I am in the process of gettting bids on a 6" OSB SIPs house with triple pane Fibertec windows. I want to make sure and size the HVAC system correctly and have read lots of your stories of oversized systems and the issues surrounding this. Does anyone have any experience with building scientists or have an opinion if consulting one would be a good use of money to take a look at my plans and calculate the size of the HVAC system? I plan to get bids from about 3 HVAC contractors, including investigating geothermal. I am concerned about being able to have all the facts I need to decide which kind of system makes sense in my energy efficient home. Any suggestions would be much appreciated. Ellen

Ellen,

As it appears no one else is going to answer you i will be the voice of inexperience.I have been planning my own home but am about two years out of starting it.I want to do icf if possible.In all my looking and searching i am convinced that a person doing icf,sips or even spray foam must have a hvac person experienced in this type of construction.I personally plan to use energy wise structures as my source for calculating the hvac load and how the duct system is sized etc.You can look up their web site and it is fairly informative.I believe for a fee they will take your plans and give you this information and if you pay a higher fee will guarantee your first years bills.I have been impressed with articles i have read about them in the past.Anyway as is said this is who i plan to use and hope it  helps.

 

I'll take a stab.  Keep in mind that I'm not a contractor..  From what I've learned, any hvac contractor worth his salt should be doing a manual j calc.  This factors in type of insulation, windows/doors/exposure, etc. & calculates heat load & heat loss - in other words, what will it take to get it warm & keep it warm.  I've talked to a couple contractors that told me, "the manual j seemed really low so we recommended going with a bigger system".  Uh...then what's the point of doing it?  I agree that it helps to work with someone familiar with sips, icf's, etc. so that they trust the numbers they see.

For comparison, before we sold our house this year we had to put in a new furnace.  It was a small house, but old & not well-insulated.  When the numbers came in, I was between sizes.  the contractor I ended up going with told me that that there is a bit of a "fudge factor" built into the calculations to ensure they don't undersize.  He said that I would be ok with either.  The bigger system was more expensive, and would also cost more to run on a regular basis.  The smaller one was cheaper & would cost less, but on days that got REALLY cold, it may struggle a little to keep up.  All that meant is that on those rare days, it would run more.  I opted for that, figuring that there were a lot more "sort of cold" days where the furnace would keep up just fine.

As it turned out, it was the coldest winter we had in several years, and we were plenty warm.

Ellen - Since this is new construction, I suggest having a HERS rater perform an Energy Star rating (and a Manual-J). I would believe that you already have the Energy Star 85% target met, so the tax rebate would more then pay for the rater services. The HERS rater will look at more then a Manual-J, but provide that as well if you make that part of your service agreement.

http://www.natresnet.org/ to find a rater near you.

Energywisestructures.com

You may want to consider using thicker SIPS panels as well - the more insulation you have the smaller the furnace will need to be and the less fuel you'll need to use. Also, and at the risk of sounding like a broken record, look for a very low air infiltration rate - should be easy with SIPS - and plan on an HRV.
Bob

If you're going to get 3 bids anyway, insist that they come with room-by-room heat load analyses, and compare them against each other. Most will use software that uses a Manual-J type methodology, and they should all be fairly close to one another, and they'll all overshoot by ~25-35% from the true peak heat-load if the house is actually built correctly with proper attention to detail. Make sure that they understand the U values of the windows, etc.

I'd agree with Bob I that thicker SIPs are likely to be cost-effective in MN- particularly when it comes to being able to down-size a geothermal system. (Natural gas, not so much.)

Thank you all so much for the suggestions regarding consulting a building scientist and the best way to correctly size my HVAC system.

Ellen

I admire you for having an energy-efficient home. You are really thinking about the environment and how to cut the costs. We all know that HVAC systems tend to make your electric bills higher and it is imperative to find the right system that suits you.