You must not have a service background. Im guessing 25% of the systems out there have 250 cfm per ton. Drawbacks are they are rough on compressors with pistons for exp device, and rough on gas furnaces in the heating season.

 

Just because a system is running 250 cfm or less a ton doesn't make it a SDHV system.
A SDHV system is designed for structures with limited space.The ducts are intentionally small and run under a fair amount of pressure.

 

The air handler is designed for the small duct systems. Fat coils, big motors. The air handlers have a freeze stat on the coil since the coil temp can easily drop below freezing. The capacity and efficiency is lowered. Often the AHRI ratings is 1/2 ton below what the outdoor unit can usually do. Getting much efficiency is rare. For example, the Lennox XC16-036 runs not 3 tons and 16 SEER but 11.55 to 12.15 SEER and 29,000-30,000 BTU output.

The systems work IF done properly. If you can't use conventional ducts, it's an alternative. Pricey but functional.

 

There is no work-around required. The evap coil is very thick (six or eight rows) and runs at just about freezing - about 33-34º F. The coil TD is about 30º.

The only con to them is that you can't match a furnace to them for heating. Because of the 2" diameter air ducts and the lower-than-standard air flow. But using them as a heat pump, or with electric elements, or with a hydronic coil in the air return, works very well.

Pros are - easy to install, forgiving of ham-handed installation, they remove more moisture, and are unobtrusive. Typically just one small small hole in each ceiling corner.

Why not request some installation literature from Unico and pore over it? Makes some interesting reading. Well; it would for me anyway. <g>

PHM
--------

As they are intended to run at the lower cfm
But ideally could be made more efficient by keeping the cfm the same and using a smaller condenser to bring the cfm per ton back up.
Thats kinda what I am tossing around here..I dont see a reason why it wouldnt work.
250 cfm a ton is to low for some areas such as AZ...It would work here in Mo..but is still low for most residential applications.

 

There is no work-around required. The evap coil is very thick (six or eight rows) and runs at just about freezing - about 33-34º F. The coil TD is about 30º.

The only con to them is that you can't match a furnace to them for heating. Because of the 2" diameter air ducts and the lower-than-standard air flow. But using them as a heat pump, or with electric elements, or with a hydronic coil in the air return, works very well.

Pros are - easy to install, forgiving of ham-handed installation, they remove more moisture, and are unobtrusive. Typically just one small small hole in each ceiling corner.

Why not request some installation literature from Unico and pore over it? Makes some interesting reading. Well; it would for me anyway. <g>

PHM
--------

 

I recently serviced a high velocity system. Worked great it had a drain issue. The down side zero room to work on it. As said in previous post that's what they are meant for, limited spaces.

 

How was the noise?...Intake and supplies both

 

I did see one that was noisy but it still worked well.

 

Spacepak

 

I havent heard anything bad about Spacepak yet...other than a little pricey,but if the equipment is good then the extra cost balances out.

 

Compressor efficiency or system efficiency?

 

What would happen? Well; the air flow would be greater (per ton / constant for the AH) so the suction pressure would trend higher. The compressor would pump more BTU's per minute and the compressor HP would increase.

Less latent cooling would be done - but would more sensible cooling be done?

I am having a little trouble picturing the net cooling effects in the space. A HV system is based Not on moving all the conditioned space's air volume through the evaporator - but rather on only moving a portion of it across the evaporator coil, but supercooling that portion, and then using that supercooled air to cool the remainder of the air volume in the conditioned space.

Once you stop supercooling the evaporator air flow - how will the remaining air in the conditioned space be cooled?

Tell me what you think will happen.

 

I'm not sure the normal 250cfm/ton of the typical SDHV system is any worse in a dry environment that a wet one. I'm not sure what they use for their freeze prevention controls though. If they simply use a temperature limit, then yes, it could be wasteful - but if you could run with a dewpoint based control (sensing return air dewpoint) and limit discharge air temp based on that, it might really work well in dry climates (dewpoint here at my house is 14°F right now, but 86°F outside - really don't have to worry about icing up anything this time of year).

SDHV is never going to be as efficient as large duct units, due mostly to the blower hp needed - but that is the tradeoff for the space limitation.

I'd like to point out that unico also makes another great alternative for limited space installations, that doesn't lose the efficiency the SDHV DX systems do, and has amazing zoning capabilities.

 

What Unico (and the rest of them that I have seen) use is a freeze stat which opens at about 30-31º.

What I use is an EPR valve set to about 31-32º evaporating temperature.

I like to run the airflow lean and the discharge air temp as low as possible but I don't like cycling the compressor on and off for no good reason. Unico makes a nice system but an EPR makes it better and only costs $100. or so to add.

If you were to use a dewpoint based control system - to limit discharge air temp - what would you be controlling exactly?

Have you used "the other great alternative" from Unico?

PHM
------

 

I keep seeing people mention HV system air velocity noise. But I don't find that to be an issue so long as air flow is set up properly. The systems seem to be pretty much silent in operation. And you can always use more sound attenuating tubing anyway. I have done installs where the full length of all the 2" ducts were done with sound attenuating tubing. I do always use a 24" by 24" return air filter grille and 20" insulated flex for the return though.

If the units start to make noise to me it means that it's time for pressure washing the blower wheels clean and new bearings in the blower motor. Fresh bearings and clean wheels - or a new motor and wheels makes them quiet again. But most people don't let me do that - they just put up with the increasing noise until something eventually scrambles; usually a refrigerant leak or a drain pan leak. And then I replace the air handler. The Unico AH in my own house only lasted about 20 years or so. Actually a little less as I just added refrigerant for the last two years I had it. <g>

 

Put in a TXV (if it doesn't already have one) and a controller to throttle back the fan speed if the evaporating temperature (suction pressure) goes too high.

 

Thats just it, we don't know if there is a specific reason SDHV has to operate at 250 cfm per ton.
I cant think of a reason why it has to...but cant find anything definitive.
I dont think you are supposed to operate it beyond factory fan speed..due to the high static.
But I am proposing using a smaller condenser with a larger air handler to get the suction back up.

 

10,255 is the total BTU difference between 15,000CF (250 per ton) and 19,980 CF (333 CFM per ton).

HV systems are designed to have a 32 to 36°F coil. They are slab coils, not A coils.

The compressor over heat will occur on the outdoor design temp days, or when the outdoor temp is above the design temp.

Didn't come up with 40,000 at 250CFM per ton.

 

10,255 is the total BTU difference between 15,000CF (250 per ton) and 19,980 CF (333 CFM per ton).

Whch you claimed that by increasing the load {25% increase in cfm} increased the load by 10,255 btu

Didn't come up with 40,000 at 250CFM per ton

I did as you said 10,255 btu load increase would results which meant the original load would be over 40 k as 10 k is a fourth of 40k
Simple algebra.
Why does the ton and a half system have a 40k load at 250 cfm per ton?

HV systems are designed to have a 32 to 36°F coil. They are slab coils, not A coils.

The shape of the coil doesnt determine what temp you want to operate it at.

The compressor over heat will occur on the outdoor design temp days, or when the outdoor temp is above the design temp.

How is operating at 333 cfm per ton going to cause this?
What design conditions are these?Where?

 

Unico's claim is that the losses in compressor efficiency are compensated for by some other factors of their system's operation. I think they say what it is on their website but I can't remember what they say exactly right now.

I have a Unico system for my second floor - somewhat "frankensteined" now - a 2430 (2 to 3 ton) but with a 1.5 ton heat pump condensing unit outside for it - despite the fact that the attic is large and could easily accommodate a large-duct system. I did originally ponder the "efficiency" issue but somewhat rationalized it to myself with the thought that, with the greater moisture removal and resulting lower humidity, the setpoint temperature on the stat could be likely be set higher and the compressor efficiency losses could be regained by the higher comfort-point of the space.

The supply duct/plenum is 8" square ductboard - so 10" outside. And all but four of the outlets are in the ceiling (one in each corner of each room) so the 4" OD register ducts lay on top of the 6" rockwool in the ceiling joists. The AH has it's own direct-access panel from the hallway below and the platform the AH sets on has a plywood 'dam' around it. There is about 10-12" or so of cellulose which buries all the air ducts so all but the '3 by 4' AH platform is pretty well insulated.

Installing it was so fast and so easy that I am always surprised that Unico systems aren't almost universal. Picture that even in a cellar install only the 9-10" supply duct would be below the floor joists. The supplies could all come off the top and so be completely within the joist bay spaces. The register ducts and outlets get assembled all at once on a workbench with pinch clamps and then fed like a slinky into the outlet hole. Pop the other end onto the 2" take-off collar and you're done. One guy could easily install both floors of my two story house in a day.

It seems to have every advantage to me.

SEER rating just ain't Everything. <g>

 

HV usually claims it makes up for the lower SEER by having a greater shr. Allowing the customer to set the tstat a couple degrees higher then they would with a conventional system.

 

If that were the case why have manufacturers been steadily increasing SST's in order to squeeze every bit of SEER they can out of their systems? HV systems use deep multi row coils to keep SST above freezing while keeping airflow low as possible. HV systems are great for small spaces, but there is some efficiency loss due to the low SST's. There is also some capacity loss, lower SST's will result in lower capacity from the same compressor. Their ductwork *may* be less susceptible to poor connections/leaks/kinks since it's easier to use "hard pipe". IMHO ductwork is the big killer of installed efficiency for most systems.

 

The whole point of the unico system is to be able to install central air in a space that couldn't fit a conventional flow'd system. Since a system is sized for cooling capacity, not air delivery, in reality, you wouldn't ever be 'downsizing a condenser', but instead would be upsizing the air handler. Since doing so also increases duct size and/or count, you are talking about diminishing the only real reason to ever install a SDHV system.

 

Since doing so also increases duct size and/or count, you are talking about diminishing the only real reason to ever install a SDHV system.

True but it still gives the person the option and not be forced in at 250 cfm per ton.

 

a bit different than possible icing, but this shows a relationship of the s/t to face affects of an A-Coil


1979 TETCO Thermal Energy Transfer Corp OEM formerly Columbus Ohio 3rd Owner: Enertec, IL

A-coil Downflow in cabinet 29 x 29 x 27h ~7.5 Ton of 7.2sqft 3row .016wall/ 5/16 OD 12FP-inch

ew

4 GPM
40F 45F 50F
CFM BTUh EER S/T BTUh EER S/T BTUh EER S/T
800 37,400 68.9 0.643 32,500 59.9 0.679 25,700 47.3 0.757
1000 39,700 64.5 0.681 34,500 56.0 0.724 27,200 44.2 0.818
1200 41,700 60.5 0.717 36,200 52.5 0.769 28,600 41.5 0.878
1400 42,900 56.3 0.759 37,300 49.0 0.816 29,500 38.7 0.941
1600 44,000 52.7 0.797 38,200 45.7 0.864 30,200 36.2 1.000
1800 44,900 49.4 0.837 39,000 43.0 0.910 30,800 33.9 1.000


6 GPM
40F 45F 50F
CFM BTUh EER S/T BTUh EER S/T BTUh EER S/T
800 43,600 65.2 0.613 37,900 56.7 0.640 29,900 44.7 0.703
1000 47,000 63.3 0.638 40,900 55.1 0.672 32,300 43.5 0.748
1200 50,000 62.1 0.661 44,000 54.0 0.698 34,800 42.7 0.786
1400 53,900 60.7 0.681 46,900 52.8 0.725 37,000 41.7 0.821
1600 59,000 57.1 0.724 51,300 49.6 0.777 40,500 39.2 0.890


8 GPM
40F 45F 50F
CFM BTUh EER S/T BTUh EER S/T BTUh EER S/T
800 46,500 58.6 0.603 40,400 50.9 0.627 31,900 40.2 0.684
1000 51,100 58.9 0.621 44,400 51.2 0.651 35,100 40.5 0.717
1200 55,750 59.3 0.638 48,500 51.6 0.672 38,300 40.7 0.744
1400 60,000 59.3 0.652 52,200 51.6 0.691 41,200 40.7 0.774
1600 63,500 58.5 0.669 55,200 50.8 0.711 43,600 40.1 0.801
1800 66,900 57.7 0.684 58,100 50.1 0.730 45,900 39.6 0.826


10 GPM
40F 45F 50F
CFM BTUh EER S/T BTUh EER S/T BTUh EER S/T
800 47,500 51.6 0.600 41,300 44.9 0.623 32,700 35.5 0.678
1000 53,500 53.9 0.613 46,600 46.9 0.640 36,800 37.1 0.704
1200 58,500 54.9 0.627 50,800 47.7 0.660 40,200 37.7 0.731
1400 63,300 55.6 0.641 55,000 48.3 0.676 43,400 38.1 0.753
1600 69,300 57.2 0.647 60,300 49.8 0.684 47,600 39.3 0.765
1800 73,500 57.2 0.658 63,900 49.7 0.698 50,500 39.3 0.784


12 GPM
40F 45F 50F
CFM BTUh EER S/T BTUh EER S/T BTUh EER S/T
800 49,500 47.4 0.595 43,000 41.1 0.615 34,000 32.5 0.667
1000 55,300 49.5 0.607 48,100 43.0 0.633 38,000 34.0 0.693
1200 60,800 51.0 0.620 52,800 44.3 0.650 41,700 35.0 0.717
1400 66,700 52.8 0.629 58,000 45.9 0.661 45,800 36.2 0.734
1600 71,600 53.6 0.639 62,200 46.5 0.675 49,100 36.7 0.752
1800 76,000 53.9 0.650 66,100 46.9 0.688 52,200 37.0 0.770


14 GPM
40F 45F 50F
CFM BTUh EER S/T BTUh EER S/T BTUh EER S/T
800 50,700 43.3 0.592 44,100 37.7 0.611 34,800 29.7 0.661
1000 57,300 46.1 0.602 49,900 40.1 0.626 39,400 31.7 0.683
1200 63,500 48.2 0.612 55,250 42.0 0.639 43,600 33.1 0.703
1400 69,000 49.6 0.622 60,000 43.2 0.653 47,400 34.1 0.722
1600 75,100 51.3 0.628 65,300 44.6 0.662 51,600 35.3 0.735
1800 79,700 51.9 0.639 69,300 48.9 0.674 54,750 35.6 0.752

 

the columns that were in line were the three sets across
under

BTUH EER S/T in 3 column sets on my end. Entering Water of 40 , 45, 50F respectively at
various
GPM
to demonstrate just a relationship to cooler or warmer face of coil at low CFM per 'TON'
at your choice of s/total cooling

Entering Air of 80/67

 

Wait a minute Mr. Missouri !

I just realized that if I had any real interest I could work on your question empirically. Because I already do, in fact; already have a 2-2.5 ton Unico AH piped to a 1.5 ton condensing unit. <g>

But I would have to do all the measurements as my 'start-up' consisted primarily of charging to about 10º SC, adjusting the air flow so as to provide a 100 lb. suction pressure, and adjusting for about 16º SSH at the condenser. <g>

Of course I have no real positive idea - but I suspect that I have about 375 CFM through the AH as I now have 9.33 outlets per ton. <g>

 

Arghhh the man has the very system we are talking about and just now realizes it....Remembers the pressures and temperatures as well....When I forget I forget..but when I remember I remember well.:grin2:
Get the airflow increased so you have about 50 degree supply air and lets see if the world ends...you can then go back to just above freezing with that coil and keep lining the pocket of the electric company...{that is if you dont forget you still have the darn thing.}:grin2:

 

MG you are fixated on steady state conditions, load is dynamic, refrigeration effect is dynamic. Comfort level is personal thus dynamic.
With mass produced equipment, it has to be designed to "work" in a wide variety ambient's, for a cost, while looking for a high averaged efficiency.
If you want to focus on efficiency and comfort, then look into control methodology, knowing what your variables are, temps, enthalpy, CO2, then having the control variability to achieve a required set point, "refrigerant mass flow, saturated pressures, air flow, force fresh air make up".
In a technical discussion you can not just use a simple CFM/ton without a reference to temperatures, and the effect on the net refrigeration.

 

The question is easy.
Can we up size the air handler and supplies on an HV system so were are not running 250 cfm per ton but rather 333cfm per ton
If not why not.
While we wouldnt want to run 333 cfm per ton if the return air was 95db 85 wb that goes without saying.
Just like we wouldnt want to run 250 cfm per ton if the return is 69db 53 wb..also goes without saying.

So other than very unusual conditions is there any reason we can not use a larger air handler
If not why not.

 

Hammer high ratings with adjusted REAL NEEDED DEHUMIDIFICATION

MG:
Presume PHD (phm) and BThere are accurate in mind and now share expressions like mine that you are just not reading because of a couple of things left out, I believe I see in my own gramantics to outline:
1) For a 'fit' in a situation
2) Like exactly the same as by-pass with conventional only discharging 275 per "ton"
3) Note my posts HAMMER ahri rated "tons" just as ACCA has for years and Dr Steve Kavanaugh (start searches for high seer and efficiency) is just not dehumidifying---
4) A "ton" thrown all over this site rarely speaks of 275 per 40k compressor in a 4 "ton" rated whatever
[VFD and Inverters aside: why? Same reason a VFD 3 hp water pump is very efficient for 2.5 HP in and 2.6hp of relative related work done, compared to usual fixed speed 2hp doing 1.9 hp of good work done... % wise we will not enter VFD Iq here yet...
SOOOOOOOOOOOOOOOOOOOO
at 275 x 4 ton = CFM related to say Unico Spec, etc (and you can call them tomorrow for better semantics than ours here read...-) 1100 then OEM cfm / 40k*12k per compressor ton LABEL === a whopping 330 per actual TON that is very very close to MANY OEM's allowing 340/c-ton for continuous Ent.Air76/64 conditioning
5) Then stack coils in series and loose all the super high SalesEER's (SEER- our bias in sales) BUT get MORE net results by the higher s/t ratio's even per kwh than losing to poorly designed by-pass attempts, because out of the box, even a Spacepak worked well , ~ 20-25% labeled compressor-tons shy of the systemic rating of the SDHV
[[anyone Queried to writing a Q to me, use the number of the outline, please...]]

6) VIEW (as I see you really are in a different angle/ like a problem in algebra, here) VIEW the allowances for extended series heat exchange
and
6B) I think you will find velocity per face area sqft IS HIGHER in sdhv with deep series plating, and as CAN BE in by-pass across a supposed 400 cfm-rated-ton coil. (a bit) (quite different than the coil I tried to table to see any relationships: but had near 150 cfm/sq ft of coil at one listing- just try to expand it under three sets...look at a first column under 40 deg EW, closer to DX, relationships remain the similarly-ly-same-
per sq ft coil face CFM and VELOCITY and using net discharge air comparing to actual across the coil face, et al:
I believe velocity through the fins of greater DEPTHS as I understand what I saw in the field, SDHV face velocity IS HIGHER like a by-pass yields across the coil (can be actualy/ sqft speeds) too, as it does that compared to all the "net cfm per c-Ton or rated ton or actual net cfm per coil cross section in the airstream of a by pass (not counting the actual cfm looping-bypassing discharge, over-running the coil) would "LOOK" slow per coil sq ft going by net again at the outlet, just like a Unico, and "per ton" to net in the supplies (I used 5 to 6 supplies per compressor ton of the ~ ? 2" ID)

7) THEN a tiny cross-section piping was allowed any way AFTER the design of the HIGH V across the coils/ wetter-coils, /-very very dehumidifying coils: out of the box

8) Again call Unico; and then "see" their trade-offs have a "fit"

9) Where those failed a contractor and were ripped out was when RULES of THUMB and STANDARD HVAC cooling were confused with OEM SDHV desired layout... -[of 20 tons of 23 net cooling tons I said was required] [result of the "know" competitor selling over the GT W:W fan-coils I was called to examine... longer story than my posts...] installer failed to accept the 27 (twenty-seven) x ~5 to 6 supply runs per ton for the 275 cfm/ rated TWENTY-SEVEN box-tons to get to that net 23 tons required... AGAIN: 330 cfm x 23 c-Tons for the 275 x 27 Rated tons to just meet minimum air conditioning and dehumidifying

PLEASE AGAIN shoot any Q by line number as I am tired now. Good Day. Forgive a math fault , if I missed, as I did not show all my calcs.

 

YES, I have reason to believe, you CAN put higher loading on the condenser than imagined-not, in having a higher-btuh exchanger designed for a lower-than-fewer-rowed heat exchange(r).

 

.

 

qualifying the "more dehumidification" per hour dependent on the same all around comfort if a lower sensible to Total cooling (s/T)