Taco Zone Valve Controller & Power Stealing Thermostat: A Curiosity

PGB1

Hello All,
Recently, I bought an electronic thermostat (ecobee3 Lite) that required a 24-Common (aka Neutral). I ran new thermostat wire to supply the neutral. But I was curious about factory made zone valve controllers, so I looked at some and found something interesting...

I looked at Taco's ZVC403 zone valve controller. The instructions (attached below) mention if one has a "power stealing thermostat" and no 24-Common at the thermostat- placing a 1000 Ohm resistor across the thermostat's W (heat) & C (common) terminals will work instead of running a neutral. I assume they mean that the resistor fools the thermostat into thinking a Common from the transformer is present.

Out of curiosity, do you all know how this resistor "creates" a Common (24-Netural) for the thermostat? (I'm not in the situation, just curious.)

Thanks For Explaining!
Paul
PS: Oddly, the existing 40v transformer caused the thermostat to shut down when it called the zone valve. (Honeywell 8043 series). A 50va solved the problem. All I can guess is that the 40va wasn't really 40va. The voltage drop was from 26.3 rms to 22.1 when the valve motor was called. With the 50va it is steady at 26.8 vac RMS with all 3 zone motors moving at once.
Do heating transformers get weak over time?

EdTheHeaterMan

This sounds like a job for @SteveSan

However I like to point out that when talking about a C wire, we refrain from calling the C terminal on a 24 VAC control circuit Neutral. The will get confusing when talking about control circuits when there are both 120 VAC and 24 VAC circuits in the system that we are wiring. I like to keep L1 and L2 and N as a separate 120 VAC or 240 VAC (Line Voltage) system. That line voltage is separate from that of a 24 VAC system

I like to use C or Com for the common side of the low voltage system. This way the apprentice electrician will have less trouble when connecting a transformer that has all 4 terminal designations. It can be confusing. In the same way as the ground wire and the Neutral wire on the Line Voltage systems should never connect in the house wiring except for the source where they are both connected together in the circuit panel

Don’t use a bare or green wire as a neutral, Don't use a Neutral as a COM. and don't get the gray area and the blue area confused by calling them both common or C . You you might just let the factory installed smoke out of the control.

109A_5

Hello @PGB1,

PGB1 said:

Out of curiosity, do you all know how this resistor "creates" a Common (24-Netural) for the thermostat? (I'm not in the situation, just curious.)

The "power stealing thermostat" types can usually receive their needed power a few ways by design. R wire and C wire is the best way. In the absence of a C wire they can charge their internal battery when there is no call for heat via the R and W wires. The problem is the battery charging and operational current on the W wire can be insufficient for normal thermostat operation and can also confuse the logic state of the heating equipment by elevating the W wire Voltage above the C wire. C wire and W wire should be basically at the same Voltage potential when there is no call for heat.

The resistor provides an additional current path for the thermostat battery charging and operational current. It also makes the W wire Voltage lower (closer to the C wire) to minimize any logical state confusion of the heating equipment.

More explanation is this thread.
https://forum.heatinghelp.com/discussion/195345/nest-with-two-wires?utm_source=community-search&utm_medium=organic-search&utm_term=NEST

109A_5

Hello @PGB1,

PGB1 said:

PS: Oddly, the existing 40v transformer caused the thermostat to shut down when it called the zone valve. (Honeywell 8043 series). A 50va solved the problem. All I can guess is that the 40va wasn't really 40va. The voltage drop was from 26.3 rms to 22.1 when the valve motor was called. With the 50va it is steady at 26.8 vac RMS with all 3 zone motors moving at once.
Do heating transformers get weak over time?

Since the output Voltage of the 40 VA transformer was less than the nominal 24 VAC I believe your situation was trying to draw more that 40 VA of power. Apparently the additional 10 VA capacity solved that problem.

A transformers VA capacity usually does not fade with time.

PGB1

Thank You both for taking time to reply & for good information.

Your reasoning for not calling Common a neutral is sound logic, Ed. Keeping things clear and as simple as possible is always appreciated when diagnosing a problem, so separate terms for line and control voltage certainly will help.

I am a master electrician by trade (industrial primarily) and am used to the primary and the secondary (control) both using the term "neutral" for each voltage. But, that's motor, lighting and similar controls. Those secondary voltages are often 120, 277 or 480 volts. Until I read what you wrote, I never did understand why climate systems used the term "common" instead of "neutral". If the transformer does not have a grounded secondary (isolation transformer), your explanation makes it critical that the common not be called "neutral". My transformer is certainly isolation type, so "neutral" would be a bad term to use.

When you mentioned that the thermostat can receive power a few different ways, I had no idea that there was potential storage in the thermostat, nor that it would be able to use power non-traditional (24-R and 24-C). The ecobee also has a magic work-around for no common wire. They supply what is called a Power Extender Kit. I can only guess it works similar to how you described.

Thanks, too for the article link. It's interesting.

Upon reading your words about an internal battery, suddenly how the resistor works in the circuit made lots of sense, 1095_A. If not a battery, perhaps there is an ultra-capacitor (super capacitor) instead.

Something must be in there to maintain CMOS memory settings during a power interruption. Electronics are certainly helpful in getting things accomplished! (And in confusing me...)


Oddly, the old 40va transformer, with the old thermostat, had no problems with all 3 zone valves open, or even moving, at the same time. It wasn't until I connected the ecobee thermostat that the problem arose. Even with one zone valve opening, the ecobee thermostat shut down and the voltage was low at the thermostat's R & Common terminals.

The fact that the ecobee could not handle one zone valve on 40va, but the old (non-smart) thermostat worked makes me wonder if there was always a voltage drop to the thermostat. Perhaps a poor splice or weak spot in the old wire caused just enough resistance to drop the voltage below the ecobee minimum, but the zone valve motors didn't mind reduced voltage. Bench testing an old valve showed it opened with as low as 11.5 vac.

When I changed to the 50va, I also ran new wire to the thermostat, so I'll never know if there was a wiring problem.

Reason for new wire: The old wires were separate cables for hydronic and for for cooling equipment. Plus, they had wrong color coding. I wanted to make things as standard as possible. This way, it would be easier for any technician to diagnose just in case there's a problem & I'm not around and my wife has to call outside.

Thank You both again for helping. I very much appreciate the education.

Paul

hot_rod

What type of zone valves? Some of the thermoelectric type pull a lot of current. If 3 or more call at once your transformer may be too small.

PGB1

They're plain, old Honeywell VF8043F1036 series. If I remember correctly, they require 7 va each. We have three, but the ecobee could not operate even one.

Using the 40va they used to operate fine, even with all 3 motors moving at once. But maybe the valves are more tolerant to voltage drop than the ecobee.