Replacing old C/Mach air/furnace stat with digital. Fan speed issue.

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Dougie Brown

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I'm about to replace the original Coleman Mach air/furnace stat with a Honeywell programmable digital stat and have familiarized myself with the possibility of adding a hi/lo switch to control the air fan speed (the stat doesn't come with that capability).

What's the consensus regarding a low or high fan speed when the air is operating?  I've read a few opinions - some say a low fan speed is better as it produces cooler air, whereas others think a high speed operation is more efficient.  My instinct tells me that the "low" argument is the more feasible, but I'd like some knowledge on it please.  Thanks. :)

Dougie.
 
Dougie:

I can't say there is a lot of science to this but it "feels" cooler when we run the Coleman on high fan to cool the RV down. We also like the low fan speed for quiet operation.

I'll be interested in how it works out as I would like to replace our t-stat with one with a remote sensor.
 
Neither our furnace nor a/c runs on high very often. Ours have automatic fan speed control and it doesn't shift to high unless the temperature differential is large, about 5 degrees.

High speed gives the feeling that you are cooling or heating faster, but it's mostly an illusion anyway. The a/c btus are the same regardless of speed, so the high fan just blows it around more. A furnace with a two-speed fan will have two btu settings to match the fan speeds, but not many RVs have that model of furnace.
 
For the air conditioner the high/low is nothing more than a SPDT switch, it selects between two different wires that run up to the air conditioner.

For the furnace it won't work, the furnace must always be on HIGH, thus there is no provision for it.

NOTE: that if the original A/C does not have a provision for high/low, then the wire you need does not exist and the motor most (blower motor) most likely won't know what to do with it either so there is no point in attempting. 

The motor much have high/low ability for the switch to work A new dual speed motor would set you back 200-300 bucks plus the added wiring.
 
Yep, I know about the SPDT switching which I do have the option to fit. The question for me though is more one of hi/lo efficiency on the air side. I only have a single speed Suburban furnace.

The auto setting on the new stat will provide the Auto setting.  Thanks for the science, Gary re. not making any difference to the actual cooling. Interesting.

Dougie.
 
The ONLY question is the air conditioner (heatpump) side, the fan switch on the Coleman/Mach thermostat only controls the AC fans and has nothing to do with the furnace fan. The furnace fan is solely controlled (automagically) by the furnace itself in most cases.

I would think that most would use that switch by putting the Auto switch to Manual and then use the HI/Low switch to simply blow air around independently of the air conditioner being on or off.  But... that's me!!! :)


For the cost of a miniature SPDT switch why not go for it???
 
This may not be germane to the original post...

When i switched out my t'stats to Hunter 2 stage stats I used the Y-2 connection for the high speed fan...
My A/C starts on low fan on a call for cooling and if it calls for 2nd stage [Y-2] than the fan goes to high speed.

As was previously posted the furnace has 2-wires and goes to RH & W [which is the heating circuit and is isolated from the cooling circuit...unlike some home systems that use the same fan for both]

This has been working just fine... it may not be correct for other users.

I believe the BTU transfer is increased by the higher fan speed and more air flow across the evaporator coil. And yes it is also noisier [that doesn't look like I spelled it correctly ..but spell check is happy ans so am I] ..

George

 
Hi
Heat transfer to air is a very inefficient process as compared, for example, to steam or water. The higher speed will defintely cool better (I've tested this) at the cost of higher noise levels. I'd bet that BTU ratings, if even based on an actual test, are established at the maximum fan speed and probably with no filter or other obstruction.

Ernie
 

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