CFM / ECM Motor Question

[tomwk]

My current furnace/blower setup has a 1600 maximum cfm ability in cooling mode according to lennox.

I have finally narrowed down my new furnace/ac search to 2 contractors. One recommends lennox, the other ruud. One is a Lennox G61V and the other is a Ruud UGRM. Both have ecm motors and list a 2000 cfm ability in cooling mode.

I feel that my current system could use a bump in airflow so is it safe to say that the extra 400 cfms will make a substantial difference?

The contractors will install a return in each bedroom - currently there are none - and I will have the contractor increase a few ducts from 4" to 6" (the rest of the ducts should be sized correctly). The lennox contractor is really stressing to me that the ecm motor at 2000 cfms will make an enormous difference provided returns are added. Just want some other opinions...

[Ed Janowiak]

What size AC do you have , and how do you know that your current fan is moving 1600 CFM?

If you live in an arid climate increasing the airflow that much can make the system work better, but if your live in an area where moisture removal is part of "air conditioning" then I can't think of how moving more air will be better.

[beenthere]

Yes it will. Very well could make your system sound like a jet when it runs.

As above, what size is your A/C(in tons, or BTUs)?

Your current up to 1600 CFM blower, may only deliver 1200CFM because of duct work issues. Changing a couple runs from 4 to 6" is not going to make your duct system capable of handling 2000 CFM.
Meaning that a 2000CFM VS blower trying to move 2000 CFM would be a mistake.
Have your contractor verify how many CFM it is actually moving.

[udarrell]

Call a PRO :callpro: & have them run a Manual D for 2000-CFM.

On a residential system getting 2000-cfm airflow delivered through the evaporator coil & through the supply & return at the proper FPM Velocities is a monumental task.

I would also have a Manual J load calc performed so you know how to size the SA ducts & diffusers & RA duct & filter areas for proper air delivery to each room.

A manual D will also tell you whether the air handler has the capacity to accomplish the airflow task. Again, Call a Pro > :callpro: - Darrell

[tomwk]

My current ac unit is a 19 year old ruud - all of the external labels are unreadable so I cannot tell the size. I am going to have to open it up. My current lennox furnace and coil are 4 years old.

One contractor recommended 4 tons of ac, and the other 3.5 tons. Neither one performed a load calculation, but they stated that they are familiar with my size and style house - not very reassuring. 1 other contractor did perform a load calc but never gave me the results - just an estimate without any explanation. Its as if he doesn't want me to see the results and then choose another contractor.

I had called lennox in regards to my current furnace (Model G50UH-48C-110)and they advised me that it is a 110,000 btu's and the coil (Model C33-50/60 C-2F1-1) can handle up to 4 tons of cooling. Lennox told me that the current blower has 3 speeds - low for fan, heating mode and cooling mode. Lennox stated that heating mode blower moves about 1100 cfms and cooling mode about 1600 cfms. Does blower speed vary according to ac tonnage?

I live in NJ which can get hot, but obviously it is not a desert climate. Heating in my house is never really a problem, but cooling is in the upstairs bedrooms - about a 4-5 degree difference on very hot days.

Since the current ac is 19 years old I am looking to replace it before it breaks. But since everything is going to the new 410 refrigerant I would have to change not only the ac but also the indoor coil. And without a variable speed furnace I won't qualify for any rebates. So I figure change it all and improve the system performance.

The contractor that is pushing lennox has steered me away from zoning systems and insists that a more powerful ecm blower and adding returns to the bedrooms will almost completely fix my problem. He said it is not worth the cost of zoning because my upstairs temperature swings are not that horrible. I do doubt that I need a 4 ton ac though - the house is a split level about 2000 sq ft. I am sure I don't need more than 3.5 tons. I am going to have to open the old ruud unit to find a label just to see what I am currently working with.

I am almost ready to buy a computer program that will perform a load calculation for me.

[beenthere]

Well. I would be leary of using the contractor that want to use a blower rated for 2000CFM as a cure for your uneven temps.

A zoning system may or may not be the best solution.
Can't tell from here.

Many contractors won't give their Manual J results to you. So you don't show them to another contractor. And that contractor then doesn't need to include the cost of doing a load calc. And beats them up on price.

[kls-ccc]

I would be careful with the guy the wants to change heat run size just ot move more air. If a room by room load calc is done and it shows that the runs need to be increased then go for it, but just to move more air can cause a greater imbalance than you have now. I reworked a system that had 6" run to every room in the house, even the 4'x7' bathroom and walk-in closet. Too much air going to these room caused the great room to be short.

[tedkidd]

Spend some money diagnosing and sizing your home, or expect unsatisfactory results for the life of your new investment.

Your furnace is a heat replacement device. Reducing the amount of heat you need to replace and downsizing equipment can often make undersized duct work properly sized... and save a tremendous amount of $ on energy bills.

[superheatbob]

Well the first problem you have is one system for two floors.... you need two systems or zoning as the BEST cure. But thet may not be practical for your budget though.

Do change your thinking...MORE CFM is NOT better! too much CFM often blows water off the coil and into your duct system... let's all say it together M-O-L-D !!!

You can get a variable speed ECM motor in may brands, most will adapt to the static pressure in your system with minor adjustment - and deliver the CORRECT airflow for the tonnage in your system. Your current PSC motor probably does not do that now. Many of the brands use the GE Think Tank motor for this, there are several versions of this motor.

With the variable speed motor, you can also set it to constant fan and run at 50% of high speed. This will slowly move all the air in the rooms and your temperature differnces will BE GONE. Seen it happen many times this way.

You can run that motor 24/7 and your electric bill will go DOWN....that's right ....DOWN. The motor uses almost no power in this mode.

This moves the hot air out of the rooms when the AC is not running, no stagnant air ever. it will be a different world when you run it.

[Cold Feet]

With the variable speed motor, you can also set it to constant fan and run at 50% of high speed. This will slowly move all the air in the rooms and your temperature differnces will BE GONE. Seen it happen many times this way.

No cigar.

Circulating air will not solve inter-room temperature differences unless the ducting was nearly adequate to begin with. Circulation won't help with major temperature differences caused by bad duct design.

[tedkidd]

As you can see, without having the actual numbers you'll get a lot of opinions that don't tell you much about YOUR situation.

Get a room by room heat loss, and measure airflow from each register. Then you can match load to ability to carry. You'll know the problems and have a starting place for solving problems.

Spend a few 100 to KNOW what's going on and solutions will not be guesses. Or you can guess, throw money at possible solutions, and hope.

[udarrell]

As you can see, without having the actual numbers you'll get a lot of opinions that don't tell you much about YOUR situation.

Get a room by room heat loss, and measure airflow from each register. Then you can match load to ability to carry. You'll know the problems and have a starting place for solving problems.

Spend a few 100 to KNOW what's going on and solutions will not be guesses. Or you can guess, throw money at possible solutions, and hope.

You said a mouthful & hit the nail on the head!
Best Practices, is the only way, & also the most cost-effective way, to do it... - Darrell

[superheatbob]

No cigar.

Circulating air will not solve inter-room temperature differences unless the ducting was nearly adequate to begin with. Circulation won't help with major temperature differences caused by bad duct design.

Good point I agree !

I missed the part where he said the ducts were too small.

But if the ducts are right, slow speed constant air circulation can be a wonderful thing in many - but not all - cases