simplified space conditioning - energy

Simplified Space Conditioning

Duncan Prahl, RAIBACOS, Inc.

Building America Technical UpdateApril 29, 2013


Rethinking HVAC Design

• Traditional Method – Assume envelope losses dictate the load – Room by room load analysis – Pick Equipment and distribute to meet the load in each room

• New Method – Consider how the occupants live in the building– Seriously consider internal gains in both heating and cooling– Consider ventilation strategy– Design system


If you are:

• A production builder• Participating in “above code” programs• Following ACCA Manual RS or ASHRAE 55• Need to prove “delivering heat to each habitable room”• Concerned about litigation

• Play it safe, Use Manual J, S & D and condition every space– But you still might have comfort complaints…


Research Results

• A Typical Production Home, Central CA


Assembly SpecificationsConcrete Slab R-10 vertical slab edge insulationFramed Walls 2x4 16-in. O.C. with R-15 cavity plus R-3 sheathing

Roof R-49 blown attic insulation at floor, radiant barrier at roof sheathing

Windows U = 0.28, SHGC = 0.26Building Air Leakage 2.45 ACH at 50Pa

Windows U = 0.28, SHGC = 0.26Building Air Leakage 2.45 ACH at 50Pa

Heating Load 23,891 BtuhCooling Load 17,193 Btuh

System 95% AFUE Gas furnace with 16 SEER ACDistribution Insulated Flexible Ductwork located in conditioned

spaceVentilation Bathroom exhaust fans, Fresh air duct into AHU Return

Roseville, CA. ~1800 s.f. single story


Single Story, 4 Bedroom, Central CA

• Distribution to Each Room

• Compact Duct System in Conditioned Space

• Right Sized• Never 100% balanced by

the HVAC contractor• Probably represents a

“typical” good HVAC job

MechCoffer


Room Temperature Performance

Room to t’stat temperatures over 2012/13 heating season mostly within +/- 2oF, cooling within +/- 3oF


Room Temperature – late winter


A “client-tuned” HVAC system:

• Do you want temperatures in each room to be uniform with one another?

• Do you want individual control in each room?• Do you keep your doors closed frequently?• Do you like to adjust your thermostat a lot?• TV’s, computers, etc. in the bedrooms?


One House Many Systems


Assembly Specifications Concrete Slab R-10 continuous below slab Basement Walls R-25 finished portion, R-19.5 unfinished portion

Framed Walls R-30 cavity insulation - framing fraction of 15% R-10 continuous exterior sheathing

Roof R-60 blown insulation at floor of vented attic Windows 306 sq. ft., U = 0.24, SHGC = 0.22 Building Air Leakage 0.54 ACH at 50Pa Heating Load 18,526 Btu/h (5,430 W) Cooling Load 11,236 Btu/h (3,293 W)

Distribution Systems“Right Sized” Fully Ducted Full Volume; Partially Ducted Low Volume; Dual Point

Ventilation ERV - Exhausted from bathrooms - Supplied into AHU return or to bedrooms

Pittsburgh PA


Winter 2012 – Cold Days


Single Point on 1st Floor

Supply

Return


Single Point on 1st Floor – Winter Cloudy Day

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Coast Down Steady StateResponse


“Right Sized” on 1st Floor

Return


“Right Sized 1st Floor – Winter Sunny Day


Single Point on 2nd Floor


Single Point on 2nd Floor –Doors Open Sunny Day

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Master Bedroom (Northeast)Comfort Band

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Doors OpenSystem Off

Doors Open1st Floor System On

Doors Open2nd Floor System OnDoors Closed


Single Point on 2nd Floor –Doors Closed, Transfer Fans, Cold Cloudy Day

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At 1st Flr Thermostat

At Hallway


Single Point on 2nd Floor -Doors Closed & Transfer Fans, Cold Sunny Day


Multi point 2nd FloorDoors Closed 15 CFM/Room


Multi point 2nd Floor - Doors Closed , Cold Sunny Day

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“Right Sized 2nd Floor – Supply in every room


“Right Sized 2nd Floor - Doors Closed T’Stat in Hall, Cold Sunny Day


Why?

• Room to wall area ratio– “Panel Heating”– Need Highly Conductive

Walls?• Location of Single Point

supply relative to transfer fan / door locations

• Solar gains not equal


Summer 2012 – Hot, Sunny Days

Supply

Return


Single Point , 1st Floor –Transfer Fans Sunny Hot Summer Day


Single Point on 2nd Floor –Transfer Fans, Hot Sunny day


Ventilation Airflow, 1st Floor –15 CFM/Room Sunny Hot Day


Ventilation Airflow, 2nd Floor –15 CFM/Room Sunny Hot Day


“Right Sized” 1st Floor –Distribution to Each Room, Sunny Hot Day


“Right Sized” 2nd Floor –Distribution to Each Room, Sunny Hot Day


Conclusions

• Large open spaces can be effectively conditioned form one point– Proper location in house, register selection / throw pattern

critical• Set up / Set back is hard to do uniformly• Factors to consider when designing a HVAC system for

a low-load house:– Sunlight (windows)– Internal gains and interior door positions (occupants lifestyle)– Ventilation strategy


What Should Grandma Do?

• RIP• Slow and Steady wins the race

– High sidewall supply outlets work well

• Comfort uniformity is enhanced by periodic fan circulation – like what you need for fresh air– Multiple temperature sensors to control fan only

operation– Locate sensors in “worst case” rooms

• To truly zone you probably need a unit in each room, or match rooms with equal solar gains– Zoning by floor with multiple temp sensors per system

or per zone


Thank You

• Questions?

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