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Old 07-22-2011, 08:16 AM   #1
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Questions to ask the HVAC tech?

Greetings everyone. I'm pretty good with most mechanical aspects of my house but when it comes to our central air system I'm fairly ignorant.

Three months ago we moved into a 1925, 2000 sq. ft. bungalow in Washington DC. The existing system, a Carrier 38ED036300, is at least 22 years old and I would suppose near the end of it's service life. The air handler is in the third floor attic and the compressor is outside at ground level.

The air handler has vibrated when operating since we moved in to a degree that was almost certainly not normal, and recently it has started to make a fairly significant rattle which I am going to attribute to a bearing in the process of failing.

Also, while the unit blows cool air and the air pressure appears to be adequate, when the temperature is over 90 degrees (which is extremely common in DC in the summer), the system needs to run almost constantly maintain 74-76 degrees. The house is old and leaky and probably barely insulated at all. A testament to that is the fact that all of the hot water radiators in the house were at one time swapped out for larger ones (the floor holes are all closer together with pipe extensions to fit the replacement radiators. Clearly the original system as designed was not adequate to the needs of the house.

I have a tech from a fairly reputable HVAC company coming on Monday. I hope the system lasts that long. We're in the middle of a fairly bad heatwave and I'm running the system about 50% of the time to try and nurse it through the weekend. If I did not turn it off it would just run endlessly in this heat.

I have a few questions for you all, the answers to which I hope will prepare me to have an intelligent conversation with him and make some follow-on decisions.

First of all, on the assumption that a bearing in the handler is failing, am I at a point from a repair cost perspective where I should consider replacing the whole system?

How much improved efficiency can I expect from a new system?

The attic in our house in the summer is brutally hot. What impact does this have on the performance and/or reliability of the system? I'm contemplating upgrading the vent fan.

Is there an easily apprehended formula for sizing a system to your house?

If you were me, what else would you discuss with the tech when he is here, or ask him?

In evaluating HVAC companies that you solicit for estimates, what questions would you ask them to help you determine your selection?

Thanks so much. This is needless to say a big expense for us and I want to arm myself with a modicum of knowledge before I am neck-deep in the decision making process.


Last edited by Ironlight; 07-22-2011 at 08:19 AM.
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Old 07-22-2011, 05:20 PM   #2
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No pro, but I'd make sure a Manual J is used for heat/loss calculating. There is one you can buy <$50 on line to use yourself as a back check.
At 22 years of age, a new machine will be much more efficient and should cut down on electricity bills; however, lack of insulation and weatherstripping is an obvious fix. Depending on how long you plan on living there might determine how much you want to invest in insulating for payback; comfort is another issue.
I know a lot of units are placed in the attics but I'd want mine in a basement or other cooler place; doesn't make sense to me to try to cool down air in a machine is a space that is 20-40 degrees hotter than outside. Also, ductwork needs to be adequately sized (both supply and returns) and insulated.
Fixing what is there might be a bit costly but I wouldn't sink money into a 22 year old unit IMO. Get three or more estimates and ask for some recent customer references; will be hard this time of year with the current heat wave and how busy they are. I'm two for two in picking contractors from my local church's weekly bulletin advertisement section; I always go with someone who is a parish member. I also check them out on Better Business and google their company name for complaints. Neighbors might give you the name of their person but that is no guarantee of good work.


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