Blower speed and zoning question

[aw]

I have a multi-speed air handler on a two zone system. Is there an advantage to have the installer slow the air speed down when only one zone it called for?

[beenthere]

No.
It will cause more harm then good.
Your A/C still needs X CFM per ton.

[JG Nothing]

No.
It will cause more harm then good.
Your A/C still needs X CFM per ton.

When you say it will cause more harm then good, do you mean in terms of cooling efficiency or mechanical problems? If there is a single-stage compressor, wouldn't slowing down the airflow extend the run time to the zone? The single-stage compressor is sized for the entire house, so it would be oversized at full blow for one zone. It would seem that running the air handler at a lower speed would lower the amount of heat being removed, thus making the unit run longer.
This, of course, is all speculation on my part. I am just a customer trying to better understand how heat pumps and home climate control works. Thanks.
- Johnny

[jstjohnz]

If the ductwork is sized to permit full airflow to a single zone then I would say don't slow it down. If the ductwork can't handle it then you may have to slow it down to keep from having a too-high static pressure in the ducts, which is hard on the blower and noisy.

The problem with slowing down the airflow is that you will get an excessive temperature drop across the coil. This could cause the coil to freeze up among other things.

You need to find a happy medium where the reduced one-zone airflow is low enough to keep static pressure reasonable, but high enough to get sufficient airflow over the coil.

[JG Nothing]

If the ductwork is sized to permit full airflow to a single zone then I would say don't slow it down. If the ductwork can't handle it then you may have to slow it down to keep from having a too-high static pressure in the ducts, which is hard on the blower and noisy.

The problem with slowing down the airflow is that you will get an excessive temperature drop across the coil. This could cause the coil to freeze up among other things.

You need to find a happy medium where the reduced one-zone airflow is low enough to keep static pressure reasonable, but high enough to get sufficient airflow over the coil.

So, if the coil does not get enough airflow, the freon gas will not absorb enough heat and the coil literally freezes. If you can reduce the airflow somewhat without the coils freezing, does that in effect extend the run time of the unit and remove more humidity? Thanks.
- Johnny

[motoguy128]

So, if the coil does not get enough airflow, the freon gas will not absorb enough heat and the coil literally freezes. If you can reduce the airflow somewhat without the coils freezing, does that in effect extend the run time of the unit and remove more humidity? Thanks.
- Johnny

Yes, lower airflow will increase latent capacity, at the cost of a reduction in total capacity. So basically the total capacity drops and the percentage of latent capacity compared to sensible capacity goes up.

FYI - Latent Capacity is the amount heat transfered to condense the water out of the air (remove humidity). Sensible capacity is the heat transferred to drop the temperature of the air. Combined, it is the "Total Capacity".

However, too low airflow as mentioned above will cause the water condensing on the indoor coil to freeze. The ice further reduces heat transfer and pretty soon you're no longer vaporizing enough refrigerant. The compressor is only designed to compress vapor.

Some zoning systems measure the supply air temperature to prevent freezing conditions on the coil.

[JG Nothing]

Yes, lower airflow will increase latent capacity, at the cost of a reduction in total capacity. So basically the total capacity drops and the percentage of latent capacity compared to sensible capacity goes up.

FYI - Latent Capacity is the amount heat transfered to condense the water out of the air (remove humidity). Sensible capacity is the heat transferred to drop the temperature of the air. Combined, it is the "Total Capacity".

However, too low airflow as mentioned above will cause the water condensing on the indoor coil to freeze. The ice further reduces heat transfer and pretty soon you're no longer vaporizing enough refrigerant. The compressor is only designed to compress vapor.

Some zoning systems measure the supply air temperature to prevent freezing conditions on the coil.

I understand your explanation. This is all very interesting. Thanks so much for the response.
- Johnny

[beenthere]

Good chance your going to have a bypass damper.
If you slow down the blower yet. The return air temp will be too low. Cause liquid slug back to the compressor. And damage it.

The bypass damper permits full air flow through the coil. And reduced air flow to teh xone.

[jmastron]

But doesn't the bypass damper just send the supply air straight into the return, resulting in the same extra-cold temperature across the coil? Or is there a difference between a cold coil due to low airflow and a coil with more (but still cold) air flowing over it?

I'm having a similar question after getting some quotes on upgrading our builder-special undersized 2 Ton unit, and have a comprehensive quote for a 3.5 Ton unit (seems to be the proper manual J size), including duct upgrades. One quote is for a single stage Carrier compressor with a dual stage furnace/blower. I don't know whether to ask for more quotes on dual-stage compressors; most of the salesmen I've talked to weren't jazzed about them in this climate.

This is in Sacramento, CA, where humidity is low and temps high (average mid-high 90s in the summer), so humidity removal isn't a concern. However, we have 2 different-sized zones -- upstairs with only ~350sf, main/lower floor with ~1400sf. With the current 2T system, upstairs-only is somewhat loud but acceptable; with a bigger system (even adding ducts) this may be a problem. Is it possible to safely bypass enough air in this case, or do we really need a dual-stage compressor in this house?

Thanks,

Josh

[coolerinfrederick]

dumping the excess supply air into the return is ok if mixed properly, but most installers come right off the plenum and straight into the return, its just cheaper on material and labor

[beenthere]

If you slow the blower and have a bypass. You can freeze the coil if not set up right.
Since not only will you have lower air flow, but now also colder entering air temp, with reduced humidity.

Going from a 2 ton, to a 3.5 ton. Will take a lot of duct alteration.

[coolerinfrederick]

beenthere, the contractor can always install a hotgasbypass, sh** shoulda kept my mouth shut LOL

[beenthere]

beenthere, the contractor can always install a hotgasbypass, sh** shoulda kept my mouth shut LOL
Not to be insulting to anyone.


But, most contractors won't know how to install HGB right, or set it up right.

[coolerinfrederick]

yeah i know, thats why i shoulda just kept my fingers off the keyboard, hey you never know, could be a cool project or a nightmare, either way the co. will gain some experince from it

[beenthere]

yeah i know, thats why i shoulda just kept my fingers off the keyboard, hey you never know, could be a cool project or a nightmare, either way the co. will gain some experince from it

LOL...

What did the OP do to you, for you to want his contractor to gets lots of experience changing out compressors, and other parts at his house. :)

[patchesj]

How low is too low for LAT? At what point to you risk freeze up?

[motoguy128]

How low is too low for LAT? At what point to you risk freeze up?

When the coil surface temp reaches around about 32F... ice will form.:p

I think supply air temps much below 50F indicate you're getting too cold.

[patchesj]

When the coil surface temp reaches around about 32F... ice will form.:p

I think supply air temps much below 50F indicate you're getting too cold.

Unless your air is really salty I guess.. :rolleyes:

But seriously, how even are coil temps? Could you have 50F LAT but ice in the corners of the coil? 45F? 40F?

[beenthere]

How low is too low for LAT? At what point to you risk freeze up?
Varies with how much air flow you do or don't have.