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How to decipher tonnage

Robert Jones

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Yes, 1.5 tons or 18,000 BTU's as shown by the 18 in the middle of the model number. You can double check (ballpark range) it against the RLA, but the formula has been changing as the efficiency has been increasing. An RLA of 8 seems about right for a unit that age. An older one of the same size would be somewhat higher, a newer unit would be somewhat lower.

That would be undersized here in eastern PA, but I don't know how hot it gets in your neck of the woods.

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That would be undersized here in eastern PA, but I don't know how hot it gets in your neck of the woods.

Not very.

A couple of years ago I did a house for a young couple moving up here from southern California. They asked me how much it would cost to add AC to their home. I guessed it would cost around $6K and asked them why in the world they'd want AC. They said something like, "We're from California and we're used to having AC."

They were buying a home up in Issaquah Highlands, hundreds of feet higher above sea level then here and cooler in the summer.

I said something like, "Well, put it in if you like, but I don't think you're going to need it up here. I live down near sea level and I'm plenty comfortable there."

The following winter the wife called up to tell me that her furnace had been working okay but then it stopped. It took about a minute to figure out that she had her thermostat on the wrong setting. She got it running again and all was well.

I then asked her if they'd had an AC system installed and how much it had cost them. She responded that they had and it had cost them $6500. I then asked her how many days they'd used it the previous summer. There was a pregnant pause and then she said, "Uh, I think it was only about two days in the very hottest days of the summer." "Two days! Man, that's going to be an awfully long payback period," I said.



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There's the Manual J Calculation which computes heat/cool loads, but I haven't used it for many years. I just ask the customer a few questions about the past performance of their existing system in the summer (and consider it's condition) before suggesting a tonnage value for the new system. The way I see it, if the unit runs non-stop, all day long, but cools adequately under worse case conditions on the hottest day of the summer, it's the right size.

Like Kurt said on another thread....different building methods interacting with new and different building materials just doesn't always yield to textbook calculations. He was referring to humidity behavior but I think it applies to heat also.

I hope I quoted him right.


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I would be careful in calling a unit too small if I had not done Manual J calculations. You really need to address alot of factors (insulation, house siting, overhangs, windows, etc) before calling it undersized. Oversized is more of a problem especially in regards to removal of moisture.

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Trane is one of the better manufacturers for ease of finding data online

If you searched google for

trane ttn

The first result is a pdf with all of the information on deciphering the Model Number.

Trane always calls this "model number nomenclature" so you an always add this to the end of a search. Only enter the first 2 or 3 characters of the model number though...

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