Two millivolt gas floor furnaces using only one thermostat?


  #1  
Old 10-11-11, 07:45 AM
D
Member
Thread Starter
Join Date: Oct 2011
Location: USA
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Two millivolt gas floor furnaces using only one thermostat?

I currently have a single floor furnace controlled by a thermostat (furnace #1). A second floor furnace has a gas valve suitable for use with a thermostat but has been wired for manual operation (furnace #2). I want to have it thermostat controlled and can of course add a second independent thermostat, but was wondering if there is a SAFE way to use the existing thermostat to control both furnaces.

Furnace #1 uses the thermopile generator to both activate the pilot and power the main valve (no thermocouple). Furnace #2 uses a thermocouple to drive the pilot valve and a thermopile for powering the main gas valve.

I have wrestled with various schemes on paper to allow the single thermostat to control both, but each seems to be at risk that if the pilot light fails on Furnace #1 the thermopile generator of Furnace #2 might open the main gas valve of Furnace #1 when there is no pilot flame.

Is there any way to handle this?

Dennis
 
  #2  
Old 10-11-11, 12:15 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
Received 39 Upvotes on 31 Posts
You might be able to use a battery-powered two stage thermostat and wire the two furnaces independently to the two separate stages. You would, of course, need to tie one end of the thermopile of each furnace to a common input (R terminal) of the thermostat but still retain independent action of the gas valves. You would still have staged action through the thermostat but should be able to program the stages fairly close.

As stated, this might work but if it were me I'd just use two totally separate thermostats.
 
  #3  
Old 10-11-11, 02:15 PM
D
Member
Thread Starter
Join Date: Oct 2011
Location: USA
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks. That is a great idea! The second furnace would come on just like the high stage of a two stage system. I've been thinking about the thermostat location relative to the furnace it currently controls and it's not too good. Though left open almost all the time there is a door that can separate the two and someone could easily close the door and the thermostat would be very slow to respond, meanwhile the furnace would get its area very toasty.

I'll probably take your last advice and use the current thermostat on the furnace that is not blocked and mount another in the space served by the current one.
 
  #4  
Old 10-11-11, 05:37 PM
GregH's Avatar
Super Moderator
Join Date: Jan 2001
Location: Manitoba
Posts: 9,500
Received 68 Upvotes on 62 Posts
Heating thermostats are not just simple switches!

What you need to consider is that these thermostats would normally have a fixed heat anticipator.
The heat anticipator creates a small amount of heat within the thermostat housing to minimize room temperature swings.
When the thermostat is satisfied and the switch contacts open, the anticipator begins heating with the open circuit voltage.
This is why you must always have the cover installed on a bi-metal thermostat.

If you add another gas valve to the thermostat you will mess up anticipation and cause very poor temperature control.

You would do much better with two thermostats.
 
  #5  
Old 10-12-11, 10:10 AM
D
Member
Thread Starter
Join Date: Oct 2011
Location: USA
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks GregH. Yes my plan now is to use two thermostats. Despite the simplicity of bimetal thermostats, I have had great results from the battery powered programmable thermostat on the existing furnace and plan to duplicate the set up for the second furnace. Being battery powered with no 24VAC available, the batteries require replacement about once per year. Does anyone happen to know whether there is a code-standard battery fail-safe that turns the furnace off in the case of a thermostat battery going dead? These thermostats seem to have a demand switch that holds its condition until an alternate command is received, sort of like a light switch with an electromagnet providing the movement force. I'm wondering if there is any risk that an 'on' furnace could be left that way when the batteries become weak. Or are the these devices designed to automatically turn off before the battery gets too low? I tested one thermostat that the homeowner didn't like and when the batteries are removed it reverts to the 'off' state, but I don't know if this is universal operation.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: