Can Anyone Help Me with Sizing A/C

Kal Row

ask how well can it handle the voltage spikes of a thunder storm - i dont know – nor can i find any docs or test reports on their site, I would surge protect it to death, and expect a lot of false “trips”, from the protection equipment – do you have spare parts?, does your rep?
how good are you at troubleshooting electronics? They insist on training before you can buy it, fair enough, but one day training, is inadequate to the task, for the avg ac contractor to really learn it all – electronics have a way of flustering troubleshooters, so that they forget their basics

oh yeah – you forgot to mention the electronically controlled expansion valve too – this is not your papa’s ac – but they do have a real simple liquid line press/temp charging chart

electricity out here ranges at 20+ cents per kw and we don’t see a big clamoring for them

however if you do put one in - ground everything - make like you are an anal inspector,
and test – make sure that you have 0v from neutral to ground everywhere – and do post your result - we’ll be waiting ;)

PeterGriffin

So confused..,.


Hey guys, you might remember, I'm the second year apprentice who's still on the receiving end for help...

My boss is confusing the heck outta me here. We were sent to a job on Thursday, to replace a unit, where last year he had installed a 2t setup into a 2800sq ft home. The job was done for a friend, and the friend was complaining it ran all day, cooled the house 2*C, and his electrical bill was $400/month. On Wednesday, the day before, we were installing a 2.5t unit into a mobile home, not a double wide or extra long, just a pretty basic 20ft wide mobile home.

In another instance, about 6 weeks ago, we were replacing an older mobile home Intertherm furnace with a 90k BTU furnace, and he said we'd need a 1.5t or 2t if they had none of the first choice, upon arriving at the wholesaler, I was informed that if it's 90k btu, I'm going to need a 2.5t unit. I brought up the inconsistency with my boss 6 weeks ago, and he said, well, generally, I just size them by the formula "half a ton does 500 sq ft, maybe a bit better on newer homes."

To which I accepted then, but after being in this 2800sq ft home, even by his formula, is way undersized is it not?

Is there something I'm missing?

Thanks!

Empire_2

Hello Rankin

The only was to truly size A/C is with load calculations Period.... The trailer you had in mind?, well I have seen 3.0 ton in these units, which is way over sized, but there is such small duct that some people try to JAM the air through. Here in the NE, we size for 80* day (which by the way I am upping to 90*) and the unit if properly sized has a tendency to run all day. Be careful not to oversize because now you will end up with a clammy house. No pun intended "Clammy" :-)

Mike T.

Brad White_179

Mike T. is right-

Calculations are the only real way. This starts with orientation. The very same structure rotated 90 degrees could use double or half the tonnage of your "control group". Solar exposure, external versus internal shade are huge contributors. Internal loads tend not to vary too much in a house, maybe 1.0 to 1.25 Watts PSF (3.413 BTUH sensible per SF) but break it down by light fixtures, computers and what have you, if you possibly can. See what that comes out too.

From the sounds of your array of structures and system sizes, you have seen first hand what a Scientific Wild **** Guess (SWAG) can achieve.

Mike's point about not over-sizing is particularly important. Under-size slightly if you possibly can. This is critical for what are typically "fixed" (on-off) capacity systems. My rule of thumb is about "20 percent off", meaning if I calculate a 5-ton system, a 4-ton will serve better for more hours by dehumidifying ("running to catch up" as it were). 3 tons? Maybe a 2.5 ton system depending on your evaporator to condenser match-up.

Two-Speed condensers are a blessing by the way. I am waiting for inverter driven compressors to hit the split system market the way the Mitsubishi City-Multi and Daiken offerings have revolutionized the "Ductless Split System" market. Alas, those condensing units only match up to their evaporator product line.

If sized for an extreme peak which occurs 20 hours per year, you will cycle a lot when it is off those peaks. Outdoor temperature has a lot less to do with it than solar gains via glass and on a roof.

don_185

Its out.

Nordyne offers IQ drive.Ac only system as of now however they are planning on the heatpump in the future.

The boys and I went to school on them last month.Talk about high tech.Varible speed indoor and outdoor blower motors.
Variable speed rotary compressor along with electronic step expansion valve.

I cant wait to sell and put one in.Would you like one for your house Mr White?

PeterGriffin

Excellent


Awesome info guys, so if I calculate the watts/PSF off all the lighting and heat emitting appliances I should have a pretty rough idea? How do I compare this up against the average outdoor temperature of 80*/90* and the sun exposure to make a proper estimate? Do I use 3.413 btuh/psf for my exposed outdoor walls as well?
Would the shared wall on a duplex/townhouse be treated the same as an outside wall or an outside wall at ~70*?
When I'm done with my BTU calculations I would then use 1 ton/12k btu?

Sorry to bombard you with questions, I know these should be directed at my boss, but I lack one with the knowledge you guys seem to have acquired.

I've looked around on a few sites that give me a general idea, but all of them refer me to a Local Competent Contractor...

Thanks guys

Brad White

Interior Loads

in Watts per SF are calculated based on floor area, not wall area. Remember, these are really "ballpark" numbers to be used only if actual counts of lights and equipment are unknown.

Add to the power and lighting loads your human occupant loads which average 225-245 BTUH (sensible now, we will get to that in a minute) per person. Those human sensible loads are for women and men respectively, seated at rest. Throw in a Thigh Master and the numbers go up quite a bit.

After you are done with internal gains, the sun hitting glass, entering the building or the space between windows and drapes is next. Sun hitting a roof or wall with any mass may not show up until later in the day due to mass and time lag. External loads are best done with a decent computer program (I use Elite Software's "CHVAC" but there are others). The sun's path, thermal and mass properties, time of day, time of year, all contribute greatly. Sometimes November solar gains, when the sun is lower, exceed summer gains. SW glass can impose a peak load on some buildings. See how complicated that can get?

Sensible Heat: What we are talking about here is all sensible heat, dealing with temperature only. The goal is to figure out "how much air at what temperature difference" is needed to cool a given space. If you have 12,000 BTUH of sensible heat, that is not a "ton" per se. It is a sensible heat number only. A "ton" in general AC use as a term, refers to the total heat capacity, sum of latent and sensible heat.

"Latent load" or moisture-based cooling, is, as far as packaged equipment or matched condensing unit and evaporators are concerned, what is left over after sensible heat is taken care of. For example: A 12,000 BTUH air conditioner (one ton) might have a sensible heat capacity of 9,000 BTUH. The remaining 3,000 BTUH is latent heat, dedicated to removing moisture. The sensible heat ration (SHR), 0.75 in the above case, can vary between 0.70 and 0.90 depending on the entering air conditions of temperature and humidity, the saturated suction temperature and coil characteristics.

Did I leave anything out? You bet I did. A lot to cover in a small post.

PeterGriffin

Complicated indeed

But well received!!

I really want to get to learn all there is to know about this stuff. Thanks so much for all the help so far, do you need apprentices?

jim lockard

I cheat

On mobile homes (the new ones) they have a heat load/cooling gain plate (must be in every trailer) so I find that plate, in the closet or under the sink and i match up my Heating/cooling to the plate and it works. J.Lockard

Brad White_181

Dang, Jim....

For years I have been searching every retrofit building I was working on for one of those name plates... You think all of that crawling around with a tape measure was to actually do a take-off? Uh-uh. It was to find that legendary BTU plate, yes sir!

In the end, I would find lots of them, on every computer, light, centrifuge... then you have to add them up....

Brad White_181

Rankin

Check your e-mail. I sent you some reading material

John MacGregor_2

AC Sizing

Rankin, The ONLY way to size AC is by doing a heat loss/gain, either by hand or computer. Here in Michigan the building code stipulates that 'M1401.3 Sizing. Heating and cooling equipment shall be sized based on building loads calculated in accordance with the provisions of ACCA Manual J-1987, as listed in chapter 43 or other approved heating and cooling calculating methodologies. Ductwork shall be sized in accordance with the provisions of ACCA Manual D-1995, as listed in chapter 43.' Any other rule of thumb is suicidal and bound to leave unhappy customers. You can contact WrightSoft.com for the software.

bill phillips sr_2

heat gain programs

Has anybody tried Thomas Associates EZ Heatloss program?

jim lockard

well brad

I do not know about all your retro fit buildings. I do know that in a mobile home the duct work comes preinstalled from the factory. I know to Brad that the system size is going to be bound by the size of the duct work, so maybe you will need to add a ductless system to cool or heat the trailer, cause you only going to get so much from the duct work. Brad we have done right many trailers and so far everyones been pleased, including the "Spam cans in the sun". Keep your returns high. Best Wishes J.Lockard

PeterGriffin

Got it!!


Thank you so very much, sir! I have read over two of the papers already, but I think I'll need a refresher or two. I think architects have a bit of a head start on me


I'm sure I'll have some questions over the next few days.

That's great stuff, I appreciate it.

Barry Hanold

needing gas nipple

I need to locate a brass nipple for Nitrogen gas cylinder to air conditioning charging hose. On the nipple - one side is inert gas threads and one side is air conditioning charging hose threads. Does anyone know each thread specifications? Does anyone know a nipple source?

Barry Hanold

sorry... wrong thread. I reposted.

tim smith

Brad, Regarding city multi

I would expect you know this but just to inform you their are 4 ton city multi available now. Inverter drive but no heat reclaim like the commercial 3 phase unit that starts I think at 7 tons. I am installing a Fujitsu inverter drive with compact ducted air handlers as we speak, pretty interesting. We shall see how she does. Tim

Brad White

Thanks Tim

I am familiar with that and the Mitsubishi and Daiken line. Yes they do have inverter technology but only within their system. My hope was for the inverter technology to find it's way into the regular split system market. I asked my Mitsubishi rep and their system is not interchangeable with other systems/products.

For now I have to settle for 2-speed I suppose.

Eugene Silberstein 3

Learn the Trade...Not the Tricks

My students are constantly asking me to let them in on the "tricks of the trade". As many have said before me, "Don't learn the tricks of the trade... Learn the trade".

As Mike T. and Brad have mentioned, the ONLY way to determine the heat gain or loss of a particular structure is to actually calculate the heat gain or loss of the structure.

Unfortunately, there is no short cut to doing things right.