One Filter or Two

[cpw]

My AC tech recommended removing the filter in the air handler, because I have one on my return and an extra filter restricts air flow. Obviously the guy who installed it thought there should be filters both places. What do you think?

[gear junkie]

If your system is designed for the pressure drop of 2 filters then it should be no problem. I've seen evaporators freeze up because people install the hepa filters which restrict flow. For you, I would go with what you been doing all this time. If you've been using 2 filters, then keep the same and vice versa.

[Old Grunter]

G J is correct that two filters, one after the other may well cause too low of a total air flow and lead to freezing of the evaporator coil(s). Also, if you have a heat exchanger inside the air handler it was setup for a given air flow. Restrict the air and it can overheat.

I like to have one really good air filter with a high air flow capacity just before the inlet of the blower. Depending on the ducting setup you may need more than one filter but not so they are acting in two stages.

For what it's worth were the inside of the air handler, the coils and the blower wheel(s) nice and clean?

If in the past fiberglass (economy type) filters have been used, it really would be wise to have things cleaned by a true professional. Hacks will do far more damage than good.

[plumberscrack]

I agree keep using two

Some blower motors will deliver the required CFM no matter the flow restriction in the ductwork.

[HVAC HAWK]

I agree keep using two

Some blower motors will deliver the required CFM no matter the flow restriction in the ductwork.

just check the amps to make sure your not over amping the motor


i have a 1" pleated filter return and i check it about every 3 weeks ,and i still have some fine dirt coming out of the supply register ,im thinking about getting an electric air filter

[gear junkie]

For what it's worth were the inside of the air handler, the coils and the blower wheel(s) nice and clean?


I've never seen clean air handler interior. Cut yourself with a blower blade and that will get infected faster than if you cut yourself and dunked your arm in a toilet. Nasty things those air handlers.

I like the electronic air cleaners. Trane just had a recall of theirs due to possibility of fire fyi.

CPW-your poll is skewed a bit. 2 places to have a filter; on the air handler itself or in a return grill that's on a wall. But both options still have a filter before the evaporator so anyone that says no is thrown off. Forgot to mention, location of filter greatly depend on the air handler's location. Most air handler's here are in the attic so a return grill is put in. If the air handler is in the garage then the filter is in the air handler. In your case, since the air handler is in the attic, go with the return grill.

[Old Grunter]

I've never seen clean air handler interior.

I fully agree unless people have them professionally cleaned and also have great air filters installed. I love my SpaceGuard 2200. Simple yet pretty effective.

HVAC HAWK (and others) - Please take a look at these. I find mine to work very well and it's simple. Changing the filter element requires that you be careful and take your time, but it's pretty easy once you get the hang of doing it.
http://www.aprilaire.com/index.php?znfAction=ProductDetails&category=7&item=2200

For people that smoke this won't totally remove enough. You might use one (largest size) in front of an electronic air cleaner as one heck of a pre-filter.

[Ruudacguy]

If your filter grille is a long way from your air handler, having another filter at the AH would catch anything that comes in through any holes in your return ductwork. I'd keep both, and change both as needed.

Andy

[NHMaster3015]

I have a question for ya. If you restrict either the inlet or the outlet of a squirrell cage blower, what happens to the amperage at the motor

1 - It will increase

2 - It will decrease

3 - It will not change

4 - None of the above

Think carefully before you answer

[Ruudacguy]

Depends on how much you restrict it. :nyaa-nyaa4:

Andy

[cpw]

CPW-your poll is skewed a bit. 2 places to have a filter; on the air handler itself or in a return grill that's on a wall. But both options still have a filter before the evaporator so anyone that says no is thrown off. Forgot to mention, location of filter greatly depend on the air handler's location. Most air handler's here are in the attic so a return grill is put in. If the air handler is in the garage then the filter is in the air handler. In your case, since the air handler is in the attic, go with the return grill.

I've got a return in the ceiling with a 20x30x1 filter (~20-ft of 16" duct between the coil and grill); and a 20x20x1 filter on the attic air handler a few inches away from the coil. I thought the consensus seems to be to put back a filter in the air handler [because it worked that way before], but I think in this last one you are saying to just use the return grill.

[Old Grunter]

I have a question for ya. If you restrict either the inlet or the outlet of a squirrell cage blower, what happens to the amperage at the motor

1 - It will increase

2 - It will decrease

3 - It will not change

4 - None of the above

Think carefully before you answer

My answer is 2

Try measuring the motor current and only open and then restrict the inlet. Then leave the inlet open but block off the discharge. Watch out for overloading/overheating. There's no need to burn up windings.

[cpw]

1 - It will increase
I know it can't be #4, so if this were the SATs answering makes sense. My guess is #1, because like a saw motor when you add load to it you're going to draw more current. Or like your vacuum if you plug up the hose it will draw more current and overheat.

[gear junkie]

It's number 2. The blower has no air to move so it has less resistance so it doesn't work as hard, amps drops.

[gear junkie]

I've got a return in the ceiling with a 20x30x1 filter (~20-ft of 16" duct between the coil and grill); and a 20x20x1 filter on the attic air handler a few inches away from the coil. I thought the consensus seems to be to put back a filter in the air handler [because it worked that way before], but I think in this last one you are saying to just use the return grill.
Only because most people will not climb in an attic every month to change a filter. If you don't mind that, then go with both filters.

[spodelee]

CPW,

In this case, the return register filter is the preferred location because:

A) It is larger and will offer less pressure drop as the filter becomes dirty. This will translate into more consistent performance and improved energy efficiency.

B) By filtering at the return register, the whole return duct as well as the air handler will be filtered,. If you operate without the return register filter, the return duct will load with particulates.


Also, the most likely consequence of having a second air filter in the air handler is that you will sacrifice energy efficiency (1-3% if both filters are clean, more if they are dirty - IMHO)

I recommend removing the air handler filter and making certain the return duct(s) is sealed at both ends so that particulates can not enter the duct after the return register air filter.

It is also not a bad idea to apply weatherstripping inside your filter rack at the register so that a seal is created between the air filter and filter rack. This way you stop particulates from bypassing (going around) the air filter.

[NHMaster3015]

close the air right off

[Old Grunter]

Not to rub salt in wounds but the reason a home use vacuum cleaner motor burns up if you block the inlet is because it relies on the incoming air for cooling. This is what's known as a pass through motor-fan. The filtered (hopefully filtered) air blows through the motor to cool it. With a wet/dry or good industrial they use a bypass motor which has cooling air enter through a different inlet. In some cases there's a special outlet for the motor air.

PLPLPLPLPLPLPL :poke:

[cpw]

Not to rub salt in wounds but the reason a home use vacuum cleaner motor burns up if you block the inlet is because it relies on the incoming air for cooling. This is what's known as a pass through motor-fan. The filtered (hopefully filtered) air blows through the motor to cool it. With a wet/dry or good industrial they use a bypass motor which has cooling air enter through a different inlet. In some cases there's a special outlet for the motor air.

PLPLPLPLPLPLPL :poke:
No wound to rub salt in. It wouldn't have been as fun not to answer, since then you can't really be right or wrong unless you are willing to commit yourself to paper. It is cool to learn something that is counterintuitive.

[NHMaster3015]

And the WINNER IS GearJunkie. Congratulations.

Do you have any idea how much money I've made over the years with guys willing to bet on this one? Even electricians, who should know better.

I have another on on venturi tee's that I'll save for a later date.

[NHMaster3015]

We have a Saturday morning radio show up here called ask the experts. It's sponsored by a local mechanical company and this restricted air subject came up from a homeowner. Believe it or not, both " experts " think the amps on the motor increase. So I called in to explain that they do not and that in fact the rpm's increase and the amp's decrease when the load is restricted. They hung up on me. See how popular I am in just about any format:D

[Old Grunter]

I wish I had access to a good video camera with sound and a good editing program in case it's needed. I would like to setup my 0-10 AC Ammeter (analog panel mount) so it would measure current of my shop vac and also my furnace blower (but not at the same time) and actually show what happens with changing air flow to the current drawn in each case. With sound on you should be able to hear the blower run slightly faster with the air flow blocked and we all know about the sound changes with vacuum cleaners.

As for the air filters, be sure all ducting on the return side is nice and clean. Remove grills and vac the dust out. As for the actual ducts that may require some professional help. Can you see the blower wheel(s) without taking too much apart? Duct leading to the blower? Is it clean? I've seen where people change things only to suck a load of dust-dirt into the blower and blast it into their heat exchanger and then evaporator coils. Then you do have work cut out taking things apart and cleaning up the mess.

Now I really am getting too - :soapbox:

[Ruudacguy]

I agree about 95% NHMaster. I think there are a couple exceptions and I will try some experimentation with a couple blowers I have.

Andy

[gear junkie]

I agree about 95% NHMaster. I think there are a couple exceptions and I will try some experimentation with a couple blowers I have.

Andy
What kind of exceptions can you think of? Are you thinking of smart blowers?

[gear junkie]

Don't forget that a blower and a fan are 2 different things and their applications are not interchangeable. A fan moves air or just circulates, while a blower puts air under pressure in your duct systems.

[Ruudacguy]

The test subject was a 5 ton drive with a 3/4 hp motor. Brand new last year when it came out of a damaged furnace. I already had about 2/3 of the wheel side intake blocked off with a piece of metal so I checked the amp draw on high speed. Reading was 3.53.

I removed my 2/3 block off piece and was surprised at what I saw. The amp draw was exactly the same at 3.53. I really thought the amp draw was going to go down a little. They seem to run much more efficiently and move alot more air with a little restriction. I have even had motors get hot and shut off on thermal overload without some type of restriction. Which lead me to believe that with a little restriction the amp draw would drop slightly.

The third test was something we all knew. I blocked off 100% of the outlet and the amp draw dropped to 1.2.

Andy

[spodelee]

We have a Saturday morning radio show up here called ask the experts. It's sponsored by a local mechanical company and this restricted air subject came up from a homeowner. Believe it or not, both " experts " think the amps on the motor increase. So I called in to explain that they do not and that in fact the rpm's increase and the amp's decrease when the load is restricted. They hung up on me. See how popular I am in just about any format:D

I did something similar on a "mold" talk radio show once -- same result and I didn't even mention your name! Feel better now?

:joyful::joyful::joyful: