ENV-VST-C Thermostat Power

Spire_Jeff

I was looking at TechNote 977 (http://amx.com/techsupport/techNote.asp?id=977) and they are calling for 196VA power for 2 thermostats without the furnace equipment taken into account. Most companies in my area are using only 40VA or 50VA transformers. I was speaking to one of the HVAC contractors we work with today and mentioned the latest power requirement documentation. He looked at me like the President looked at Dr. Evil when he asked for $100 billion dollars. He said that 96VA to power one thermostat was ridiculous as a 40VA transformer is enough to run multiple furnaces with quite a few dampers. I would have to agree as we have thermostats installed and running on WAY less power than AMX is now specifying.

I am thinking that there may be a case of AMX being technically correct and the HVAC transformers using a different measurement on the transformers? Does anyone here speak fluent HVAC? I am looking for conversion from AMX's VA requirements to the VA ratings found on HVAC transformers

I thought it was much simpler in that the thermostats require 2amps at 24VAC, so to be safe I was using 4amps per thermostat. On a system with 5 thermostats, I would need 20VA of power from the transformer. (4amps x 5 stats) The ratings in the () on the tech note seem to jive with these figures a little closer.

Any help appreciated,
Jeff

ericmedley

I'm sorry, but as a guy with an electronics background, that tech note makes no sense. You don't up the voltage, just the amperage draw when you add T-Stats.

Think about your house. You don't start adding up the hair dryers and add to the voltage feed by 120VAC every time.

ericmedley

On another note, we were early adopters of the color Viewstats and I was told specifically to use the power from the HVAC unit and not provide a separate power supply.

Y'all may recall during the days of the older AMX Viewstat/Aprilaire 8870 that we needed to use a separate power supply. when I called AMX tech support asking where the power hookups were for Axlink and/or whether or not to use them, I was instructed to use the HVAC system only. Are we to assume that that has changed?

I will say I've had no issues with the HVAC unit power to date. They've all worked fine.

vining

Wow that tech note is a bit confusing and you have to read it a few times to make sense of it. Basically what I get from it is each T-Stats draw 2 amps @ 24v (VAC). Now they want you to double that in your caculations to determine xformer size. So figure each T-Stat requires 48 volt amps (not VAC). Typical HVAC 24 volt xformers are as previously noted 40 or 50 volt amps so you can only power 1 T-Stat. Hmmm, WTF is that!

Lutron has some 100va/24vac xformers that they use for shade power but it sounds like you either need to power each T-Stat seperately from each other and the HVAC system on standard 40va xformers or you get a lighting xformer 12/24v buck/boost type that typicaly is used for low voltage lighting that can range from 100va to 750va @ 24 volts. You could use one of these and use this to power the HVAC as well. Then you could condition/filter the 120 volt power feeding it and put it on some sort of power brick to cycle power.

Q-Tran makes some nice fused xformers but so does Acme.

Spire_Jeff

Keep in mind that AMX says that the ENTIRE system needs to be on one transformer, or you will encounter problems (As Eric said). This means that a single zoned furnace needs to have enough capacity to power the furnace, the furnace electronics, the zoning dampers, and all of the thermostats attached. According to the HVAC person, you could use multiple transformers as long as the commons are tied together, but who knows how well this will work.

I have seen some 500VA transformers from Grainger, but I think those are normally used on commercial systems and they might even need 220, not 110.

Jeff

vining

Spire_Jeff wrote: »

Keep in mind that AMX says that the ENTIRE system needs to be on one transformer, or you will encounter problems. This means that a single zoned furnace needs to have enough capacity to power the furnace, the furnace electronics, the zoning dampers, and all of the thermostats attached. According to the HVAC person, you could use multiple transformers as long as the commons are tied together, but who knows how well this will work.

I have seen some 500VA transformers from Grainger, but I think those are normally used on commercial systems and they might even need 220, not 110.

Jeff

If that's the case just use a standard low voltage lighting xformer like the Q-Tran or Acme, etc. Any electrical supply house should have them in either 12v or 24v since low voltage lighting can be either (the buck/boost can do either depending on how you wire it). It's typcally 12v but when we have large runs with high loads we'll go to 24 volt systems to 1/2 the amperage draw and reduce the conductor size required to feed them.

So now there's a service call for no heat and the HVAC tech comes out and the transfomwer is cooked what does he do, say sorry, call your systems integrator for "no heat" calls? There got to be something better and why isn't the Brain's side power of the T-Stat isolated from the HVAC switching side like the Aprilaire Stats.

edit/added:

According to the tech note referenced earilier you can't technically have 1 T-Stat on a system at all since a standard HVAC xformer is 40va and just one Stat requires 48va not to mention the needs of the HVAC equipment itself. Fortunately if it's a single zone air handler there shouldn't be any dampers to open or close but they may be humifidiers to deal with, air cleaners, etc.

So why should it be on the same power? What did they screw up when they designed it? They had the perfect T-Stat to model after in the AprilAire Stat so why the power issues?

Spire_Jeff

vining wrote: »

According to the tech note referenced earilier you can't technically have 1 T-Stat on a system at all since a standard HVAC xformer is 40va and just one Stat requires 48va not to mention the needs of the HVAC equipment itself. Fortunately if it's a single zone air handler there shouldn't be any dampers to open or close but they may be humifidiers to deal with, air cleaners, etc.

So why should it be on the same power? What did they screw up when they designed it? They had the perfect T-Stat to model after in the AprilAire Stat so why the power issues?

If you read closer, you will see that a single tstat requires 96VA as you have to double the requirement to give 50% headroom. This is precisely why I think there is a flaw in the technote. I understand that these thermostats were designed for use in hotel rooms and designing them for a single power source tied in with the furnace makes sense in that application. Requiring the hotel to use a power supply that costs 3-10 times as much per unit doesn't make sense in a bid process... but it wouldn't surprise me if this really is the case.

Jeff

vining

Spire_Jeff wrote: »

If you read closer, you will see that a single tstat requires 96VA as you have to double the requirement to give 50% headroom. This is precisely why I think there is a flaw in the technote. I understand that these thermostats were designed for use in hotel rooms and designing them for a single power source tied in with the furnace makes sense in that application. Requiring the hotel to use a power supply that costs 3-10 times as much per unit doesn't make sense in a bid process... but it wouldn't surprise me if this really is the case.

Jeff

Yeah you're right it is 96va per stat. The specs say the TStats have a max surge rating of 2 amps and a max output rating of 2 amps (all outputs), 1 amp any single output. To me this means these would be the switching outputs of the HVAC control side of things and have nothing to do the the T-Stats draw at all but what the HVAC sub systems would draw through the TStat switching. Figure an NI-4100 draws .98 amps @ 12vdc so why would a TStat need 2 amps @ 24vac to operate which makes me think it doesn't. The TStat itself probably needs less than 1/4 amp to power itself if I were to make a guess.

Now if you oversize the transformer above 48va or a 40va/50va standard xformer you're putting a potential on the TStats greater that its max rating so if there were a dead short downstream of the TStat on wires or connected equipment the TStat would fry before anything else so I would think any fuse feeding the TStat should be equal or less than the max current rating of this TStat device or don't suppley the TStat more current than it can handle. How about that AC condenser low voltage wire on the outside of the house getting smashed by a landscaper and you're running on a 750va xformer and then the TStat calls for cooling. Poof!

This technote contradicts itself on every level and too me makes no sense.

Now if this 2 amp rating is indeed the TStat switching ratings then on a multizone air handler all these outputs are common. Since they share the same air handler fan, the same AC condenser, the same boiler relay, just different damper motors but those are usually controlled as is everything else by a zone controlle panels so the only thing going through the TStats is enough current to pull in relays on the zone control panel.

On a single zone system the TStat would switch the air handler, ac condeser or bolier directly and there the 1 amp per output rating might be pluasible.

Also if you put a potential on a wire the wire has to be sized large enough for that potential. Now if I do my Ohm's law right a 750va xformer would require you to run #10 awg wire to your TStats. That's the size wirre that feed your electric clothes dryer at home. Think about!

vining

Another thing (if you were to believe anything that technote says), if this system is supposed to be fed by the same xformer what are you supposed to do, rip out the xformer that's built into the air handler and re-wire it to use an external lighting xformer? Makes no sense. Who owns the air handler then? You also typically have seperate xformers for heating and cooling, one on RH and RC so putting the entire system on one xformer isn't plausible in these common systems unless you add isolating relays which is or isn't done on systems depending on the type of air handler and/or if they use aquastats or time delays to pull in the heat.

Since IMHO the TStat should only require a minimal amount of current to power it and the amperage limitations are strictly for the HVAC equipment being switched through the TStat then a single TStat (zone) system should have no problem and forget the 50% headroom BS. On a multi TStat (mulit-zone) system your draw through the TStat should actually be less than a single zone system since you're now only passing enough current through the TStat switching to pull in the zone controller's relays. Maybe on the high side 100ma. Probably closer to 50ma.

What they should tell you is what the actual draw of the TStat is and that can't be the 2amps they use in this technote. How much more do these TStat draw than any other power stealing programmable TStat with display. I'm sure it's more but what I can't find it.

IMHO I think this technote is going to cause more problems and solve nothing.

TurnipTruck

There was an issue with the earliest revs of the env-vst-c where if one side of the AC supply was ground with the other side high, some part(s) of the stats would go up in a blaze of glory. I do believe this has been corrected. Some of their recommendations may still take that into consideration.

Spire_Jeff

First, I did verify with tech support that the stats do require a power supply that is rated at 48VA. I also got confirmation from a friend I trust in the HVAC business that according to the values I provided, 48VA is appropriate. He also said that it is not common to find such a power supply in the HVAC market

I just did a search and Altronix seems to sell power supplies perfect for the job. The ALTV248600 is 8 fused outputs with a total capacity of 700VA. It costs around $200.00 online (on a quick search).

That leads me to the real reason I am posting I have created a PCB that will isolate thermostat power from the furnace. The board is also capable of keeping Y1 and Y2 active when set to a heat pump and emergency heat is activated. (Your HVAC unit must remain responsible for determining the ability for Y1 and Y2 to actually function ) I am wondering if anyone is interested in purchasing these boards? I am about to order a batch for a client and there are setup and order fees that are fixed, so it is better for all involved to get a larger run I am looking at selling the boards separately (you source and assemble the parts yourself). I am also considering selling the boards as a kit that include all of the parts (you just have to solder everything together). The kits would not include any warranty (expressed or implied) and would be a use at your own risk type deal. I have a prototype installed and functioning with a Water Furnace geothermal unit, but I am not able (or willing) to go through the testing process, the certifications, and the lawyers required to provide this as a complete product ... unless there is great interest

Approximate pricing of the PCBs is $29.00 each if I can order at least 24 boards (I currently need 16). The price of the components is approximately:

Floor Heat Only (no fan, only W1 functional): $20.90
1 Stage Furnace with A/C (single fan control): $32.60
2 Stage Furnace with A/C (single fan control): $44.30
1 Stage Heat Pump: $43.31
2 Stage Heat Pump: $55.01

If anyone is interested, let me know. I am expecting to place an order in a week or two. Depending on the size of the order, it is generally filled within a few days.

I am attaching a bitmap image of the board layout. The boards measure roughly 6in x 3.5in and you will probably want to put them in a plastic project box or mount them in a metal cabinet of some sort. There are mounting holes in the board.



I am open to suggestions as well

Jeff

kholm

Stupid but true

Yes they are requiring you use ridicules amounts of power for there thermostats. It basically boils down to poor design. While the stats are fine for a single or two stat installation. But they become unruly in Large systems. We have a system with 17 stats and have a 2000VA power supply to run them (per AMX). We spent hours and weeks just getting them all to run properly. We were told it must be the power supply. 5 Power supplies later we found one that worked ok. We were thinking about scrapping the boilers and just heating the house from the thermostat power supply.

vining

Is that 2kva xformer being fused for each t-stat run? If not that's an 83 amp potential (@24v) on wire that's most likely only rated for around 16 amps of non continuous load. Could be a fire hazard if the wire becomes shorted or damaged. Of course if you're using 4 awg copper your fine but that's a little hard to fit in the terminals.

If the smaller TPs can run on POE which has to be 500ma or less (?) then why do these need so much power? Other than the draw of the t-stat itself it only needs to pass through enough current to pull in the equipment relays which should be minimal. I don't get it and it is completely stupid. How can they design such crap and put them into production with such a flawed design and if they can't get a simple t-stat right then .......

TurnipTruck

If the stats run on something less than 24 volts for a period if time, they will fail.

I oversaw an installation with 13 of them. I ended up using a "Buck-Boost" style 250 KVA transformer. And yes... very fused! So far so good several months later.

Sent from my MB865 using Tapatalk

Spire_Jeff

I hesitate to say this for fear of the inevitable hex, but the board I designed has been running successfully for a few months now. It allows you to separate the power from the furnace system. I have 15 stats running on 4 different power supplies. They are also working with Water Furnace without issue. If anyone is interested in the boards, I have a few left from the production run and I can supply a list of parts from mouser.

Jeff

vining

I was just thinking about the Lutron SVQ-10-PNL power panel which has 10 individually fused 24vac outputs @ 3.15 amps each (75.6VA) from a single 15 or 20 amp, 120v supply circuit. Not quite the 96VA spec'd but it's gotta be good enough again since most HVAC systems with multiple zone dampers and other contactors only require 40VA xformers. Still needs to be isolated or used in lieu of the built in HVAC transformer but at least you're starting with a quality pre-built power distribution center.

edit added link:
http://www.lutron.com/TechnicalDocumentLibrary/085-0003%2012-3.pdf

DHawthorne

AMX will not support an external supply, only feeding it from a furnace/air handler. I just had one blow on me, and they won't cover it under warranty because of my external supply, even though I have the outputs relay-isolated, and a new one in the same location, same wiring, and same supply is just fine.

But I think the 2A per stat requirement *must* be an error. I think what they really mean is the furnace should have a 2A (48VA to be precise) transformer to handle the stat *and* the furnace solenoids, etc. I put an amp meter on a supply that was, at the time, powering 6 thermostats. As you know, the nature of an amp meter is that it completes the circuit, so I can be assured when I connected it all the stats were powering up at once. It pulled a grand total of 0.5A for six of them ... a *far* cry from 2A each. I realize they have six relays on them, but you can't tell me the relays need that much current to throw. I am utterly convinced the requirements are to allow for spikes cause by the HVAC system, since they are intended to draw from it's transformer. But you won't get anyone in tech support to get behind that, they only read off the paper in front of them in this case, and have to tow the company line (which is understandable, but frustrating when you are certain it's wrong).

I've found other errors in the documentation ... one that stands out is it says they power off the C and RC terminals, which I verified in the field is not the case, they power off the C and RH. If there is one verifiable error, how can we be assured there are not more?

TurnipTruck

I agree that 48 VA is overkill. The issue is that if the supply voltage is something less than 24 volts for a period of time, the stat will die. I have verified this through my own experiences. Perhaps the available current specification is an indirect way of ensuring ample supply voltage?

Sent from my MB865 using Tapatalk 2

vining

DHawthorne wrote: »

I've found other errors in the documentation ... one that stands out is it says they power off the C and RC terminals, which I verified in the field is not the case, they power off the C and RH. If there is one verifiable error, how can we be assured there are not more?

Some systems require a jumper between RH & RC if they share common power but other systems require seperate feeds one for the cooling side (RC) and another for the heating (RH).

There was another thread started by SpireJeff (I believe) that went back and forth about this stipud design, the specs, etc. He actually created an isolation circuit board in order to be compliant.

Most HVAC xformers are only 40va so to be compliant you'll need to re-wire the air handlers (most cases) which hosts the transformer but you owns the HVAC system then.

Unless the air handler is zoned and driving damper motors there is no real draw the the standard 40va xformer and besides the draw through the t-stats contacts are extremely minimal since they just need enough current to make the relay contacts in the air handlers that handler the larger current loads.

DHawthorne

vining wrote: »

Some systems require a jumper between RH & RC if they share common power but other systems require seperate feeds one for the cooling side (RC) and another for the heating (RH).

There was another thread started by SpireJeff (I believe) that went back and forth about this stipud design, the specs, etc. He actually created an isolation circuit board in order to be compliant.

Most HVAC xformers are only 40va so to be compliant you'll need to re-wire the air handlers (most cases) which hosts the transformer but you owns the HVAC system then.

Unless the air handler is zoned and driving damper motors there is no real draw the the standard 40va xformer and besides the draw through the t-stats contacts are extremely minimal since they just need enough current to make the relay contacts in the air handlers that handler the larger current loads.

I understand that. My point is if you are powering it separately (against AMX recommendations) you need your transformer to be on the RH, not the RC like the docs say. Jumper doesn't apply in that case.

vining

vining wrote: »

There was another thread started by SpireJeff (I believe) that went back and forth about this stipud design, the specs, etc. He actually created an isolation circuit board in order to be compliant.

I should have looked at where this thread originated since it appears that this is the thread i referenced.