View Full Version : load calc's vs rules of thumb
05-01-2007, 08:11 PM
I got 3 estimates on replacement furnace/ac - American Standard, Rheem, and Trane. 2 story house in Kansas with about 2000 sf above ground.
The A/S guy made lots of measurements of windows, asked about wall insulation, etc. and ran a number back in his office. Recommended 3.5 ton AC and 100K variable speed furnace. Other guys walked around but came to the same size estimates. price from low to high was Rheem, Trane, Am Std, each about $1000 higher.
any recommendations? thanks
So far only the A/S guy appears to have done a load calc. You can ask the other two, who may tell you they will do one if/when they get the job, but they don't want to invest the time and money to do one beforehand. That is normal operating procedure, not to be dissed.
Did the other guys recommend VS furnace? That ought to add to comfort level and with AC can add about 1 SEER point to efficiency, and improve humidity removal significantly. By the way was it a 1-stage furnace they recommended? You live in Kansas and I am a homeowner in S.Texas so I imagine your winters and summers are serious weather. But I have lots of experience with the hot-humid part of the year<g>.
You are probably wondering about what brand but that is *not* the important part of the job, quality of installation is. Can you identify any factors which suggest one of the companies will do a better job? If you can that company is the clear leader, even if the bid is highest. You really don't want to live with a poor installation.
FWIW I just got installed an A/S air conditioner and am using a 2-stage 80% furnace of 80K input capacity (64K output). The furnace is way oversized for our winters but was the smallest unit which could also handle the required AC airflow. The equipment is fine but I would rather have a Goodman (the most disrespected brand) with good install than any other with poor installation.
Best of luck -- Pstu
05-01-2007, 08:35 PM
Yea, probably ought to post model #s of what they guys are quoting so we can make sure all are like products. No pricing though. The A-S guy gets points for doing the load calc even though he came up with the same size as the others. At least HE cared enough to measure and crunch.
Are you looking at dual fuel? Good climate for it!
05-01-2007, 08:59 PM
Doing the load is important. The actual heat gain/ heat loss in wether total or sensisble including latent can be very important in determaining the comfort the home will have. Here, we rely a lot on the sensible capacity of the system matched with the sensible gain. For example a total heat gain of 36k may lead one to think a system with a capacity of 36k (3 ton per se), but if the sensible gain is 20k and the 3 ton system is rated at 17k sensible- you just may have problems. And don't forget the latent requirements.
We know all things relative to heat load calculations are not exact as many small variances can affect the final and actual conditions, but a combination of known and proven calculations and common sense (in experience) is hard to match.
I would preffer to choose the company that takes the time and concern to use this proceedure and feel that may reflect the same attitude in doing a quality installation.
05-01-2007, 11:03 PM
They are all recommending 2 stage variable speed furnace. The A/S standard guy has been our service guy on our existing Lennox and we have been happy with the service. But . . . his service tech's told us the unit was undersized during 2 different repairs - we requested they have someone from the office give us estimates and they never did. My bad for not following up either. Also, they didn't follow up on getting a back-ordered expansion valve last summer. A/C started working OK but is freezing up this year. He also discovered some disconnected duct work in the attic that may be the cause of our upstairs being very hot in summer and very cold in winter. I told the Rheem and Trane guys about the problem to try and make sure everyone was bidding the same job.
The Rheem guy did say that a 3.5 ton A/C was the biggest our existing ductwork could handle. The house is completely finished so revised ducting would probably be quite expensive. He had said one rule of thumb was 1 ton per 500 sq ft of house - which would suggest 4 ton. He also said with the variable speed blower and a 14 SEER unit, he might be able to show the system was equivalent to a 15 SEER A/C and be eligible for a $300 tax credit. Neither of the other guys mentioned that.
Very hard for a layman to evaluate who knows what they are doing - all 3 seemed quite knowlegable.
05-01-2007, 11:47 PM
listen carefully --
if you have duct problems, the best unit will not cure that!
you need to buy a system, not just a "buy & carry" appliance --
else get portable units --
you can pay for a total system,
or for part of a system, then high energy usage from then on --
05-02-2007, 12:55 AM
The guy who did the load calc gets my vote.
With regards to the ductwork, think of an AC system in terms of a car. The actual AC equipment is just the engine. The ductwork and refrigerant lineset are the transmission. You can drop a new engine in a car with 200,000 miles, but if the transmission's shot (or wasn't done right in the first place, which is VERY common in residential construction), you're just wasting a lot of money.
05-02-2007, 07:41 AM
I vote for the AS guy. The 500 sq ft per ton is the same rule of thumb they use here in Myrtle Beach, SC. I have a 2000 sq ft house. Using rule of thumb, my heat pump should be 4 ton. But I have a single 2-ton heat pump doing the whole house.
So go with the guy who did the calcs. Besides, what else will he check that the other guys will just thumb?
05-02-2007, 08:03 AM
So how do I know if there are ductwork problems? I'm counting on the experts to tell me if something needs to be fixed.
05-02-2007, 08:42 AM
To determine the sufficiency of your current duct system they should measure the airflow and the ESP (External Static Pressure), which is the resistance of the air moving through the duct system. Both are done with special equipment. Only then will they know how good or bad the duct system actually is and whether or not it can deliver the air that is needed based on the calculation of the heat loss/gain.
In many cases residential systems are oversized on the unit and undersized on the ducts. With a proper load calculation (known as Manual J) they will know how much heating and cooling the house will need. No guessing, rules of thumbs, or over sizing. Plus, it will tell them exactly how much airflow is needed in each room. It may be that a smaller unit will be properly sized and the duct work sufficient for that smaller unit. A unit should not be sized to the duct work – the duct work must be sized to the unit.
05-02-2007, 08:58 AM
Unfortunately, you still have to be careful even if they do a load calc. I have seen many calcs that were done, and they added a fudge factor at the end or they used an extremely high/low design temperature instead of that directed by Manual J. I would ask what summer and winter design temperatures he used, what the results were in sensible and latent btu's, and if there were any "adjustments" made to come up with the tonnage. I am always suspicious if a load calc gets the same answer as the guy that just uses a rule of thumb. Find out that information and post the results here and you can get some additional guidance.
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