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tearing hair out over robertshaw or invensys specs

so they aren't the only company that doesn't publish good specs on their equipment, they just are the only one that can't tell me the missing specs when i call.

i had to call TACO to get the VA capacity of the transformer on their SR501 control (15 VA for the curious)

it is in their documentation that the SR-501 consumes .18 amps when energized so that means about 4.5 VA (i don't why we don't say watts when it is 24V load but that is a stylistic question i guess).

That leaves about 10.5 VA to do something with, but as Joe Matiello pointed out me, you also have to account for the thermostat load, whether it may be a power scavenger or runs off the common terminal as a 3-wire.

so i go to the robertshaw documentation and of course they don't indicate what the load for operating the thermostat is. So I call to ask and they keep reading me back the load limits for the switching contacts. Finally i got someone who understood the difference but told me they don't have that info and i could submit a written request but he wasn't sure they would release it.

Maybe i'm not smoking the good stuff, but i would think this should be a specification of the appliance.

Indeed, TACO should specify theirs because they mount one of the larger transformers compared to the competition.

These days there are lost of ancillary loggers and controls that you might want 24FV for, short of higher consumption load like a zone valve.

But maybe i'm just being crotchety and retentive and i should just assume that the thermostat load is next to nothing. when i get back there i'm going to put an ammeter in line and trip the thing and i trust i can reverse engineer its consumption in different phases of operation but strikes me as a failure of documentation that the theoretical info isn't available.

Don't know that anyone would know the value although you might know a typical value or have some experience that would be illuminating.




  • Jamie Hall
    Jamie Hall Member Posts: 20,500
    First off,the difference between a volt-ampere and a watt is not a stylistic thing at all. They are different (the difference is the power factor) and the difference can be important.

    The power consumption of a switching type thermostat (the old two wire sort) is zero when it is off, and small (depends on whether there is an anticipator or not) when calling for heat. The power consumption of a three wire thermostat with electronics will also be small; it would not be all that hard to determine if you had a good milliammeter, of course.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • archibald tuttle
    archibald tuttle Member Posts: 990
    right, this a three wire digital. i used a milliamp meter and got about 4 when it was not on and 6 when switched on. not sure if it uses an onboard SSR but it uses something to switch that actually consumes a little power the entire time it is on.

    gone are the heat anticipator type -- at least on this job.

    so very small current is correct although the solid state control i'm running with it uses even less at under 2.

    one of these should make much difference to the onboard transformer on the SR-501 and we found the problem elsewhere but thanks for your reply.

    And when you say that volt-amps goes to the power factor, how would a 15 VA and a 15 watt transformer differ in that respect, albeit that may be a trick question because all the transformers are rated in VA i think?

  • Jamie Hall
    Jamie Hall Member Posts: 20,500
    Transformers and most power transmission equipment is -- or should be -- rated in volt amperes. The reason is that what one is interested in is breakdown characteristics and heating effects, rather than the actual amount of power being transmitted. The power factor is the actual power generated or used divided by the volt amperes. If the voltage and current (amps) in a circuit are precisely in phase -- a pure resistive load -- the power factor is 1. If they are out of phase, as they will be with inductive and some solid state loads, the power factor is less than one (sometimes a lot less!) -- but the heating effect in the equipment is still related to the current alone.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • wmtandson
    wmtandson Member Posts: 62
    75va and done!