furnace filter - in vs out

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tnx for all the comments.
My main line of thinking was - A - air pressure on fan motor B - air flow capacity & delivery to rooms C - runtime to get room temps up to thermostat turnoff
Had not considered what little bunnies might be gathering on the A-Coil.
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It's designed for that.

See my other response. Your thinking is absolutely opposite of reality.
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On Tue, 16 Feb 2010 18:52:38 -0600, ps56k wrote:

wear a sweater
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All of the forced air furnaces that I have seen had the filter on the supply side of the furnace. The fan sucks air through the filter. It does not blow through the filter. Therefore, removing the filter wouldn't change the back pressure on the fan. It would increase flow because there would be no filter losses. I wouldn't do it though because dust and dirt would collect on the heat exchanger and reduce both airflow and heat transfer efficiency.
If you need more airflow you could try playing with the fan pulley sizes to speed up the fan.
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fan motor - didn't know if blocking off more of the return air flow into the squirrel cage fan housing would cause the motor to work harder or not...
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The furnace is designed to operate with a filter in the return air. It may restrict the airflow slightly, as does the ductwork, but all of that is taken into consideration in the design. That is one reason most furnaces have mulitspeed blower motors-- to allow optimum airflow with vith various amounts of restriction to that airflow. Larry
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Lp1331 1p1331 wrote:

The air handlers are designed for a certain amount of back pressure from the ductwork. If there is not enough back pressure and the multi speed motor is set at one of the upper speeds, the airflow will be too high and will result in condensation being blown off the A/C coil instead of flowing down into the condensate pan.
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And, because different speeds and HP needed for heating versus cooling.
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wrote:

Of course the fan motor is going to need more power the more restrictive the filter element is. It doesn't matter if the restriction is after the fan or before it, whatever you do that restricts air flow will increase the work the fan must do to move the air. And that equates to slightly higher operating cost.
However, I don't see why in your original question you made this sound like a seasonal issue. With any reasonable filter that is not clogged up, the house should heat or cool pretty much the same as it would without the filter. Let's look at heating, let's say with a filter the air leaving the furnace gains X degrees. Without the filter, because there is more air flow the air will be slightly cooler, let's say X-1 degrees, but there will be MORE air flowing, so the amount of heat going into your house is about the same. I say about the same, because with a higher temp delta, the furnace becomes slightly less efficient, with a little bit more energy going out the exhaust. But unless you have a clogged filter, it should not be enough that it is even remotely connected to how long it takes to raise your house temp 5 degrees.
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wrote:

Block off the suction on your vacuum. What happens?
The motor runs FASTER.
you do the math
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Load decreases, amperage drops.
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On Feb 17, 10:53am, snipped-for-privacy@rochester.rr.com wrote:

I would agree that at some point, with very severe restriction, that indeed happens. But I would think based on logic and science that this occurs do to a phenomena like cavitation. That is to say, at some point, the fan is no longer acting like a fan or moving any air at all, just spinning around. But this is a special condition and not a linear one.
Let's take a shop vacuum. Here's how I would expect power would change versus air flow. With the hose wide open, the power would be at minimum. As I start to slowly block off the hose opening in increments, the power will INCREASE. At some point, with the hose almost blocked or totally blocked, then the power may decrease again.
If it doesn't draw more power when air flow is choked off, why do circuit breakers trip on a circuit that shop vacs are on when they have a heavy load? For example, I've had my shop vac on a 15 amp circuit that had other various loads, so there was just enough left for the shop vac. I could let it sit running with the hose wide open all day and it won't trip. But if I hook up the wand and start vacuuming the floor, I can hear the motor starting to slow down, strain under load, and eventually the circuit breaker will trip.
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A filter on the supply side would not protect the furnace. I've never seen a filter on the supply side.
A fan sucking air in through the filter is on the return side.
The dirty filter would starve the fan for air, and reduce the air flow. That creates some other problems.
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On Wed, 17 Feb 2010 10:33:32 -0500, "Stormin Mormon"

"supplies" the air is the same as your "return side".
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Only in the southern hemisphere. (ha-ha)
Between the furnace and the conditioned area is the duct that supplies the conditioned area with air. The low pressure duct that goes toward the furnace is the return air (returning from the conditioned area).
HVAC terminolgy is that the supply side goes from the unit to the conditioned area. When discussing furnace and AC, it's wise to use the terms properly.
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ps56k wrote:

I use mounted EAC and coarse metal mesh pre-filter in front of it. Works fine. I have two sets of elements. One is in use. One is cleaned and sits stand-by.
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I've seen both heat only ,and heat / AC systems really badly clogged by dust. So, the missing filter may provide better heat today. But nwarly guarantees expensive repairs later. I'd leave the filter in. Might try a different brand of filter, though.
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More air flow is not better.
What you will notice is that instead of a steady flow of toasty warm air, you will be feeling a COLD DRAFT the entire time the furnace is running.
What happens is the air moves across the heat exchanger too quickly, and doesn't get warmed up.
The same thing happens if you open up all the dampers in the hot air ducts. Instead of warmer rooms, you get a cold draft because the airflow is too high.
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In typed:

Just look at all the "experts" who climbed out of the walls here.
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Newsgroups are great places to get assistance.
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