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Topic: New Thermostat for furnces. (Read 1028 times) previous topic - next topic

New Thermostat for furnces.

  I'm experiencing the same inside temperature swings that so many of you all have experienced in the past.  Did a quick search and it appears that its a simple matter of swapping out the OEM thermostats with a higher quality off the shelf unit from Lowes.
  Don't really need/want a set back type unit - but I know the price difference is tiny compared to the nice warm coach in the AM.
  Real question is simple:
  How many wires am I going to have to deal with, and do the colors of the wires conform to HVAC conventions?

Re: New Thermostat for furnces.

Reply #1 –
I assume you still have the original mechanical Hydro Flame thermostats.  I moved the thermostat for our front propane heater.  I can't remember for sure, but I think it only had 2 wires.  It is basically just a simple analog ON-OFF switch.  See the thread linked below.  It may have some helpful information:

Atwood Hydro Flame Thermostat Relocation

The online Atwood service manual has wiring diagrams.  Identify your furnace model, then refer to the appropriate schematic:

http://bryantrv.com/docs2/docs/hflamefurn04.pdf

Re: New Thermostat for furnces.

Reply #2 –
Moby,  it would be of interest to me whether a more sensitive/better quality? thermostat can lessen the temp swings that  most felt were inherent in the basic design.



Re: New Thermostat for furnces.

Reply #3 –
Very tough to buy a non programable tstat any more. As mentioned before its a 2 wire on/off switch to a furnace that is grossly oversized to deal with 0 deg. worst case conditions, so temperature fluctuations are common. digital tstat have a higher deadband built in for energy savings so less comfort, so you raise the temperature setting to feel comfortable. The smaller the furnace btuh size that you can deal with the more the quite it's going to be, the more comfortable it's going to be.

Re: New Thermostat for furnces.

Reply #4 –
Ah yes, the oversized furnaces to deal with zero degrees.  Been there many times.  Did work. Sort of.

Too hot.  Throw off the blankets.  Too cold.  Put them back on.  Repeat a hundred times.

Dual, different sized furnaces in each zone might help this but I doubt it.

Versus undersized units that would not heat enough for zero degrees

1989 beaver Marquis had a four zone webasto hydronic heating system installed with variable fan speed rheostats on the lower cabinets.

And an dash like the bluebird including a fancier digital overhead console.

91 went to a 3176 cat motor.  Way ahead 

Re: New Thermostat for furnces.

Reply #5 –

Moby,

Reading the responses above, and thinking about how my heaters function, I don't know if replacing your thermostats will accomplish your goal.  Seems like you would be removing a mechanical ON-OFF switch, and installing a digital ON-OFF switch.  I suppose you might be able to tighten up the "dead band" between ON and OFF, but I'm not even sure about that.

However, it certainly wouldn't hurt anything to give it a try!  If you have any success, I (and others) might follow your lead.  So keep us informed.

A quick Google search returned the 2 hits below.  They both are "2-wire" devices, so should hook right up to your existing wires.

Amazon.com: Honeywell TH1100DV1000 Pro-Digital 2-Wire Heat Only: Home...

LuxPro Digital 2 Wire Heat Only Thermostat - PSD010B - Programmable Household...

As an Amazon Associate Foretravel Owners' Forum earns from qualifying purchases.

Re: New Thermostat for furnces.

Reply #6 –
Just for kicks and giggles, I may try one of the old style Honeywell units.
Basic design that's worked for years, may be adjustable, and cheap.
Will report on success or failure.

Re: New Thermostat for furnces.

Reply #7 –
I vaguely remember that these old OE furnace thermostats has an adjustment for sensitivity.  Might pull the cover and check.

Re: New Thermostat for furnces.

Reply #8 –
Quote from: wolfe10 –
I vaguely remember that these old OE furnace thermostats has an adjustment for sensitivity.  Might pull the cover and check.
Yes.  On the thermostats in our coach, the "Anticipator" adjustment is a sliding metal tab.  The range of adjustment is incremented from 1.0 (the factory default setting) to 0.2 (minimum).  However, the owner's manual cautions that it should never be set below 0.48 (the closest mark on the scale is 0.50).  I have played a bit with ours, and find that moving it does seem to narrow the dead band slightly.  Ours are set at 0.70 (see photo).

Re: New Thermostat for furnces.

Reply #9 –
The more common issue with the thermostats was the original "4 button" Dometic where you had to push two buttons, simultaneously, in order to switch zones.  The upgrade was to a 5 button version that FT went to.  While in NAC recently, I wanted a newer programmable thermostat, but had to settle for the 5 button due to incompatibility with the A/C units.  Each of the upgrades required changing the circuit boards in the A/C units, but according to Old Town Motorcoach, Dometic said the new digital thermostats would not work with the  OEM A/C units.  I ended up spending a lot for a minor upgrade.

Re: New Thermostat for furnces.

Reply #10 –
Quote from: Sven and Kristi –
The more common issue with the thermostats...
Sven,

The thermostats you are talking about is "newer" technology.  The OP's original question pertains to earlier systems, where the heater controls are entirely separated from the A/C controls.  No matter what age the coach, it seems the OEM climate controls can certainly be improved upon.

Re: New Thermostat for furnces.

Reply #11 –
The Dometic 4 button thermostats with faulty buttons are very repairable. I have done six hundred of them for folks.
Jim

2002 U320

Re: New Thermostat for furnces.

Reply #12 –
Quote from: wolfe10 –
I vaguely remember that these old OE furnace thermostats has an adjustment for sensitivity.  Might pull the cover and check.
Looked, but did not see something obvious.  When we stop for the night, I will post pictures.

Re: New Thermostat for furnces.

Reply #13 –
The design of a propane furnace for safety mandates the temp swings.

Disabling some of the safety protocols may less the swings.  You would be foolish to do so.

On startup the fan comes on first to make sure that any propane buildup is disbursed, then the flame heats the unit.

Upon reaching the thermostats high level setting the flame cuts off but the fan stays on till the box cools down a bit.

As was noted a smaller capacity unit running more often might mitigate the swings to some degree but would lose the serious winter use btu output needed.

External venting of the first cold air until the flow warms up and  the flow after the high temp cutoff would seem to be the only solution.

Does not exist.  Would have to be engineered.

A diverter valve would seem safe as it would not disable any safety systems just the outflow to either inside or outside?

As long as the bays are vented so to not build air pressure the diverted flow could go there?

Or maybe just the initial cold air output?  Or switchable between hot or cold diversion.

Furnace kicks off diverter open till the fan off switch operates? 

Re: New Thermostat for furnces.

Reply #14 –
Quote from: Caflashbob –
The design of a propane furnace for safety mandates the temp swings.

Disabling some of the safety protocols may less the swings.  You would be foolish to do so.

On startup the fan comes on first to make sure that any propane buildup is disbursed, then the flame heats the unit.

Upon reaching the thermostats high level setting the flame cuts off but the fan stays on till the box cools down a bit.

As was noted a smaller capacity unit running more often might mitigate the swings to some degree but would lose the serious winter use btu output needed.

External venting of the first cold air until the flow warms up and  the flow after the high temp cutoff would seem to be the only solution.

Does not exist.  Would have to be engineered.

A diverter valve would seem safe as it would not disable any safety systems just the outflow to either inside or outside?

As long as the bays are vented so to not build air pressure the diverted flow could go there?

Or maybe just the initial cold air output?  Or switchable between hot or cold diversion.

Furnace kicks off diverter open till the fan off switch operates? 
Or mount a separate motor to operate draft inducer fan, instead of using one motor to run both the house fan and the draft inducer fan. Once heat exchanger is at an acceptable temperature, then turn on furnace "house" fan. My front furnace fan is too weak to move enough air, so efficiency is severly reduced, discharge air temperatures way too high 190+, so it runs until the high limit safety switch shuts it down. Suburbans new SHD series is a two stage furnace, but only available in 40 mbh size so it really does no good.

Re: New Thermostat for furnces.

Reply #16 –
Had a moment to pull off the cover.  I'm going to assume that the part indicated is the adjuster?
It appears that if I move the lever, I'm going to break any existing electrical circuit.

Re: New Thermostat for furnces.

Reply #17 –
Quote from: Moby –
I'm going to assume that the part indicated is the adjuster?
Yes, that is the "anticipator" adjustment lever.  Obviously a different design than the ones in my coach.  There should be some sort of numerical scale next to the sliding lever.  If it is set on 1.0 then it is on the factory default.  You could try moving it a bit, and see if it makes any difference in heater operation.  Or just buy a digital thermostat and try that.

Your choice.

Re: New Thermostat for furnces.

Reply #19 –
I've been reading a bit about thermostats and the "anticipator" function.  As I understand it, the anticipator is actually a small resistance heating element.  I copied the pertinent page out of the Atwood manual linked in a earlier post, and will add it below.  In the diagram, I have marked with arrows the ends of the very thin little heating wire.  In the photos of my thermostat and Moby's thermostat, you can see the wire very clearly.  In both cases, it runs behind the bimetal coil.  On mine, it is straight.  On Moby's, it is in 2 sections so it is kinda curved.

The purpose of the anticipator (as I understand it) is to "fool" the bimetal coil into thinking the air temp is hotter than it really is.  The bimetal coil normally reacts to the air temp inside the thermostat housing, which should be roughly the same as the nearby air temp in the coach.  When the anticipator lever is set on 1.0 the resistance wire is not active (it is bypassed), and therefore the thermostat waits to sense the actual air temp before it decides to cycle on and off.  If you move the anticipator lever off 1.0 and set it on a lower number on the scale, then the resistance wire is placed into the power circuit.  As power flows through the thermostat circuit, the resistance wire heats up.  This additional heat is sensed by the bimetal coil, causing the thermostat to cycle on and off more frequently.  The lower you set the anticipator lever, the shorter the resistance wire.  When carrying the same amperage, a "short" wire heats up more quickly than a "long" wire.  So, the lower you adjust the anticipator lever, the more frequently the furnace cycles on and off.

If you shorten the resistance wire TOO much, then the amperage flowing through it can heat it up enough to melt it.  That is why the technician in Stump's video (above) is measuring the amp flow in the power circuit to determine the proper anticipator setting.  The .pdf page from the Atwood manual states that .48 is the lowest safe setting for the anticipator, because that equates to the amp draw of the gas valve.  The manual further states that optimum performance occurs when the furnace cycles on and off 5-6 times per hour.

SO, all you are really doing, by adjusting the anticipator, is forcing the furnace to cycle on and off more frequently.  You are trading more fan on-off noise for smaller temperature swings between cycle times.  With a simple 2-wire switch (the thermostat) that can only do one thing (turn furnace on or off), this is really the only parameter that can be adjusted.  As discussed above, more sophisticated heater units would employ a multi-speed fan running all the time (but at variable speeds) plus a propane burner that would cycle on and off as required to maintain a constant air temp.