upgrade OPS300 TO VSTAT

karabo

We recently upgraded OPS300 thermostat to V-Stat thermostat with lot of difficulty. One of the major problems that we face is that as soon as we add 2 or more V-stat on the same bus we loose communication and the port shuts down. The only way to get the port up is by resetting power on the accent3 pro. Here is a brief description of the system:

Accent 3 pro
6 V-stat thermostat connected on port 1 RS422
Addressed for the thermostats are 2, 3,4,5,6 and 8.
24 AC Volt power supply for all thermostat (looped).
3 VRG card to control voltage to VAV.
Axcessx program.


The same setup was used with OPS300 and all worked fine. The program and wiring was not changed when we replaced OPS300 with V-stat. We tried different solutions like diving the thermostats into 2 groups using 2 RS422 ports with no success. As far as I know OPS300 and V-stat use the same commands. Do we need to terminate which thermostat and how.



Any advice will be most welcomed.

DHawthorne

There are some particulars you have to observe for the conversion. The command structure is compatible, but the hardware is not exactly the same. There is a tech note available form April Air about it (the AMX product is just a branded April Air device), but I don't have a copy of that to send you. As I recall, you cannot share power supplies between the two types, the Viewstats need to have their own; and the Viewstat addresses have to be contiguous and at one end or the other of the range (either the first devices in the address list, or the last).

alexanbo

You would definetly want to make the address's contigious. Also you can't send out commands to fast over the bus or that causes problems.

Using the April Aire distribution panel with that many thermostats makes the install a lot cleaner and allows for easier troubleshooting.

DHawthorne

Another thing I've discovered the hard way is that there is not proper isolation between the comm circuits and the thermostat relays that fire the furnace/compressor. If there is any wierdness at the furnace, it can knock out the entire comm buss. I was on a site where a furnace was being replaced, and the instant the HVAC guy fired up the new furnace, every stat on the job went into an endless reboot cycle. Turning off just the comms and back on again fixed it. I assume they have a common ground, because I was using seperate power supplies for the stats, not running them off furnace power.

April Air also makes an isolation relay for the output of the stats, and I strongly recommend putting them on every job.

flcusat

DHawthorne wrote:

April Air also makes an isolation relay for the output of the stats, and I strongly recommend putting them on every job.

Dave, is there any place where I could find information about this isolation relay? I went to Aprilaire's web site and I couldn't find it. Also AMX doesn't offer this product.

DHawthorne

flcusat wrote:

Dave, is there any place where I could find information about this isolation relay? I went to Aprilaire's web site and I couldn't find it. Also AMX doesn't offer this product.

It was years I last used one, and my boss told me the same thing when I tried to get another for a new customer; my guess is they don't make it anymore. Just toss any old 24VAC coil relay in there ...

ericmedley

DHawthorne wrote:

It was years I last used one, and my boss told me the same thing when I tried to get another for a new customer; my guess is they don't make it anymore. Just toss any old 24VAC coil relay in there ...

We always provide our own 24 volt power supplies with an UPS backup and filter. the 24 volts coming of the HVAC system is notoriously awful.

DHawthorne

ericmedley wrote:

We always provide our own 24 volt power supplies with an UPS backup and filter. the 24 volts coming of the HVAC system is notoriously awful.

I'm not talking about the supply voltage. I'm talking about isolating the relay outputs from the furnace, because the stats themselves share an internal common. Problems on the output can and will mess up comms on the stats.

ericmedley

DHawthorne wrote:

I'm not talking about the supply voltage. I'm talking about isolating the relay outputs from the furnace, because the stats themselves share an internal common. Problems on the output can and will mess up comms on the stats.

That's interesting. I thought the relays were isolated... I'll have to check into that. Not so good if it's the case. Thanks!

shr00m-dew

So you need a relay for every output? H/C/F/etc? Just trying to put my brain around it. We have just taken over a job with 15 VSTAT's on two DIST panels. The HVAC always had issues and the previous AMX guys were replacing thermostats left and right.

I haven't dived into them yet, but judging from everything else they did I would imagine the V-stat's are not the core issue, but would like to protect them as much as possible.

Kevin D.

John Gonzales

flcusat wrote:

Dave, is there any place where I could find information about this isolation relay? I went to Aprilaire's web site and I couldn't find it. Also AMX doesn't offer this product.

Pedro

The isolation relay is Aprilaire P/N 8024. There's some information about it on page 35 of the Viewstat installation manual. The diagram is for a radiant floor heating application, but the pinouts of the 8024 are shown. They're readily available through Aprilaire.

shr00m-dew wrote:

So you need a relay for every output? H/C/F/etc?
Kevin D.

The 8024 Isolation relay pack is 3 relays on one board, so you'd need one 8024 per stat for a standard application, and maybe two if you're doing multiple stages of cooling AND heating that require actuating more than 3 devices.



Dave, your idea is interesting about the on-board relays sharing some sort of internal common. It seems almost counter intuitive since the purpose of the relay is to isolate the two circuits. There are other issues that can occur with relays like a huge voltage spike when the magnetic field collapses in the coil (e.g. when the relay is turned off). I wonder if that might possibly have been a cause, but that wouldn't explain why adding isolation relays fixed your issues.



--John

DHawthorne

John Gonzales wrote:

The isolation relay is Aprilaire P/N 8024. There's some information about it on page 35 of the Viewstat installation manual. The diagram is for a radiant floor heating application, but the pinouts of the 8024 are shown. They're readily available through Aprilaire.
...

Dave, your idea is interesting about the on-board relays sharing some sort of internal common. It seems almost counter intuitive since the purpose of the relay is to isolate the two circuits. There are other issues that can occur with relays like a huge voltage spike when the magnetic field collapses in the coil (e.g. when the relay is turned off). I wonder if that might possibly have been a cause, but that wouldn't explain why adding isolation relays fixed your issues.

--John

Thanks for the part number, I just couldn't find it.

My reason for believing there is not proper isolation comes from a job where I actually watched the comms while the HVAC tech fired up a furnace. The stats went haywire as soon as he turned it on, and I had to reset them. They all had their own power via the Disty, and the only connection between the stats and the furnace was the output leads of the stats. I talked to an Aprilaire tech and browbeat a confession out of him that there is not full isolation between the comms signal ground and the output common. On that particular job, i did not need the isolation relay because it turned out the HVAC guy had hooked something up wrong; however, it still should not have backfed into the comms.

John Gonzales

DHawthorne wrote:

I talked to an Aprilaire tech and browbeat a confession out of him that there is not full isolation between the comms signal ground and the output common.

Nothing better than an old fashioned interrogation to get the answer .


--John