Damage is internal. Such as the valves, motor windings, bearings. In the A/C mode the as the air flow is reduced by closing dampers, especially 70% of them, there is less heat available to transfer to the refrigerant. With less air the refrigerant returns to the compressor in a liquid form instead of vapor. This causes damage to the above mention components inside the compressor. If you used this in the A/C mode already one sure sign of a problem is the copressor sweating.
You will need to learn the refrigerant cycle inorder to complete your project or have someone to help you.

Lorne:

What you are saying is making sense. However, there's a couple of things:

- This project has been activated just very recently, and the A/C was working exactly the same way before, with the difference that people were shuffling the registers, not the computer

- Liquid coming to the compressor instead of a vapor would have destroyed it in a few weeks, if not sooner (that's my guess based on understanding of vapor compression vs. liquid compression, or should I say, non-compression) - can someone please verify this?

- Side note, I wonder if my 30 year old unit has a TXV...

- As far as I can tell, the compressor is not sweating. I'd appreciate, though, if you can elaborate on "sweating" - this is a HVAC jargon term, what's a technical explanation or the symptoms?

As for refrigeration cycle, I've known the details since I was a kid. It' be stupid to start a project like this without studying the basics first.

Now, let me modify the question (original one being: What is the tolerable percentage of open airflow that doesn't stress the compressor). Though, I'd appreciate if someone could answer that.

The question now becomes "how do I know if I'm overstressing the compressor, airflow aside".

My guess (like I said before) is that this should be based on the analysis of:

- temperature at the coil - if I know the temperature under normal operating conditions (100% open airflow), I can figure out when it gets really wrong
- head pressure - same method

Actually, I believe I've covered that in the answer to Robin Boyd (on page 1).
__________________
--vt

Sweating= The compressor is getting wet.

Baldloonie has posted a superheat chart on this site. I would think it would show up from a search.

TXV everything changes but chart will still help.
Superheat is taken at the TXV bulb.

I am seriuos about the independant testing on your system from Enalasys. Why spend years on figuring out that your system is not installed well enough to cool the house in the first place. You mentioned duct leakage before. That could be and probably is the main factor in the discomfort to begin with and undersized ducts. Even that package unit on the roof leaks like crazy. Seal up the doors.

VT;

Lorne's reference to compresor sweating is the condensing of moisture on the compressor while in the cooling mode. This would indicate insufficient superheat in the vapor. (If you set the glass filled with ice on the table in the summer, it "sweats.")

As to your question; the allowable percentage of airfliow reduction will depend on how far off the airflow is already. For the sake of explanation, let's assume your airflow is already perfect. So, a three ton unit moving 400CFM/ton needs 1200CFM. Anything below 300CFM will cause icing and prompt compressor damage, so we can use the simple assessment of 25% reduction in actual airflow to be the maximum.

But; as you close off vents, your static pressure increases, causing more air to come out the remaining open area. Up to a point. The ducts have a maximum capacity for airflow at the unit's maximum capacity for creating static pressure. I know that is a confusing sentence, and a confusing concept, so let's reassess the "perfect" airflow situation.

You still have a three ton unit, moving 1200 CFM, so it's still perfect, right? But, the ductwork is a little undersized, and the unit is generating .5 inches of water column in static pressure. In a situation like that, when you close off 25% of the vents near the unit, you have actually cut off 1/2 the airflow.

But, if your ducts are designed especially with zoning in mind, and they provide an even distributed 1200 CFM at .01 inches of water column external static pressure, you could conceivable close off 75% of the open grill area without reducing the airflow 25%.

I hope this makes sense, because it is the combination of several concepts not previously discussed in this context. But, this is what Lorne has been trying to say.
__________________
There are no stupid questions. . .just a lot of inquisitive idiots.

BamaCracker:

Rephrasing your words:

- the ductwork for a multizone system has to be redone with a multizone system in mind;

- the proper ductwork allows high airflow while creating little static pressure;

- if whoever was installing the original unit oversized the ducts (unlikely), I'm lucky;

- otherwise, I better redo the ducts when installing a new unit. Could use this chance to get servo controlled dampers instead of registers - less noise, pressure gets used to direct the airflow instead of being wasted etc.

Right?
__________________
--vt

Everything you have said I said is correct. I might add, one of the benefits of a VS system is the ability to generate higher static pressures, allowing you to move more air through the same duct system.
__________________
There are no stupid questions. . .just a lot of inquisitive idiots.

Vt:

When we use air to remove the heat from the heat exchanger and transport that heat to the conditioned space we have to live with the relationships described mathematically by the following formula.

Qs=1.08 * CFM * TD
Where Qs is Sensible Heat in BTU's, CFM is the Volume of Air through the heat exchanger expressed in cubic feet per minute and TD is the temperature difference in degrees Fahrenheit of the Air on to Air off of the heat exchanger. The 1.08 is a constant that is derived from the properies of air.

To apply this formula to your AC if you know the units BTU rating and the rating of sensible heat to latent heat (SHR) then you can determine the sensible BTUs that are being extracted from the air stream by your unit.

For example if you have a 36,000 BTU rating and a SHR of .72 then you should have
36,000 * .72 = 25,920BTU (of sensible heat) the additional Latent BTU's are being used to remove moisture from the air.

The following variation of the formula can then be used to determine how many CFM are needed to hold the TD to a given value.

CFM = BTU / (1.08*TD)
CFM = 25,920/ (1.08*20)
CFM = 1200 [what do you know! 400CFM per Ton of rating]

With normal operating AC systems we look for TD's in the 18�F to 21�F ranges for a normal operating system.

If the AC has drops greater than about 21�F then you should begin find other ways to verify the airflow. Another check that you can make is to check the superheat to make sure that liquid is not getting back to the compressor. Got to make sure we don't slug that compressor.

The next thing you need to do is look at the blower curve or chart from your air-handler so that you can determine at what ESP you can work at before you are in trouble.

You might find a chart that gives values similar to this example.
With the Blower connected on the high speed:
Total External Static is given in inches of water column.
1200 CFM @ .81 "wc.
1250 CFM @ .72"wc
1300 CFM @ .64"wc
1350 CFM @ .51"wc
1400CFM @ .39"wc

If this table represented your air handler at a point where turning of registers begin to cause the external static pressure to begin to exceed .81 inches one option would be to provide a "dump zone" somewhere in that house located in an area where you don't care about the temperature and you could dump the excess air into that location without allowing the total CFM of the system to drop below safe minimums. Most of the non-variable speed systems that I can remember will usually deliver adequate CFM until you get to an ESP somewhere between 0.5"wc and 0.9"wc.

As Bama has said you can probably work down to about 300 CFM per ton of AC rating but you need to find that data on your blower and determine some method to dump some air when the ESP begins to adversely effect your air flow.

The same formulas are used for the heating mode but you need to find supporting data from your heat pump to plug into that equation. If you lower the CFM too much in that mode your TD is going to raise, bringing up your head pressure which can put the compressor into an overloaded condition (especially at higher outdoor ambient).

Hope some of this makes sense Tom R.

Tom R:

Thanks a lot. It does make perfect sense.

Right now, I'm overloaded with information that you guys unloaded on me, and it's gonna take a good week for me to digest it.

__________________
--vt

More food for thoughts:

I've done some more modeling - don't know how close is the model, plus again, this is a quick and dirty draft, but here, take a look:

http://freehold.crocodile.org/model.png - original model, no dump zones;

Below are the models with the allowed minimum open percentage in a range from 30% total to 60% total:

http://freehold.crocodile.org/d03.png
http://freehold.crocodile.org/d04.png
http://freehold.crocodile.org/d05.png
http://freehold.crocodile.org/d06.png

I'd say 40% is marginal...

Moral: you better have the ductwork that allows your full CFM to go to one zone

Anybody can confirm/deny these figures?
__________________
--vt

Another question: what were the reasonable velocities for acceptable noise? I've made some calculations (just the blower CFM and the areas of the registers and the return) and came up with the following:

Return: 512 FPM
Registers: 320 to 940 FPM, depending on the number of registers open

Does that sound normal?

__________________
--vt

450 fpm is really considered the upper end of acceptable noise generation. 600 fpm will begin to be very noisey. Reynolds numbers in excess of 3500 begin to be achieved in metal pipe at about 500-600 depending on length, and in flex duct at about 400-450. Noise propagation really becomes bothersome to most people once turbulent flow is achieved.

And that does not even consider noise associated with termination hardware (grilles, boots, collars, etc. etc.
__________________
There are no stupid questions. . .just a lot of inquisitive idiots.

BamaCracker:

Then I guess I have one of the four (or any combination thereof):

- ducts leak like hell (noone looked at them yet)
- the actual airflow is less than necessary (I was assuming 400CFM/ton, the unit I have is 4 ton)
- The blower is more noisy than the air (and it *is* noisy)
- I'm really noise tolerant

It's strange, though, under worst circumstances (most of the registers closed) nobody in the house notices any noise whatsoever. OK, one more argument in favor of having it checked. Thanks.
__________________
--vt

BamaCracker:

Afterthought: If I were to bring the velocity down to acceptable level (940 to 450), then the size of the ducts at the registers will have to be increased from 14*8 to approximately 20*10 - does *this* sound normal?
__________________
--vt

I'm not 100% sure I'm tracking with that last question, but I think you are talking about the vent size. 14x8 is much more common size than 20x10. Normally, on a 4 ton unit, I'd expect to find between 12 and 20 outlets, ranging in size from 8x4 (inches) to 16x8. Which sizes are most common seems to be a function of where you live; in the south where I learned this crap, all outlets were rectangle or round, with 10x6 being the most common size. Here, square is the most common, with 8x8 being used pretty much exclusively.

If I am answering the wrong question, please expound.
__________________
There are no stupid questions. . .just a lot of inquisitive idiots.

I remember reading an article about the thermostat that is on my wall right now - the Honeywell Chronotherm III, can't find a link anymore but the point of the article was that it was TOO complicated.

OK, here's my shot at things. The following is a sequence of snapshots of the prototype - I wonder, does it look clear enough? The functionality it's supposed to deliver is way more complex than just the thermostat... What would be your questions if you saw this thing on the wall in the house you've just bought?

Keep in mind - this is a prototype, not everything is perfect. In particular, button sizes fluctuate depending on the context.








__________________
--vt

"... What would be your questions if you saw this thing on the wall in the house you've just bought?"

My first question would be, "What the F is that?"

My second question (after you told me it was the thermostat) would be, "How do you work it?"

My third question, "What the heck is a throttle?" (I though that was something you do to your wife and kids.

Chronotherm is not hard to operate if you have ever sat at a computer and knew what you were doing, but millions of people have never set at a computer and do not want to. My 10 year old son can program (and reprogram) a Chronotherm III, but my wife can't. He can also set the VCR clock.

I think the display you are showing us here will fall in the same catagory.

__________________
There are no stupid questions. . .just a lot of inquisitive idiots.

BamaCracker:

OK, that's just about the kind of response I was expecting

throttle - yes, should have mentioned that. Will go away, as well as the tabs at the top and the bottom - those are debugging artefacts. The throttle shows the register position - on the picture above it's closed.

How do you work it - eight rectangles at the sides are the soft keys. If this is implemented as a touch screen, then it should be obvious, if it is a regular screen, then it'll be just rectangles with keywords, and the buttons along the sides. Just like ATM. My 13 year old was able to figure it out.

What's P.F.?


__________________
--vt

Reading those posts about the dampers from smarthome.com etc. and feeling the pain of losing $$$ again...

Here's what you get for $10.95 for a servo plus a stock 14x8 register, plus 15 minutes of labor:



PS: opens/closes in about 0.5 seconds at 6V, but of course, can be made to move as slowly as required and can be set to an arbitrary position between open and closed - this is done in software.

[Edited by vt on 02-25-2002 at 12:10 PM]
__________________
--vt

Nice idea, the amount of stress that servo will get is nothing like what it was designed for. It also gives total control. I'd use one of the heavy duty ones in a duct like a S3302 or a S3801.

Nice job VT, nice job.


[Edited by n6ber on 02-25-2002 at 04:34 AM]

http://hvac-talk.com/vbb/showthread.php?threadid=11015&pagenumber=2

It's my punishment for being nice.
__________________
There are no stupid questions. . .just a lot of inquisitive idiots.

n6ber:

Don't even bother with high torque servos - standard Futaba S3003 is more than enough, provided you use at least 16 gauge cable to supply power. It's been working for several months now without any sign of insufficiency. The only thing I could think of is ball bearing servo, but so far the standard plastic gears work just fine - oh well, for ten bucks I might just upgrade one when it fails
__________________
--vt

Finally, tha last backordered parts came

Which brings up the question I forgot to ask before: what kind of electrical load do the A/C control circuits represent? Is it purely resistive, inductive or capacitive?

I'm talking about Y, O/B, G, W and C wires.
__________________
--vt

quote:

---

Originally posted by vt
Finally, tha last backordered parts came

Which brings up the question I forgot to ask before: what kind of electrical load do the A/C control circuits represent? Is it purely resistive, inductive or capacitive?

I'm talking about Y, O/B, G, W and C wires.

---

n6ber:

Can't do. I'm using triacs and optoisolators; this setup is sensitive to the nature of the load. However, solid state switches are way more reliable than relays.
__________________
--vt

quote:

---

Originally posted by vt
Finally, tha last backordered parts came

Which brings up the question I forgot to ask before: what kind of electrical load do the A/C control circuits represent? Is it purely resistive, inductive or capacitive?

I'm talking about Y, O/B, G, W and C wires.

---

It usually contains all three. The contactors offer inductive loads with their electro-magnetic coils, the heat sequencers and heat anticipator are resistive loads, and the circuit boards that control defrost and lockouts contain all three types. The preponderance of the load is inductive, through the contactor and relay coils, but all three exist in multiple forms.
__________________
There are no stupid questions. . .just a lot of inquisitive idiots.