[Etaz]

First, a brief description of my dilemma. In an effort to reduce heating/electric bills (going green!), I am in the process of replacing two mechanical line voltage fan coil thermostats in my condo with digital programmable thermostats. The building was built in the 70’s so the heating/cooling is hydronic (I think, after doing research) with a 2-pipe system. In April, the hot water starts flowing through the pipes and causes the switch connected to the pipe to switch the thermostat to “COOL” mode and vice versa. Many hours of research produced the major obstacle of finding a programmable, line voltage (120 vac) thermostat that wouldn’t break the bank. I have found (obvious to you, I am sure) that Home Depot, Menards, Sears, etc do not stock these. So I turned to the Internet and contractor supply channels. After finding one that fits all criteria and ordering, it is on back order and won’t be shipping for over a month. In addition, they weren’t cheap :-/. Ugh.

So, instead of trying to find a digital, programmable thermostat that can handle an input of 120vac, I instead want to place a 120vac to 24 vac step down transformer (where a “normal” HVAC system takes care of this) in between the house line voltage and a 24 vac thermostat. My thinking is this would afford me a much wider array of consumer programmable thermostats that I could purchase from the aforementioned retailers. I would expect these transformers would be inexpensive and found something on Radioshack.com that does this.

Question: Am I on the right track and if so, what kind of product would I be looking for and where would I find it?

Thanks a ton and I look forward to your response.

JP
Chicago, IL

[KISS]

There is some missing info like: How many wires, what are the terminals called, what model whatchamacallits it connects too and the anticipated current at 120 it has to switch.

Is this an auto-changeover type application? Is a fan involved? How is it controlled? Is it part of a zone controller?

[Missouri Bound]

You are on the right track, it's a very common way of solving line voltage switching... BUT as KISS said, you need to supply much more information before any of us can help you. WE're waiting.

[letmetellu]

I think you are going to have to have a relay that is operated by the 120 volts to send line voltage to a transformer that will then send 24 volts to the thermostat, so that as it makes or breaks it will send the 24 back to the really to then switch the 120 volts telling it to open or to close the water valve.

[Etaz]

There is some missing info like: How many wires, what are the terminals called, what model whatchamacallits it connects too and the anticipated current at 120 it has to switch.

Is this an auto-changeover type application? Is a fan involved? How is it controlled? Is it part of a zone controller?

Thermostat(7): International Environmental TC126
7 wires total. 3 from fan (blower)Red(HI),Blue(MED),Black(LO). 3 from sensor attached to water pipe Yellow(? ),Light Blue(COOL), Red(HEAT). 1 from hot house
line(14GA),Brown(COM).

Fan Coil(4): International Environemntal 4PMOD3-2 (3 speed)
4 wires. Blue(MED), Red(HI), White(Neutral bus house line 14GA), Black(LO)

Wires from House line(2):
1 Black (HOT) to thermostat. 1 W (neutral) to fan White

Now, in terms of the terminals, there is no wording at all, this is an old mechanical thermostat with just soddered connections. I made the table below to orient the connections.

9pin connector (house side-->thermostat)
Color Function Connects to:
---------------------------------------------------------------------------
Black 120 V AC Thermost Brown(COM)
Red (pipe sensor) ?? Switches heat/cool Thermostat Yellow
Black Fan Hi Thermostat Blue
Blue Fan Med Thermostat Black
Red Fan Low Thermostat Red
Black (pipe sensor) Heat Thermostat Red
Blue (pipe sensor) Cool Thermostat Light Blue
N/C
N/C

The house White (Neutral) connects to the fan White directly bypassing
The above mess.

It is auto changeover in the respective seasons. When the bldg maintence staff switches from boiler to chiller (vice versa) the sensor on the water pipe communicates to the thermostat switching to HEAT or COOL. (at least this is how I see it happening)

The water pipes feed into a series of "fins" that cools/heats the return air. I can't see well enough to determine the mechanism of how air is returned but there is a filter we replace that sits in front of the "fins".


Hope this helps. Thanks for your expert opinions.

[KISS]

Is there any way you can confirm what the 3 wire sensor is.

Based on 3 wires, it's probably an RTD
2 wires will read zero
And between any one of these and the other; guess around 100 ohms.

What is the typical loop temp for heating and cooling?

[KISS]

Your thinking that the fan speed is the stage and the sensor determines heat or cool based on loop temperature?

So, I'd like to know the loop temp in heat and cooling modes?

Some systems are Heat pump based and the loop temp isn't that much different. Your thinking that it's just a fan coil with the loop temp determining heat or cool mode?

[Etaz]

I will have to track down a maint staffer to answer the loop temp. I'll get back asap. Would a pic of the archaic system help? Being able to visualize always helps me. Not sure if I can attach low-res pics on this site since I am a newbie.

[KISS]

Their website is IEC International Environmental > Home but it looks like they are not too helpful. In their stat literature they do mention a changeover sensor, which matches what you suggested. It may measure temperature or could be an SPDT switch.

3 heat/3cool stats might be difficult to find although you can only use 2 cool.

You can wire a stat like you would for a damper, so it does auto changeover (Y and W are combined), so heat and cool activate the same fan speed. Then you will have to do something with G.

Which means you won't have choice of fan speed using "FAN".

Or you can buy their programmable stat?

[KISS]

Pics can be attached in the "go advanced" section. The limits are specified. IrfanView - Official Homepage - one of the most popular viewers worldwide resizes images.

Any thought about going communicating stats? I'm not sure what's available.

[Etaz]

Loop temps are 42-48 and 120-140 respectively. The maint staff called it the Water FLow Sensor, which indeed does measure temp but HOW it works is beyond the scope of my objective, what it does is paramount.

I assumed I would lose the fan speed option but who cares. Now, back to the question... can I use a transformer in my scenario so I can open up the range of programmable stats I can purchase? (i.e. install a 110-120v receptacle and plug-in a 24vac transformer and connect it's leads to the C terminal?

[Etaz]

If by two-way communcation between the stats is what you mean then, originally, communicating stats were considered but budget and simplicity overrode that option. If you meant something completely different than please fill me in.

[KISS]

It's probably out of budget: Aprilaire Communicating Thermostats &#124 HVAC Home Automation System Thermostats

[Etaz]

You are on the right track, it's a very common way of solving line voltage switching.....BUT as KISS said, you need to supply much more information before any of us can help you. WE're waiting.

MB or KISS,

Do you have enough information to render further assistance? You state it is a very common way of solving the line voltage issue but I am still seeking a feasible, practical solution to my obstacle. Thanks in advance for your help.

JP

[Missouri Bound]

Is this heating system just for your (personal) condo? Is the system actually 30 years old? In order to keep the original line voltage controls, you will need a relay to independently switch each line voltage circuit. These relays would need to be a low voltage type (typ 24ac) and they would need their own power supply and control transformer. Then a low voltage thermostat would in actuality control the relays which in return would control the line voltage load. What you have is two systems (correct me if I am wrong) that are currently controlled by one thermostat for each zone. There isn't a simple way to fix this... other than replacing the HVAC equipment. But it can be done, probably not simply or economically. You need to design a low voltage control panel to switch the line loads. You won't find one at Menards. (where are you that there is a Menards available?) You just me better off pursuing a live voltage programmable thermostat.

[KISS]

Assuming it switches a low current 120 V, these relays would be fine: https://www.ecomfrontier.com/asi/mem...on=Y&cid=11981

They are about $10 ea. You'll need a piece of T35 DIN rail cut in two and 4 end stops. The WEB data says 24 VDC coils, but the PDF says AC/DC coils. This is about as cheap as your going to get. 3 relays, one for each speed.

I guess the option of fan only for circulation makes no sense? Help me here.

It's going to cost you about $40-50 just to convert the signals from 24 VAC to relays assuming two places.
You can use two transformers or one. 1 location or two. Use DIN terminals to engineer a solution. You can put it in a box or not. All will drive up cost, but increase reliability in the long run. You can get 24 VAC DIN rail mountable power supplies.

This diagram might clear things up a bit: http://www.white-rodgers.com/wrdhom/.../0037-6808.pdf

You could supply power to Rc or Rh depending on the season to avoid blasts of the wrong air. You could also measure the temp of the line and switch to cool or heat mode by applying power to Rc or Rh, but that may not be the only way.

You'll need at least a 3 heat/2 cool stat.

See: Thermostat signals and wiring - Transwiki

1st stageheating, will connect to low fan speed
1st stage cooling to med fan speed 24 VAC relay coil
2nd stage heating to med fan speed 24 vac relay coil
2nd stage cooling to hi fan speed coil
3 rd stage heating to hi fan speed

It doesn't really matter if the tstat is set to electric or conventional because your not able to control the fan separately and you won't use the G terminal.

[KISS]

MB:
A line stat doesn't come close either.

The speed of the fan is the stage.

I did point out that the manufacturer has programmable fan coil stats, but lacks useful online information.

[KISS]

Here is a fan coil stat:

http://cgproducts.johnsoncontrols.co...df/2497150.pdf

It controls the line voltage directly for the fan speeds.

Just means the price went up:

SC-E03-24VAC Products

Might work?

Don't know if you can mechanically interlock 3 relays for each fan speed. You can probably do it electrically with the aux contacts. Mechanically is safer.

[KISS]

The cool and heat wires could be used to switch the mode of the stat. Basically pick one. Make heat the failsafe position.
Use the existing stat (you can move it out of sight) and it's sensor to determine heat or cool mode.

I'm still assuming that there is no FAN only mode.

[KISS]

If you take a look here: Honeywell VisionPro Thermostats - iaqsource.com

There are a number of stats. What's interesting is that Honeywell offers a gizmo that will provide and interlock the 120 control relays and, I think, provide the 24 VAC to their FAN coil stat.

This fan coil stat does offer an speed control.