Replies

1. 
   skader1 | May 24, 2002 12:03am | #1

   I worked with an HVAC supplier who told me "400 sf/ton". I had run some load calcs using one of the computer programs for doing heating and cooling sizing and come up with less total tonnage. He told me that using 400 sf/ton will ensure that you can cool the house on the hottest day. The additional tonnage was not a big cost push in the big picture. I live in a fairly hot climate, so I would imagine that the rule of thumb would change by region. I worked with someone I had recommended to me and he gave me confidence during our dealings, so I went with his experience and recommendation. I have heard both 400 and 500 sf/ton for rough sizing of units, I have lived in CA and AZ, though, both hot areas in the summer. With a house that big, I would assume that you will zone it with multiple units so that the east side isnt hot in the morning and the west side hot in the afternoon because the thermostat is in the middle of the house...

   Figure out which one of your bidders you trust and use their recommendations.

   Sami

2. 
   rjw | May 24, 2002 12:17am | #2

   Word is that HVAC contractors err on the side of over-sizing, if no other reason, no one ever called to complain "HEY! this thing cools the house down too fast!"

   Units can be mis-sized though; too big and it won't dehumidify properly.

   There are too many factors which can affect the load calc; sq. footage alone often isn't suffient information.

1. 
   mammasboy | May 24, 2002 01:05am | #3

   There are too many factors which can affect the load calc; sq. footage alone often isn't suffient information.

   That's why each of the contractors got a set of plans and toured the frame.  And they STILL differ by 25 to 30% in their total load calcs.  I don't want to go by "rule of thumb", but I'm getting the impression that some of those guys go by it when they do their calcs.  Impossible to know what I'm buying and impossible to compare pricing.

1. 
   rjw | May 24, 2002 04:00pm | #6

   "And they STILL differ by 25 to 30% in their total load calcs."

   As I understand it, even with the rigorous formula, there are subjective factors in it: how tightly built is the house, how ell shaded.

   I don't know if that's enough to lead to a 25% difference, but believe it probably could.  Get one guy who asssumes a very tight house and a lot of summer shading, and another guy who assumes no shading, looser construction, and who's had some complaints in the past about undersizing, and you'll probably get some large differences.

   And there are probably some guys who glanced at the plans, closed their eyes and siad "X tons."  One of them might be the right one!

   As a home inspector, I don't do load calcs, but I see a lot of central A/C units.  Tell me the neighborhood, square footage, overall maintainance & condition of the house and whether the current owners are old and I can tell you the size A/C we'll find, being right 90% of the time. 

   (Actually, that's a little game I play with myself, but I base it on neighborhood, size and apparent condition, and I get it right most of the time.  In my area, most houses up to about 2,000 sg feet will have 2 ton, unless they're really old where it might be 2 1/2.  unless it's a lower income type of neighborhood or a badly maintained house, where we'll drop 1/2 ton or so.  Real small - 1 1/2.  Etc.)

3. 
   DaveRicheson | May 24, 2002 02:26am | #4

   Call your local utility company. Many of them will run load calculations based on your prints if detailed enough with window mfg. numbers and insulation type. They run a computer program that is based on standard construction practices for your area. around here it can take from a week to three weeks to get thier report. They may model several different systems if you ask, i.e. gas, heat pump, geothermal, propane, etc. Their models are based on the average heating/cooling degree days for your service area. Alot better than any ROT.

4. 
   TLRice | May 24, 2002 02:29pm | #5

   Several folks in recent times have asked this question and it is a fairly common problem/question.

   Did you see their calculations? I would bet you that one of those numbers is not actually based on a calculation, but is just a guess. There are two common methods of calculating heating and cooling loads, and if done by a knowledgeable individual, will result in similar values. Ask them how they arrived at those figures, what are the design conditions they used, how did they figure inleakge, etc? If they know what they're doing they will be able to answer there simple questions.

   I don't know your location or construction or exposure, so I won't guess at what the numbers should be. For the last two houses that I calculated loads for in northern IL, for exaple, they worked out to be 800 sf/ton.

   
   

   ---

   Edited 5/24/2002 7:51:20 AM ET by Tim

1. 
   mammasboy | May 26, 2002 01:24am | #7

   Tim, I'm in southern NJ, which is a heck of a lot warmer in the winter than northern Illinois, so I agree with you that 800 sf per ton should be about right to heat this house.  I think the problem is that I am talking to a couple of geothermal guys, and IMO they are using the increased tonnage calculation to justify why their prices are so damn high.  The guys that are quoting me for gas fired units are way lower in their calcs.

1. 
   TLRice | May 29, 2002 03:23pm | #9

   I am being nitpicky, here, but so that you get the "lingo" right, the following conventions are fairly standard:

   Cooling is quantified in tons, heating is not. A ton is equivalent to 12,000 BTU/hr of cooling is is based on the cooling effect produced by 2000 pounds of ice melting.

   Heating is quantified in BTU/hr (btuh) or thousands of btuh or MBH.

   As far as the rest goes, in the commercial world, cost for jobs is estimated on the tonnage, more cooling, more money. For a house, an ESTIMATED cost is anywhere from $2000 to 3500 per ton. I recently paid $10,000 for a 4 ton system, (100 MBH gas forced air heat) installed.

   Gas-fired furnaces, rated by input and efficiency or AFUE (annual fuel utilization efficiency) usuaully come in 50, 75, 100 and 125 MBH INPUT sizes.

   Residential style AC units/coils typically come in 1-1/2, 2, 2-1/2, 3, 3-1/2, 4 and 5 ton sizes, though a competent designer or installer can mix and match to meet any load.

   In addition to the installed cost of the equipment, consider the operational costs. If natural gas costs $0.50/therm then electricity would have to cost $0.02/kwh to compete as a heating source.

1. 
   ablejack | May 30, 2002 03:21pm | #10

   Hi Tim,

   Mama's boy was lamenting the huge variation in prices and specs one contractor to the next.  One or both of those specs is wrong.  Physics does not change day to day.

   I wanted to reply and re-state that first, an accurate 'number' of what you are cooling [or heating] is the place to begin.  Anything else is what they call a 'Hail Mary' in the game of football when sizing a cooling or heating system.

   BTU's or British Thermal Units is an agreed upon unit of measure to look at temperature, or more specifically, the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit.  Calorie is another, and joule is another that is better still.

   The BTU's relationship to cooling is probably very similar, but I can't confirm that it is the amount of energy needed to cool one pound of water one degree Fahrenheit.

   Regardless of wether you are driving in miles or kilometers, the most important thing to know is where you are heading to. 

   /P

1. 
   TLRice | Jun 04, 2002 03:01pm | #11

   Pablo,

   A BTU is defined as the quantity of energy required to raise one pound of water (at 60 deg F), one degree F. It is the same quantity of energy regardless of the direction it moves (i.e. into or out of a system).

   "BTU's or British Thermal Units is an agreed upon unit of measure to look at temperature, ..." I know what the rest said, but this part of the statement is wrong. Temperature is a measure of the average kinetic energy of the molecules of a particular substance/object. The common units are Fahrenheit, Celcius (or centigrade), Kelvin or Rankine.

   "Calorie is another, and joule is another that is better still." Better for what? I've been a mechanical engineer for over 14 years. Never yet (outside of a lab) used a calorie or a joule in any energy or heat transfer calculations.

2. 
   ablejack | Jun 07, 2002 02:24pm | #13

   Hi Tim,

   You are right in your critique of my specific use of the word ‘temperature’.  My emphasis in this discussion is to indicate that Mama’s Boy has a specific heat loss that needs to be calculated…..and that the BTU is the unit of measure that we [this country] uses to look at quanifying heat loss.  Don’t get caught up in the miles or kilometers……it is exactly 'that' far, however you want to describe it.

    

   My understanding of this topic you ask about is that we and the UK use BTU’s.  Everyone else does their calculations in Joules. 1000 joules = 1 kilojoule = 1 Btu.  

   Based on that thinking, I would say that metric is better then the king's inch, foot and yard, but they both work [and I have to beat the english system into submission on a regular basis].

    

   Back to what counts in the realm of not getting jacked up on your new HVAC system……get a proper heat/loss calculation competed!

5. 
   ablejack | May 26, 2002 08:17am | #8

   A proper heat load calculation is a very complicated process.....not in theory, but in practice.  The calculation is looking for the amount of heat loss [for heating] or heat gain [for cooling] that happens to a given space.  Factors such as cubic volume [square footage times room height], insulation R value, and wall penetrations [windows and doors] all factor in.  It also gets right down to the orientation in the room a given window or door faces, south raises the heat load for cooling, and north does the same for heating.

   Bottom line is you can't do this in five minutes so both your estimates may be guesses.  That said, if you did enough installs, you would start to be able to 'look' at a place and make a guesstimate.

   I suggest if you want the real numbers, do the calculation yourself.  This site used to let you do your own:  http://www.heatload.com/forcedair/  It looks like they might have changed their policy....check it out.

   An earlier post has it right. Too large a system and the unit can't run long enough to dehumidify, too small, and it runs too much wearing on the system and creating high energy bills.

   Good luck!

6. 
   TooManyTools | Jun 05, 2002 06:19am | #12

   I think part of the difference is in what you said in one of the later messages (I hate this new system)  that the geothermal heat pump boys were coming up with 25-30% larger systems.  Remember a heat pump is a reversed air conditioner so it is often necessary to size the compressor, evaporator, and condenser for the heating load rather than the cooling load.  This being the case I am not surprised by a 25-30% larger tonnage quote.  Did they quote both heating and cooling capacities?

   Depending on the relative costs of gas and electricity a geothermal system can be very attractive long term.

7. 
   TLRice | Jun 07, 2002 03:14pm | #14

   Mamasboy,

   So'd you ever get some feedback on your contractors on this? What did you decide? I saw your other post on the ductboard. Did you get more estimates? Who proposed the ductboard? Curious to know where this all endesd up, for you.

   Tim