ReplyThey both may be.
Regular Guest
I got two quotes for replacing my 4 ton outside unit that has a 40 Amp service wih a 5 ton unit. One quote for the Trane XL19i said to connect to existing electrical. Second was for a Carrier Infinity 18 model 38TDB but stated that a 50 amp service is needed. Who's right?
Banned
They both may be.
Professional Member*
Have each contractor show you the manufacturers specification sheet on the particular unit (model and size) that they are installing and it will indicate the minimum circuit amps that are required for the unit. There should be no doubt. Make sure if a larger breaker is installed in your panel, that it doesn't create an overload for your panel. Also be aware that in some areas, A LICENSED ELECTRICAL CONTRACTOR is required to change that breaker. Sorry, but that is the code (law) in some areas and it's that way for your protection. Some HVAC contractors may also have a licensed electrician on staff, but make sure whoever does the work acquires all the necessry permitting and the work is inspected upon completion. If in doubt, make a clarification that you will hold 10% until local code enforcement performs their inspection and gives it their seal of approval.
Next question. Why are they upsizing the system, and can your duct system accomodate the added airflow required with the larger system? Remember, bigger is not necessarily better.
Each contractor, particularly in changing the size of the system should have perform a load calculation (manual J) to determine exactly what size system SHOULD be in your home.
Regular Guest
Trane guy did perform load calculation and measured all rooms and size of ducts. His calculation said 4.6 tons. Current sytem during hot weather would kick on at 11am and run continious until 11pm at night. System could not keep up and indoor temp would creep up to 82 degrees. Current system checked on two ocasions by defferent companies for correct pressures and if condenser and filter was clean. Both said that unit was too small. Having two large shade trees on West side of house die on me did not help.
Banned
I have a problem with a unit that would run from 11AM to to 11PM and let the temp creep up to 82 degrees, and then hear that that unit is a four ton and the load calc said it needed a 4.6 ton, I'm not going to sit here and do the math but I can tell you that that does not make any sense common or otherwise. If the load calc was correct then there is something wrong with the existing unit and that should be addressed first before you spend a lot of money and end up with the same problem again, ask the Trane guy to clear that up for you.
My point is that you are going to switch from a 4 ton to a 5 ton and the problem is probably in your ductwork.
[Edited by fat eddy on 02-18-2005 at 10:55 PM]
Professional Member
The math is easy to do, the load calc says a 4.6 ton will run nonstop at design temps {hot out}. That means a 4 ton will run nonstop and the temp will rise in the house.Originally posted by fat eddy
I'm not going to sit here and do the math but I can tell you that that does not make any sense common or otherwise.
Professional Member*
Ten bucks says that fats is right. Many believe Manual J oversizes. A study by Proctor Engineering Group showed that J7 oversized by 30% and that contractors added another 20% of unnecessary fudge factors. J8 is rumored to oversize by more. Of course those systems did not output 50% more due to poor installation practices, etc. My crystal ball says that airflow is pathetic and the ducts are leaky. There's probably some minor to major building envelope issues as well.
On the other hand, my crystal ball has been wrong before. You could be right Mr. Smith. But if fats is right, then it could be that if there are leaky ducts and poor airflow then adding a ton of capacity might net a delivered increase in capacity of half that amount.
Regular Guest
Fat eddy, I understand what you are saying, there's only so much air that can be forced through a certain size duct. The entire system is being changed out, condenser, A coil and furnace so it will be a matched set. Same duct work though. Condenser is 23 years old and furnace is 18. Entire first floor is vaulted ceiling with no attic and little ventilation up there. Ceiling gets pretty warm in summer. Downstairs is walk out basement and 1/3 size of upstairs. Going to install two zones so this may help also.
Professional Member
one needs to know the full load amps, the locked rotor amps, the length of run, how | where any circuit is to be installed. The use of other than 60C rating of wire is wasteful each & every time the unit runs.
Regular Guest
Originally posted by cem-bsee
one needs to know the full load amps, the locked rotor amps, the length of run, how | where any circuit is to be installed. The use of other than 60C rating of wire is wasteful each & every time the unit runs.
cem-bsee
first sentence made alot of sense. second sentence got a little off track. if you only used 60C wire, you would have to upsize wire, some of the time, after you derated it for the hot attic space. 75c & 90c wire might allow you to use the normal wire size after derating. not always, but those attics can be really hot in the summer. in other words, for example, I would rather use #6 75c, than use #4 60c. would be cheaper and easier. copper cost more than insulation.
Professional Member
Isn't it ironic how most people think that changing all the equipment without consideration to the ductwork will be all that is needed. Manufacturers make very efficient equipment. Contractor and homeowner agree on complete equipment changeout. The homeowner now has the best efficiency in equipment but a very poor delivery system because no one considered the duct work. Its like delivering pure clean water in a dirty leaking bucket. Oh and another irony, All new equipment and still the same ol $1.25 air filter, I call it an air strainer.
Saddle Up!
Professional Member*
Electrician Needed~~~~~~~~~Licensed
The Trane catalog states max fuse 60 with MCA 38 amp.
'Life begins with the journey each day'
Regular Guest
-80guru: You may want to read my second post. He did measure the duct sizes.
aircooled53: Thanks for the info. From that is it 60A surge and 38A continious? That would mean that my 40 Amp service needs to be upgraded, right?
Professional Member*
60A max, MCA of 38 means that your 40 amp service is "ok".
I prefere to go with the maximum rated size whenever I can though.
HVAC equipment is hard on breakers due to the high inrush current of the compressor. The 40A breaker is designed to handle that, but the breaker is much more likely to fail a few years down the road than a 60A breaker would be.
Professional Member*
No, it means that you should install a 60 amp (2)pole breaker at the main panel,then have 40 amp fuss disconnect at the condenser. That is if you want to do it according to the manufacture...
This will protect the compressor from over amp, and you will be able to find shorted problems at the condenser before over (amp) occurs.
'Life begins with the journey each day'
Banned
RSmith> "hot out" is relative, design is db\wb two definite parameters, in my area those are usually 95\73. We actually reach those about twice a year for a matter of hours not days. The problem with the unit running all day and never satisfying can be due to a lot of things including undersized but more than likely the problem will be something else with airflow being the most likely,apparently the two service techs came to the same conclusion that the unit was to small. If they had said that they were seeing a higher than normal suction pressure and that the compressor has lost capacity then it would be more beleivable that the new unit is going to do fine, but they can not possibly be accurate with their diagnosis if the load calc was right also. Jan did not specify which type of days this happens on so I'm assuming that it is a regular occurence on warmer days, not just the two days a year that it runs at design conditions. There is more than likely an RH problem then too because theoretically an overloaded system will have a high DA and not be able to do very much dehum.
On another note from what Jan wrote it sounds as though the Trane guys load calc was done based upon existing ducts, and not necessarily the heat load on the house. Why was he measuring the ducts to determine the unit size. Don't you do the load calc first then see if the ducts are compatible ? Did he do the calc right then and there or did he do it and then come back to measure ? just curious of the method.
Professional Member
Oh no I wasn't referring to you or your guy. I was just making a statement in general. Sorry for the mix up my good man.
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Regular Guest
Fat eddy: This is a normal occurance on very hot days above say >92 degrees. Thermostat set on 75. Have been here 18 years and did not have a problem until after losing the shade trees. Would only have the problem before if outside temp got above 96-98 degrees which did not occur but a couple of times during the summer.
The current system seems to do a good job DH and there is always a constant trickle of water out of the condensate drain.
I will have to take back what I said about the Trane guy measuring the ducts. I do remember him mesuring the size of the main supply ducts leaving the furnace but not any branch sizes or lenghts of runs.
He did not do the calculation right then but came back a few days later and showed me the figures but did not leave them with me to study.
I had one guy come out but have not heard from him yet. He asked me everything for his calculation. He mesured the outside of the house then the inside rooms, measured windows and asked if they were low E glass, which side of house faced west, what R value inslation in walls and ceiling, what size and type of exterior doors, height of vaulted ceiling, how high walk out basement was below gound. Hope to hear from him.
It's alot of money and I'm very thankful for everyones help with this and don't want to throw in a new system if it's not going to correct the problem.
Regular Guest
-80guru: Sorry, I didn't mean to jump on you. I'm just a little stressed over this.
Professional Member
I suppose ther is alot of guessing but until your guy does a capcity check of your current system you can guess all you want as to why the reason is. If you currenly have a 4 ton system and you are only getting 38,000 btuhs of total cooling out of it, then the system has a problem, if you are getting a close to nominal 48,000 then your system may indeed be undersized.
Manual J8, unfortunately doesnt account for trees, it does for whatever reason care if your bug screens are inside or outside of your windows, go figure.
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