9.      Mechanical Systems

Mechanical systems include the equipment needed to heat, cool and control humidity in a house, and also the plumbing system. The primary reason for mechanical systems is occupant comfort. The building envelope keeps out the worst of the weather, but humans are wimps and we like the indoor temperature and humidity to be within a certain range. For the most part, the building itself doesn’t care how warm or cold a house is. Most people prefer the indoor temperature to be 65°F to 75°F in winter, and 68°F to 80°F in summer, and for the relative humidity to be at least 20% and no more than 60%. High humidity levels exacerbate the risk of mold and fungal growth, so we try to limit indoor RH to 50% or less. People also experience radiant heat loss or gain, the opposite of the radiant heat you feel from the sun or a woodstove. Even when there is no draft, or air leakage, being near cold surfaces, particularly windows, will make a person feel colder than being in a room the same temperature but with surfaces the same temperature. This is one of the biggest arguments for triple-glazed windows; their energy savings may (or may not) be hard to justify, but they always provide greater comfort than windows with higher U-factors.

Performance-based. For the most part, the Pretty Good House concept seeks to simplify the complexity of high performance building by providing rules of thumb and guidelines. When it comes to heating and cooling systems, though, rules of thumb don’t work very well, so some form of energy modeling is necessary. Building codes usually require ASHRAE Manual J calculations for room-by-room heat loss, which can be accurate but often are fudged to make things easier for the supplier or contractor. Oversizing equipment is often not desirable, as heat pumps work most efficiently near their maximum capacity and their efficiency drops off dramatically if oversized. Dehumidification is an important part of air conditioning does not happen with oversized cooling equipment.

Energy Sources. Burning fossil fuels on site has a long history, but it comes with risks to health and safety, and a lot of embodied carbon as well as carbon emissions. With the drastic reduction in the cost of photovoltaic (PV) panels for on-site energy generation, the efficiency of electricity-generating plants, and the option to buy “green” power produced by wind turbines, dams and solar farms, most Pretty Good Houses aim for all-electric energy sources.

Choosing a Mechanical System. Pretty Good Houses in cold climates are usually a good fit for air source heat pumps (ASHPs) for heating and cooling. The most efficient type is the familiar units mounted high on a wall, but at some reduction in efficiency you can also choose from ceiling-mounted units, ones mounted low on the wall called floor units, or various ducted systems. In some situations ground source heat pumps (sometimes erroneously called geothermal systems) can make sense, but they usually require a lot of energy to pump water through long lengths of pipe or deep wells, and risk contaminating groundwater in the case of failure. Plus their initial expense is usually much higher than air-source heat pumps. ASHPs are available that will operate down to -20° or lower, so don’t believe contractors who say they don’t work in cold climates—find one familiar with this type of system. When there is a good (or Pretty Good) building envelope, it’s not necessary to have an ASHP head in every room—it’s more cost-effective to use small amounts of electric resistance heat in auxiliary rooms like bathrooms, mudrooms and sometimes even bedrooms. Woodstoves or fireplace inserts can provide supplemental heat, but they come with several concerns, including indoor air quality, keeping the building airtight and the particulate exhaust of the woodstove, which is a potent greenhouse gas.

Ventilation. In a Pretty Good House, mechanical ventilation is necessary to have good indoor air quality. The best approach is to use balanced ventilation with heat recovery. Heat Recovery Ventilators (HRVs) exhaust stale air from the house and they use a simple radiator-like system called a core to pre-heat or pre-cool the incoming fresh air. With an Energy Recovery Ventilator (ERV), moisture is also transferred. A couple of systems are available that can switch between the two types. Most systems are ducted from a central unit to the main spaces in the house, exhausting from areas that create moisture and odors, and supplying to living and sleeping areas. Others are point-source ventilators, meaning they ventilate single rooms. Which system is best depends on many factors, including climate, occupancy, and whether bathrooms are on the same system as the rest of the house. Some homes use exhaust-only ventilation, essentially leaving a bath fan running on low speed continuously. This usually saves money initially, but the energy saved using balanced ventilation eventually pays for itself, and because the incoming air is filtered instead of coming in through random gaps in the structure, the indoor air quality should be higher as well.

Plumbing. Use low-flow fixtures, especially in drought-prone areas, but with climate change areas that traditionally have plenty of water may see extended droughts, so learn to conserve this precious resource. When possible, design rooms that use water—kitchens, baths, laundry—so they are near each other. This saves a little on installation costs, but the greater impact is that it reduces the amount of water wasted before hot water reaches the tap. Insulating hot water lines saves energy, even when in conditioned space, and insulating cold water lines prevents condensation. Systems that recirculate hot water so it’s always available are usually not a good alternative because of energy consumption, though some are better than others. For creating hot water, the best systems for Pretty Good Houses are air-source heat pump water heaters. In creating hot water they cool and dry the air around them, so they need a lot of air flow, which doesn’t work in every house with the typical all-in-one units. Split systems use an indoor water tank and an outdoor heating unit, which cost more but have other benefits: they fit in confined spaces, their CO2 refrigerant is much more benign than typical refrigerants and they avoid the complaint some homeowners have that the units cool down the room they're in uncomfortably. On-demand or tankless electric water heaters use huge amounts of energy and are not a good choice in a PGH. Solar thermal systems, which use solar energy to heat domestic hot water, are usually less cost-effective than PV panels with a heat pump water heater, but may be worth considering in some situations. If you can’t use any of those systems, opt for an electric resistance water heat and wrap it in extra insulation.