HVAC for a ‘Pretty Good House’
Planning a well-insulated but not over-the-top retirement home in Oregon, a homeowner looks for the best heating and cooling plan
Matt Mesa is looking ahead to retirement in a new, one-level house in Hood River, Oregon. It's going to be a Pretty Good House, a phrase coined to describe a well-insulated house of an appropriate size.
The “pretty good house” approach usually calls for above-code levels of insulation, but stops short of the energy performance or airtightness requirements of the PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard. Mesa wil be using the 10-20-40-60 rule of thumb for insulation, a formula describing, in order, the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of insulation beneath the slab, on foundation walls, on above-grade walls, and in the roof.
The floor plan of this 1,500-square-foot house will have a great room of roughly 700 square feet, along with kitchen, dining, and living areas, a master bedroom, and two smaller bedrooms.
Mesa has ruled out radiant-floor heat, at least for the main part of the house. But he's thinking about a gas fireplace insert heater in the great room plus a single ductless minisplit head in a central location. His contractor suggests installing an air duct from a spot near the fireplace to the bedrooms.
So, he asks in a Q&A post at Green Building Advisor, what's the best HVAC option for a house like this?
That's the topic for this Q&A Spotlight.
It doesn't matter what you use
GBAGreenBuildingAdvisor.com Senior Editor Martin Holladay thinks Mesa is focusing on the right thing: the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. "The key is to create a very well insulated envelope with low levels of air leakage," Holladay says, "and it sounds like you are doing that."
And just about any kind of heating system will work, he adds, providing its designed and installed correctly. That might include a natural gas furnace, ductless or ducted minisplits, or a conventional air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps..
It sounds like a minisplit would do the trick
From the sounds of it, writes Dana Dorsett, a single minisplit with a three-quarter ton output (9,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h) would be able to handle the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load.. "Your heating load is truly tiny," Dorsett says. With R-60 worth of insulation in the roof, heat losses there amount to only 1,275 BTU per hour, while losses through windows and doors are an estimated 3,500 BTU/hour. Walls would add another 2,800 BTU/h, for a total of about 7,575 BTU/h.
"Even if you posit a hefty 3,000 BTH/h for air leaks, ventilation, and slab losses, you're only at about 11,000 BTU/h, and that's before subtracting off plug loads and the mammalian body heat inputs, let alone adjustments for thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. effects," he says.
In fact, Dorsett says, Mesa might start thinking about the cooling loads. "Your cooling loads are likely to exceed your heating loads by more than 2:1 unless you manage your gains (particularly late-day gains from west-facing windows) very carefully," he says.
As to ducting warm air to the bedrooms, Dorsett advises against it. Instead, he suggests electric cove heaters paired with an occupancy sensor control. These units are are mounted on the wall near the ceiling (one brand is made by Radiant Systems Inc.).
"Unlike electric baseboards, cove heaters heat the objects in the room directly rather than heating the air, which makes it comfortable even during the recovery ramps," Dorsett says. "And unlike electric panel radiators, cove heaters have no thermal mass to speak of, so the radiant output comes on in a few tens of seconds rather than minutes."
Gas is probably not a good idea
No matter what the main source of heat, more than one reader cautions Mesa not to introduce a gas-burning appliance in the house. Forget the ambiance a fireplace would bring — think about the downsides.
Kevin Dickson, for example, suggests that avoiding a natural gas connection has a couple of advantages. Mesa can skip the initial expense of running the plumbing and the tap fee, plus he'll never have the service fee, which might be as much as $20 a month. "If you install PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow., that savings cannot be used to offset your gas bill," Dickson continues. "You shouldn't cook indoors with gas. A gas fireplace requires another large hole in your thermal envelope. You won't be able to use the gas fireplace for long before that room gets too hot."
He's seconded by Bob Irving, who writes that a gas fireplace will be more expensive to use than a minisplit, and it will "provide waaay too much heat."
AJ Builder suggests that if Mesa decides to install a gas fireplace, a sealed combustion unit would be a way to keep the thermal envelope intact. "A way to have the gas fireplace, without the monthly costs, which in some areas are ridiculous, is to go with propane," AJ Builder writes. "And get a sealed unit to deal with what some say may be a hole in your home. I like fires myself... if not inside, you could do a nice setup outside your home to enjoy. Life should be enjoyed."
Our expert's opinion
Here's how GBA technical director Peter Yost sees it:
The collective advice given makes a lot of sense to me.
(1) A high-performance enclosure allows a wider, simpler, and more economical range of conditioning options.
(2) In my experience, by the time fireplace-loving homeowners are “forced” to select a sealed gas fireplace for efficiency and air-quality reasons, they are so far away from the original “ambiance” they sought that the option has lost its appeal.
(3) Minisplits are really attractive in climates that need both active space heating and cooling. And this technology is proving to be both efficient and robust.
(4) I am partial to quick-response electric-resistance “task” radiant heating, but the efficiency of this type of heating (lower air temperature and elevated mean radiant temperatureMean radiant temperature (MRT) is roughly the average temperature of all the objects or surfaces that a person "sees" inside a building, with the surface temperatures being weighted by their area. A surface or object's contribution to MRT is also based on its temperature in comparison to the person (temperature difference or differential) and the viewing angle between the person and the surface.) is pretty dependent on occupancy patterns. It’s not hard to stay in a room long enough that the initial dominance of radiant heat transfer fades as surfaces warm up and conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow. and particularly convective heat transfer sneak up. For more information on electric-resistance radiant heat panels, see the ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. paper I co-authored with Ed Barbour and Richard Watson a number of years ago: “An Evaluation of Thermal Comfort and Energy Consumption for a Surface-Mounted Ceiling Radiant Panel Heating System.”
- Erik Becker