Replies

1. 
   danski0224 | Oct 08, 2009 01:19pm | #1

   The heating and AC units need to be sized properly for the load, or else the system will not work right.

   Typical "rule of thumb" or "I have a truckload of this" residential equipment sizing is oversized (and ducts are undersized) before foam is factored in.

1. 
   junkhound | Oct 08, 2009 02:29pm | #2

   "I have a truckload of this" residential equipment sizing

    

   LOL

    

   No need to downsize the heating.

   Since the OP is in New England vs. Houston, would not worry too much about AC either.  Easiest low cost way to 'downsize' AC to remove more humidity would be to simply reduce the fan speed.

2. 
   MHolladay | Oct 08, 2009 02:40pm | #3

   Maguire,

   Your HVAC contractor shouldn't arbitrarily downsize equipment by 50% based only on insulation type. Instead, your HVAC contractor should do what is required by code: perform a heat loss and heat gain calculation for the house. That means inputing values for air leakage, R-values, window sizes, and window glazing choices, room by room. No shortcuts. No rules of thumb. That's a code requirement.

   Martin Holladay, senior editor

   http://www.greenbuildingadvisor.com

    

1. 
   maguire | Oct 08, 2009 03:09pm | #4

   And he has done this as well as every house we do . These were just comments of the building inspector . Thanks so much for your input , Martin

3. 
   renosteinke | Oct 08, 2009 03:51pm | #5

   Sizing AC is a contradiction in technology.

   The courses spend a lot of time in teaching you the importance of doing proper calculations, and point you to an extremely involved ASHRAE manual. This procedure would, somewhere in it, allow for different levels of insulation.

   HVAC guys, by contrast, seem to pull sizes out of their hats. What's amazing is that their off-the-cuff selections almost always work right.

   Ditto for short-cycling. While the books preach endlessly on the evils of this, the trade doesn't seem concerned about it in the least.

   Part of this might be simply because the available equipment is either - according to the books - either way too small or way too big. It just might be that (say) a 2-ton unit is too small for anything larger than a garage, while the next size -3 ton- is too big for anything smaller than the Taj Mahal. At least, according to 'the books.'

   I'd say, leave it to the pros; their opinions are moderated by their pocketbooks; call-backs are profit killers.

   For an existing house with new insulation, I'd look at the way the unit operates on a moderately hot day. Does it 'short-cycle? Only after knowing there was a problem would I worry.

1. 
   edwardh1 | Oct 08, 2009 03:57pm | #6

   they (contractors) are afraid to downsize as if it does not cool properly, that problem is blamed on the lack of capacity

2. 
   danski0224 | Oct 09, 2009 12:32am | #8

   What's amazing is that their off-the-cuff selections almost always work right.

   "Work right" how? A system that produces warm and cool air may be "working", but it usually isn't working "right".

   Working "right" would be an AC unit delivering its rated capacity... and in order for that to happen, it must be charged properly (check subcool and superheat) plus the filter and ductwork must be capable of moving enough air.

   The guys that aim for 70 psi on the gauge... or a "beer can cold" suction line aren't charging it right.

   The homeowners that use those 1" pleated filters are losing 30% or more airflow due to restriction... when the filter is clean.

   The contractors undersize ductwork because it is cheaper. 

4. 
   DanH | Oct 08, 2009 04:00pm | #7

   The heating unit generally isn't a problem. Yes, it may "short cycle", and that may cost you a small amount of efficiency, but small in proportion to the savings the insulation produces.

   As for AC, the problem is that if the AC doesn't run as long in a cycle it won't remove enough humidity to maintain comfort and reasonably healthy conditions in a humid environment. Probably not a problem in the desert, and likely not AS MUCH of a problem in temperate areas where the AC is only used occasionally.

   In any event, it's not a problem that must be addressed immediately. One can take some measurements with the current equipment (HVAC people may even have recorders for this) and get a better idea as to what the proper sizing should be post-insulation.

   As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
5. 
   barmil | Oct 09, 2009 04:08am | #9

   The conventional wisdom is that an overpowered AC unit will shut off before it's ridden the house of humidity, yielding a clammy environmet. I've never personally experienced that. I've always had my fan run 24/7 at a low speed and then kick up when the unit came on, heating or cooling. Humidity has always been stable through the seasons. I guess you just have to try it, but I doubt that the insulation yielded a 50% reduction in cooling requirements, otherwise it would be in all the news.

1. 
   maguire | Oct 09, 2009 08:51pm | #11

   I tend to agree with you 100% Barmil, Thanks maguire

6. 
   rdesigns | Oct 09, 2009 06:25pm | #10

   That inspector is guilty of repeating a rule-of-thumb approach, but he is using one that is on the opposite end of the scale from rules-of-thumb that HVAC contractors often use.

   HVAC contractors very often size equipment on the basis of 1 ton per 500 sq., and then add another ton, "just to be safe".

   What the inspector said could well be true in many instances, but the only professional way to accurately size the equipment is to do exactly as MHolliday said.

   IMO, the greatest flaw in the industry is poor duct design and installation--poor ducts will defeat the best equipment, regardless of its size or its efficiency rating.

   A 3-ton unit with inadequate airflow will perform as a 2-ton unit, or worse. It may indeed cool the house, but the short-cycling will not only degrade comfort, it will cause premature compressor failure, so your HVAC contractor gets to sell you another unit in 5 years instead of 10.