# Thread: 2-stage 80 vs. 1-stage 90

1. Professional Member
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Originally Posted by weber
I see it more often then you would think!

I have calculated 90% running at 67%, with no problems other then gas consumption. Thats what a combustion analyzer is for. And the customer loves you next month when they get thier bill!

But yeah, for this scenarios we are assuming that everything is operating properly.

I think its understood what you were implying here.

2. Professional Member
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Originally Posted by weber
I see it more often then you would think!

I have calculated 90% running at 67%, with no problems other then gas consumption. Thats what a combustion analyzer is for. And the customer loves you next month when they get thier bill!

But yeah, for this scenarios we are assuming that everything is operating properly.
A derated furnace can still have efficient combustion. The furnace could still be technically utilizing 90% of the fuel for heating the space. I think in the case of a derated furnace, gas pressure readings and a meter test would be more useful tools to employ.

3. Originally Posted by Frostie
I think in the case of a derated furnace, gas pressure readings and a meter test would be more useful tools to employ.
You still wouldn't know the efficiency it was at. All you would know is the input rate. You wouldn't know how high the excess air was. So you wouldn't know how much you were cooling the flame.

4. Originally Posted by Frostie
A derated furnace can still have efficient combustion. The furnace could still be technically utilizing 90% of the fuel for heating the space. I think in the case of a derated furnace, gas pressure readings and a meter test would be more useful tools to employ.
You're right that it can still have somewhat efficient combustion....however how would you know that if you weren't using a combustion analyzer??? Most furnaces will tell you to set the gas pressure at 3.5 IWC ... assuming that it is either a single stage or in high fire ... however, I have rarely seen then left at 3.5 when they are set up properly with an analyzer. Pressure readings can get you kinda close....but never truly dialed in.

5. Back to the original post - I think the OP answered his own question. Throw away 20 cents on each dollar of gas, or 10 cents on each dollar of gas. Which is more efficient?
Get a 90%(or better) furnace, with variable speed blower, properly installed, and forget about the 80% piece of junk.

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