New Thermostat Question

Looks good, and sounds like it's working out well. I'm glad I can be of some help every once and again!

This thread actually got me thinking about doing something similar, but I probably won't simply because it's relatively rare for me to camp in sub-freezing temperatures (though I have a few times).

Well Drew, I ordered the temperature controller pictured above, and installed it.



The way I have it programmed, the target temperature is 40°, and it will let the temperature at the probe drop 5° below that before it will turn the furnace on. If the temperature gets 6° below the target temp, it will flash the display as a low temp alarm. The thermostat wires from the furnace are connected in parallel to the temperature controller and the wall thermostat. When the temperature controller is powered on, it will turn the furnace on regardless of whether the wall thermostat is in “Heat” mode or “Off”.



The instructions that came with the controller say that the “Rst” button also functions as an on/off button, but you do have to hold it down for 3 seconds. I installed it with a real on/off switch instead. I installed it where I did because it’s right above the furnace, and I had easy access to the power and thermostat wires. This is the backside of the unit. That’s the furnace below it.



The temperature probe is in the black and gray tank area, near the valves. It’s reading air temperature, but I considered whether I should have it in contact with the gray tank or maybe the pipe just before the gate valve. The primary intent is to prevent the gray tank from freezing, so it may take some experimentation to determine the best location.



The furnace duct that’s heating that area can be seen in this image (silver flex duct). I wanted the probe to be far enough away from it that it wasn’t immediately influenced by it, but close enough to the gray tank gate valve that it can prevent it from freezing.



Drew, I appreciate your help and suggestions with this. I didn’t even consider connecting two thermostats in parallel. I think it’s going to work just fine.

That’s why I like this place. Extra eyes, extra brains. :W

:):)

Yes, the "wrong wiring" would actually be correct in this case. The load output of these units is just a set of relay contacts--a switch--as is a normal thermostat. You connect the control lines for the furnace to that switch and it turns the heat on and off.

As I said, I don't have any specific personal experience using any of these devices, but based on what I've seen and understand I think this one would work just fine. Presumably most of them come from the same few factories in China, even if they have a dozen different brand names stamped on.

(If the included wire for the probe isn't long enough, you can extend it with pretty much any old wire that is convenient.)

Thanks Drew. I hadn’t considered connecting a second thermostat or controller in parallel. I’m going to mull that over a few days, but it seems like it should work.

What do you think about this unit?

Digital Temperature Controller



One of the images with this unit shows some suggested wiring examples, and they are all showing a “load” device being controlled. In my case, the t-stat really isn’t a load as it doesn’t require voltage, so it seems to me the correct way would be to connect it like the example they label as “Wrong Wiring”, except with the temperature probe connected as well.

The probe wire length should be long enough to reach the tank area through the bathroom cabinet that’s just to the left of the furnace. There’s already pass-through holes for the plumbing that I could route the wire through.

Thanks for the suggestion.

:):)

I'd look up Amazon or Ebay digital temperature controllers, which are basically thermostats but more designed for process control applications than room heating and cooling. Most of them have a remote sensor of some sort--a thermistor most likely for the temperature range we're talking about--and can be set to have whatever setpoint, hysteresis, etc. one may wish. Putting the sensor in the wet bay and wiring the controller in parallel with the thermostat would seem to me to work reliably and simply. The furnace would come on whenever either the room thermostat or the wet bay temperature controller called for heat, and go off only when both are satisfied.

The obvious advantage to this over a time control is that you get the heat you need, but not more or less. If the temperature drops too much, the periodic run might not be enough heat; and if it's only a little below freezing, it could be a good bit more than is necessary.

A couple of examples of these sorts of controllers (no idea if these are any better or worse than others, nor how well they'd cope with "12V" supplies that are much more or less than the nominal voltage): Cheap and simple bare-board, More finished looking panel mount unit

Fifteen minutes of running the furnace when you already have the interior at room temperature is going to make it a fair bit warmer, I suspect. Shorter and more frequent cycles would probably be better overall. That's not much more than a semi-educated guess, though.

Well, finding a digital thermostat with enough control over the cycle time that it would do me some good is proving to be impossible, so I’ve switched gears and decided to come at this from another direction. I still need to replace the RiteTemp t-stat because the up and down buttons have gotten difficult to use, but I’ll find a simple, non-programmable digital unit to replace it with. Had the Orbit t-stat not been so inaccurate on controlling the set temperature, I’d use it. I need the t-stat to have the +/- 1° swing that it claimed to be capable of. The RiteTemp is doing that now, so anything I replace it with needs to do that as well.

Just to clarify, my need is to have the LP furnace run “once in a while” during extreme cold periods (below 30°) so the waste tank and valve area stays above freezing, while the living area of the camper is being heated primarily by an electric heater. If I have access to shore power, the electric heater will be running on its HI setting, if I’m using my Yamaha 1000 for power, it will be on its LO setting. The electric heater has its own thermostat that works pretty well.

Under those conditions, rather than have the LP furnace be controlled by the digital t-stat, how about controlling it with a infinite cycle timer that has adjustable “ON” and “OFF” times? I’m thinking the furnace would need to run 10-15 minutes, then stay off for 1-2 hours. I would want it to do that endlessly, until I switched it back to the digital thermostat. I would monitor the temperature in the waste tank area with a wireless thermometer, and adjust the ON/OFF times so that it stays above freezing. The LP furnace will of course be adding heat to the living area while it’s running, but the electric furnace will react to that and not run as much. I see very little chance of overheating the living area while using the cycle timer, once I determine what the ON and OFF times need to be.

My LP furnace has a two wire connection to the thermostat. One is “+Thermostat”, and the other is “Thermostat”. I believe this is what’s referred to as a “millivolt system” as there is no power on these two wires. If you connect them together, the furnace starts a heating cycle, and when you disconnect them, the furnace turns off.

Functionally, what I think I need is something like this:

Adjustable Cycle Times Switch Module



From the description, it looks like if you supply the cycle timer with 12v DC, then you will have 0-120 minutes of ON and OFF time adjustability. I’m not locked in to this particular cycle timer though. I would prefer one in a case that could be mounted out in the open. In addition to the cycle timer, I would install a switch on the two thermostat wires so they were connected to EITHER the wall thermostat, OR the cycle timer. Never both.

The ON and OFF times need to be easily adjustable. That’s one thing I do like about this unit, is the simple, mechanical way of setting the cycle times. I would also install an on/off power switch to give the timer 12v, and jumper the trigger circuit on. The board would power up in a “triggered” state.

I would install this on the wall next to the furnace, where there’s easy access to the thermostat wires, and 12v power.

So, any recommendations on a cycle-timer like this, from a reliable brand? This one can be had for about $10, but the reviews (of course) range from “It’s great” to “it doesn’t work”.

Can you think of any reason why this wouldn’t work? It seems like a fairly simple, and inexpensive way to achieve what I’m wanting. Another option may be to just install another thermostat in the waste tank area, and a switch to control which thermostat is controlling the LP furnace. Thermostats aren’t typically designed to control a heating system below about 45°, and that’s about the highest I would want to keep that area. I see more of a chance of overheating the living area trying to use a thermostat like that than with a cycle timer.

:):)

Since starting this topic, it occurred to me that the “Minimum Cycle Period Setting” on this thermostat could actually do me a lot of good when I’m trying to keep the waste tanks from freezing, and not run through my LP too fast. It would introduce a bit of automation into a cold weather routine that I’m doing manually now by fiddling with the thermostats on the LP furnace and the electric heater so the furnace runs occasionally.

In really cold weather, I can always find or supply myself enough 120v AC to keep the inside of the camper comfortable, but as the temps fall into the teens, it becomes more and more important that the furnace run periodically to keep the waste tanks from freezing. Like I said, I can do it all manually, and have been for years.

But, if the thermostat would allow a once-every-hour single heating cycle to the set point temperature, that would help me in several ways.

Today I did some testing of the Orbit t-stat I installed, and found some issues. First, it overshot the set temp by 5-6 degrees before turning the furnace off, and then it turned it back on before the temp fell to the set temp. That’s not any better than the old analog stat used to do. The specs for the t-stat say it’s supposed to keep the temp within 1 degree of the set temp. The RiteTemp stat did a better job maintaining the set temp.

The fatal flaw for me though, is that the “Minimum Cycle Time” adjustment didn’t work. I set it to 60 minutes, but the t-stat continued to call for another heat cycle after 12 minutes (the default for a gas furnace). I called Orbit technical support, and they told me the stat isn’t compatible with RV systems. It’s a battery powered t-stat, like the RiteTemp I’m wanting to replace, so I don’t understand what’s incompatible with it.

I’ve seen the programmable version of this unit on lists of “Best Thermostats for RV’s” before. I wasn’t going to argue with them about it though, so I’m looking for another digital, non-programmable unit. I’d like to find another one that has an adjustable cycle time if possible.

If anyone has a recommendation, I’d love to hear it. Ive got some important things to do for the next few days, so I may not be able to look back at this right away. I’ll get back to it though.

Thanks.

:):)

Thank you pianotuna

wa8yxm wrote:
If it does not turn on the a/C fan then something is wrong

It’s been wrong for over 20 years then. I don’t need it to do that, and I wouldn’t use it even if it did. It’s of no value to me.

I do however need to keep my tanks from freezing when we’re traveling in freezing weather. I was out just recently as a winter storm system moved through the TX panhandle and western OK, we had several nights in the twenties and mid-teens. Nothing froze because I know what to do with my TC as the temps get colder and colder. That’s what almost 20 years of experience with the same RV gives you.

I’m trying to fine tune my cold-weather processes by taking advantage of the features some of these new digital thermostats have, like the “Minimum Cycle Period” this thermostat has. I’ve run into a problem with it though. I’ll explain in my next post.

:):)

If it does not turn on the a/C fan then something is wrong