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Old 09-23-2011, 09:37 AM   #1
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HVAC sizing snafu


Hello,

I've been working on my foreclosure house, out of which former owners pulled all the appliances and HVAC.

The original system was a pair of 2.5 ton 13Seer R22 Paynes. Both condensers were removed, and also the furnace in the garage. The attic furnace was left in place ( and still works ).

Last week, I ordered a pair of Goodman 2.5ton condensers to replace the
ones that were removed.

Yesterday, I went down to the county and pulled a permit for this work. While there, I got to look at the original plans for the house. At the back of the plans were the Title 24 calculations ( Title 24 is the California law pertaining to energy consumption, insulation etc ).

According to the extremely detailed computer model, the house only needs 30068 BTUH *total* of cooling and 57540 BTUH *total* of heating. The contractors had grossly oversized the system.

Is this a bad enough snafu that I should trade these condensers in for smaller units? One lucky thing is that we are not a high-humidity area, so dehumidification is not such a concern as back east.

And I do have a use for a little bit of extra capacity - I'd like to somehow add a zone for the 3-car garage so it can be conditioned space - but only when I'm using it and the door is closed.

- JerryK

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Old 09-23-2011, 12:30 PM   #2
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HVAC sizing snafu


Garage can not be tied into a unit serving the house. Against code.

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Old 09-23-2011, 12:39 PM   #3
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HVAC sizing snafu


I thought your system sounded oversized when I first started reading your threads, but I just assumed that you had done a proper manual-J load calculation.

You won't be happy with an oversized system. Your comfort will be compromised, especially if there is high summer humidity in your locale and your equipment will suffer due to short cycling.
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Old 09-23-2011, 05:56 PM   #4
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I thought your system sounded oversized when I first started reading your threads, but I just assumed that you had done a proper manual-J load calculation.
*** I was going to. But things just got really busy, what with moving and getting the old house ready for market. So I just punted and ordered what was in there before.

[quote=fabrk8r;734532]
You won't be happy with an oversized system. Your comfort will be compromised, especially if there is high summer humidity in your locale
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*** That we don't have. That's why our hills are brown.
and your equipment will suffer due to short cycling.
*** No doubt.
What burns me is that the original builders had all these beautifully detailed energy calculations, and they stuck in these oversized monsters. Why?

-JerryK
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Old 09-23-2011, 06:11 PM   #5
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I have put in a call with the engineering firm that did the load calculations - just to make sure I'm not misunderstanding something.



- JerryK
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Old 09-23-2011, 06:30 PM   #6
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If these are/were heat pumps, then you were sized to the heating load.
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Old 09-24-2011, 07:13 AM   #7
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Originally Posted by jerryk1234 View Post
I have put in a call with the engineering firm that did the load calculations - just to make sure I'm not misunderstanding something.



- JerryK
Just because the indoor units are label as 2.5 ton units, doesn't mean the condensers were 2.5 tons.

And as said earlier, the garage is not allowed to be on the same system that doesn't any other part of your house.
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Old 09-24-2011, 08:15 AM   #8
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[quote=jerryk1234;734764]*** I was going to. But things just got really busy, what with moving and getting the old house ready for market. So I just punted and ordered what was in there before.

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You won't be happy with an oversized system. Your comfort will be compromised, especially if there is high summer humidity in your locale


*** No doubt.
What burns me is that the original builders had all these beautifully detailed energy calculations, and they stuck in these oversized monsters. Why?

-JerryK
2.5 tons will do 500 sq feet per ton/8 foot ceilings.
just an old rule of thumb/we never go by the calcs for fear of low sizing and call backs.just my 2 cents but I have installed several hundred units using this formula/and not one call back yet
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Old 09-24-2011, 08:19 AM   #9
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Just because the indoor units are label as 2.5 ton units, doesn't mean the condensers were 2.5 tons.
*** Other houses in the tract - the exact same size, same builder, same plans - have two 2.5 ton units. Payne PA13NR030.


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And as said earlier, the garage is not allowed to be on the same system that doesn't any other part of your house.
*** Makes sense. Cars, exhaust. How about a mini-split for the garage?


- JerryK

Last edited by jerryk1234; 09-24-2011 at 09:12 AM. Reason: mini-split
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Old 09-24-2011, 08:36 AM   #10
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[quote=Marbledust;734987]
Quote:
Originally Posted by jerryk1234 View Post
*** I was going to. But things just got really busy, what with moving and getting the old house ready for market. So I just punted and ordered what was in there before.


2.5 tons will do 500 sq feet per ton/8 foot ceilings.
just an old rule of thumb/we never go by the calcs for fear of low sizing and call backs.just my 2 cents but I have installed several hundred units using this formula/and not one call back yet
That just over sizes the units, and makes the customer pay higher cooling bills then needed. A load calc, and a proper Manual S equipment selection will make sure the system isn't under sized, and that they aren't paying higher then needed cooling bills.


My place is 1650 sq ft, by your rule of thumb, I would need 3 tons. My system was in before I was here, its 2.5 tons, an grossly over sized. I have the blower slowed to 280 CFM per ton, and can still cool my place to 72 when its 98 outside. A 1.5 ton would be the proper size for my place.
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Old 09-24-2011, 09:11 AM   #11
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2.5 tons will do 500 sq feet per ton/8 foot ceilings.
just an old rule of thumb/we never go by the calcs for fear of low sizing and call backs.just my 2 cents but I have installed several hundred units using this formula/and not one call back yet
*** Hmm,

By that formula, I would need 7 tons. Maybe 5 isn't that bad.

- JerryK
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Old 09-24-2011, 10:23 AM   #12
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The old rule of thumb was started by Trane years ago as some sort of guideline to sizing a house. Its not a rule, or anything else based on scientific theory, practice, or methodology. If you applied that rule of thumb to one of these new styrofoam houses, you would end up with a walk-in cooler.
As far as tract housing goes, you can take two identical houses, turn one sideways, and end up with dramatically different heat loads.
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Old 09-25-2011, 06:34 AM   #13
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[quote=beenthere;734994]
Quote:
Originally Posted by Marbledust View Post

My place is 1650 sq ft, by your rule of thumb, I would need 3 tons. My system was in before I was here, its 2.5 tons, an grossly over sized. I have the blower slowed to 280 CFM per ton, and can still cool my place to 72 when its 98 outside.
*** And that's a bad thing? 72 sounds nice to me. BTW, does slowing the blower give you any problems with evaporator icing?

I had a discussion about this with my Fresno HVAC guy - he takes care of my apartment complex there, we do something like $5K/year of business. Year after year. Great guy. Fixes it once, fixes it right. Doesn't just top up the units in the spring, but finds and fixes the leaks. In the off season I have him walk the roofs and clean the condensers. One of our big tenant draws is that we have real, working AC.

He said that I would be reasonably happy with the system as designed. He admitted at a pair of 2-tonners would be better, but not $640 better. He said that I would have a lot of versatility with two complete systems - a lot better than zoning.

I plan to install a couple of wifi-enabled thermostats. These will be controlled by Perl scripts running on my Linux server. I think I might be able to mostly cool the house using the upstairs system, and mostly heat it with the downstairs system.

- Jerryk
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Old 09-25-2011, 08:36 AM   #14
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I can only run my A/C for 20 minutes if its below 74 outside, before it will freeze up. Sometimes less then that. So running it at 280 CFM per ton does have a few draw backs.
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Old 09-25-2011, 09:45 AM   #15
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Quote:
Originally Posted by beenthere
I can only run my A/C for 20 minutes if its below 74 outside, before it will freeze up. Sometimes less then that. So running it at 280 CFM per ton does have a few draw backs.
I have the hot gas bypass selonoid and valve off of an addison unit. We can strike a deal!

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