HVAC sizing for ICF block house

[madcowscarnival]

I'm looking for some reference documentation to point a client towards for sizing an HVAC system. The situation is they built a house with manufactured ICF blocks, two story, 2500 TLA. In this area, residential utilities are simply sized/installed per code (no engineering design required), however, the HVAC company they are using has no experience with ICF blocks and a net insulation value of ~R-24. Anecdotally, the ICF block rep has a similar ICF home with a 2/3T two-stage HVAC but at 3800 sq. ft.; the HVAC company has suggested a 5T for this (2500sq. ft.) house.

I've looked at the Mass HVAC sizing for concrete homes HUD document, and the referenced program (now "EnergyPlus"). Is this an acceptable industry standard to direct them towards? It seems the data entry is pretty straightforward, similar to the HERS program. Are there some other methods/programs/documents that would apply more toward the installer side of things. I can help them with basic data entry, but am not an engineer in that field, nor have any experience with it. Suggestions appreciated.

 

[EnergyProfessional]

The HVAC load calculation should be like for any other building with a load software. Depending on how sophisticated the method is, it takes into account thermal mass (especially for cooling) and obviously windows etc. The fact it is ICF shouldn't really make a difference beyond the R-value and the thermal mass in the concrete poured into the ICF blocks. It could be concrete wall with insulation attached to outside and inside, would be the same thing for sizing. For any material, have a good understanding of actual R-value due to thermal bridging (especially framed installation). But once you determined actual R and thermal mass, it is all the same physics.

Since you refer to contractor "not designing "and just "somehow knowing".... I have no idea how to provide a good design value without designing.

Please verify actual R-value. I saw ICF reps on exhibitions that claimed their R20 (based on material thickness and minor thermal bridging) actually is R70 due to "radiation". That R70 value is BS and should not be used for design.

Don't just size based on "someone has a X-ton system in that size house". First, you don't know the exterior surface if they have complex footprints. Second, you don't know type of construction and airitightness, windows (size, orientation, type)etc. And lastly, you don't know their internal loads. One person never having visitors and only one LED bulb vs. a big-party family with running their gas fireplace and icnadescent light palaces all summer makes a big difference.

 

[nuuvox000]

Having done load calcs for years, I honestly think it might be a waste of time without experience and engineering knowledge. Even my junior engineer comes up with vastly different values sometimes than I do because of the many assumptions you need to make and all of the settings you need to check. Just one box checked in error could give them the wrong data and then they've wasted their time and money. I would definitely recommend hiring someone who does this for a living. I would offer to do it myself if I had the time!

That being said, we use CHVAC software for the calcs and I don't really have anything bad to say about it.

Just a tip, if they're not able to get accurate load calcs for whatever reason, my recommendation would be to go with the 5 ton like they suggested but get the variable capacity air conditioner and modulating gas furnace. That way it can turn down the capacity and keep you comfortable instead of blasting you with cold or hot air when you don't need that much capacity. Downside is cost (expensive) and future repairs (expensive). Even better, get an HVAC system for each level of the house but then you're talking even more cost.

 

[EnergyProfessional]

The biggest variable that has to be guessed is infiltration. You can get pretty good values for actual U-value (with thermal bridges), windows and doors. But infiltration is a big unknown even for new construction. With a massive house, like ICF and concrete core, I would be comfortable to assume a relatively low infiltration unlike what I had to assume for any stick frame construction.

For a same quality stick frame building you probably assume twice the infiltration even when they do a blower door test. So that alone would downsize equipment somewhat even if the U-value is the same.

If they do a blower-door test, you can have some assurance they do a decent job in sealing around openings and penetrations. But also consider the roof and wall/foundation interface.

 

[miningman]

Not wanting to hijack this thread , but how does one exaluate the results of a blower door test?? I will be having this test done next month prior to replacing windows in a 2000 sq foot home built 20 years ago. My energy costs arent atrocious but theres a govt rebate plan available which requires a blower door test. Cost of windows will be around $21,000 and Id like to identify any other thermal weaknress areas before next winter

 

[EnergyProfessional]

Blower door tests show how much leakage you have at a standard pressure difference. The Fans maintain 50 Pa (residential) or 75 Pa (Commercial) pressure difference and measure the flow required. The more flow required to maintain the dP, the more leakage. they test infiltration and exfiltration since some cracks can behave differently depending on direction.
I'm sure whoever makes you do that will provide thresh holds for what leakage is acceptable.

It does NOT tell you the infiltration rate required for load or energy simulation. That is because over the normal year you don't have 50 or 75 Pa. Just an indication. I use 3 different values for tight, lose or leaky buildings. so a blower door test would tell me which to use.

 

[miningman]

EP, could you please advise what are the three values that you use , so I can have an intelligent conversation withe HVAC technician when he is testing my house??. I am well aware that most houses leak more than the homeowner recognises. I have a strong background in mine ventilation which should make for producytive discussions.

 

[EnergyProfessional]

Some load and energy simulations will adjust infiltration based on weather data (wind), height above ground (stack effect, impact of wind) and temperature difference. So make sure what it actually end up using for design conditions. Make sure you understand the numbers like cfm/ft² of floor space or wall area.

what I sue as fixed numbers for load only in cfm/ft² of wall:
tight building: 0.04
Improved building: 0.08
Average Building: 0.12 (I use for existing masonry or similar when no data on sealing is available)
Leaky building: 0.25 (I use for existing framed construction when no sealing is evident)

These are very conservative. But for unknown construction, I rather have too much. We had some projects where we had a sealing company test before and after air-sealing and two buildings that appear to have similar construction can have very different values before sealing. And you can drop infiltration to 25-30% by sealing bad buildings. There are typical trouble spots, like wall-roof interface, penetrations, openings etc. You also find insulated stud walls without air barrier, exposed fiber insulation etc.