The larger blower would only be needed for a much larger AC than you are considering. The smaller one is more than sufficient for rooms rooms added later. Additional rooms raise the question of heat capacity, not blower capacity.

A communicating stat can work better if you add an outdoor temp sensor to whatever AC you have. But I think that is true of better thermostats as well.

Even forgetting the AC, a communicating stat will help your modulating or two stage furnace to make intelligent decisions about heat. Normal thermostats are kinda dumb, they only send the furnace a call for heat. They don't know your actual temperature. Lets say you set back the thermostat temperature and later turn it back up. How does the furnace know that it has to heat 1 degree or 10 degrees? Communicating that info to the furnace helps determine what stage it starts at, and how it ramps up and down based on the results over time. More comfort, optimal run times.

Sounds like you are really sold on the SLP98. I hope because you found a great installer. If so, they should be able to tell you the rationale to help you with some of these equipment decisions. I'm not suggesting not to ask here, but it is good to get your questions answered in person as well. You are handing them a chunk of business and you want to develop a good ongoing relationship with them. If they are not able to answer your questions, you may have some issues if you need service later.

 

Thanks, I see now how my original post made absolutely no sense, in the context of heating. (can I take it back? :yes

I guess the larger blower would be useful if we ever did go to a bigger AC unit.

Are there ever situations where the larger blower would be beneficial, even if just planning on 3-ton AC? Could it help with any issues with poor ventilation / circulation in the summer? I realize that the existing ductwork would be the key issue there. But can the larger blower help overcome some of the ductwork issues (does it produce a larger static pressure to help push more air throught he ducts, and hopefully not too noisy), or could it make things worse?

And I would still be interested in understanding what would go wrong if the furnace had a much larger blower than needed, if the SLP98V allows for an adjustment?

 

No, the way to fix duct deficiencies is to fix the ducts, not add even bigger equipment. If you can upgrade your house envelope and go to smaller equipment, sometimes that is like upgrading your ductwork.

Most ductwork is undersized, and going larger with the equipment makes it worse. If you are upgrading from an old furnace, often the ductwork that was adequate for the old furnace, is now undersized for the needs of modern furnaces.

Without looking up the specs for the SLP98, the smallest blower motor might handle a four ton AC. Again, ask your installer.

No, there isn't usually anything much lost with a bigger motor. Slightly higher upfront cost. Possibly a small increase in static pressure (a few hundredths of an inch). Even if a small increase in static, virtually no difference in electric costs to operate at a given CFM. Only caveat is if the furnace board can work with the smaller AC to set the appropriate CFM but there is no good engineering reason why it won't.

 

A larger blower used to over come poor duct work, would tend to make the duct system noisy.

They don't have higher static ratings.

 

Ok. So no real harm in oversizing the blower sounds like, but no benefit either. And costs extra $ to get it. (so doesn't make sense to do it)

Thanks !

 

if using a 3 ton 2 stage unit you can't really get the high speed blower setting as low as you want with the 5 ton blower package.

lowest high speed blower on the 60c model is 1320 cfms.

 

wow, I didn't realize that.

So how does this work? I imagined a low range for heating and a high range for cooling, but the center of both ranges could be adjusted by the installer by a lot.

Are you saying that after the installer adjusts the cooling range to be as low as he can, that 1320 cfms is the "top" of that lowest adjusted range ( and the controller can lower the cfm further for humidity control) ? Or is 1320 cfms the lower limit for cooling for the 60c blower, even for dehumidifying? Either way, it makes a good argument for not oversizing possibly.

With the variable speed fans, like on the SLP98V, does it do everything it can to achieve whatever cfm it is set to? Does it know approximately what the cfm is? Maybe if these do a better job of delivering the intended cfm compared to the older blowers, then the tendency to want to oversize the blower should go away ???

 

1320 cfm is the lowest speed you can get on 2nd stage cooling with a 5 ton blower package in that furnace. if dehumidifying it would run on 1st stage blower. 1320 is more than you want for a 3 ton unit though.

 

The above was with regards to the SLP98v.

Now that I'm taking a 2nd look at the Carrier ICS furnace, it looks like the blower size jumps from 3.5 ton to 5 ton. Do you know what the lowest cfm would be for the 5-ton blower when cooling? Is it also limited to 1320?

 

Thanks! How accurately is the furnace able to determine the actual cfm it is producing? (and how does it know?)

 

not sure how it knows, but the ones we have checked have been dead on the displayed cfms.

 

iComfort requirement for useful dehumidification

I have been looking at similar Lennox equipment. Lennox is amazingly bad at providing useful information on their equipment. Lots of marketing fluff but no specifications or anything of substance.

The icomfort thermostat WITH an SLP98 and WITH the compatible A/C unit will control humidity by reducing fan speed as appropriate to balance cooling versus dehumidification. Carrier's top of the line system was first with this. It is far more efficient than using a dehumidifier. This coordination seems like a simple idea, but is only available on the top end systems.

Icomfort thermostat does not support G61 furnace for instance. Anything short of the icomfort thermostat such as the 7000 is marketing B.S. The thermostat simply sets for a larger temperature range between on and off. A additional 2 degree temperature swing between A/C on and A/C off allows longer compressor runs-> better dehumidification at the expense of occupant comfort!. One of the big gains of variable speed furnace is exceptionally stable room temperature. Using the comfort sense 7000 with the SLP98 blows the main gain from this furnace over the G61. The up to 3% efficiency improvement is hardly worth the price. (Efficiency is measured at a fixed speed so there is no guarantee for any improvement over most of the operating range)

 

Icomfort thermostat does not support G61 furnace for instance. Anything short of the icomfort thermostat such as the 7000 is marketing B.S. The thermostat simply sets for a larger temperature range between on and off. A additional 2 degree temperature swing between A/C on and A/C off allows longer compressor runs-> better dehumidification at the expense of occupant comfort!.

Not quite. None of the 7000 configurations use over cooling as the sole means for dehumidifying. They're a combination of overcooling and decreasing the blower speed or just decreasing the blower speed.

 

honeywell IAQ will dehumidify with any Lennox v drive blower.:yes:

 

All green grass climates have months of fairly high outdoor dew points with very low cooling loads.
With this in mind, I suggest having enough cooling capacity to handle the hottest of weather or visitors or the decline in capacity as the equipment ages.

That just needlessly over sizes A/C equipment. No reason to do that.

Sized right, most places don't need a whole house dehumidifier. And can easily maintain less then 60%RH, all year.

 

Having an a/c that handles my cooling loads to my satisfaction is not over-sizing.
Back to the point of fresh air infiltration and minimal fresh air ventilation should be our concern when providing a heating/cooling system. Check todays dew point for the U.S. When the outside dew point is <50^F, we get a drying effect on a +70^F home. When the outside dew point is >60^F we get a wetting effect on a 70^F home. 100 cfm of fresh air removes or adds a specific amount of moisture if we maintain 50%RH in home.

Oustide dew point---%RH--70^F home____100cfm drying/wetting Lbs/hr
32^F----------------25% RH__________-1.8 lbs per hr.
50^F----------------50% RH___________0 lbs per hr.
60^F----------------72% RH_________+1.4 lbs. per hr.
70^F---------------100% RH_________+3.6 lbs per hr.

Clearly, you need 1-4 lbs. of moisture removal with any significant infiltration/ventilation. During rainy cool weather, +60-70F outdew points are to be expected. Also you have the moisture from the occupants which expect .5 lbs. per hour per occupant.
This is old high school physics. During high cooling loads, the a/c handles the moisture. During low/no cooling loads/+60^F outdoor dew points, a good dehumidifier makes it easy to maintain <50%RH. During low dew points a good humidifier with 2-3 lbs. of humidifying capacity may be needed.
Chech todays US dew point. Most of the green grass climates need dehumidification yet in Sept.
http://www.weather.com/maps/maptype/currentweatherusnational/uscurrentdewpoints_large.html

I am dehumidifying 0-30 Lbs. of moisture per day to maintain 50%RH in WI during Sept. Make your case for dehumidification only from an a/c.
I used to live in a damp house also.
Regards TB

 

I am dehumidifying 0-30 Lbs. of moisture per day to maintain 50%RH in WI during Sept. Make your case for dehumidification only from an a/c.
I used to live in a damp house also.
Regards TB

Quit opening the windows.

 

I remember when you posted after your A/C was first installed, you complained it was oversized.
then later you changed it to. It was a correctly sized A/C.
Now, its sized correctly to meet your temp desires.

You keep changing the description of it to sell dehumidifiers.

If green grass climates truely "needed" dehumidifiers. then either everyone would already have one. Or they would be dead.

While a good dehumidifier may be a nice accessory for a home. It is NOT needed(not like you make it sound). Since people were living in those green grass climates a lot longer then what A/C's or dehumidifiers have been around.

 

Nice of you to remember Me?? I had about 10 days this year that when my a/c was properly sized this year. Most day the a/c gets to 4-6 hours per day. Pulls out 18-20 lbs. of moisture on those days.
I see you chose not help me with those 60-68^F outside dew points with no cooling load and how to provide <50%RH inside the home.
You are right, high indoor %RH may be uncomfortable, grow mold, and make your home smell musty, but does not kill many.
I do like If green grass climates truely "needed" dehumidifiers. then either everyone would already have one. Or they would be dead.
Lets us keep on promoting, you do not need a dehu and me- you would be more comfortable and have less mold and dust mites. It is ok with me.
Thanks for participating in the discussion. The really nice premium homes with ideal indoor conditions do have ventilating whole house dehumidifiers as part of the space conditioning ventilation system.
We all keep doing what we believe. I can keep your home dry and have fresh air ventilation during wet cool weather.
Just be honest and tell people- no cooling load with outside dew points +65^F means a damp home not a <50%RH home.
Regards TB

 

he really nice premium homes with ideal indoor conditions do have ventilating whole house dehumidifiers as part of the space conditioning ventilation system.

I'll remind my customers with their 3/4 plus million dollar homes that they have terrible shacks, because teddy bear said nice premium ones have whole house ventilating dehumidifiers.

Oh, thats right, they're not bringing in high humidity air so that they need to run a dehumidifier.

 

Better not bring up the fact that they are not getting fresh air thier homes. First thing you know, they will want fresh air to purge the indoor pollutants and renew oxygen. Like the ASHRAE, Am. Medical Ass., EPA, American Lung Ass. suggest, Than what? Whole house ventilating dehumidifiers, God forbide.
Ok, truce. Lets call it a draw for today and get a good nights sleep. We start all over again tomorrow. The dry air is on the way across the Northern US.
Regards TB

 

Few homes have less then .2 ACH nat.

I did just recommend using Thermostore for 2 places. 1 a residential, the other a commercial.

The resi, said no way.

The commercial is still debating. But, its a bad environment. And the A/C's have to have coated coils. So they are trying to decide if the dehumidifier would be a worthwhile addition.

So do you coat your commercial coils.

 

Well, was just hoping to find out the range of AC settings / CFM that the ICS 5-ton blower hardware can support, in comparison to the 3.5-ton blower package, if nothing else but to satisfy my curiosity. Since no answer, I did some digging and was able to get some additional information. Not 100% sure I've captured it correctly.

Randomly picking 0.4" external static pressure to look at, the ICS 080-20 version (5-ton blower) can support settings down to 2-ton, with 630 cfm at 0.4".

The ICS 080-14 version (3.5-ton blower package) can support settings down to 1.5 ton with 475 cfm.

What I don't know is, if you configure the ICS 5-ton blower for use with the 25HPA6 3-ton 2-stage, and set the blower to the 3-ton setting, then in the low stage, will it not be able to go as low in CFM via Infinity controller as say the 3.5-ton blower package also configured for a 3-ton outdoor unit?

By the way, any advantage of using a 1hp blower motor instead of 1/2hp, in terms of longevity?

 

The 3.5 ton blower can slow down more.
And no advantage at all using a 1 HP over a .5 HP motor.

Don't expect your ESP to only be .4", a wet evap coil and the air filter alone will be more then .2" PD.

 

Ok, thanks. For the moment let's say the blower has plenty of range from high and low. Then when the 25HPA6 3-ton is in low-stage, is there some lower CFM limit to what is practical for performance, especially dehumidifying?

I don't really know the capacity of the 25HPA6 3-ton in low-stage, but I think I remember reading somewhere 60% (which would be 1.8 ton). So what's the lowest usable CFM for 1.8 ton, when trying to dehumidify?


By the way how do basements that are heavily used fit into the overall blower sizing picture? It doesn't seem to factor into the load calculation very much, but what about ventilation. Guess there is no way to provide a lot of ventilation to the basement from the central air without either over-heating in winter or over-cooling the basement, since it has a more even "outside" temperature ???

 

It doesn't seem to factor into the load calculation very much, but what about ventilation. Guess there is no way to provide a lot of ventilation to the basement from the central air without either over-heating in winter or over-cooling the basement, since it has a more even "outside" temperature ???

How could ventilation of an additional space cause overheating or overcooling? It should merely trend towards the thermostat setting. Since you are right that basements do not generally affect the load much, conditioning (ventilation) of the basement should not affect the equipment capacity.

 

Well, I guess you're right, since everything gets mixed in the return air.

But what I was thinking originally was the following, and I'm still thinking there may be something to it.

Let's say there are two rooms of the same size, with the same duct balancing (getting the same CFM I mean). If one room is a lot better insulated than the other, and they are both getting the same CFM from the furnace, and let's say the thermostat is in the less insulated room. Won't the better insulated room see bigger tempreature swings, because the furnace is trying to satisfy most accurately the less insulated room that has the thermostat?

Normally the above situation gets corrected by not pushing the same CFM to the two rooms. In this case, the much better insulated room is the basement. So wouldn't it need a lot less CFM to avoid the temperature swings? If it is Really well insulated, then it would need very little CFM relative to the size of the room (and thus not much ventilation).

 

Anyone here will freely admit that there can be temperature imbalances between rooms for many possible reasons. Those imbalances would be minimized with a room by room load analysis and a total HVAC design based from that. That could well include different sized ducts and CFMs in different rooms, more than one system, or zoning. That would include the basement.

I'm not a professional, but I would think that best design of HVAC would include adequate air exchange in basements (finished ones at least) to help control whole house humidity. Others here may be able to answer your hypotheticals. However, as you've seen, they prefer to answer based on real life situations. Do you have a problem with environmental control in your basement?

 

A blower motor uprgrade called the Concept3 has a dehum setting it runs the fan at 85% capacity meaning a 3 ton system would need to have about 1020 cfm for proper dehum. I

 

Hopefully, if he is getting a 2 stage outdoor unit, he also getting a ECM VS blower unit.

 

Why would you have the same air flow to a less insulated room, as you do a more/better insulated room?

You size the supply to what the room needs. Not to its sq ft.

 

The infinity control will determine what CFM to use to dehumidify.

So in first stage, the CFM it uses, will vary greatly. And is not comparable to other units.

Thats why people don't get blowers bigger then what they need.

 

Two critical issues.
It is critical that the air flow be low enough to have allow the coil temperature cold enough to provide 45-50^F dew point air during high cooling loads. This is usually about 325 cfm per ton of cooling capacity. Two speed a/cs have poor circulation on low speed and make ducts sweat. The low speed a/c dehumidifying thing will not dehumidifying when there is low/no cooling load. They are slightly better are a half load condition.
Get the best dehumidifier you can afford. The high efficiency units like Ultra-Aire/Honeywell/Santa Fe/Rheem etc are very durable. Also they remove much more moisture at low temps and 2-3 X more moisture per KW.
I would put the extra money of the 2 speed a/c toward a good dehumidifier. The salesman that sold me my VS furnace, claimed to be able to pretty much dehumidify. When I asked the tech about the lack of dehumidification, tech says he is just a saleman. Ok, I knew the a/c would not be able to keep my basement dry. We both had a chuckle. I remove 2-3 gallons of moisture per day when the outside dew point is +65^F.
Regards TB

 

TB, you have my attention.

How does the whole house dehumidifier work? Does it have its own compressor, so that it cools the air on the evaporator coil and heats it back up on the condenser coil?

Does it need to be installed at the same time as the furnace, or can it be installed later?

Is electric bill from it pretty small? How much noise added to the basement with his thing running? If it's noisy, then when spending time in the basement, any reason we can't just switch it off for a while and then turn it back on later?

With a whole-house dehumidifier, is there really much reason to get 2-stage AC anymore? Like you said, the cost savings of a 1-stage could be used to offset the dehumidifier.

Why aren't they more common (or maybe they are and I just didn't know).

 

TB, you have my attention.

How does the whole house dehumidifier work? Does it have its own compressor, so that it cools the air on the evaporator coil and heats it back up on the condenser coil?

Does it need to be installed at the same time as the furnace, or can it be installed later?
).

It has its own compressor. It has an evaporator and a condenser with a couple extras for efficiency. The WH dehu can be added later. It probably cost less to have both installed on the same trip.

TB, you
Is electric bill from it pretty small? How much noise added to the basement with his thing running? If it's noisy, then when spending time in the basement, any reason we can't just switch it off for a while and then turn it back on later?

With a whole-house dehumidifier, is there really much reason to get 2-stage AC anymore? Like you said, the cost savings of a 1-stage could be used to offset the dehumidifier.

Why aren't they more common (or maybe they are and I just didn't know).

They remove 3-6 lbs. of moisture per KW. Typically the remove 30-50 lbs. of moisture during days of high outdoor dew point and low cooling loads. On real hot days, the dehu does not to operate because the a/c removes enough moisture to maintain <50%RH. I would expect $15-$20 per shoulder season month.
The noise is like a refrigerator. You can run them on a timer to avoid any noise when you are routinely near. 4-6 hours of interuption is not a problem.
A WH dehu eliminates over-cooling or even running the a/c when the home is unoccupied. I would opt for the dehu instead of the 2nd stage a/c. The ducts are more prone to sweating and the low air flow provides poor air circulation. The large cooling coil retains many lbs. of moisture which reevaporates back into the via the a/c ducts.
The main reason there are not more is that they are an additional investment. A/c contractors do not promote them because they are afraid to admit the weakness of the a/c not being able to maintain <50%RH during low/no cooling loads. Also this is a recently introduced product. We have been providing them for 8 years. I hope this helps. Keep us posted.
Regards TB

 

Specs for Lennox are hard to find. All my info is based on GV71 since I have not found SLP98 manual though I suspect they are very similar.

Oversized blower:

I believe either a 1/2(-36) or 1 hp(-60) motor is used depending on CFM required. Being a variable speed motor helps, but larger motor will always consume more power. Not a big deal, as long as can limit output (which can be done thru DIP switches on main circuit board). These DIP switches can both select regular and constant fan speeds separately. E.g. continuous blower mode can be 28%, 38%, 70% or 100% of max fan speed. If the Harmony III(Zoning) is attached, minimum blowing is 250CFM for the 1/2 hp motor and 450CFM for the 1hp versions. This can be significant if you have zoning with a very small zone that wants less than 450CFM. In summary, being a little off probably not a big deal. But affects the lowest speed possible, the highest speed possible and power consumption.

RE: icomfort thermostat without compatible A/C unit:

icomfort thermostat is grossly overpriced. It is a very nice thermostat, but if you do not need the special control logic, can probably do as well with nonLennox thermostat and save money. Lennox is quite vague about many things. Much of the control logic is in the circuit board for the SLP98 furnace. The question is how much of this is used without the control thermostat. A great deal of staging decisions are made through DIP switches on the furnace circuit board including fan speeds under heating/cooling and ramp up of motor/burners during heating/cooling. The furnace can even act as a two stage furnace(70%/100%) when hooked up to a single stage thermostat letting the thermostat make staging decisions (a real waste IMHO).

Per specs: used with single stage A/C and ComfortSense 7000 thermostat, will run A/C on full with fan at 70% on dehumidification call.

with dual stage A/C configure to:
stage 1+ 70 fan (low cooling)
stage2+100% fan (high cooling)
stage 2+70%fan(dehumidify)