HomeOwners : Dont play 'the register game' .

[pstu]

Just so I can get to understand this better...

What would have to be done so that a homeowner could be free to open and close the registers freely? I'm thinking of my parents' home where 1) they do close registers for little used rooms, and 2) there is little room under the doors, the carpet fits snugly so there is virtually no return air path when those doors are closed. Despite these faults, they have long lasting equipment and supernaturally low electric usage.

Just trying to figure out what is right and wrong in principle, plus how to apply that to practice.

Thanks in advance -- P.Student

[docholiday]

Originally posted by JoeSix
I'm curious, but doesn't a TXV help with this? (e.g. if the coil gets very cold, doesn't it meter less refrigerant into the coil?

(No, I'm not advocating blocking registers, just wondering about those dehumidifying stages of the air handler operation which also reduce air flow over the coil)

Fist off, no it doesnt help significantly for several reasons. TXV's are sized for nominal capacity and when you reduce the capacity the Valve does not work properly causing it to hunt flooding and starving the coil. Also, by lowering the amount of refrigerant, you also lower the pressure which lowers the saturation temperature. Now you have a freeze condition which will eventually cause floodback.


Now, the reason its not a problem with ECM motors is because they are proper air flow more often than a PSC motor. If you have it set for 1200 CFM, it will deliver 1200 CFM as long as your static isnt above 1" or so.
A PSC motor at higher static has a tremendes loss of airflow to begin with. With the ECM you can knock it down 15% because you know the airflow will be there. With a PSC, you have literally no control over the airflow and you are at the mercy of the curve. You may think the design is 900 CFM only to find out you only got 600 and want to reduce it futher.

[JoeSix]

Originally posted by docholiday
Now, the reason its not a problem with ECM motors is because they are proper air flow more often than a PSC motor. If you have it set for 1200 CFM, it will deliver 1200 CFM as long as your static isnt above 1" or so.
A PSC motor at higher static has a tremendes loss of airflow to begin with. With the ECM you can knock it down 15% because you know the airflow will be there. With a PSC, you have literally no control over the airflow and you are at the mercy of the curve. You may think the design is 900 CFM only to find out you only got 600 and want to reduce it futher.

OK, but we aren't talking about 15% here in some cases even in properly operating systems.

Both Trane and Lennox (that I know of -- I'm sure the others do this too) offer a dehumidification stage that runs the system blower at 60% or so of nominal for the first 7 or so minutes or so at a time (and I have a Lennox Dehumidistat that will do this on demand, which lasts the entire cycle when it's damp).

So, instead of 800 CFM with my 2-ton case, I'm only getting around 500 CFM.

Why doesn't this cause a problem?

[hvacfella]

Originally posted by perpetual_student
Just so I can get to understand this better...

What would have to be done so that a homeowner could be free to open and close the registers freely? I'm thinking of my parents' home where 1) they do close registers for little used rooms, and 2) there is little room under the doors, the carpet fits snugly so there is virtually no return air path when those doors are closed. Despite these faults, they have long lasting equipment and supernaturally low electric usage.

Just trying to figure out what is right and wrong in principle, plus how to apply that to practice.

Thanks in advance -- P.Student

REPLY: It largely depends on the size of the trunk ducts and branch ducts , plus, the size of the furnace drive package ; if the trunk and branch ducts are adequately/generously sized (which few are..) then it becomes less of a problem to close some registers off. Same applies to a generously sized furnace blower/motor that can handle the added static of some registers being closed to divert the air to the other open registers . You have to take each residential system into account, but, the vast majority of duct systems (especially in large developments, condos, townhomes, etc...) are cheapened by cutting corners on the size, the number of branch ducts, the thickness of the sheetmetal, the tightness of the connections, and the overall design (unfortunately).

[alinnova]

Very interesting thread that nicely explains why my Trane XL19i A/C with the Trane XV80 furnace, with both have the ECM or "variable speed" fan and blower motors work so nicely.

Two years ago, my contractor who knows my house, TOLD me to shut most of the register capacity downstairs in my two story house and that the static pressure would NOT be a problem.
Al

[54regcab]

Originally posted by JoeSix

Originally posted by docholiday
Now, the reason its not a problem with ECM motors is because they are proper air flow more often than a PSC motor. If you have it set for 1200 CFM, it will deliver 1200 CFM as long as your static isnt above 1" or so.
A PSC motor at higher static has a tremendes loss of airflow to begin with. With the ECM you can knock it down 15% because you know the airflow will be there. With a PSC, you have literally no control over the airflow and you are at the mercy of the curve. You may think the design is 900 CFM only to find out you only got 600 and want to reduce it futher.

OK, but we aren't talking about 15% here in some cases even in properly operating systems.

Both Trane and Lennox (that I know of -- I'm sure the others do this too) offer a dehumidification stage that runs the system blower at 60% or so of nominal for the first 7 or so minutes or so at a time (and I have a Lennox Dehumidistat that will do this on demand, which lasts the entire cycle when it's damp).

So, instead of 800 CFM with my 2-ton case, I'm only getting around 500 CFM.

Why doesn't this cause a problem?

Doesn't it take about 10 minutes for a system to reach full efficiency? I belive the 7 minute delay just allows the coil to get cold a bit faster. I personally put a timer/relay on my A/C system to switch the blower from low to high speed @ 7 minutes and it helped a lot with dehumidification Delta T only decreases about 1 degree when high speed kicks in. Normally it won't hit the high blower speed unless it's over 95 outside or the thermostat setting has changed. Oddly enough our power bills don't seem to be any higher either.

I have had ot go behind the previous homeowner and increase the size of a few ducts to get tempatures relatively even in the house. The heat is about 2X oversized and the A/C is about 50% oversized. We are the 2nd owner of the house, the people we bought it from were HVAC contractors!! You would think an HVAC contractor would have his own system set up right ...

[Edited by 54regcab on 08-06-2005 at 09:37 PM]

[JoeSix]

Originally posted by 54regcab
Doesn't it take about 10 minutes for a system to reach full efficiency? I belive the 7 minute delay just allows the coil to get cold a bit faster. I personally put a timer/relay on my A/C system to switch the blower from low to high speed @ 7 minutes and it helped a lot with dehumidification Delta T only decreases about 1 degree when high speed kicks in. Normally it won't hit the high blower speed unless it's over 95 outside or the thermostat setting has changed. Oddly enough our power bills don't seem to be any higher either.

If your system is cycling off before 7 minutes, then it's oversized.

Also, the timer approach is simply a convenience. The "better" approach is to get a dehumidistat so that the low-speed blower operation is activated on demand rather than whether it is needed or not.

Lennox makes one of these ("EfficiencyPlus") and I had one in my old house. On a damp day with low-demand, the system would run it's entire cycle (typically 12-15 minutes) at 275 CFM/ton or so and do this all day, which is what prompted my question (e.g. why is this any different than closing some registers).

[54regcab]

The unit is in fact oversized by about 50%. I can't justify replacing a working system when a $5.00 timer does a decent job at controlling the humidity (I already had the 24V DPDT relay on hand). I looked into dehumidistats but I have yet to see a standalone digital model or one integrated with a thermostat at a reasonable cost (the analog ones just aren't accurate IMHO)
When the existing equiptment (2000 model) wears out we will be getting a 2 ton instead of a 3, and a 40K BTU furnace instead of an 88K. Until then we'll just pay the increased utility costs. I really do wish more HVAC contractors would use HVAC-CALC instead of using "rules of dumb" to size equiptment. As a HO I would have no problem paying the same price for the smaller equiptment knowing that the contractor took the time to size it correctly.
IMHO Chances are also good if a contractor went through the trouble to engineer the system correctly, it will most like be installed to actually get the rated efficiency

[JoeSix]

Originally posted by 54regcab
The unit is in fact oversized by about 50%. I can't justify replacing a working system when a $5.00 timer does a decent job at controlling the humidity (I already had the 24V DPDT relay on hand). I looked into dehumidistats but I have yet to see a standalone digital model or one integrated with a thermostat at a reasonable cost (the analog ones just aren't accurate IMHO)
When the existing equiptment (2000 model) wears out we will be getting a 2 ton instead of a 3, and a 40K BTU furnace instead of an 88K. Until then we'll just pay the increased utility costs. I really do wish more HVAC contractors would use HVAC-CALC instead of using "rules of dumb" to size equiptment. As a HO I would have no problem paying the same price for the smaller equiptment knowing that the contractor took the time to size it correctly.
IMHO Chances are also good if a contractor went through the trouble to engineer the system correctly, it will most like be installed to actually get the rated efficiency

Wow, you sound like you had the same experience I did in my old home. Contractor used a 2-ton when a 1.5 ton would have worked just fine (and based on the 2-ton's 50% duty cycle at 95 degrees, I'd bet even a 1-ton would have worked. It seems crazy to try to cool 1400 sq feet with that, but it's a townhouse with a load only on 40% of the exterior wall space).

They put in a 66K/44K 2-stage furnace, too. Even on low, the thing would only cycle 2 times an hour with 5-6 minutes of burner operation. Not comfortable at all -- roast you out and then you endure cold spots for 20-25 minutes until the next cycle. Ugh.

[jramunni]

Originally posted by ct2
ok so if blocking the regesters is a bad thing (and I understand why it is) what does a zoned system do differently. The zoned systems that we make will either close one side or the other or supply air to both outlets ( on a two outlet plenum)

but by closing off one zone you are diverting all of the airflow to the other, and by closing all of the regesters on one floor you would force the airflow to the other.....ooooo smaller ducts going to the regesters increases static and reduces airflow through the system--- right?

Allow me to shed some light here....here is some data from our test lab. Rob Falke from the National Comfort Institute was doing some training in our lab last week and decided to measure the changes in system performance when closing 50% of the duct capacity. We operated the 5 ton Heat Pump on 2nd stg and closed a 10 x 16 trunk damper leaving open eight six inch branch ducts. (by design criteria that would be 100 cfm x 8 = 800 cfm total) What many people forget to realize is that when you zone or close registers you change all the dynamics of the air flow. Design criteria is no longer relative. By closing outlets we increase supply static which increases velocity which increases cfm delivery through the open outlets. We are allowing the system to react to a peak load in one zone.
Here is the test results;

-----------------------100% open-------50% open--------Diff
Total delivered CFM-------1816-----------1412---------(-22%)
Enthalpy change-----------4.86-----------3.82---------(-21%)
D.B. Change---------------17.1-----------12.6---------(-26%)
Total BTU---------------39,715---------24,272---------(-39%)
Sensible BTU------------34,910---------19,214---------(-45%)
Latent BTU---------------4,805----------5,085----------(+6%)

This was a quick test done at the end of a long day...I will be utilizing the NCI performance testing forms and doing some further testing in the next few weeks.

jr


[Edited by jramunni on 08-09-2005 at 04:57 PM]