VST Thermostat installation

hodeyp

Hi,

I have a VST thermostat, connecting up to the netlinx looks straightforward. Having no experience of heating systems I asked my plumber to take a look and he is unsure how to connect.

I have a very simple system, just a boiler (no aircon).

Does anybody know which wiring diagram I should use?

Thx,

Phil

GSLogic

Phil

There is a detailed diagram in the Viewstat Installation manual.

hodeyp

thanks, sadly i've taken a look at the manual and am unsure which diagram to look at, all seem to be dual stage this and ac that but all I have is a simple single cable with dual wires coming out of the boiler which i presume acts as a switch.

All of the diagrams in the manual seem to involve a much more complex setup...

GSLogic

Use diagram #4 (Boiler with AC) but disregard all wires coming from the AC unit. Did you show the HVAC installers the diagram?

hodeyp

thanks, I did show them the diagrams - I think they were a little nervous as they hadn't dealt with anything like this before.

shr00m-dew

One thing I found out the hard way with some Vantage thermostats (same thing, different controller) is to make sure they see the diagrams *BEFORE* wiring the unit. At the last house they were planning on their thermostats that needed 4 wires and ran on a battery. There was no 24v wire to power the thermostat! Luckily we were able to fish a 5 wire cable there, but it could have been bad.

Kevin D.

DHawthorne

The HVAC side of a Viewstat wires exactly the same as any standard thermostat, allowing for variations with power. I typically have the HVAC people install a temnporary cheap standard stat, and they I have my installers go around and swap them out. The color codes on the wires are usually standard, and if not, the terminals are named the same, so it's easy to match which goes where.

Here is something I have discovered about Viewstats that I really don't like at all: there is no definite isolation between the communications lines and the HVAC lines. I have seen the entire comm buss wig out because an HVAC tech disconnected the furnace, and that on a system where the stats had their own power transformer, so it wasn't a power issue. I believe the lack of isolation is on the ground - it's common throughout the stat, so the common line on the HVAC lines is also the signal ground on the comm buss. I strongly advise buying the external relay cards from April Aire to isolate the furnace and compressor from the thermostat.

2nd Rick

DHawthorne wrote:

Here is something I have discovered about Viewstats that I really don't like at all: there is no definite isolation between the communications lines and the HVAC lines. I have seen the entire comm buss wig out because an HVAC tech disconnected the furnace, and that on a system where the stats had their own power transformer, so it wasn't a power issue.

We sat through the Aprilaire training session on this system about a year ago and they specifically warned against letting the HVAC unit's transformer supply power.

I believe the lack of isolation is on the ground - it's common throughout the stat, so the common line on the HVAC lines is also the signal ground on the comm buss. I strongly advise buying the external relay cards from April Aire to isolate the furnace and compressor from the thermostat.

Do you happen to have the part # for that piece??

I have a whole system going in shortly.

DHawthorne

You misunderstand me. I am talking about powering the thermostat with it's own transformer. There is still an isolation issue between the output relays of the stat and the comm buss. Activity on the output relays to your furnace/compressor can mess up communications.

There are several outboard relays available, most of which are intended for specific multi-stage furnace types the thermostat does not support directly. The old Enerzone number for them was DF2 and DF3; that's how I have them in my database, and I have no idea what the new Aprilaire number is. You could probably do the same job with another make relay - just need a 24V trigger.