how to calculate VA

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Old 12-08-08, 09:59 AM
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how to calculate VA

i have a 24 vac transformer for 4 honeywell ta87 stats. the anticipators are all set at 400 milliamps.
do i just multiply 4x.4x24 to get the total VA for this setup??
if so, i get about 38.4 VA. if that is correct do i need a 40 VA transformer??
 
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Old 12-08-08, 10:43 AM
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Your math is correct for computing VA.

But, what is the current requirement of the devices that the thermostats are controlling - relays, valves, etc.? It is possible that the current draw is less or more than what somebody set the heat anticipator for.
Doug
 
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Old 12-08-08, 02:45 PM
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doug
i'm running 4 erie zone valves that say: "24 vac 100 ma."
i assume this is the draw of the end switches. the actual internal device that opens the valve, whether it is some hot wax or a motor or whatever, is run off of a separate transformer integral to the aquastat. the black wires connect to the thermostat transformer; the red wires to the aquastat transformer [honerwell r8182]. is my thermostat transformer draw the total of 100max4x24=9.6VA?? but that seems dwarfed by the 400 ma draw of each tstat anticipator. i'm still confused about all of this.
werner
 
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Old 12-08-08, 03:19 PM
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Sorry, I'm confused here. You seem to be saying that the transformer for the zone valves also supplies another transformer for the aquastat? Are there two 120V-24V tranformers or one? Or maybe one with two secondary 24-V windings?

The circulator (in addition to the zone valves) isn't turned on by the thermostats? If not, what turns the circulator on? I would have expected a 24-V relay controlled by the thermostats.

Doesn't each thermostat have an adjustment for the proper heat anticipator current? If so, did you set it to 400ma or did somebody else?
Doug
 
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Old 12-09-08, 08:59 AM
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doug
i hope i didn't confuse thngs. it seems there are two separate transformers. one is separate and attached to ceiling joists and the 4 thermostats are connected to this one [there is no relay here]. the black wires of the 4 zone valves connect to this transformer through the red and white wires of the t'stats, but there is no direct electrical connection between this transformer and the aquastat. as i undersgtand the system, when the tstat calls for heat, the signal goes to the end switch of the zone valve. when that switch closes, it allows a relay connected to a transformer inside of the aquastat to send current to the circulator and to each zone valve [the red wires] whose tstat is calling for heat [and also to the burner, if necessary to satisfy the settings of the aquastat]. i hope i do not have this wrong. as for the anticipators, i set them to 400 ma. the honeywell tstat brochure suggests 800 ma as a starting point, but that gave bad room temp control-a 4 degree overshoot. when set to 200 ma my boiler was short cycling and the boiler temp never got hot enough to get really hot water into the loops [the supply temp maxed at 155]. at 400 ma the short cycling problem is gone and my supply water is at 180 degrees. this was trial and error.
werner
 
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Old 12-09-08, 12:28 PM
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OK, Werner - that helps.

You are adjusting the heat anticipator to give you the run times that you want. That is fine. No need to worry that the heat anticipator doesn't match what the valves or other loads are listed as. Changing the heat anticipator does not actually change the current, or if it does, not appreciably.

The 40-VA xfmr is original, right? I think it should be fine. The VA through the xfmr should not be affected significantly by your fiddling with the heat anticipator settings.

I think you are good to go - unless there is some problem you haven't told us about.
Doug
 
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Old 12-10-08, 09:14 AM
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thanks doug, appreciate the help
 
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Old 12-12-08, 04:43 PM
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By the way, please add your location to you automatic header. We don't know if you live in Miami, International Falls, or where.
Doug
 
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