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Old 03-12-2010, 02:53 PM   #1
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2nd stage, please


Had my system replaced 2 years ago with a 2 stage Rheem RARL 4 ton unit. This replaced a SS 4 ton unit (builder's) which was obviously oversized.

I had run a heat calc which called for 3.5 tons, but was talked into going with the 2 stage unit. You know the drill; longer runs in first stage; better dehumidification and comfort.

It does a decent job with humidity, by reducing CFMs to 350/ton, but in 2 summers, it has NEVER gone to 2nd stage (don't set it back). There are times when 2nd stage (with higher airflow) would be welcomed to better cool the 2nd story, especially since the airflow is reduced.

I have heard where people have locked their 2 stage furnaces in first stage (switchable), and thought about the possibility of being able to 'switch off' first stage.

So, is this possible? Would wiring in a switch to the Y1 line accomplish this?

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Old 03-12-2010, 04:43 PM   #2
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2nd stage, please


I assume you have a proper 2 stage thermostat? Is it wired up properly? Try manually set it 2-3 degF lower on A/C than room temp to make sure the 2nd stage works. May have a wiring problem if it doesn't. Usually they kick into 2nd stage after the temp has increased more than 1 deg F than the 1st stage cut in point (can't keep up). That deadband can be set at 1.5 degF in some of the menus of the higher end tstats. Bypassing Y1 wont necessarily help as it will take 2 to 2.5 deg before the A/C starts. Then it will run full blast and not dehumidify as well. Does your furnace/air handler have a ECM variable speed motor? Welcome other opinions.
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Last edited by yuri; 03-12-2010 at 05:36 PM.
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Old 03-12-2010, 06:30 PM   #3
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2nd stage, please


How about another single stage thermostat on second floor? Run Y1 from
the downstairs stat to the furnace Y1 terminal and the upstairs rc terminal. Y from upstairs back down to y2 at the furnace. Upstairs can turn on second stage if it's hot up there and only if the main level calls for cool.
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Old 03-13-2010, 05:07 AM   #4
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2nd stage, please


4 ton Rheem in first stage is between 2.8 and 3.2 tons capacity in first stage.
So only on the hottest days will you really need second stage.

You can lock it to always go to second stage. But I wouldn't.

Have you tried balancing the system yet?
Do you have enough return for the system(few new homes had/have enough supply, let alone return)?
If your system is set for 350CFM per ton. Might want to have it set to 400, and get an IAQ thermostat installed. So that the blower slows down if humidity is high, and runs at 400 CFM per ton if humidity isn't high. That way you still get the advantage of the 2 stages, and better air flow in first stage.

Unfortunately. Lots of 2 stage systems are sold and installed by contractors that just listen to their distributor on why they should sell them. But never bother to learn how to set them up properly.
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Old 03-13-2010, 06:26 AM   #5
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2nd stage, please


Thanks to all who have responded. I'll try to address all of your comments.

The unit is paired with a Rheem Modulating furnace and stat, which is set up for 2 stage cooling and minimum deadband. Setting the stat higher will bring on 2nd stage after several minutes, but we keep the stat at setpoint.

It's not a question of the unit keeping up, but of airflow. Even with the old a/c, airflow to 2nd floor had suffered. Now, with reduced flow, it seems worse. During installation, the return drop was increased from 8 x 22 to 12 x 25. I know, I need more return.

I've thought about adding a return, but doesn't seem like any place to put it. It would still pull from the first floor which has 2 returns already. The contractor cut in low grills (no dampers) on them which seems to have enhanced comfort (no stratification). I have adjusted the dampers (runs) and think I've found the 'sweet spot'.

With regard to returns, would cutting a grill into the return trunk be helpful (could close it in the winter)? Some time ago, I had a leak in the return (fixed) which I became aware of by feeling air being pulled under the door leading to the basement. If nothing else, it would relieve a very slight oil-canning in 1st stage; somewhat more (IIRC) in 2nd. But, nothing like with the old system (NOISY).

I have the option to set airflow to 400 CFM + 10% and since I'm married to the Mod stat (no DOD), I can wire a humidistat to the board for DOD.

One other thing: A house to the rear and west of mine just turned over and he had a lot of trees (late afternoon shade) removed. Also, my next door neighbor had a tree fall (same shade) during a recent storm. I'm thinking that my heat load will increase.

Should I hold off making any changes to see if any changes?

Thanks for your comments.

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Old 03-13-2010, 06:39 AM   #6
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2nd stage, please


I wouldn't use 400CFM +10%, might get a bit noisy. And even with DOD. You won't dehumidify, since the air flow will be too high, and you'll end up just over cooling and increasing the humidity.

If you have a gas fired water heater. Or if they didn't run an intake for the furnace. I wouldn't pull return from the basement. Could take combustion air away.

They enlarged the return drop, but didn't add any additional return to the house?

You'll find balancing the air will do you the most good.
Doesn't take long to install dampers in the first floor supplies.

What type of supply registers do you have on the second floor?
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Old 03-13-2010, 08:09 AM   #7
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2nd stage, please


Quote:
Originally Posted by beenthere View Post
I wouldn't use 400CFM +10%, might get a bit noisy. And even with DOD. You won't dehumidify, since the air flow will be too high, and you'll end up just over cooling and increasing the humidity.
Noted.

Quote:
If you have a gas fired water heater. Or if they didn't run an intake for the furnace. I wouldn't pull return from the basement. Could take combustion air away.
The furnace has outside air. I think I tested the WH vent for back-drafting (WH was on) before the return leak was fixed. I did not detect any.

I think there is enough air leakage to support the WH.

Quote:
They enlarged the return drop, but didn't add any additional return to the house?

You'll find balancing the air will do you the most good.
Doesn't take long to install dampers in the first floor supplies.

What type of supply registers do you have on the second floor?
Just added secondary grills as mentioned. Registers throughout the house are 4 x 10 w/no identifying information. All runs have dampers. There are 11 x 5" runs downstairs; 7 x 6" upstairs.

I probably could close off downstairs (a little more) but am concerned about noise/static.

My thought about cutting in a grill (return) near the furnace goes back to one contractor I interviewed, who opened the blower door slightly while the blower was on. The air poured out of the 2nd floor registers. This was with the old 8 x 22 trunk/drop.

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Old 03-13-2010, 10:13 AM   #8
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2nd stage, please


The way I am reading things, the problem isn't with the equipment, but how to get the second floor comfortable during the cooling season. Instead of dealing with the air flow issue, the company sold you fancy gear.

You mentioned how air-flow improved drastically when the blower door was opened. Cutting in more return at the basement may be nice, but wouldn't you rather bring in the hot air from the upstairs?
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Old 03-13-2010, 10:55 AM   #9
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2nd stage, please


That was your old furnace that he took the blower door off of though, wasn't it?

VS ECM motor, it would only make a real difference if you are extremely short on return.

I think if you just have the blower reset to 400 CFM per ton. You'll notice a big difference.
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Old 03-13-2010, 11:43 AM   #10
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2nd stage, please


Exactly. There is NO perfect system unless designed new by a Professional. Always has to be some compromise. I would go with the 400 cfm and all things being equal you will be more comfortable, sleep better and enjoy the system. I have units which are set for 3 stage timed ON and same furnace set for modulating. Most people don't know or notice the difference unless I tell them or explain it to them. Starting to "split hairs" with the technology we have. Even more of it is on the way. Lennox has a I Comfort line of stats and a 98.2 % efficient unit coming out this Summer. Lots of fun for the few techs who can figure it all out.
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Old 03-13-2010, 12:49 PM   #11
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2nd stage, please


Yes, Beenthere, that was with my old furnace.

Flash: Yes, it would be nice to have more return upstairs, but there's no way to do it.

A little more info re: my return system. The return consists of 2 wall sections (3 studs). The upstairs grill spanned the 2 sections. Downstairs, the return is split; half covers the front of the house; the other covers the back end. They are joined at the return trunk.

The upstairs opening was enlarged and the grill upsized.

Downstairs, low openings were placed under the existing grills. They are always open.

While I don't think I could gain much airflow by adjusting dampers (already dampened some), I thought about blocking one of the 'lower' grills. This would still give me more return than before, plus more pull from upstairs.

Currently, on high speed fan (1200 CFM in heating) there is a slight hum (not objectionable), but duration of high speed is slight. Second stage air is a little louder, but not bad.

Comments please.

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Old 03-13-2010, 05:08 PM   #12
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2nd stage, please


Restricting a return will make the VS blower speed up to pull more air. Increase your electric bill. And give you more noise from a grille or grilles elsewhere.

The same can happen when dampering down the supply, so you can only close them so much.
VS blowers vary their speed to move set air flow.

If your first floor are 5" and your second floor are 6". Then its a good chance that the trunk line is the main hold up for air flow to the second floor.
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Old 03-14-2010, 10:41 AM   #13
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2nd stage, please


Usually, there is a way to bring a return "line" to the furnace, and a good hvac guy could figure out how (with being as minimally evasive as possible). However, if there isn't any way to tap a return line from the second floor, I would try focus on exhausting the air in the heat up there.
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Old 03-14-2010, 10:56 AM   #14
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2nd stage, please


Quote:
Originally Posted by Flashheatingand View Post
However, if there isn't any way to tap a return line from the second floor, I would try focus on exhausting the air in the heat up there.
The word "exhaust" implies removing the air from the house/structure.

Which would mean bringing in fresh high humidity air, and causing the A/C to run longer then need be.
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Old 03-14-2010, 11:41 AM   #15
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2nd stage, please


Even if you are bringing in fresh humid air, at least you are getting rid of the stagnant in the second floor. Which seems to be the source of the problem. Obviously, we aren't there, we don't know all the details.
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