View Full Version : LOW CFM effect and limits on evaporator/AC system ??

08-01-2012, 07:46 PM
I am trying to figure out all possibilities for my installation,
and i was wondering what implies to run lower cfm than recommended
on an AC system evap ??

problem is that i'd need something like 4ton
but i've only got 6 fans of ~150cfm each ( rated ~180cfm i believe .. )

and i wanted to have some of them run on lower speed when AC is not required
so other could run full and balance temp for each section

what do you think ?

08-01-2012, 07:54 PM
If there isn't enough airflow over the evaporator coil when the a/c is running, you won't remove enough heat and the water will freeze as it condenses on the coils, blocking airflow. Lower airflow can help for awhile before that happens because it will extract more moisture as long as the fan speed increases before it starts to freeze up. On mult-speed fan systems, they generally start slow, then ramp up. This will be especially problematic with an oversized compressor since it will freeze up the coil much faster than one properly sized.

08-01-2012, 11:43 PM
Thanks JAD for the quick reply.

What i thought ...

Problem is that house has concrete walls and setup is in basement, almost impossible
to add more ducts to get higher cfm ( i used 6 4"PVC runs for all different section ..could add 1 or 2 at basement if required..but it seems to stay within confortable range for now ..22-24c )

Also i wanted not to run all fans on full speed all the time,
were supposed to run on reduced speed ( let's say half speed..just enough so we can't hear in rooms )
and go to full speed when AC is on because a specific room thermo asked for cooling

so let's say 1 out of 3 fans is running at full on the 2nd floor setup
that would make for something like 250-350cfm total at max ..not much

house would need 4-5 tons ( 1600sqft on 2 floors + basement of same size )

so now i'm wondering if i should only get a 4-5 ton unit and install it within my 1000liter water reservoir
that i use for GEO cooling and let the fans ask for what they need

or go with 2 smaller AC setup ..something like 2 2tons
or 2 1.5ton

what do you think ?

08-02-2012, 08:21 AM
You are getting into engineering calculations that are critical for proper operation. Get the install/spec sheet for the air handlers. They will lay out how many cfm you need. A typical dx air handler in the 4~5 ton range will typically use between 1500 and 2100 CFM, so I see problems with your numbers.

08-02-2012, 09:01 AM
Unless this house has a lot of east & west facing windows with high solar gain glazing it's unlikely to need even 4 tons of cooling. Basements rarely have anything like even a quarter the cooling load of the top floor (with it's roof-gains), and the middle story will likely have half that of the top floor. The 1-ton/1000' rule of thumb is usually "close enough" for single-story houses, but not for a 3 story where the bottom is mostly below-grade.

A real Manual-J type cooling load calculation is called for. It's possible that even a 2 ton unit could handle the whole thing if you don't get into mid-day thermostat setbacks, etc.

08-02-2012, 08:47 PM
hi Dana,

no roof ( flat roof with r40~56 insulation ) and a very reflective steel deck :p

house as very large windows all around, but all 3pans double lowE double argon
unfortunately not all sunshades are in place, so i do get some SHG from 8 to 11 am and at late evening
but we have many many high trees all around the house ( trees on 2.5 sides of the house starting at 10ft from walls .. good deciduous coverage on south side )

anyhow, it will all get better when i am finished with sunshades and real doors on all openings :p

i read also a few moments ago that a 1 ton for 1000'sqft for a recent house is more realistic

so i guess i will be looking for 2 1.5ton units
which would only be a little more than what required
but i really need 2 seperate systems

i need you guys to tell me one thing

i've been looking for units for some time,
and i've only read bad reviews about all the large brands ( lenox,goodman/amana, comfortzone etc... )
with many failures within the first years

now i ma puzzled, my old house has a lenox system that has been in used ( heat pump complete system ) for heat and cold for about 11 years now
( parent's house..still running )
never required ANY intervention ..worked 100% of time
and they put the AC ON in may and take it off only in october
( lots of unshaded windows in the wrong direction :p ..but beautifull river view ...neway )

Should i be looking for a more uptodate brand like Fujitsu, Mitsubishi etc ???

Can we use the mini split systems AC condensor with a regular evaporator ????
the price for the condensors are almost the same but they are seer 15-18 compared to 13-14
also looks easier to install ? ( fitted valved lines?? )

08-03-2012, 04:48 AM
Mini splits are matched pairs. You cannot use a different evaporator. The are NOT DIY. The system must still be evacuated, then turned on a super heat/sub cool set using gauges and thermometers.

Just like a TV set you buy today probably wont last for 20 years like your grandpa's philco did, so it is with air conditioning. PRICE WARS, caused by YOU ( and me) the consumers, have forced compromises. Trane prides itself on quality, and that comes with a price. And you could still have a failure. With any brand, the most important aspect is getting from a very solid dealer, who is well supported by the factory, has well qualified techs, and a reputation for standing behind his work.

Your house is rather unique, and screams for a proper manual j calculation. Winging it will not go well. Undersized will leave you unhappy. OVER sized will result in almost as many problems.

08-03-2012, 11:05 AM
1000' of floor area per ton works pretty well in most instance for slab-on-grade new construction in climates like Florida or Georgia.

Montreal's 1% outside design temperature (http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf) is 83F, compared to 94F for Augusta GA, or 100F for Miami, FL, which is enough of a difference to matter.

The high midsummer solar angles in the southern US drive much higher roof gains that you'd likely see in Montreal too. Your roof gains are pretty negligible despite being a flat roof due to your relatively high R factor. (R30 is code in FL & GA). It's the solar gain through windows (and your shading factors) that will drive your cooling load numbers.

I haven't used it recently enough to remember, but you may be able to get both heating & cooling peak load numbers out of Hot2000 (http://canmetenergy.nrcan.gc.ca/software-tools/hot2000/84), which is a freebie download energy-use simulator.

HOT3000 (http://canmetenergy.nrcan.gc.ca/software-tools/hot3000/246) is a more sophisticated energy model that allegedly renders hourly load numbers, but I've never used it.

If your floor plans are relatively open you may be able to cool adequately with ductless multi-splits, but it will be more expensive up front than working with an existing ducted system.

Ductless systems need compatible compressor/evaporator units that use the same communication protocols, since the embedded control software is constantly updating the compressor speed, blower speed, interior head blower speed and adjusting the refrigerant volume for optimal efficiency. While there are a few evaporator units for some mini-splits that work in ducted systems, it's probably not going to be the right solution for you.

08-04-2012, 09:26 PM
WOW very very nice program from NRCanada :)

but i'll need to take time to fill in the info correctly as it gave me a 10 000$ yearly cost of heating ( we just completed first year and first winter period with something like 3000$ of total electricity cost, that includes all heating
and i have leaks everywhere, missing some doors ( taped tyvek/foam in its place..leaking with no end!! )

this gives me a 40 000 but cooling required
so probably will end up much less than that like heating cost :p

I understand that mini splits are not designed to operate with another evaporator now,
done some reading and i've been pointed out to this also a few times now
( unfortunately )

I believe we have waited too long now and will probably buy the AC for next year
since only 2-3 weeks left of heat here, and it wouldn't be installed before that time neway
so not worth it to rush

Do you guys know if Daikin makes a ac compressor that can be used with regular evap ?
seems like it is now the only reliable company for compressors ...and not that much more expensive !!

Too bad that most model now use an "inverter" with variable speed compressors etc...
would've been nice to be able to use those

But seriously, i don't see how it couldn't be use
other than having to bypass the electronic control board
( unless the compressor cannot be turned on regularly )

Will have to think my design again ...
i think i might have enough CFM if i can get out with 2 1.5ton or less units

worth it to consider buying some cheaper brands like goodman lennox etc...
and get on with it ??

not that there is much of a difference as i've found some mitsu and daikin units at less than 300$ more than the same size of goodman/amana

would still be intrigued to find and look at electrical schematics for one of the split units compressor system

jimbo: i understand the analogy and i agree with it
Please do not include me within the CAUSE as i have NEVER purchased based on price solely and i seriously despise the late POS products that all brands are manufacturing in the recent years
i always design and or purchase based on quality and performance factors

that is the same reason why i am considering getting something like daikin or mitsu compressor even if more expensive
and more of an hassle to install

08-05-2012, 01:51 PM
The compressor unit is intimately matched to the evaporator unit and mixing and matching rarely will work. When it comes to the mini-splits, it's even more complicated as each end is 'smart', and they communicate, rather than there just being an a/c on signal for a conventional system.

08-06-2012, 09:03 AM
"would still be intrigued to find and look at electrical schematics for one of the split units compressor system"

Better yet would be the source-code for the control software! These systems sense incoming and exit air temps on both the compressor & evaporator end, and adjust both the blower speed(s) & refrigerant volume to match the sensed load at the highest efficiency possible. Many ductless systems from different manufacturers use the same compressors and digitally controlled refrigerant valves etc (Fujitsu is a major supplier to brands) and are nearly identical under the hood from a component & electrical wiring point of view, but the control algorithms are proprietary, and they matter in terms of getting better efficiency & capacity out of essentially the same system. There is some evidence from field testing done for the NEEA Northwest Ductless Pilot program that newer versions of exactly the same model sometimes outperform the older ones by a significant margin on efficiency, despite IDENTICAL hardware & wiring: The "special sauce" is in the control software (which is constantly being tweaked/improved/upgraded), not so much the hardware.