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Old 12-30-2011, 12:27 AM   #16
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Originally Posted by biggles View Post
might want to relieve that discharge air with a duct looping back to the return right off the discharge with a damper if this is a seasonal happening
Is this an advisable setup?

Originally Posted by how View Post
Age/brand & model of the furnace would help. The indentification plate inside your furnace will list the recommended temp rise for the heat exchanger of your furnace. That is the difference in temp of the air entering the furnace compared to it's temp when leaving the furnace.
By placing a thermometer into a small hole in the plenum beyond the line of sight of the heat exchanger and subtracting that temp reading from the room temp, you will see if you are within the recommended furnace temp and if adjustments will need to be made. (like how much dampering of your ducts is possible)

We don't know here if your installed ducting was over or under sized for the furnace. We also don't know your real gas output or fan speed but the measured temp rise compared to the furnace listed temp rise will give us the best info to work with..
Tell me if I have this right.
The Danger is that if I dampen to much the change in temperature is two extreme, i.e. the air leaving the furnace is to hot compared to the intake?
So, I should use the thermometer and the damper to find an airflow that makes the change in temperature agree with the printed recommended furnace temperature.

And practically since I live in a climate where the ambient temp. in the house rarely goes below 50, the change in temperature will always be rather small. That means I can get away with a more restricted airflow than a furnace system in a colder climate?


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Old 12-30-2011, 01:28 AM   #17
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We can probably assume that you should not dampen down your heating outlets and certainly not your return but the real proof of all of that will be the temp measurement of your temp rise across the heat exchanger. If your measured temp rise goes above the recommended temp rise you'll have a proportionately faster wearing rate for parts & the exchanger and lose fuel efficiency.

This test should be done when your furnace has been firing for a while. This means that the return air temp will be close to the average temp your thermostat is usually calling for. Let's say a 70 F average so a hot climate or a cold one doesn't matter. (I actually fudged the test to make it simpler but it will still tell you what you need to know.)
The temp that this air has been raised as it exits through plenum from the heat exchanger is the variable that is determined by gas input and the moving volume of that air. The rating plate on your furnace that lists the temp rise is the manufacture's engineering recomendations for safety, efficiency and furnace longevity. The temp you measure can be adjusted to fall within the manufactures temp rise by adjusting the gas input (leave this to a pro), the choice of fan speed and anything that affects the available volume of air through the furnace. ( most commonly duct size, dampers, type and cleanliness of the air filter, coil and fan cage.)

Measure temp first, subtract your average home temp and see how it compares to the rating plate. Tell us what it is.
Could be that all you ever needed was open heat registers and some more water for a thirsty tree.


Last edited by how; 12-30-2011 at 01:59 AM.
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