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Space Conditioning Systems

see Reducing Emissions on the Cheap,  Incentive Programs, and Heating/Cooling Ducts, further down this page

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In California, for more than the past half-century, the most popular house heating system has been a natural gas burning central furnace.  In areas that experience hot summer / fall weather, an air conditioning (AC) system is often paired with the heating unit.  In this case, a condensing unit is placed outdoors.  Refrigerant piping ties the outdoor AC unit to a cooling coil which is placed inside the furnace.  In this way, the furnace fan and ducts do double-duty as the AC air circulation system.

 

Other space heating systems include gas wall heaters, gas boilers providing heat to floors or radiators, and electric wall heaters (usually only found in bathrooms). 

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Phasing out the use of fossil fuels (i.e. natural gas heating) is essential in order to make some progress in slowing down global heating.  One alternative, solar space heating, consists of heating a liquid in solar panels, then circulating that warmed liquid around a home.  But such a system is expensive, in part because it will need a "conventional" back-up heating system for when there are excessive cloudy days and cold nights.  Another space heater type, electric resistance heating (such as used in wall heaters and plug-in heaters) technically could heat an entire home.  But straight electric heat is so inefficient that it is banned by California's building energy code (except in rare, limited uses).

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The good news is that there is a technology that can efficiently heat homes without relying on fossil fuels, and do so economically!  The technology is called a heat pump.  Heat pumps have been around for decades.  But until the past decade or so, they were no more efficient than gas furnaces.  And until the climate crisis started becoming clear to designers, builders, and building owners, there was no big incentive to change from the common gas furnace to using heat pumps.

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SPACE CONDITIONING HEAT PUMPS

(see the Water Heating Systems page for Heat Pump Water Heaters)

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Heat pumps use only about 1/4 to 1/3 as much electricity as electric resistance heating.  This is because heat pumps use a refrigeration cycle to squeeze heat out of the outdoor air.  In both refrigerators and conventional air conditioners, the heat is rejected, leaving colder air to cool a refrigerator or a building.  With a heat pump, when heat is needed, the refrigeration cycle is reversed:  Hot air is 'squeezed' out of the air, collected, and is used to heat homes (and other types of buildings).  The cool air is rejected.  All heat pumps also provide cooling for hot days by reversing the refrigeration cycle.  When heating is needed, what happens if the outside air is very cold?  Heat pump technology is getting better at squeezing heat out of colder air.  But some heat pumps, in some cold air conditions, will need to revert to electric resistance heating.  This is allowed under the energy code, as such conditions are expected to be the exception, not typical, in the operation of a heat pump.  Consult with manufacturers and heat pump installers as to whether your microclimate conditions will necessitate an electric resistance coil for the heat pump model you are considering.  You may find that some models will need the coil, and other models that are engineered to work at lower temperatures will not need this feature.

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TYPES OF HEAT PUMPS​

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  • Through-the-Wall Heat Pump:  This type of heat pump has been popular in vacation lodging for many decades.  Sometimes these units are used in apartments.  And they could be used to heat/cool a room in a home, in a situation where a separate system was desired for that space.  However, compared to other types of heat pumps, these are somewhat noisier, and less energy efficient.   Caution: Manufacturers of through-the-wall heat pumps often make cheaper versions that provide conventional cooling, but the heating is electric resistance.  This type of unit would not be allowed under CA's energy code, unless it is replacing a like unit.  And, as noted above, electric resistance heating uses much more energy compared to using a heat pump.​

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  • Package Heat Pumps:  These combine the condenser and compressor functions in one box, so they are typically installed outdoors, and typically placed on a flat roof (although they can also be located on the ground).  Package HP units are especially popular on commercial buildings.  They can be used for a residence, but would have to be situated in such a way as to connect to the home AC air ducting system.  Package heat pumps are often less energy-efficient than "split" heat pumps.

 

  • Split Heat Pumps:  These consist of an outdoor unit (condenser unit) and an indoor unit (compressor unit), connected by refrigerant piping.  As with conventional home AC systems, the outdoor unit is usually placed on the ground, or on a deck or balcony (although small units may be wall-mounted). The indoor unit contains a coil and fan, for heating the building's air.  This heat pump type is available in several configurations (see Types of Split Heat Pump in next section, below), and come in a range of efficiencies - with the highest efficiency models being significantly more efficient than the minimum efficiency models.  The energy code minimum heating efficiency varies by system size and power type, but for most applications, the minimum efficiency is 8.2 HSPF.   High efficiency heat pumps can have HSPFs of 10 or higher.  As for cooling efficiency, for most applications, the minimum allowed SEER efficiency is 14.0.  However, there are split heat pumps with SEERs of 21 and higher.  Note that units with higher SEERs also have higher HSPFs, and vice-versa.

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split heat pump outdoor unit and wall-mounted indoor unit

Incentives
Cheap

  Types of Split Heat Pumps:​

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  1. Wall-mounted indoor unit.  This is different than the through-the-wall design mentioned above, because of the separate outdoor unit. These are only used to condition (i.e. heat and cool) a single space, or small apartment. 

  2. Above-ceiling indoor unit with register grill in ceiling.  If conditioning multiple spaces, the outdoor unit could be connected to multiple indoor units (see system at item 4 below).  This design is more expensive than the wall-mount type, but doesn't take up wall space.

  3. Above-ceiling indoor unit connected to ducts.  This design is close to that of the conventional gas furnace/AC system, in that it is a central system with conditioned air routed to various rooms through ducts mounted above the ceiling (or occasionally below the floor), with air directed to ceiling, wall and/or floor registers.

  4. "Multi" split system.  In this design, one outdoor unit is connected to two or more indoor units.  The advantage of this design is that there doesn't have to be any ductwork.  Instead, each indoor unit could be the type with its own ceiling air register.  One benefit is that ducts tend to leak air, which may increase over time; leaky ducts can waste a lot of energy.  And leaky ducts are often not noticed for many years (until the ducts are inspected - if they ever are!). 

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  • Hydronic Heat Pumps:  Hydronic heat pumps generate hot and cold liquid (depending on season) that can be used for "hydronic" heating and cooling, typically using the home's floor system as a 'radiator'.  Some people like heated floors.  But they add significantly to construction costs, and may result in energy waste if the floor is not carefully insulated.  Alternatively, the piped fluid could be directed to either radiators or fan coil units.  Consult with a space conditioning system designer for details about these system choices.

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  • Geothermal Heat Pumps:  These systems have significantly increased energy efficiency as compared to air-based heat pumps, because they exchange heat with the earth instead of the air.  The earth maintains a fairly stable temperature as compared to the air, warmer in winter, cooler in summer.  By burying long lengths of refrigerant piping in the earth, greater energy efficiency is winter and summer is achieved.  However, the installation cost for this type of system is significantly greater than for the more typical 'air-to-air' heat pumps.

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Note about air conditioners:  If you have an old AC system, but are not inclined to change your heating/cooling system to a heat pump, consider upgrading your air conditioner to a high efficiency model.  Old models may have a SEER rating of 12 or even much lower.  The current minimum allowed efficiency for new models is 14 SEER.  The cost of higher efficiency models - SEER 16 or even higher - may not be a lot more. 

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INCENTIVE PROGRAMS  Here are some agencies that offer and/or list programs (e.g. rebates, tax credits) to make more affordable home upgrades to change from gas to electric heating equipment and to increase energy-efficiency:

  

  • BayREN (Bay Area Regional Energy Network, a coalition of the Bay Area’s nine county governments)  https://www.bayren.org/  

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  • The Switch is On, a project of the Building Decarbonization Coalition.  They list both rebates and tax credits.  See https://switchison.org/​

                       

REDUCING GREENHOUSE EMISSIONS on the CHEAP

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When we moved into our townhome around a decade ago, we replaced a very old gas furnace with a new one.  At that time, replacing a furnace with a heat pump was a substantial cost.  And our thought was, given that space heating is only seasonal, and that we thermally improved our home, we wouldn't use all that much gas.  Now we know that we need to do more to reduce emissions than simply turning down the thermostat to 66.  However, a new heat pump is still not within our budget. What we did do to significantly reduce gas use, starting in the winter of '22-'23:  Created a "hybrid" heating system.

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Our hybrid heating system consists of the following:

  1. Two simple plug-in electric resistance heaters.  Ours have 3 settings, from 500 to 1500 watts.

  2. Changed our winter gas furnace thermostat setting from 66 to 64 degrees.

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Operation:  When it is cold enough to require heat, sometimes the electric heaters are adequate.  But on winter mornings and winter evenings, the furnace is set to go on and heat to 64 degrees.  When the furnace starts on cold  mornings and evenings, we also turn on the electric heater located near where we will be, set at high heat.  With this setup, never once during the entire winter did the furnace cycle back on after turning off when the air temperature initially achieved the thermostat setting.  Not only do our electric heaters keep the furnace from cycling back on, after a while we typically turn the electric heaters down to the middle or low heat setting, and the furnace still doesn't turn back on. 

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Are we comfortable at 64 degrees?  Surprisingly, yes.  Actually, the electric heaters boost the air temperature to around 65 to 67, depending on conditions.  We make sure that the electric heater airflow is directed at us, which helps.  Does this raise our electricity use and costs?  Yes, but not very much (as noted above, improving our home insulation and windows helps lower heating energy needs).  What about electric resistance heaters being so inefficient - doesn't that extra energy use negatively affect the climate (they do use about 3 times the energy that heat pumps use per unit of delivered heat)?  No, because we use MCE 100% renewable electricity - at about the same cost as PG&E electricity (for more on this, see the Low-Cost Actions page).

 

HEATING / COOLING DUCTS​

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In new construction, and in remodels and additions where there will be more than 40 linear feet of new heating duct length, the CA energy code requires that ducts must pass a leak test performed by a third party.  This is a good thing, as heating your attic or crawl space is quite a waste of energy (and doesn't improve your comfort level!).  These duct leak tests typically only cost a few hundred dollars.  So even if you are not going to upgrade your space conditioning system, if you have a duct system that was constructed under a building permit issued before July 2014 (when duct leak tests became mandatory), having a duct leak test that follows the Title 24 energy code protocols is recommended.  All new ducts are required to be insulated, including ones that are part of a remodel.

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Ducts
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