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HVAC question: condenser size?

mitchedo on Thu July 24, 2008 10:11 AM User is offlineView users profile


I have a question on a central home AC. Those HVAC guys forums are not very helpful. They just want to poke fun at Mr. Do It Yourself.

To make a long story short, My sister has an ancient (30+ years) Lennox central AC in the house she just bought. Home warranty is replacing it. HVAC tech did not size the new unit based on house measurements, just based it off the old one at 4 ton. Based on the size of other systems in our family in the same town, I think 2.5-3 ton would be right.

The condenser housing outside is so old that there are no legible numbers on the tag. There are some legible numbers on the compressor.

How did he determine it was a 4-ton unit? Can you tell by measuring the condenser itself? It's just a flat, rectangular one, like in a car.

If anyone has gotten this far, they're putting in a 3.5 ton evaporator, cuz they couldn't find a 4-ton. It doesn't seem right that the evaporator should be sized smaller than the condenser. ???

On the auto AC front, last year, my stupid Suburban only blew cold air if the rear unit was on. Well, I finally got around to looking at it, and the cycling switch is not right. It cycles the compressor off just fine, but it doesn't cycle back on until the low side is over 75 psi. When the rear unit is on, the compressor never cycles off, so it cools fine. Well, not fine, but some. ...stupid R-12 ban.

Home of the "Tunex Suburban Disaster"

bearing01 on Thu July 24, 2008 11:47 AM User is offline

I would imagine that the larger the condenser coil the better with regards to improved efficiency and energy consumption. The only problem is that, with a more efficient condenser, there may be times where the system won't cool as well. On a capillary tube / critically charged system you could have a scenario where it's now evening and you're just getting home. The house is really hot inside from having windows closed all day & AC off, but outside the temperature has dropped and it's now cool outside. Rather than open a window you turn on the A/C. The condenser, in the cool outside air, will be so good & efficient at condensing the refrigerant that it condenses quickly and a lot of it can back up in the condenser. Also, because the condenser is operating at a cooler temperature (because outside temp is lower) the condenser (high side) pressure will be lower. This will force less refrigerant into the evaporator and can cause the evaporator to starve for refrigerant. As a result, the system won't cool well and behaves as if it's undercharged. The solution is to have a temperature controlled variable speed condenser fan that will reduce air flow over the condenser when it's cool outside. That way the condenser pressure & temperature will have to rise and this will help keep refrigerant fed to the evaporator.

With putting a smaller evaporator in, I would imagine that could then make the compressor appear slightly oversized. It may quickly pump down the evaporator pressure and perhaps cause the system to cycle more frequently. Whether it's behavior will be acceptable or not I don't know. I'm sure that if these guys are reputatable then it will work just fine.

Dougflas on Thu July 24, 2008 5:11 PM User is offline

No No No!! You can not mix equipt. You can't use a 3.5 ton Air handler on a 4 ton condensing unit. you can use 4 ton Air handler on a 3.5 ton condensing unit but never the reverse. You have to balance the sensible and latent. To properly size equipt. you need a load calc. You need to know the direction of the building reference sun load, location of the building,type of windows, size of overhangs, r value of ceilings and walls, area of roof, color of roof,inflitration construction, location of ductwork, r value of ductwork, # of berooms, appliances, Type of window treatments,and a lot more. You can take 2 houses of 12oosq ft and have a differnt load. You need to know the sensible and latent loads to size a hvac system. You do not want short cycling nor high humidity levels.

mitchedo on Fri July 25, 2008 1:27 PM User is offlineView users profile

Well, after much cajoling with the warranty company, the good info from this site, and some input from the previous company they sent out to look at the AC, we have been able to fire the company who was going to install the mixed, grossly-oversized system today. The bad thing is we have to start the whole 3-week process all over again with the new service folks. It's mighty hot.

Home of the "Tunex Suburban Disaster"

NickD on Fri July 25, 2008 2:00 PM User is offline

In my neck of the woods like to oversize the A coil as not to restrict the airflow as it sits on top of the natural gas furnace that essentially the same thing Dougflas is saying. Our heating season is over twice as long as our cooling season, but seen dingbats install a very small evaporator even with reducing plates because the claim you don't need it. So their furnace is cranking all the heat out the chimney instead of the house.

If they install a five wire thermostat, make sure the put in a five wire cable so they won't have the problems my oldest son is having. Younger son paid a fortune for his electrician to run a 10-3 w/ground out to his garage for later 240 VAC to run his air compressor, guy charges a hundred bucks to add an outlet. So he runs it to the garage a foot about the floor and leaves it two feet long, code states at least four feet high. His plumber did a hell of a job in not chalking sinks so water can drip down inside the cabinet to rot them away. Probably why I hate contractors, but once in a great while, run across a good one. Should write a book about stupid excuses contractors make when questioned.

Spent six years of my life as far down south as you can get without AC, survived, hopefully they can for three weeks. Can they take cold showers? Ha, I was much younger then, took cold showers for other reasons.

befuddled on Fri July 25, 2008 10:15 PM User is offline

Glad you canned this bloke.

In the old days, you could buy condensers and evap's seperately and mix and mathc them. Not today. The government's energy rules and regulations have changed everything. The units must be matched, and the unit must be sized for your house. The general idea with HVAC is to slightly undersize it and keep it running all the time. The last thing you want is an oversized unit that cycles on and off. It is not like a furnace that cycles givine you hot and cold, annoying enough. Do that with AC and you never get the humidity out of the house.

There used to be a fellow who sold a HVAC Calculator for something like $39 that would let you plug in your building and location and the program would figure the heat loads for you. It works well. It takes into account exposure to the sun, amount of insulation, glass area etc. It is worth it to keep the contractors honest.

NickD on Sat July 26, 2008 9:44 AM User is offline

Government seems to be getting everywhere into our lives, and with immigration, even into our love life, so why not HVAC as well.

Never made sense to me why OPEC oil crude prices have direct effect on our electrical and natural gas energy costs, my questions to my congressman go unanswered to this subject, but they likewise have more than doubled in the last two years.

That only leaves comparing the efficiencies of either using gas or electrical for HVAC. I read they are working miracles with heat pumps lately and if there is an energy gain of even 1.5 say at 33*F, would be far cheaper to heat my home with a heat pump than using natural gas, just going back a few years, the price jumped from around 60 cents a therm to over two bucks a therm on the last bill. Where the last electrical bill only doubled over the last several years.

I did write to a number of heat pump manufacturers and receive nice colored literature, but all BS, they only rate the energy gain at the best ambient temperatures. I recall back in the old days, whenever that was, they showed a chart as to how this gain decreased with ambient temperature, wrote back and they don't do this anymore. I know they are worthless at subzero temperatures but would just like to change my outside unit for a heat pump, design my own control circuits and switch from the heat pump to gas at the best point. But maybe even with the specifications, the government won't allow this anymore.

A new subdivision is being built outside of town claiming to use geothermal on one acre lots with huge savings on energy bills, found out who makes the system, but again zero specification on exactly what the tradeoffs are and the recovery period. Waste of time for me anyway as my lot is not large enough and against the government law for me to drill two wells to get warm water from one and return it to the other, have to put in a bed.

So what can a guy do when we have a government like this? If the Boston Tea Party kicked off a revolution, what's going on today should completely wipe off Washington DC off the map or just the guys in there. But you still can write to your congressman.

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