough to turn on the boiler control but is is enough to hold it on after the circuit is already energized. Looks like that thermostat is not going to work for you.

If you want that LOOK of 1960s round thermostat for some reason, i expect you can purchase the old T87F on ebay as something call New Old Stock. That is an obsolete part that has never been out of the box, so it is NEW, but out of production and out of warranty because it's OLD.

109A_5

Hello @Hydronicz,
I suspect the newer T87 has some parasitic current when not calling for heat. And the Navien ncb 250- 150H or a Zone Valve controller is sensitive to that parasitic current. Measure the Voltage R wire to W wire with the system intact and no call for heat, what is the Voltage ? It should be 24 VAC or a bit more. You probably need to install the resistor C wire to W wire similar to the ones that are use with the NEST, Ecobee and others.

Hydronicz

I believe it is the t87k, heat only round. Not the good old mercury type. It only uses 2 wire no common so i dont think its power stealing. Thanks ed i didnt think these had a battery built into it. Makes sense though 

109A_5

Hello @Hydronicz,
Measure the Voltage R wire to W wire, it will tell you what is going on. Compare the Voltage, no thermostat on the base plate to the thermostat installed correctly with no call for heat, the Voltage should be the same. If the Voltage is different there is a parasitic current draw.

EdTheHeaterMan

If you only have 2 wires... IT IS POWER STEALING... If it has 3 wires then there could be a common to feed the power all the time. R to C would always have power. W would would only get connected to R on a call for heat.

On a 2 wire R gets connected to C on a call for heat. when there is no call for heat, you can measure 24 volts with a meter between R and C at the thermostat. That is the power that is being stolen. But it can only be stolen when there is no call for heat. And that is when you are having the problem, when there is no call for heat.

Believe me on this ... I have done this once before. or maybe more than once

Get a different thermostat.

Hydronicz

I understand,  I was just under the assumption that only the nest and wifi thermostats were power stealing enough to causes issues. Never ran into a t87k causing any type of issues 2 wire hooked up to argo or taco or directly to a cad cell relay ever. Thanks for all the feedback guys. Much appreciated