Single stage vs 2 stage furnace, what's the difference


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Old 11-06-13, 04:07 PM
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Single stage vs 2 stage furnace, what's the difference

hope someone can help, recently had my Goodman furnace stop working, heat exchanger cracked. Rather than replacing I am going to go with a new unit. I am being told that we are better off with a single stage vs 2 stage unit and the reason why is because the temp in the return duct was too hot and the theory was that a 2 stage may make it worse. any thoughts would be helpful.
 
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Old 11-06-13, 04:23 PM
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Pure unadulterated hogwash! Even if the problem WAS too high a temperature in the return air (it wasn't) the two-stage would be an improvement.

I have to go out for a bit but I will write more this evening.
 
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Old 11-06-13, 04:31 PM
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The max temp rise was 65 deg, actual was 78 deg
 
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Old 11-06-13, 04:42 PM
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That indicates too low of an airflow. It could be from inadequate return ducting or too low of a blower speed or too restrictive of an air filter or too high of gas pressure causing overfiring of the burners but installing a two-stage furnace would more than likely alleviate many of these conditions rather than make them worse.

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Old 11-06-13, 04:50 PM
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I have been through 3 induced draft blowers, 3 rollout sensors, and now the cracked heat exchanger. There was an original problem that we discovered the contractors placed a piece of cardboard in one of the return vents that prevented all the upstairs air from being returned from the upstairs return duct. There was a theory that I did not have enough resisters in the house or they may need to be larger. This system was only 8 yrs old and maintained yearly, my last contractor missed some key issues such as soot which should not occur in the propane furnace, so. I want to be sure what I eventually go with is correct.
 
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Old 11-06-13, 06:34 PM
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I have to agree with Furd --- being told a 2 stage furnace would cause more problems is hogwash.
The HVAC contractor should be checking the ductwork airflow to see if it meets the CFM requirements for both supply and return.
 
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Old 11-06-13, 07:10 PM
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how many return registers should there be in a 2300 sq ft home?
 
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Old 11-06-13, 07:30 PM
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There should be as much return as supplies.
Here in Canada we have returns in every room there are supplies.
 
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Old 11-06-13, 10:27 PM
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What size is the existing furnace? (check the model number & post it)

U might be able to downsize rather than make changes to the ductwork.

Is the ductwork sheetmetal or flex? Do you know the sizes?

Static pressure measurements can be done to see if there's a problem with the ductwork.
 
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Old 11-06-13, 11:35 PM
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With all the repairs that have been needed AND the magic word, propane, I am wondering if this furnace was ever properly set up for propane or if a natural gas furnace was installed. A natural gas furnace connected to a propane system would be horribly overfired at THAT would definitely cause problems with the induced draft fan, the roll-out switch and the heat exchanger. It would also cause the house to heat up really fast. If indeed this is the cause then your first order of business is to find a new heating contractor.

It is also possible that both the supply and return ductwork is way too small for the output rating of this furnace. What IS the BTU rating, both the input rating and the net BTU rating? Do you as a regular course of action close off registers in unused rooms? How often do you change (or clean) the return air filter? What size and type of filter are you using?

Now here is why a two-stage furnace would actually be better. Under "normal" conditions a two-stage furnace only fires on the first stage. That first stage is generally around 60% of the maximum firing rate and along with the decreased gas flow there is a corresponding reduction in the exhaust stack temperature and also the output temperature to the room ducts. A properly installed Two-stage furnace (including the two-stage room thermostat) will ONLY go to the second stage under two conditions, the first is when the room air temperature continues to fall (with the furnace running on the first stage) and it drops so low as to energize the second stage switch in the thermostat OR if someone manually turns up the thermostat more than the second-stage differential. As an example: My thermostat is set up with a 3 degree differential between the first and second stages. My normal daytime setting is 68 degrees. That means that the inside temperature would have to drop below 65 degrees before the furnace went to the second stage. Even in 25 degree weather (pretty cold in my area) the furnace will cycle on and off maintaining the inside temperature at 68 (or 70) when set to that temperature. I normally set the thermostat down to 62 or 63 when I go to bed and the ONLY time the furnace will go to the second stage is IF I turn the thermostat more than 3 degrees higher than the inside temperature the next morning. If the inside temperature dropped to 65 and I then set it up to 68 the second stage will NOT fire.
 
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Old 11-07-13, 04:52 AM
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Those are good ideas, here is the information I have for the specifics of the unit. Goodman model number gms90904cxa. max outlet 170 deg, actual 150 deg, max temp rise 65 actual 78 deg. Outgoing duct from furnace measures 19.25 inches wide 20 inches deep, return cold air to furnace 10 inches wide 20 inches deep. Input BTU 92,000 output BTU 86,000. I have a total of 5 return air ducts about 6 by 12 each, the heat registers are never left closed, and are aprox 3 by 6 inches. The furnace was never strong enough to heat the upstairs.
 
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Old 11-07-13, 12:30 PM
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The furnace was never strong enough to heat the upstairs.
This tells me the ductwork is inadequate.
 
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Old 11-07-13, 02:43 PM
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Would a 2 stage unit be a better option, is the duct work likely the cause of my first failure
 
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Old 11-07-13, 06:42 PM
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Input BTU 92,000 output BTU 86,000.
A pretty large furnace for 2300 sq ft unless you've got minimal insulation.

You posted the size of the plenum and return drop - what about the trunk and branch lines?

How many vents are there? What are the sizes? (duct size leading to vent)

Are there dampers in the branch runs that you can adjust?

Is the ductwork in a finished basement?

Are the branch lines metal or flex?

-----------------
Did the contractor check the secondary heat exchanger for dirt/dust buildup? That can reduce airflow, as can a 1" pleated filter.

-------------
What you need to do is find a contractor who can actually measure static pressure* and diagnose duct problems.

*A critical piece of information to see what's going on
 

Last edited by user 10; 11-07-13 at 09:19 PM.
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Old 11-07-13, 06:53 PM
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"Would a 2 stage unit be a better option, is the duct work likely the cause of my first failure."

Low airflow or overfired for sure. A two stage furnace is nice to have but isn't the fix.

Overfiring is caused by using the wrong burner orifices or having the gas pressure too high.

Many things can cause low airflow -> blower speed set wrong, undersized ducts (or a bottleneck in the duct system), undersized filter, dirty evap coil if you have a/c, etc.

The temperature rise should be 50F give or take 10 on a modern furnace - 78f is through the roof.
 
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Old 11-08-13, 06:35 PM
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The home is super insulated, an energy star home. Initially when the furnace would fire, it would create a vacuum that would close doors. There are about 20 ducts and I am guessing about 6 inches in diameter, vents are 3 by 6 inchees. 5 return ducts 6 x12 inches.
 
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Old 11-08-13, 06:41 PM
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^That doesn't sound too bad.

I'm beginning to think that Furd is right and the furnace was severely over-fired from the start. (you mentioned soot)

If the design temp in your area isn't extremely low (less than 0F =/- 10F on coldest night of the year), you should definitely downsize to 75k btu or less. Heck, if it's as well insulated as you say, 60k may do the job with capacity to spare.

With a furnace that needs to move less air, you'll be able to partially close some vents on the main floor to get some more heat upstairs. (all exposed duct joints should be sealed with foil tape or mastec as well)
 
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Old 11-08-13, 06:45 PM
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Trouble is always got a weak stream of hot air upstairs about. 10 deg colder than downstairs...
 
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Old 11-08-13, 06:52 PM
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Your system probably needs some balancing and sealing. (if u have an unfinished basement, you're in luck)

Naturally vents closer to the furnace are going to get more airflow - provided that it's moving enough air, you might be able to damper them down a bit with a smaller furnace. (with your current furnace every damper may need to be left wide open to get proper airflow)

One thing to look for in a new furnace is a variable speed blower motor, which automatically adjusts itself to maintain proper airflow as the pressure in the duct changes. (for example from partially closing vents in rooms that are getting too much heat)
 
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Old 11-08-13, 06:55 PM
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Thinking American Standard, think I a done with goodman and no basement finished
 
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Old 11-08-13, 06:59 PM
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^no basement finished

Seal all the takoffs after the furnace is changed - they tend to be very leaky...

 
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Old 11-14-13, 03:55 PM
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I did have another company here and it was recommended to add another cold air return and would cut into the floor and use the joist space to add return cold air, as well as add some transfer grilles over some doors upstairs.
Is it odd to add a cold air return after construction has been completed, turns out that the first goodman unit was not vented properly and that is likely one underlying cause of early failure
 
 

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