Variable speed/ two-stage compressor and loud ducts

[danielh]

In a 50yr old rambler I have an 18yr old central AC with attic ducting that I recently upgraded from a mix of hard ducts and flexible duct to all hard duct (as part of an energy-efficiency project). Well, the air flow is much stronger now, so strong that it is annoying loud (no clanging, just the sound of a lot of air moving). Note that before this upgrade, noise was not a problem.

The unit is a Carrier 2 ton (13 seer) unit. The air coil it has a low-medium-high "easy select" option. Alas switching it from high to low didn't help at all. I guess the low setting just isn't low enough (10% reduction?).

So what are the solutions? Restricting the air flow in the main supply seems counter productive. Moreover, although the system is working adequately, given its age I would seriously consider upgrading to a high efficiency (>16 seer) two-stage/speed compressor with a variable speed blower. The energy savings, and the usefulness in controlling humidity (DC suburbs) are appealing. And I can afford to wait a decade for payback.

But will that work? Will the fan speed be lowered enough (during low power operation)? It's hard to get info on this from manufacturers specs.

Lastly, the wife's allergies means we run the unit a lot in the fall. I assume that with a good thermostat, a slow fan speed can be chosen for these non-cooling times (or is easier to just use a "circ" mode?)

Thanks.


BTW: Using an approximate online Manual J calculator, I get a cooling load of around 20k BTU (using what will be the attic insulation, new windows, air sealing, upgraded ducts, etc). This suggests a 1.5 ton "low power" setting would be more then enough?

[surenuff]

The flex ducts could have been damping the noise a lot. But if your ducts are that noisy, I would say you are trying to move way too much air through ducts that are too small. The old duct could have been restictive enough that you were not moving all the air the blower was capable of moving. Now that you have ducts that are less restrictive, you are moving a lot more air. Please let us know what size your ducts are and a little more about the unit you have. Then maybe we can give you some better advice. Right now, if you put an ecm motor on a duct system that is already too restrictive, it is going to ramp up rpm to try and overcome the static pressure of the duct, and it will louder than it is now.

[danielh]

The flex ducts could have been damping the noise a lot. But if your ducts are that noisy, I would say you are trying to move way too much air through ducts that are too small. The old duct could have been restictive enough that you were not moving all the air the blower was capable of moving. Now that you have ducts that are less restrictive, you are moving a lot more air.

That was the opinion of a 2nd AC contractor I had in (that the flex was damping the noise AND restricting air flow).

Please let us know what size your ducts are and a little more about the unit you have. Then maybe we can give you some better advice.

The newly installed ducts are 6" round aluminum, wrapped with R6 insulation (the reflective "bubble wrap" stuff).The system is a Carrier 38TRA024 (2 ton, 13 seer) ACU and a FK4BNF002 fan coil (manufactured by Radco).

Right now, if you put an ecm motor on a duct system that is already too restrictive, it is going to ramp up rpm to try and overcome the static pressure of the duct, and it will louder than it is now.

Interesting -- the new ducting has got to be much less restrictive (before I had to put my hand by a ceiling register to make sure it was on, now I call feel a breeze on my face standing below one). Why would would an ecm motor ramp up even more?

Note that there is a "AC/HP Size = Select-system size installled" set of four "jumpers" on the circuit board. I suspect it would be unwise to change those (especially since the manual and the circuit board do NOT indicate what each jumper corresponds to). But it is tempting...

[Swampfox]

The variable speed blower settings are likely set wrong

[danielh]

The variable speed blower settings are likely set wrong

Thanks for the tips, surenuff

I changed the "AC/HP Size = Select-system size installed" jumper (actually, they are spade connectors), even though the manual is unclear on what the settings actually do.

There is a drawing in the manual of these jumpers that had fine print showing (from left to right) "..,36,30,24" (the .. means the print was illegible). It was set on the .. setting (the furthest left). So I moved it to 24 (the furthest right) -- which I am guessing means "two-ton".

And that seemed to solve the problem! I even upped the "CFM adjust" jumper from low to high. The air flow and noise are back to what we had before the duct upgrade (fairly gentle flow, just a low hum).

So unless those jumpers control something other than gross air flow, the problem is solved. I am guessing that the original installers (18 years ago) selected max air flow (on both sets of jumpers), to make up for sloppy duct install.

Or is there some linkage from this fan coil circuit board to the ACU, and the prior setting was correct, and by moving this setting I might damage the ACU ? At this point I am willing to take that small risk.


BTW: that still leaves open the question of whether an upgrade to a near-top-of-the-line system is a good idea (given that now is a convenient time to upgrade, and the system is 18 years old).

BTW2: The original installers cut more than this corner. They probably low balled the bid, even though I hinted that cost was not the major decision factor.

[skippedover]

As long as you have the replacement equipment properly sized (Manual 'J') and a thorough duct analysis to determine adequate sizing for the system, there is no reason at all that you should not upgrade to a higher efficiency/higher comfort system. But a variable speed blower will be less tolerant of an under sized duct system so that duct analysis mentioned is of the utmost importance, as it the load analysis.

[danielh]

As long as you have the replacement equipment properly sized (Manual 'J') and a thorough duct analysis to determine adequate sizing for the system, there is no reason at all that you should not upgrade to a higher efficiency/higher comfort system. But a variable speed blower will be less tolerant of an under sized duct system so that duct analysis mentioned is of the utmost importance, as it the load analysis.

I will make sure to keep that in mind.

Actually, a half hour ago I had tested with just the fan. I decided to try the AC (it is a cool morning so I had to crank the thermostat down to 60). And the AC just wouldn't turn on

So I moved the Select switch to the 2nd-from-right position (30?), and now it works. The air flow is a bit too strong though. So what is there in the furthest right position that won't cause the ACU to turn on? I wonder if there is something on the ACU that needs to be set to match this setting on the fan coil?

Or should I just take this as a sign that it is time to upgrade!

[George2]

Was the return air (as part of the ductwork "upgrade") altered in anyway?

That could be why you have so much airflow now.

[George2]

How many supplies and returns are in your home? Is the return(s) in the ceiling as well?

[danielh]

There is one return (1 foot x 2 foot grille in ceiling); and 10 supplies (rectangular supplies that are about 3" x 12"). All of these are on the first floor (the house has a basement that has no returns or supplies).

Other then being wrapped in R-6, the return was NOT altered. Note that the duct leading from the return's ceiling grille to the fan coil is about 2 feet x 2 feet x 6 feet long.

Do note, as I reported in a later posting, that when I switch the "AC/HP Size" jumper to what appears to be the lowest setting, the airflow and noise drop down to an acceptable level (in terms of noise and air flow, it is similar to what I had before).

The curious feature is at this lowest setting the ACU will NOT turn on no matter how low I set the thermostat. But at the second lowest setting (which has noticeably stronger air flow and is a bit noisy), the ACU WILL turn on.

[George2]

That RA grille is about a half (560 cfm) of what is needed. If all the supplies are 6" you have about 1,000 cfm.

[danielh]

So the supply ducts are oversized (assuming 350 cfm/ton x 2 tons), and the return is undersized.

That said, I got back from work tonight and retried the "lowest setting" .... and it works properly (the fan runs both in fan-only mode and when the ACU is on). I suspect I didn't seat the connector properly before.

Assuming nothing changes, I am satisfied -- quiet ducts with reasonable air flow (or at least an air flow I have lived with satisfactorily for a decade). So I don't see the point of installing a 2nd return, but I could well be missing a subtle point.

Thanks for all your help. I could keep asking questions, but at this point it would be mostly to satisfy intellectual curiosity.

[surenuff]

I will look at the info you put on here about your duct and sized when I have a little sleep. But as for now, just make sure you have not cut down air too much. It could take out your compressor. I am not conviced the your ducts are oversized at all, and the reason is this. If your ducts were oversized for a two ton, the the discharde air would not have been so loud out of them. (assuming that the factory fan dip switch settings were correct for two tons of airflow). If your return is undersized, it could and would be loud. I will do some looking into it. Check your temperature difference through your indoor coil and let us know what it is.

[danielh]

I will look at the info you put on here about your duct and sized when I have a little sleep. But as for now, just make sure you have not cut down air too much. It could take out your compressor. I am not conviced the your ducts are oversized at all, and the reason is this. If your ducts were oversized for a two ton, the the discharde air would not have been so loud out of them. (assuming that the factory fan dip switch settings were correct for two tons of airflow). If your return is undersized, it could and would be loud. I will do some looking into it. Check your temperature difference through your indoor coil and let us know what it is.

FWIW, the air flow at the ducts, now that I have reset the HP/AC select jumpers, is pretty much what it was before the duct upgrade. And before I set those jumpers it was a lot stronger (sorry, I don't have tools to measure velocity).

Also, before the reset the noisiest part of the system was in the supply right after the fan coil (which is 2' x 2' square hard duct). The return wasn't that loud. I must say that I have a hard time understanding why an undersized return would generare a noise that is directly correlated with air flow at the ducts, but what do I know?

OTOH: the fk4bnf0002 fan coil is adjustable for system sizes of 18,24,30, and 36. So perhaps the "lowest" setting is for 18 (I can't find a key on the unit). Which suggest the 2nd is for 24.

I still think the original installers did a sloppy job with the flex duct, and compensated by pushing all settings (the HP/AC select and the Hi/Med/Low CFM) to their maximum. Of course that doesn't mean I must correct it by setting it to a minimum (though it is pleasantly quiet now)

[surenuff]

The statement that I made was that if the return was undersized "IT" could and would be loud. It, being the return. I was not saying the return would cause the supplies to be loud. I am glad that you seem to have figured out the problem and the solution on your own. I hope it works great for you from here on out. Take care.

[danielh]

The statement that I made was that if the return was undersized "IT" could and would be loud. It, being the return. I was not saying the return would cause the supplies to be loud. I am glad that you seem to have figured out the problem and the solution on your own. I hope it works great for you from here on out. Take care.

But now you made me nervous about setting the air speed too low

Seriously, thanks for your help -- I can talk more intelligently to the hvac contractors now (still seriously considering upgrading to super high efficiency system).

[surenuff]

But now you made me nervous about setting the air speed too low

Seriously, thanks for your help -- I can talk more intelligently to the hvac contractors now (still seriously considering upgrading to super high efficiency system).

Generally speaking, you should have somewhere around a 15*-20* temperature difference between the return air temperature and the supply temperature. Usually, we try to measure the supply temperature at the closest outlet from the unit to avoid as much error as possible due to heat gain in the duct work. There are a lot more things to consider in evaluating a systems performance, but as a rule, that temperature split across the coil was what most manufacturers had in mind. Too much temperature difference equals low airflow rate in most cases, and too little of a temperature difference could be from too much airflow(very unlikely). Hope this helps.

[cartercrew]

With a quick search I was able to locate the attached pdf that shows the control board in your air handler's lowest setting is for a 2 ton system. Also listed are the other settings and their effects. You mentioned humidity issues, so I would set the blower to do 350 cfm per ton and let it run. As for upgrading, I would let this one run until it needs to be replaced. I seldom see payback from energy savings to be enough to replace a running system. When you need to decide repair or replace, that is the time to replace. In the mean time, keep this one serviced and running as efficiently as possible until it dies.

[danielh]

With a quick search I was able to locate the attached pdf that shows the control board in your air handler's lowest setting is for a 2 ton system. Also listed are the other settings and their effects. You mentioned humidity issues, so I would set the blower to do 350 cfm per ton and let it run. As for upgrading, I would let this one run until it needs to be replaced. I seldom see payback from energy savings to be enough to replace a running system. When you need to decide repair or replace, that is the time to replace. In the mean time, keep this one serviced and running as efficiently as possible until it dies.

Ooof, I was satisfied with the printed manual for the fan coil (which I managed to find under 18 years of paperwork), so I didn't try looking very hard for online documentation.

The pdf you point to is more thorough than the manual. In particular, Figure 4 indicates that the furthest right "quick connect" terminal is 024, which I take to be 2 ton. That agrees with the harder-to-read schematic is in the printed manual. This suggests that the furthest right position is appropriate.

My only hesitation is the manual list several FK4B fan coil sizes in a table; with model#, system-sizes, and CFM-range columns. On the row for Fk4BNF002, the system sizes are "018,024,030,036" (cfm range 400-1450). Well, perhaps I am overthinking this ... but that kind of suggests that the 4 "quick connect" terminals correspond to 018, 024, 030, and 036 -- rather than the 024,030,036,042 listed in the diagram in the pdf file (figure 4).

So which is correct??

(it seems stupid that the pcb doesn't have sizes printed on it for those connections -- esp since all the other connections have printed info on the board).

I would prefer to put it on the smallest (furthest right), to keep the sound down; so I want to belief the figure. Perhaps what is needed is a temp measurement (as suggested by surenuff), and set it to get 15-20 degree diff.

I am wondering about one tweak. It should be easy to set up a manual switch (say, next to the thermostat) that would allow me to set the fan speed (between lo, medium, and high). Say, so I can set it low for humidity control when we aren't around, or for running through the air cleaner when the compressor is off.

For example, start with a 4 prong rotary switch, with one output and 3 inputs. Run 3 wires from the 3 "quick connect" terminals to the 3 inputs; and one wire from the output to the existing female quick connect (that goes to the motor).

Is that a silly idea? For example, would switching fan speeds while the motor is running potentially cause damage?