help heat pump theory too much air flow cause high head?

[johnwas64317]

OK iam going to school for hvac and was having a discussion the other day

with a guy in the same class hes working now installing residential heat pumps

here in Iowa and hes having a problem charging the heat pumps he say that if there

is too much air flow across the inside coil in heat pump mode that it will cause system to kick out on high head

now that would mean that too much air across the condenser would cause high head ?

does not make sense to me

he and my teacher argued that the air moving across the coil would not have enough time to pick up any heat thous causing the coil to get hotter not cooler this does not

jive with any books i have read and i have reed allot . iam probably the only one in

class teacher included that has read the whole book

Evey thing i have read says that more air flow = more delta T = more heat pick up / rejected i say that more air flow at a faster rate will mean that each

cubic foot of air will have less delta T but there will be more per min

witch will mean that more heat/ BTU removed = lower temp = lower head

and i say that once you reach a certain amount of air you will reach the max amount of BTU

that can be removed from the coil with out an ambient air temp change

even if you are able to push more air through that coil .

is what they are saying right or wrong

if they are right please explain or point to to the place to find the explanation

[beenthere]

Take a heck of a lot of excess air flow to do it. Guessing you would probably need over 700 CFM per ton to do it.

Many manufacturers want 450 CFM per ton in heat mode.

How much air per ton does this guy say these units had moving through the coil?

[skorepeo]

OK iam going to school for hvac and was having a discussion the other day

Evey thing i have read says that more air flow = more delta T = more heat pick up / rejected i say that more air flow at a faster rate will mean that each

cubic foot of air will have less delta T but there will be more per min

witch will mean that more heat/ BTU removed = lower temp = lower head


if they are right please explain or point to to the place to find the explanation

The faster the air moves across the coil the less heat that it picks up. The less heat that it picks up the more heat that stays in the coil. the more heat that stays in the coil the higher the pressure.

[Lambo]

Well, you say you like to read? Here are some documents related to the subject.

[westval]

Are these installs on dual-fuel systems with the indoor coil sitting on top of a running furnace in second stage? Maybe he is missing a jumper or a solder trace that needs to be cut upon install, and the furnace is kicking on while he is commisioning the heat pump? Would like to know what you find out.

[snewman24]

.........snip........................
cubic foot of air will have less delta T but there will be more per min
witch will mean that more heat/ BTU removed = lower temp = lower head
and i say that once you reach a certain amount of air you will reach the max amount of BTU
that can be removed from the coil with out an ambient air temp change
even if you are able to push more air through that coil .
is what they are saying right or wrong
if they are right please explain or point to to the place to find the explanation

It's my opinion that you are right and they are wrong. The more air thru the condenser the more total heat will be rejected. As the Delta T is reduced, the slope of the heat rejection will lessen, but it will still reject more total heat.
See the Florida Solar Energy document that Lambo posted. Look at pg. 6 Fig. 4. That graph tells the story.

http://www.fsec.ucf.edu/en/publicati...CR-1674-05.pdf

[Lambo]

I'm with you Snewman----for now----unless someone can prove otherwise. I was also told by one of my superiors, years ago, that this could happen. In fact, we had one rooftop condenser that we thought had this issue. We had four techs up there making sure that a previous company had not replaced the motor with one having the wrong RPM, or a blade with the wrong pitch, etc. This was all after cleaning coils and evac/recharge to exclude non-condensable----bla--bla--bla. I can't recall what the outcome was, but it wasn't an excessive air quantity/velocity issue.
I do know that this can apply to water through a chiller barrel, but I can't find any published proof that it can apply to air across a fin-tube coil. I know that we have several "phenomena" to deal with in this trade. They all have an explanation based on physics, thermodynamics, or some other field of science, yet somehow seem to defy initial logic. I actually want to believe that it could be true---but can't find proof.

[johnwas64317]

thanks to lambo and snewman24 for the papers
they confirm what is said there problem of kicking out on high

head is not caused by too much air flow across the condenser coil
i agree with you lambo that it might be possible but can not find
proof so i will make proof i will prove one way or the other
form my class project and will post the results here .
in about 8 weeks ( thats when project is due )

but something else not sure what there problem is as i believe they are doing

more than one thing wrong and iam unsure what they are doing

here's what i know of there problems

they are installing in existing home with duct work designed for a split system

natural gas furnace's 80-95 % 's and standard central a/c systems though one i know

did not have a/c originally in stalled they have installed at last count 28 systems

like this

goodman air heat pump / and a couple geothermal

with gas furnace back-up

he claims goodman sends this heat pump without an accumulator

teacher said he was full of s**t that all heat pumps need accumulators

i agree that heat pump do need accumulators ( this is not from experience but from what

i have read on heat pump theory and the few i have seen apx dozen so far )

all systems are being installed with communication thermostats ( there the kicker

they say that the stat will automatically know what kind of systems - the btu and airflow

setting need to be without programing or without changing any of the jumper settings on the air handlers )

i think they are wrong about that .

and they claim that the system only need about half of the recommended refrigerant

that the specs call for .and usually the systems are shipped over charged .



if this is the kind of quality that is in the field now i shutter to think .

this is the kind of **** i saw on my job shadow where the top service

tech took 4 hrs to change a heat pump relay for back up electric heat

and did not get it hooked up correctly let the system not working

and no he would not here any thing i said he had the 20 years experience

and i was to stand there and shut up .

( that was a different company than this other student is working for )

[deux]

DO not worry to much about what your being taught.

Because: A great mind will always over come BAD teachings.

[snewman24]

[QUOTE=johnwas64317;8881811s............ .snip............................

he claims goodman sends this heat pump without an accumulator

teacher said he was full of s**t that all heat pumps need accumulators

i agree that heat pump do need accumulators ( this is not from experience but from what

i have read on heat pump theory and the few i have seen apx dozen so far )

[/QUOTE]

You need to read these threads (& others) about whether heat pumps always have accumulators:

http://hvac-talk.com/vbb/showthread.php?t=532152
http://hvac-talk.com/vbb/showthread.php?t=226012
http://hvac-talk.com/vbb/showthread.php?t=150928

[beenthere]

As above. Not all heat pumps have accumilators.

Some where down the road, you run into some that have charge compensators.

In school, you will be taught what a heat pump can, and can't do. In the field, you will learn a heat pump can do everything school told you it can't do.

[kenney t]

DO not worry to much about what your being taught.

Because: A great mind will always over come BAD teachings.

[Lambo]

Attached are some examples of refrigerant charge compensators. The one example shows the use of the compensator in combination with an accumulator. There are probably designs out there that use only the compensator, but I would imagine that they would only be in combination with the more slugging tolerant scroll compressors. Still looking for other info.