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HVAC Equipment

Heating systems have come a long way since the invention of the fireplace, sometimes combining a furnace, an air conditioner and a ventilation system all in one.  Since these systems are typically installed by a single mechanical contractor, we refer to them as HVAC: Heating, Ventilation and Air Conditioning; although depending on climate not all systems are always present.

In spite of advertising by various manufacturers, there is no such thing as miracle heat.

The traditional idea of burning something to heat the home is deeply ingrained in most people's minds, and often has a romantic appeal, especially in the case of fireplaces.  Unfortunately most of the fuel we burn is of the unsustainable variety: oil, natural gas and propane.  To further compound this problem, homes are often poorly insulated, don't take advantage of solar energy and don't use their fuel particularly efficiently.

The first step in designing an HVAC system is to design the house so that an HVAC systems is hardly needed at all by using a very well insulated structure and utilizing the available solar energy and passive cooling strategies (see solar).  In this scenario, the HVAC is used largely  as a backup for when there isn't enough solar heat (it is sometimes also used as part of a ventilation system).  Money spend on avoiding the use of HVAC systems can be offset by savings in installing a smaller system than would otherwise the necessary, reducing the up-front costs as well as saving fuel.  The one caveat to this is that most equipment on the market is designed for poorly insulted homes, so that even the smallest unit will often put out far too much heat (resulting in it running for only brief amounts of time, which lowers the efficiency of some units).

The descriptions that follow are somewhat geared toward superinsulated buildings, and so focuses on the issues that are different from conventional lightly insulated buildings.

Heating systems have two primary characteristics: how the heat is created, and how the heat is distributed. For example, typically furnaces burn either natural gas or oil, which is used to heat air, which is then forced around the house by a fan.  The choice of fuel sometimes limits the distribution methods that can be used, although it needn't be the case.

Methods of Creating Heat

Furnace - burning oil, natural gas, propane, coal, or electric a furnace heats air up.  Old style furnaces consume air from inside the house, while newer ones have a separate vent to bring in outside air to support the burning.  Older furnaces are may be a little as 50% efficient, but all newer ones (called mid-efficiency) are at least 80%.  A high efficiency (sometime called condensing) furnaces are over 90% efficient, but often cost much more.  Early condensing units had reliability problems because the condensing section of the furnace produces highly acidic water that must be disposed of properly.  Forced air furnaces heat the air hot enough to "cook" the dust in the air, potentially with some negative health consequences.

Boiler - similar to a furnace, but heating water instead of air.  Boilers can combine heat and hot water in one unit, and can be more efficient in cases where only a small amount of heat is required (since starting a furnace is the least efficient part of the cycle, heating a large tank of water avoid excessive cycling).  For some reason there is often a huge price jump from an ordinary hot water heater and a boiler, even though a boiler is essentially just a hot water heater with a high BTU output.  A superinsulated house could potentially use an ordinary hot water heater for hydronic heating, provided the HW tank provides enough heat (for example if the buildings worst case heat demand is 10k Btu/hr, a 40k Btu/hr HW tank will probably work fine - however if the heat demand causes the tank to run too much, it may burn out early.

Fireplace - burning wood and exposed to the air in the room.  Fireplaces are at best about 10% efficient, but are often take more heat out of the house than they put in due to burning a lot of the air in the house and sending it up the chimney, only to be replaced by cold air leaking into the house from elsewhere.  People love sitting by the fireplace, but from an environmental point of view, sitting in front of a wood stove with a glass door is much preferable!  Rumford style fireplace are the most efficient type that don't use and insert.

Wood/Gas stove - burning wood, natural gas or propane, the burning process is contained, and so isolated from the room. Modern stoves often at least have the option of using outside air to burn the fuel.  Older wood stoves are not especially efficient (although much better than fireplaces) and highly polluting.  Newer ones have a hi-tech design the results in very efficient combustion (around 80%) and are much cleaner burning, but in order to get truly clean combustion, a wood fire must be very hot.  In an energy efficient home, so little fuel must be burned that an efficient wood stove could quickly overheat the house.

Masonry wood stove - invented in various forms in northern European countries in the 1700's to make more efficient use of a scare wood source, masonry stoves fit well into energy efficient homes because they allow for the rapid burning of wood (hence hot, clean burning) while also capturing a very high percentage of the heat generated.  Masonry stoves combine an efficient firebox design with a long series of passages for the smoke to pass through in order to capture all the heat of combustion before it goes out the chimney.  Unlike most other methods of burning fuel, the idea behind a masonry stove isn't to heat the air, but to heat the brick the stove is made out of.  While a fireplace or woodstove give off heat mostly only when they are burning fuel, a masonry stove is designed only to burn for a short while (1-3 hours) and letting the masonry give off heat when the fire is out.  As with any thermal mass system (see thermal mass), the more mass the stove has, the longer it takes to heat it up--a large stove could take an entire day to heat up.

Baseboard Electric Resistance - electric can be converted to heat with a high efficiency (ignoring the efficiency of generating the electric in the first place), and electric baseboard heaters have a lowest initial cost.  They are often found on low cost construction and in additions where it was difficult to extend the existing heating system to the new addition.  Like furnaces, baseboard heater "cooks" any dust in the room, potentially producing some quantity of toxic compounds.

Electric radiant panels - these work the same as the more common hydronic ones, but use electric.  The most common type are used in bathrooms under tile floors to keep the floor warm.

Heat Pumps - a heat pump is essentially a refrigerator run in reverse, and operates exactly as its name indicates; it moves heat from outside to inside.  There are two basic kinds, air-source, which pumps heat from the outdoor air, and water source, which pumps heat from the ground or a pond.  Many heat pumps can run both ways, providing air conditioning and heating in one package.  While most heat pumps heat or cool air, some will also produce hot or cold water, which than can be used in radiators or an in-floor hydronic system.

Because heat pumps move heat, they can produce more heat than could otherwise be obtained with the energy to run the heat pump, and so are measured by how much better they are just burning the fuel and collecting the heat, with current good models generating almost four times as much heat energy as it takes to run the heat pump.  This is measured by their COP (coefficient of performance), which measures how many times more heat is produced compared to the energy to run the heat pump, for example, a COP of 1.5, means 1.5 times more heat comes out.

As always there is a catch: a heat pumps efficiency (and hence COP) goes down dramatically when the temperature difference is large, and most fail nearly completely when the outside temperature is freezing (to combat this, most have a electric resistant heater that turns on when the heat pump won't put out enough heat).  See below for units that avoid this problem.

Environmentally there are two issues with heat pumps: they run on electric, which currently is produced largely from non-renewable sources, and the contain Freon, a greenhouse gas.  Depending on where you live, using grid electric to power a heat pump is essentially, burning coal and gas (at maybe 35-40% efficiency, for details see the fuel section) to generate electric, then converting the electric to heat: if the power plant is 40% efficient, and your heat pump has a COP of 3, you're getting an overall efficiency of 120%: better than the 85% gas furnace, but not much better. While it is technically possible to power a heat pump without electric (for example a propane refrigerator), it's much harder.

The type of Freon used has moved from CFCs, which are both ozone depleting and a greenhouse gas, to HCFCs which are only greenhouse gases. Ideally the Freon never escapes the heat pump (and in many states you're legally required to recycle it), but in reality surely some escapes.  There is a move to other alternative gases, but what they will be and what the issues will be are not clear yet.

Ground-source (often called geothermal) heat pumps use long loops of fluid filled pipes buried deep enough in the ground to access the year round temperature (which is often above 50F),  thereby allowing them to operate at nearly constant efficiency and avoiding the problem of failure during cold weather.  The pipes can be installed vertically or horizontally, generally down 10 feet or more (which is about how deep the region of constant temperature is).  Because heat moves quite slowly in earth, the loops are typically quite long (more than 100 feet), and can be very expensive to install (if the loop is too short, the ground around the pipe could cool down, reducing the units efficiency).

Multi stage heat pumps solve the cold temperature problem by using a second stage when it is cold out, allowing these devices to keep a high COP down to much colder temperatures (currently (2009) in the range of 20F).  Although these devices are currently rare, they will probably become more common baring any reliability or cost issues.

Mini-Splits (ductless) are just wall mounted air conditioners that work as heaters (ie, wall mounted air source heat pumps).  You need one for every room (or for additions, just for the addition).  They have the advantages and disadvantages as central air source heat pumps, with the additional disadvantage of likely being noisier, but since they use no ducts, they are easier to install.  Also since there are multiple units, zoning is possible (see more on zoning below).

Methods of Heat Distribution

Forced air - the furnace heats the air, and then an electric fan pumps the air through ducts to all parts of the house.  Forced air systems are relatively inexpensive and are by far the most common installed.  While most forced air systems are part of furnaces that use a simple heat exchanger to transfer heat from a furnace to the home's air, a boiler can also be used in a configuration called a fan-coil because a coil of hot water from the boiler is put in front of a fan which blows air over the coil to heat it.  The fan-coil system runs at a much lower temperature than a normal furnace and so doesn't cook the dust.  Forced air systems can also be used for cooling and ventilation, which is a significant benefit, but also means they can have a negative effect on a home's air quality also (see ventilation). 

Radiators/Convection - a heat source (like electric or hot water baseboards) is put in every room (typically under a window) and the heat rising off it sets up convection currents in the room that distribute it around the room.  If the radiator is hot enough, a significant amount of heat will radiate also. How much heat moves by the various methods, depends on the size of the radiators, and the temperature they operate at. 

Radiant (non convection)- in a radiant heating system, one of the surfaces of a room is heated (typically the floor) and it gives off heat to both to the air and, if the temperature is warm enough by radiant heating also.  Although these systems are called radiant, much heat is transferred by conduction and convection, and in a superinsulated home where the "radiant' surface is not much warmer than the air, the system is really just a giant radiator operating a a lower temperature.  Radiant systems are sometime installed in walls or ceilings, but the downside of this is that it lowers the effectiveness of the convection heat transfer, which in a superinsulated house could lead to temperature stratification.

In-floor systems may use high thermal mass (ie in a concrete slab), low thermal mass (a thin added layer of gypcrete or equivalent), or no added mass.  Mass spread the heat better, but also slows the response down (both heating and cooling), making the interaction with passive solar heating difficult.  The conventional wisdom is that radiant heat doesn't work unless the pipes are installed in some thermal mass, but this isn't the case. Metal heat spreading plates make systems with no additional mass just as effective, and with a faster response time.  What is true, is that without mass, the floor will heat up and cool down as the system cycles.  Systems with no mass or metal heat spreader plates are relatively common, due to lower installation cost.  When the heat pipes are covered in wood (or other) flooring, the system may need to run at a higher temperature to overcome the insulation value of the flooring.  If metal heat spreader plates aren't used, the temperature may need to be higher yet, because the effective heating surface area is reduced.

Because most in-floor radiant heating systems aren't really radiant, they are generally referred to on this site as in-floor heating.

Comparing Distribution Systems

Every system has its advantages and disadvantages, and of course, even in a category some devices perform better than others.  The following should be considered as typical issues, rather than absolute advantages and disadvantages.

Air has a very low specific heat, so as a distribution method is inherently not efficient: a lot of air must be moved to move a little bit of heat.  Traditional forced air systems (as opposed to fan-coil systems) get around this by making the air very hot, but in doing so spread cooked dust around the house and tend to create a high level of temperature stratification in rooms, which generally leads to higher heat loss.  Fan coil system eliminate these problems, but heat the house very, very slowly using much more fan energy in the process.  In addition, fan coil system demand very hot water (140-160F) in order to deliver any significant amount of heat at all, and as a result if the hot water is from a tank, additional standby heat loss from the tank will occur. On the positive side, system failure does not generally result in damage to the house (leaking air versus leaking water).  If the heat source is from a heat pump, the system is effectively a fan-coil, because heat pumps don't produce high temperatures.

In all forced air system, duct losses can be very high, both due to air leakage and heat leakage. The best solution is to seal the ducts tightly with mastic (which should be done to keep dirt out anyhow), but more importantly to keep the ducts inside the heated envelope.  If they can't be kept in the heated envelope, they should be very well insulated.  In practice, a house can loose 40% of its heat in duct losses, in particular when leaky ducts are installed in unheated attics or crawlspaces because in practice it is difficult to insulate them effectively.

Fan energy use is also a factor, and that is determined by largely by the back pressure (resistance) the fan sees. In practice, the fan uses a fixed amount of energy, but produces a varying volume of air depending on the back pressure, which itself is determined by the length, number of turns and size of the ducts.  Shorter, larger straighter ducts result in less fan energy used, so a centrally located furnace is best.

For the heat to be evenly distributed in the house, ducts need to run to all rooms and be sized based on the room's predicted heat loss (or gain), or rather set so that the percentage of the heat coming from the heat source that goes to that room is the same as that room's percentage of the building's overall heat loss.  In practice, ducts only come in a limited number of sizes, and so dampers must be used to tune this.  In fact this is almost never done, at least in single family residential buildings.  Instead, HVAC contractor use rules of thumb that may not work well with super-insulted homes.  What happens is that rooms are all served by 3x14 ducts (the size that fits between 16"o.c. 2x4 walls), or maybe two ducts if the room is big.  Ducts are typically placed under windows (because that is traditionally where the coldest air is), but again in a super-insulated house, this may not be the best placement: heat may be distributed just fine no matter what wall the duct is on.  The advantage of shorter ducts is not only that there is lower back-pressure, but more importantly that it helps avoid putting ducts in insulated walls, which are voids in insulation of the worst kind.  The placement of cold air returns is equally important, because the air coming in to a room will create a small pressure rise, which is then forced toward the area of lower pressure created by the air return.2

Water holds a lot of heat, so not much water must be moved to move a lot of heat: for example 1 gallon of water raised 10°F absorbs 82Btu (8.2Lb/gal and1Btu/lb/°F). To get those same 82Btu with air (which holds .018Btu/°F/ft3), you need 455ft3.  So to deliver as much heat as a 1gpm pump, a 300CFM fan would need to run for 1.5 minutes, or to get the same heat in in equal time at 300CFM, the air temperature would have to be raised 15°F instead of 10°F.  Water pipes also take up much less space in walls than ducts, and so are easier to insulated, and presumably easier to route so that they stay fully inside the conditioned space.

The downside, is that if a pipe breaks, it does a lot of damage to the house (for this reason, its is safer to move water that is not at city water pressure).  The temperature that the water must be at in order to deliver enough heat is somewhat determined by the surface area of the radiators (or in the case of in-floor heating, the surface area of the floor).  Systems that move heat with water will generally use less pump energy than systems that move air (depending, of course, on the actual efficiency of the pump/fan).

The the other potential downside to moving heat with water, is that you still need ducts for ventilation, although these ducts are typically much smaller than the ones you'd need for any type of forced-air system (heating/cooling demands often run in the few hundreds of CFM, while ventilation requirements are as low as 5-15CFM per occupant).  For a further discussion, see "Ventilation and Heating", below

Hot water radiators that use very hot water (and electric radiators also), usually have the same temperature stratification problems that forced air systems have.  Electric radiators also have the cooked dust problem.

In-floor (usually called radiant) can often operate at lower temperatures than radiators because the heat distribution surface area is so much larger (even if the heat must move thru the flooring). Hydronic in-floor heat can be combined with active solar collectors as a way to supplement passive solar heating, although system design can be tricky.  It is generally not recommended to install hydronic pipes in thermal mass that also receives passive solar sunlight in order to avoid overheating.  If that room needs supplemental heat, use wall mounted radiators, or some other method.  Even with a massless in-floor system, response time will generally be quite a bit slower than conventional forced air systems.

When installed under hardwood floors the water temperature is often kept somewhat low (under 90F) to prevent damage to the floor.  Even in mass systems, the floor temperature must be kept low enough to avoid discomfort when walking barefoot.

There are many claims about energy savings for radiant heat, but largely they are all false because the assume that the house will be operated so that the air temperature stays cooler while the floor stays warmer--an assumption that is not true in superinsulated homes, and in practice usually not true, since people don't actually turn the thermostat down.

Zoning

The idea behind zoning is that you can save energy if not all rooms have to be at "room temperature": for example, guest room, or bedroom for people who prefer colder bedrooms.  Zoning may be harder to achieve in a superinsulated home, because the ventilation air movement may even out the temperature.  If a zoned room is allowed to get too cold, there is some chance of excessive humidity in that room, leading to mold growth.  This will only happen if the humidity in the rest of the house is high enough, and the room is cold enough.  As a result of this, it is not clear how much energy can be saved in a superinsulated house via zoning.

Ventilation & Heating

Depending on how tight the building is, the tighter buildings will need some mechanical ventilation during some part of the year (detailed discussion on tightness & energy in the infiltration section).   In forced air systems, the two can be combined, but doing so is trickier than it might seem.  The general issue is that unless you insulate to a very high level (think in terms of R50 walls for moderate (4000 degree days or so) climates, the amount of air that needs to be moved to meet the heating/cooling demand is much higher than the amount needed for ventilation.  In the Passive House design (That is Passiv Haus, a German standard: the US website is http://www.passivehouse.us), the building is designed extremely tight, and the house is insulated enough so that on the coldest/hottest day, the volume of ventilation air still delivers enough heat/cold.3

The alternative approach, which some argue is a necessity in the coldest and hottest climates is to let the heating/cooling system recirculate air to meet the heating/cooling demand, and feed the required amount of fresh air into this air stream (for example, using the aircycler controller -- see discussion below). One down side to this approach is that it incurs higher fan energy costs due to needing to more more air (in practice this may be a negligilble issue).

 Because heat/cool requirements vary with the weather, but ventilation requirements don't1,  there must be some de-coupling of the two.  In the Passive House approach the heating/cooling source must be able to be off when the ventilation system runs, and in the aircycler approach, the amount of fresh air must be limited when ventilation requirement have been met, but heating/cooling is still needed.

Duct placement of ventilation air supply is typically different from heat duct placement: fresh air ducts are often placed in closets and other remote locations, designed to diluted pollutants, however ventilation systems as part of a forced air system is probably the most common configuration.  The general technique is to tie a intake duct from the exterior into the cool air return system, using a damper to limit the air flow to the desired amount.  In this system, the heater fan must run for ventilation even if there is no need for heat: it either runs only part of the day, or uses a variable speed fan that can be run at low speed for ventilation only.

Alternatively a ventilation system can be completely separate from the heating system (for example, using in-floor heat).  These systems are discussed fully in the ventilation section.

Integration with Passive Solar

Heating systems do not necessarily integrate well with passive solar, because typically they're designed to heat the house to its comfort level first thing in the morning, even if there will be significant passive solar during the day.  Thermal mass can absorb much of the incoming passive solar, but some temperature rise will still happen, and there is a potential for overheating.  In the case of in-floor heat set in mass, the mass will already be warm, so passive solar gain will end up making it too warm.  With a bit of morning cool tolerance and manual thermostat control, the problems can be avoided as long as the heating system isn't a high-mass radiant system. 

Thermostat Issues

Nighttime setback: There was a big campaign in the 1970's to get people to turn their thermostat back at night to save energy, and as a result this has become part of many people "common knowledge".  This technique works when the house is poorly insulated and leaky, such that before the night is over it gets to the cooler thermostat setting  quickly.  As the house is cooling down, no energy is saved because it all has to be added back in the morning to reheat the house.  The energy that is saved, is during the time the house is at its steady "cool" temperature, because heat loss has been reduced during this time and the energy required to re-heat it in the morning is the same.  When a house is superinsulated and well sealed, it often only cools off by a few degrees, unless it is quite cold out, and even then the temperature drop during the night is not likely to go beyond the ten degree setback that was generally recommended.

However, in a passive solar house if there is likely to be solar gain in the morning, and you can tolerate the house being cold until the sun heats it up, nighttime setback would be a win.

Aircycler: One innovative solution for ventilation in forced air heating systems is use an auxiliary air intake (sized with a damper to the right amount of ventilation).  This system still over ventilates in cold weather (since outside air is added as a fixed percentage of the return air, the more the system runs, the more ventilation you get.)  However, during mild weather the controller will run the heating fan in "fan only" mode if the heating system has not run enough to supply the required ventilation.  This is programmed into the controller as a hourly amount, ie the percentage of the hour that the fan must run.

There are numerous tradeoffs that can be made with how much air is allowed by the damper, how the outside air is admitted (earth tube, HRV), and how to run the heater fan (which draw more power than ventilation fans because they have to move more air).

Hydronic systems: apparently its a good idea to circulate the water in a hydronic system once a day, even if no heat is used, and so the thermostat would have to do that.  Whether this is necessary or not, and what thermostats do it isn't clear.

System Sizing

Heating and cooling systems are measured by their maximum capacity.  To determine the size needed, calculate the maximum heat needed (coldest day ever recorded),  add some room so the unit isn't actually ever run at its maximum capacity, and that's the system size you need.  In calculating the heat loss, you will need to adjust for "internal gain", which is the heat put out by electrical use (virtually all electric used ends up as heat).  Solar gain is generally ignored because in the worst case there is none.

Resources

Choosing a heating system that saves energy - Home Energy, March, 1996
A more detailed description of conventional heating systems.

Energysavers.gov - US dept. of Energy, description of heating systems

Building Science Corp building primers - Including "Read this before you ventilate"

Heating Systems for Your Home, Richard Kadulski,
The Drawing-Room Graphic Services Limited, 1998

Builders Guild Series, Joe Lstiburek, EEBA, 2000


Notes

1: Some commercial systems, especially those in buildings with no operable windows,  operate on a constant volume basis: ie they move the same amount of air, but vary the temperature.  Occupants often find these systems unsatisfactory.

2: air conditioning source ducts are best placed high on the wall, while heating are placed low, and likewise the best location for the returns is the opposite.   Such complexities are beyond the scope of this document.  One thing that seems sure: super-insulation reduces the air flow demand, and hence allows more time for air mixing, and presumably less stratification problems.  Its all conjecture on my part.

3: it is not clear if this is actually a PassivHaus requirement, or just a suggestion.