Multiple furnaces on one thermostat?

Tenor · March 10, 2008, 02:25:10 PM

I may be installing 2 furnaces in my bus, both Suburbans, and wonder if I need two thermostats, or can use just one. Thanks!

fe2_o3 · March 10, 2008, 02:46:16 PM

Glenn, Hello. I have a 4104 with 2 suburban furnaces. One in the front and one in the rear. I put a thermostat on each because with just one we developed climate zones. If the sun or several people are in the front, the rear got cold or the front got too warm. With a thermostat in the front and one in the back we can keep both ends comfortable. Hope this helps...Cable

H3Jim · March 10, 2008, 02:57:15 PM

since the thermostat is just a switch, it should be no problem. The only issue could be if the current draw were to exceed the capacity of the switch, but that's highly unlikely.

Nick Badame Refrig/ACC · March 10, 2008, 03:53:37 PM

Hi Tenor,

Sometimes with electronic boards it's an issue and at worst you would need a t-stat with a built in resistor.

The ststs that come with the suburbans, usually are bi-metal. meaning expanding metal contacts.

Read the installation instructions, they should discribe "twinning" and the steps to do so.

Good Luck
Nick-

Tenor · March 10, 2008, 04:00:01 PM

Thanks everyone! Here's a follow up question - which would take more electricity - one furnace running longer to reach temp vs. two? This may be a reason not to run both furnaces off of one thermostat. Depending on the answer, this might be a bad thing to do while on battery power. What are everyone's thoughts?

Nick Badame Refrig/ACC · March 10, 2008, 04:08:15 PM

The only thing that comes to mind..

Is, depending where you place the t-stat, you could be sweating in one part of your coach and where the stat is could be cold.??

If surely would operate much more efficiently with 2 stats. This way you can keep the bedroom warm at night and vise versa

during the day. Yes, you would save on fuel that way. Or on my case, my wife is always HOT! and I'm not!

Nick-

WEC4104 · March 10, 2008, 04:10:34 PM

I will second the opinion to go with the two thermostats. In addition to Cable's very valid point about the zone heating, it also covers you better in the event of a component failure in your heating system.

I have personally experienced a thermostat failure on the Suburban installed in my 4104. Fortunately I had an alternative heating source (vented catalytic) on a separate thermostat and the weather was only cool, not cold, so I did not have to troubleshoot while on my brief trip. I isolated the problem and replaced the stat after I got home. If you install one thermostat only and it fails, you freeze or go for immediate repair/bypass.

Also, if you have a problem with one of the two furnaces, there are multiple ways you could be affected. If one furnace fails to "fire", you could end up with it blowing cool air everytime the working one is running. Not pleasant, but could be worse. Or, depending on how you wire to the thermostat, it is possible that a failure of one could completely disable both.

I am a big believer in redundant systems, and I think the investment in the second thermostat is well worth it, especially since it simplifies the installation wiring.

WEC4104 · March 10, 2008, 04:34:16 PM

Quote from: Tenor on March 10, 2008, 04:00:01 PM
Thanks everyone! Here's a follow up question - which would take more electricity - one furnace running longer to reach temp vs. two? This may be a reason not to run both furnaces off of one thermostat. Depending on the answer, this might be a bad thing to do while on battery power. What are everyone's thoughts?

I am going to start off by assuming that both furnaces are sized the same (BTU output). There should be little difference in the total heat output or the total electricity use of running one unit for 30 minutes versus two units for 15 minutes. HOWEVER, if we are talking about the electrical use in getting the bus evenly heated, two properly located units will be more electrically efficient.

It is not often that we camp in a situation where we do not have shore power or the ability to run the generator as needed. Consequently, my house battery capacity is small. Typically the only time my battery capacity becomes a problem is when I need to run the furnace. The overnight draw from fan in the furnace is my number one issue.

Tenor · March 10, 2008, 06:26:48 PM

Again, thanks for the replies! At this time, one furnace I already have is a 19.5k BTU output (model -- 25) and I am looking at another used one with an input of 30K BTU, probably putting out about 25K BTU. Since I only own the smaller one right now, I plan to use it on the drivers side, under the couch venting forward to the driver's seat, out from under the couch, next back to the kitchen and one into the bedroom. I would of course put the bigger one in this place if I get it. The other will be on the passenger side, midway back under the fridge sending heat into the front, one out the bathroom area and two into the bedroom at the back. The BTU combined output BTU will be bigger than the SF 42 Suburbans and I would have redundancy. Based on everyone's recommendations, I'll go with 2 thermostats. They aren't that expensive!

WEC4104 · March 10, 2008, 06:48:29 PM

Glenn:

If I'm reading your comments correctly, it sounds like you are not sure that you will be getting the second furnace, and if not you plan to duct the Model 25 to both the front and back of the bus. IMO that sounds a bit shy in the BTU department, especially for a Michigan based 40 footer. Hope you have alternative heat sources or plan to spend the winters farther south.

JohnEd · March 11, 2008, 12:05:28 AM

I have two thermostats for one furnace. At night I set the front to 60 degrees and the rear by the bed to "off". In the morning I turn the rear "on" and it is set to 76 and the rear takes over control. Everything is warm in a few minutes and I get up. 60 up front can yield 50 in the back if the temp outside is in the 40's but I use feather comfort and like the bedroom cold. I would go with two furnaces and two thermostats.

2 centavos,

John

baker4106 · March 11, 2008, 07:02:44 AM

Not sure what two furnaces would do to one tstat but amp draw would be higher causing fluxuation in the temperature on and off. Because you are tying two power sources together it could burn something out. You could use one and use a relay from the first to control the other. On HVAC we never control two furnaces with one tstat without an isolation relay. Larry 4106

Jerry32 · March 11, 2008, 08:59:20 AM

They do make a dual thermostat to run two AC's and two Furnaces and use two sensors. Jerry

NJT 5573 · March 15, 2008, 09:02:47 PM

Keep em seperate. Work on perfecting your air flow. I have (1) 40,000 BTU and it must think I have it hooked to a 150 gallon tank. It is also fond of electricity. Wish I had 1 or 2 units half that size. I used my Little Buddy single element last trip without the furnace. The kids got up every nite and turned the little guy off and I only had it on low heat. What a great portable heater, ventless indoor approved and safe. Not to mention 110 hours of heat on a barbecue size propane bottle.

If your coach is tight you need very little heat to be warm. Just a couple of screw holes can let your heat out if its cold out.