Heat and Air Control

Post date: Nov 3, 2014 5:18:05 PM

My plan for house is to have a heat storage slab with heat exchanged by air flow. The floor slab has air ducts beneath it formed by concrete blocks laid on their sides. Floor registers at the outer walls let room air into the ducts. A sheet metal return down the middle beneath the floor slab pulls air into the furnace fan. If needed, a gas-fired furnace raises the temperature of the air as it passes through the furnace.

These are my early thoughts on temperature control.

Sensors are located at these points:

• Temperature within the building, called “Room”
• Temperature outside the building, called “Outside”
• Temperature within the floor slab, called “Storage”
• Occupancy (usually with infrared motion or carbon dioxide sensor): This would be push-button timer by toilet plus humidistat over shower AND thermostat over gas range
• Indoor air quality (IAQ - carbon dioxide, volatile organics): Can substitute the switches described under Occupancy, above
• Window position - to detect if outside air is available to balance exhaust or fresh air intake

Set Point, here called “Set” is the one other temperature in the algorithm. Set point is the desired room temperature. This set point will have a ‘dead band’, a narrow range of temperature within which a small change of temperature does not initiate action. For instance, if the set point is 72F, then the Room temperature must rise above 74F for cooling to begin and the Room temperature must fall below 70F for heating to begin

There are only these active elements in this heating/cooling system:

• Burner in the furnace
• Fan in the furnace
• Heat Recovery Ventilator (HRV), which also has a fan
• (In the future, window coverings might be active)
• (In the future, there may be a refrigeration system - air conditioner)

Heating

Heat for the room can come from these sources:

• the sun, through the windows
• the slab, via the ductwork and the furnace fan
• the furnace burner
• gas cooking range in the kitchen
• outside air - either through open doors and windows or the ventilation system

Heat is only needed

• If Room within Setpoint headband, turn Fan off, else
• if Room < Set AND Occupied, then:
• If Storage > Room, turn Fan on, else
• If Outside > Room, replace indoor air with outdoor air, else
• if Room << Set turn burner on Hi (assuming fan will come on automatically)
• turn burner on Low

Note that this ignores heat from the cooking range. May add this later.

Cooling

Cooling can come from these sources:

• the slab
• outside air - through open windows (by opening the exhaust damper from the circulating fan)
• (in the future, possibly a refrigeration system)

Cooling is only needed

• if Room > Set AND Occupied, then:
• If Storage < Room, turn Fan on, else
• If Outside < Room, replace indoor air with outdoor air, else
• (in future, turn on refrigeration compressor)

Air Quality

There is a building code minimum ventilation requirement per square foot of floor space. If there is no one in the zone, ventilation can be reduced to meet the “floor area” ( R_a x A_z ) component of the ventilation requirement per ASHRAE Standard 62.1-2010. [60 cfm]

See Siemens “Demand Controlled Ventilation: Application Guide for Consulting Engineers”, 2013. Well explained and extensive - though with a commercial building slant.

Indoor air quality (IAQ) must be sufficient for health and comfort. Clean air at a rate of at least 20 cfm (cubic feet per minute) per person, plus more for cooking and internal combustion (such as cooking). York, Trane, Lennox, and other air conditioner makers sell HRV.

At this time, air quality can only be improved by dilution with (tempered) outside air.

• IF IAQ < IAQ_Set AND house is Occupied, then
• turn HRV on, else
• turn HRV off

Per Table 6-1 in ASHRAE Standard 62.1-2010, the air requirements for my house are 5 cfm/person * 2 person (design occupancy) + 0.06 cfm/sq ft * 1,000 sq ft = 70 cfm. Demand Control Ventilation (DCV) would only reduce this to 60 cfm, so forget DCV.

If ventilation is shut off when the space is unoccupied, it may need a purge period to remove the contaminants that have built up during this period. Either start before occupancy or run the fan faster for a bit.

Integration

HRV basically exchange air for IAQ and does not control temperature. The heating/cooling must be integrated with the HRV. There is no sense in running a heat exchanger (even at 80% efficiency) if we are bringing in fresh air through open windows (at 100% efficiency).

This means we need to integrate with a perimeter security system. Which means that the open window sensors must be working, and - from my perspective - invisible.

Also, the heating ducts and heat recovery ducts can be integrated, separated, or combined. Here is a good discussion.

Bibliography

Whole-house ventilation systems - Honeywell

Sensor product overview

The bare sensor or with wall mount and display for about $130

Air Quality Sensors

Siemens AQR series with display for wall mount

Ducting HRVs and ERVs, Fine Homebuilding, December 2014, p 86.

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