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HVAC
     My related HVAC pages      
Heating, Ventilation and Air Conditioning. 

I don't know about you, but I think this term must be one of the great misnomers of all time.  Heating... sure I got that, Air Conditioning... no problems, but what about Ventilation?  Ventilation is all about providing fresh air for you to breathe, but chances are, if you trace ALL the air ducts in you HVAC system, you won't find any that bring in air from the outside.  You have Circulation, but NO Ventilation. So where is all the critical Fresh Air coming from? It looks like we lost the V in HVAC.

In the old days, that fresh air would come in from open windows and doors, and all the leaks around pipes and cables that pierce the shell of your house.  But thanks to the new energy building codes, these openings are disappearing because they sap away all that heating and cooling energy that you pour into your home daily.  So get out your duct tape and seal up every one of those cracks.

But hey...  aren't we also meant to provide exhaust ventilation to suck the steam and humidity away from bath and kitchen areas.  If we don't, we get problems with mold and mildew that adversely effect our health.  So start cutting holes, and installing super-power exhaust fans to the outside.

So let's do a mind experiment shall we?  Seal all the cracks that let outside air in, then add exhaust fans to suck air out of all the bathrooms etc.  Can you see the result? I have an image forming in my head from a Sci-Fi movie where all the air is being sucked from the spaceship airlock, and the occupants balloon up and explode from the vacuum. Hmmm.... cool !!!

OK, so what's wrong with this picture?  The simple fact is that in order to get enough fresh air to stay healthy, and to stop those bathrooms from turning into swamps, there has to be a continuous flow of fresh air entering and leaving the house.  As fresh air enters, nasty old wet and smelly air will leave through all those exhaust vents.  So why did we bother making the house air-tight in the first place ??  

The step here that everyone misses is that we need to circulate fresh air, but we also need to extract energy from the stale air as it leaves the house and put it back into the fresh air entering the house.

This is a classic example of what I have found while exploring this whole energy efficient house design process.  Different groups are designing systems and making policies without considering how they effect the total solution.  Everyone is focused into their own specialty and don't seem to be able to look outside their box. I found this tunnel-vision with:

  • Passive Solar people who hadn't considered Hydronic (water based) systems as a more efficient way to distribute thermal energy thought a solar slab floor (everyone wants to use air).
  • Radiant Heat people who hadn't thought to utilize their systems for Radiant Cooling applications.
  • Energy Efficient home building people who forgot to include adequate Ventilation, or who overlook the energy loss issues.
  • Fresh Air advocates who didn't seem to care about all the energy being wasted through great ventilation.
  • Geothermal Exchange people who hadn't thought to use their systems for Hydronic Cooling (rather than air cooling).

There really are lots of cool ways to integrate the available HVAC components, and if you follow along I'll illustrate my Ideal system.

H is for Heat.

My house is designed with passive solar heating and cooling in mind, and it benefits from earth-berm walls on 3 sides (or should I say 2, since it really is triangular).  However, I'm still assuming that I will still need a supplemental heat source for comfort and domestic hot water.  Since my home is located in a cold climate (80 inches of snow a year on average) I'm assuming that these two heating requirements will be the major portion of my energy needs, so any efficiencies here will have maximum impact.  My HVAC system will use a Ground Source (Geothermal) Heat pump (GSHP) to generate Domestic Hot Water (DHW), and hot water to be use in the Radiant Slab.  Since a GSHP has an operating efficiency of between 300% and 400%, an energy saving of at least 60% is expected for Heat.  

Note: Although the GSHP generates all the Domestic Hot Water, it is stored offline, in a conventional 75 gallon water heater.  This storage tank is ideal for several reasons: 

  • It's large capacity ensures that there will always be sufficient hot water for human use.
  • It provides consistent source temperatures regardless of number of zones operating.
  • It is inexpensive since it is a mass-consumer item.
  • In the event of a heat pump failure, or extreme cold conditions, the built-in heating element can be turned on to provide backup/supplemental heat.

The GSHP heats the slab directly, bypassing the DHW storage tank.  This way the slab can be run at a lower temperate, therefore obtaining the maximum efficiency.  The system plumbing is such that EITHER the DHW OR the Radiant slab will be circulating hot water.  Since the GSHP can also be put into "Reverse Cycle" it's also possible to actively cool the slab in the summer. 

V is for Ventilation.

To promote a healthy and clean air environment, my home utilizes forced flow 160 CFM ventilation system which exhausts air from high humidity/odor areas, and replaces it with fresh filtered outside air.  To prevent wasting the energy used to condition the exhaust air, an Energy Recovery Ventilator (ERV) is used.  This passive unit lets the outgoing exhaust air exchange energy with the incoming fresh air to effectively recapture 60% of the energy used to condition the exhaust air in the first place.  Think of it as a "Pre-heater" in winter, or a "Pre-chiller" in summer.

AC is for Air Conditioning.

Since my house uses Radiant Heat, and the cooling needs are extremely low, the need for Air Conditioning and large air flows are minimized.  Since the Slab and 2 walls of the house are in contact (via insulation) with the earth, these are all natural radiant cooling surfaces in summer.  My original design simply called for a A 600 CFM fan and water coil, fed from the ground water circuit.  However, the HVAC designer wasn't happy with relying on this passive approach, so a second Ground Source Heat Pump was added.  Since this unit has the ability to both heat and cool, the Radiant Slab unit did not have to cope with extreme cold temperatures by itself, so the unit was able to be downsized

The diagram below shows the major components in my design concept.  This integrated system uses a Geothermal Exchange system as the primary heat source/sink.  Radiant Hydronic coils in the slab floor are used to control the temperature of the house interior, and a low flow ventilation system is used to provide fresh air, and control humidity.  The ventilation system may also be used to provide supplemental summer cooling and winter heat, but this is not it's main function.  I repeat, the ventilation system is NOT designed to heat and cool the house.  It is designed to efficiently provide a healthy living environment for the occupants by supplying fresh air, minimizing airborne contaminants and eliminating condensation (all very important for a house built partially underground).


Energy is supplied to the system efficiently using a ground water constant temperature source.  

Geothermal, Radiant, Efficient, Energy-Network (GREEN).

Geothermal because utilizing a Geothermal Exchange earth loop is key to the energy efficient operation of the system.
Radiant because the primary heating and cooling action is achieved through radiant temperature equalization.
Efficient because the year-round heating and cooling configurations of this system permit all the components to operate at their maximum efficiency.
Energy-Network because all the active energy transfers in the system are achieved through a switched network of  water pipes.

© 2000-2015, Phil and Lisa's relaxed lifestyle home.
An exercise in Energy Smart, Not So Big living.
www.OurCoolHouse.com - Ideas@OurCoolHouse.com

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This site is all about building a cool, energy efficient house, that makes maximum use of earth sheltered design, passive solar heating and cooling, geothermal exchange energy management, and right sizing of the house for it's designated use. The home's placement is on a south-facing hillside in Deep Creek Lake, Maryland. This site describes the design process, the technologies used and the expected results. We also have a comprehensive Links Page for anyone who is also interested in designing a similar project.