Duct Design in Low Load Homes

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Peak Heating and Cooling LoadsComparison IECC vs. 50%

Peak Heating IECC

(Btuh)

Peak Cooling IECC

(Btuh)

Peak Heating

50%

(Btuh)

Peak Cooling

50%

(Btuh)Phoenix 22176 28899 13099 17755Atlanta* 45469 24448 18458 13082Orlando 23184 20890 12965 11739Pittsburgh* 35379 16072 26901 14184* includes ~1000 sf conditioned, unfinished basement

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Airflow Comparison

First Floor Second Floor

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Peak heating flow ~ 2x summer flow on one floor, and visa versa on other floor

ROOM NAME Area (ft²) Htg AVF (cfm)

Clg AVF (cfm)

Htg AVF (cfm)

Clg AVF (cfm)

Basement 35 33Breakfast 142 81 49 140 38Kitchen 139 28 9 8 5Dining 132 74 53 30 40Family 421 138 82 57 55Laundry 46 28 12 13 6Powder 21 0 0 0 0Foyer 155 82 70 63 43First Floor Zone 431 275 346 220Bedroom2 160 43 59 34 43Bedroom3 152 31 123 23 111Bedroom4 185 59 92 47 72Bath2 50 10 8 7 5MasterBedroom 255 60 79 46 58MasterBath 85 20 17 15 10MasterWic 56 9 8 6 5Hall 114 9 9 5 7Second Floor Zone 241 395 183 311Total House CFM 672 670 529 531

Phoenix Atlanta

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Low load duct layout (4000+ SF house, ~2 ton load ~1,000cfm)

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Low load duct layout (4000+ SF house, ~2 ton load ~1,000cfm)

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High Mass vs. Low Mass Ducts

• Side by side houses, one with low mass central system, other with traditional sheet metal perimeter system

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Terminal Air TemperaturesHigh Mass Duct System

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Terminal Air TemperaturesLow Mass Duct System

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Low Mass Implications

• Low mass systems (flex duct and duct board) may be a key strategy in lower load houses in the 30% - 50% savings level – but the opportunity to install poorly is increased (pinched ducts, not stretched for smooth flow, etc.)

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Low Load Home Implications

• Same size house, still need to get air to each room, but quantities are greatly reduced, and vary from room to room and season to season, by as much as 50%

• Zoning airflows show high variation season to season –Peak heating flow ~ 2x summer flow on one floor, and visa versa on other floor

• Adequate flow over coil without freeze up at low speed (where unit operates most of the time)

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Low Load Home Implications

• With lower loads and flows, wider temperature swings from room to room can be expected, with more customer complaints.– Longest vs. shortest run– Thermostat location near shortest run– Impact of duct system material

• Fan continuous operation is an energy intensive way to try and even out temperature imbalances.

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Low Load Home Implications

• Higher airflow with smaller capacity?(800–1000 cfm / ton?)– Risk of “cold blow” if industry does not take register placement

seriously – and what about retrofit where register location is fixed?– Not gaining any synergy on reduced duct size, runs, etc.

• Distributed Fan Coils – heated / chilled water or refrigerant?– Easily zoned, small units with minimal duct design (if any?)

• Distributed individual systems?– 5,000–9,000 Btuh Minisplits, but aesthetics and room to room

mixing are issues• Integration of ventilation

– Unclear where ASHRAE 62.2 is going to end up

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Low Load Design Barriers

• Design methodology does not address dominance of internal loads or windows – Opaque walls and infiltration are less dominant

• Multi-point data is needed to drive comfort systems – Treatment of cooling load excursions

• Developing strategies to deal with load diversity is important– Overlaying an auxiliary zone

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Low Load Design Barriers

• Humidity control and ventilation is still an inelegant issue– Ventilation must be code mandated

• Multi-capacity / speed systems are needed to meet base and peak loads

• Duct system mass is significant• Temperature loss / gain for ducts in semi-conditioned

space is now significant

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Low Load Implementation Barriers

• Duct design (and installation) for low flows is outside the current industry capabilities.

• Low mass systems (flex duct and duct board) are prone to poor installations (pinched ducts, not stretched for smooth flow, etc.)

• Smaller duct pieces are not standard stock items• Higher performance registers are not standard stock

items and therefore more expensive• Commissioning has not been a “residential” thing but

now must become a standard practice