Honeywell VisionPro power issue

David Nadle

My system has the TT in series after the LWCO and the vaporstat. The LWCO cutting out 24V I don't mind, that's an exceptional event. But the vaporstat is a bummer.

I emailed Utica tech support and asked if it was OK to wire TT just after the LWCO and before everything else; they wrote back that 3 wire tstats are unsupported and I might "have problems." A lame response, in my opinion.

I can't see any reason not to place TT before the vaporstat, but I haven't got around to doing it yet. The tstat has battery backup so it's really more of an annoyance than a problem.

David Nadle

Is the 24V getting interrupted?

I've got the VisionPro wired with 24V common and have the constant backlight turned on. Every so often I notice the backlight is out for a minute or so, then comes back on. I think it coincides with either the themostat cutting out or the vaporstat cutting out. Maybe something is interrupting the 24V supply and the stat is reverting to battery power?

The thing is, I'm pretty sure things are wired correctly. Has anyone else seen this?

Big Ed_4

Power Source

Is the transformer after any controls ?

David Nadle

No, just the shutoff switch.

Glenn Harrison_2

You are definatly loosing 24 volts

if the backlight is going out, and I have never seen the thermostat by itself cause this, so it must be a boiler control. I'm thinking your vaporstat is wired into the "common" side of the transformer, and when it opens, you loose the common to the t-stat, and therefore, 24 volts.

David Nadle

I was thinking the same thing...

But all the exposed wiring matches the diagram. If that's the case then something's miswired in the box. I'll check it out and post back.

David Nadle

Gotta be the LWCO or the vaporstat.

Everything is wired right. The LWCO is first after the transformer, then the vaporstat, then the TT. I guess I'll have to live with intermittent loss of 24v to the stat. Not the end of the world, but my inner engineer is disappointed ;-)

David Nadle

On second thought...

Is there any rule against putting the TT after the LWCO but before the vaporstat? Electrically it's identical but are there saftey reasons/codes that mandate cut-out pressure kill power to the thermostat?

Captain Who

Isolation relay?

Maybe you need an isolation relay for the thermostat so that both the thermostat and the LWCO can see the 24V from the transformer all the time? When the thermostat calls for heat it powers the coil in the relay, which closes a switch after the LWCO in the controls circuit (LWCO, pressuretrol, damper, flameout switch, gas valve etc.) so that the LWCO cannot cut off power to the thermostat when it is testing the water level. I'm assuming you have a probe type LWCO. I just wired up a White Rodgers thermostat this way and it works well. Trying to go by the manufacturers instructions for the thermostat and LWCO was a joke, had to figure it out myself! By the way, I also had to swap out the 120V LWCO to a 24V model though (CycleGuard 400).

Captain Who

Check out the schematic on page 30 at: http://www.burnham.com/PDF/8141049.pdf (it takes a while to download - 104 pgs.). It shows how the thermostat will be able to always get 24V and never be cut off from it no matter what the LWCO or the vaporstat is doing, but you absolutely have to have the isolation relay in there. In my case I had to set it up this way because I bought a thermostat that has an optional remote indoor sensor and I now have it set up so that it averages the two temperatures of the sensor and the thermostat itself and I get more even heating this way. The remote sensor input is disabled unless you power the thermostat with 24V continuously. I will try to put just the schamatic in as an attachment.

Captain Who

Check out the schematic on page 30 at: http://www.burnham.com/PDF/8141049.pdf (it takes a while to download - 104 pgs.). It shows how the thermostat will be able to always get 24V and never be cut off from it no matter what the LWCO or the vaporstat is doing, but you absolutely have to have the isolation relay in there. In my case I had to set it up this way because I bought a thermostat that has an optional remote indoor sensor and I now have it set up so that it averages the two temperatures of the sensor and the thermostat itself and I get more even heating this way. The remote sensor input is disabled unless you power the thermostat with 24V continuously. I will try to put just the schamatic in as an attachment but I'm not sure how to do it.

Okay, I guess I did it but it is much clearer if you download the pdf, cause you can zoom in on it and it is still sharp.

David Nadle

Thanks. That's the answer.