Home A/C Condenser Questions

My home's a/c condenser is a little over 20 years old and sound like it about to fly apart. Not surprising as it hasn't shut off since May. I know nothing about these things but was told that 20 years is a "good run" and I should probably start looking for a replacement. I'd like to go in better armed than I am right now so I'd like to pester y'all w/some questions...

I'll probably just wind up w/the same size unit that I have now but I was wondering how you calculate the proper tonnage of a unit? Sq footage only? Do you take into the account the existing air exchanger?

I'd like the move the condenser farther down the house. The builder put it within 8" of the dryer vent so the coils get covered w/lint very quickly. A wall would be too close to the condenser as it sits now. Would it be a big deal to pour another slab and have it moved about 3' away from the vent?

Any makes/models to beware of? This current model is a Trane.

Thanks!

Re: Home A/C Condenser Questions

The condenser unit can be placed anywhere you want it, just remember that the lines may have to be insulated and the electrical supply will also need to be moved.

Most regions now require an electrical shut off box near the unit in order for a maintenance person to kill power without entering the home.

You can either pour a slab or pick up a ready made slab from a contractors supply co/lumber yard.

Sizing the system takes into consideration a number of things, size of the home, insulation value of the home, regional heating/cooling loads (weather).

If there is a Grainger Supply in your area, call and get a catalog, it has a lot of information on calculating heating/cooling loads for a space to size an A/C system.

You can usually oversize the evaporator coil by a 1/2 ton and increase system efficiency, but don't go overboard. Size the compressor/condenser following the heat loss/gain calculations for your insulation and regional weather. Too large a system will 'short cycle', start up and run a short while, shut off for a while and keep repeating the cycle. This tends to increase your operating costs as the major electrical draw required to start the compressor can be more than the amount used in keeping it running longer.

There are newer model variable speed compressors that match motor speed with the cooling load, minimizing start cycles. Price is a factor.

Shade the condenser, many models now have a louvered cover that keeps the condenser shaded from the sun, increasing efficiency.

Re: Home A/C Condenser Questions

To properly size the system, you need to do a complete thermal survey of the house. Few do that, so on to the next best thing, history.

How has the present system run. It appears that this year is not typical, so reach back a couple of years. Did it cycle normally? Was it comfortable? Was electricity usage reasonable? If so, go with the same size.

I find, and it is common consensus, that if in doubt, it's better to slightly undersize than oversize. The reason is that if a system is oversized, it cools down and shuts off before the moisture is removed from the air. If it is undersized, it may not quite meet the thermostat, but the air will be well dried and you will still be comfortable. It will use the least possible amount of energy to get the job done.

You can put the outdoor unit anywhere you want. It should be shaded, and have ample free air.

hope it helps
John

Re: Home A/C Condenser Questions

Dont forget the available Federal and State rebates out there too.

Re: Home A/C Condenser Questions

j_martin is dead on. Most people think bigger is better when it comes to AC. A heat loss survey is tricky....there is no rule of thumb. Do the survey,there is some good info on the net that will help....http://www.mrhvac.com/loadcalcshortform.htm

Re: Home A/C Condenser Questions

bigdee said:

j_martin is dead on. Most people think bigger is better when it comes to AC. A heat loss survey is tricky....there is no rule of thumb. Do the survey,there is some good info on the net that will help....http://www.mrhvac.com/loadcalcshortform.htm

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I used to do surveys using ASHRAE Manual J and paper forms. Now you have to buy a program and do it on your computer.

When you hit it right on, you get the sale. If someone underbids you, you get the replacement/repair down the line.

Re: Home A/C Condenser Questions

I think J_martin has it nailed, too. Only thing I'd differ with is that if you have decent history on your unit, and in 20 years you should, I'd prefer that to a thermal survey unless it truly was properly done....and that costs a few $$$$.
If your old unit ran nearly full-time during the worst weather, it was probably right-sized.

People do seem to think bigger is better.

Best thing that you could do is add insulation and ventilation to your attic////in most cases.

Re: Home A/C Condenser Questions

Great info gentlemen, Thanks!

As far as it's history goes I guess it's been a pretty good unit. Our worst utility bill ever was last month and it was only $186. My only complaints would be the lack of air in the master BR (but it's the farthest away from the exchanger) and the heat pump would cycle on and off twice per second. A tech told me that it did that when it was too cold outside but it got to where I had to leave it on emergency heat all winter.

A coworker replaced his this spring and said that he looked into that rebate. He said that it had to be a 16 (sear?) unit or something in order to qualify. Is that some sort of an energy rating, the higher the more efficient?

Thanks

Re: Home A/C Condenser Questions

CN Spots said:

My only complaints would be the lack of air in the master BR (but it's the farthest away from the exchanger)

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That is more likely to be an air balancing issue than a problem with the AC unit.

CN Spots said:

A coworker replaced his this spring and said that he looked into that rebate. He said that it had to be a 16 (sear?) unit or something in order to qualify. Is that some sort of an energy rating, the higher the more efficient?

Thanks

Click to expand...

SEER, Seasonal Energy Efficiency Ratio...higher is better, but more $$$.

Re: Home A/C Condenser Questions

20 years is really good for a unit Heres the real question can you get a r-22 condenser. Here in florida the supply houses have run out. That means your going to go to r-410a. And you will need to replace both indoor and outdoor units. The lineset and evapcoil can be flushed with cemicals but I dont trust that. Now with the seer rating you dont want an exteme diffence between the indoor and outdoor units. It throws the efficincy out the window. Im saying if you got the money git a whole new system and look at the warrentys they offer most have 10 years on the coils and compressor now and 5 years on everything else with 1 year labor. I know amana has lifetime on their compressors now. Also the tax rebate ends this year. .

Re: Home A/C Condenser Questions

right now 13 seer is the min. effiency you can buy .both indoor and outdoor coils need to be manufactured the same year or it will void your warranty and r22 is out ,everything now is 410.