Anything special about zone valve transformers? Can I use a general purpose transformer?

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CheesecakeLover

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Hi all, I currently have a single 24 V transformer powering 2 Taco zone valves. The transformer is marked as "40 VA", which means it has a current rating of 1.66 Amperes.

The problem is that I believe my Tacos have the older 0.9 A power heads; not the modern 0.3 A heads. If that's true, that means I'm slightly over the capacity of the transformer: 0.9 A * 24 V = 21.6 VA (for 1 valve). Two valves make that 43.2 VA on a 40 VA transformer.

To make things worse, I am about to upgrade one of my thermostats to a digital model, which will of course employ the use of a C wire to allow an additional 20 mA to be drawn from the transformer.

Now that the background is out of the way, I can ask my question: Can't I just use a general-purpose 24 V transformer rated for 2 A? That would be equivalent to a having a "48 VA" power rating. Or am I really better off buying one from a supply house that is specifically meant for zone valves? I honestly can't see any difference, other than the industry-purpose transformers list the power rating in VA.

Even more background: The REASON I would rather purchase a general-purpose transformer is because I can find them with a center-tap (12-0-12). I can utilize that center tap to power a couple voltmeter chips. Doing this will let me keep an eye on the thermostat activity while I'm standing in the boiler room. This helps me diagnose things at a glance, instead of pulling out my multimeter and probing around.

Thank you.
 

Stuff

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The valves only operate at the same time for a short burst so I would leave things alone. Adding .020 amps for a thermostat is in the margin of error.

If you want, yes, you can use a general purpose 24vac transformer. Make sure that it is rated to be installed in an exposed location - coils and 120v wiring needs to be protected. More common when more power is needed is to use a second transformer to split the load. Other thing with higher load is to upsize the wiring.
 

sophocles

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The valves only operate at the same time for a short burst so I would leave things alone. Adding .020 amps for a thermostat is in the margin of error.

If you want, yes, you can use a general purpose 24vac transformer. Make sure that it is rated to be installed in an exposed location - coils and 120v wiring needs to be protected. More common when more power is needed is to use a second transformer to split the load. Other thing with higher load is to upsize the wiring.
zone valve transformers have an overload link in the 120v side, regular transformers typically don't. i found out the hard way when the overload link failed (with only one zone valve in the circuit). wish i had used a regular transformer in this case - the zone valve was wired to shut off water to a hot water heater if the moisture sensor activated. (this was 1996, before any advanced electronics were available for this purpose.)
 

Reach4

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zone valve transformers have an overload link in the 120v side, regular transformers typically don't.
All class 2 transformers have that internal overload link. I don't know if those are powerful enough for this job.
 

Fitter30

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Most zone valves use a synchronous motors spring return. If valve is a normal closed the motor will remain powered till satisfied. 50 va transformer can be purchased with a built in resetable overload for the 24v. Others that are fused are soldered in under the paper that the coils are wrap in.
 
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