My Pretty Good House
By IAN MCTEERHeating HVAC Systems Building Envelope Residential Buildings Residential Design
Air source heat pumps are still a good choice for residential applications.
I will be retiring in a few more years; I want to build a small retirement home on a property I own in northern Ontario just east of Algonquin Park. It gets very cold there in the winter so I fully expect to be constructing a small building (under 1000 sq. ft.) that is between the prescriptive requirements of the latest Ontario Building Code (OBC – SB12) and near net zero. Cost is a concern and the site is too far off the natural gas pipeline, so my choices are limited.
A residential house design concept called: the Pretty Good House (PGH), which was developed in the state of Maine, will achieve reasonable energy conservation goals all around (better than code). It is also affordable for the average person to build. PGH design incorporates the latest technology to ensure the building envelope, its mechanical systems and water management systems are integrated properly. PGH’s building envelop utilizes a suitable SB12 package of insulation and air sealing levels. The design focuses on reduced thermal bridging and a blower door test will confirm the air barrier is not full of holes.
My PGH will be a bungalow over a full basement using a forced air heating system in concert with mechanical ventilation. The design must not have any atmospheric combustion, so a sealed combustion gas furnace using LP fuel might be a good choice for me. But, I have decided to use an all-electric heat pump system for several reasons, one of which is affordability.
• The ideal heat pump will be a two-stage communicating system that can deliver at least 7500 Btuh at 0F (Region V). I have predicted a balance point (BP) of 17F based on a heat loss of 15K Btuh at design. I will use 5Kw of electric heat for back-up on below BP days. I contemplated a tankless water heater using a hydronic coil in the duct for backup heat (after all, I will require domestic hot water) but I rejected that approach because of the extra cost associated with the necessary components. I intend to use a 10 gallon point of use electric water heater for domestic hot water.
• Assuming my BP calculation is correct and that Mother Nature co-operates by keeping the average number of hours below balance point in my area to a minimum, then I foresee the backup electric space heating costs coming in at less than $140 per year based on $0.15 per KwH average.
• I believe it will cost about $476 to run the heat pump for six months. The cooling side of the heat pump will likely run on first stage for most of the summer.
Things may change a bit before my retirement. I will hold to my belief that a finely honed building envelope is the key to having a miserly, yet very comfortable HVAC system. Having settled my situation, in my mind at least, let’s take a look at the practicality of residential air source unitary heat pumps for the average homeowner.
Unitary Split System Heat Pumps
Several iterations of split system heat pump designs have been used in the province of Ontario and elsewhere in Canada over the years. Westinghouse had a Hi-Rely system featuring only one expansion device and Carrier, York, and General Electric (now Trane) also produced viable products. Today, these companies incorporate many hard-learned lessons into high efficiency designs.
Because the outdoor coil will be operating below 32F during the heating season, moisture in the outdoor air will freeze onto the outdoor coil; thus, blocking airflow and reducing heating efficiency. Heat pumps incorporate a reversing (or switchover) valve to direct hot gas from the compressor to the outdoor coil in the cooling season and then to the indoor coil in the heating season. All heat pumps must have a control system that will recognize the coil is frosting and be capable of taking corrective action to de-ice the coil.
The Secret Life of Air Source Heat Pumps
When operating in a cold climate, an air source heat pump has an Achilles heel: the defrost cycle. As the outdoor coil ices, the defrost control forces the reversing valve to direct hot gas into the outdoor coil thus causing the accumulated ice to melt away. During defrost the indoor back-up heat source is enabled to temper the air being delivered to the space. Defrosting the outdoor coil is inherently inefficient: the heat pump is actually recovering heat it spent hours putting into the house to now defrost the outdoor coil.
Heat pump manufacturers spent years tweaking the defrost control so that it triggers defrost cycles only when necessary. The control will examine several different parameters, such as the temperature difference between the ambient air temperature sensor and the outdoor coil temperature sensor. Typically, the defrost control will only allow defrost under certain conditions, for example:
1. The actual outdoor air temperature is below a specific value;
2. The compressor has been running for a specific amount of time; and
3. Increasing delta T is occurring between the ambient air temperature sensor and the outdoor coil sensor.
A microcomputer logic process memorizes and constantly compares most recent defrost performance to conditions from prior defrost cycles, allowing subsequent cycles to be optimized thus improving overall system performance.
Depending on the prevailing outdoor air temperature, the defrost board could limit a defrost cycle to under six minutes.
If the outdoor coil sensor does not detect the coil has warmed adequately, the control will record a fault condition. It may allow several more defrost cycles to occur under the normal parameters. If the coil cannot achieve the design termination temperature in subsequent attempts, the control will revert to a timed cycle and inform the customer by way of an error message or by turning on a red light in the thermostat. Thus, the efficacy of outdoor coil defrosting and the access to unrestricted outdoor air can create a dilemma for the contractor and homeowner when the time comes to determine a suitable outdoor unit location. I have seen the outdoor unit installed in the least appropriate place too many times.
Secrets of Heat Pump Installation
Because a heat pump is so much more than just a cooling unit, and because a heat pump requires optimal conditions to create an ideal defrost environment, it requires extra special attention to detail from everyone involved:
• The outdoor unit must have unrestricted access to outdoor air. Manufacturer’s instructions for site location, distance to walls/fences/obstructions, and minimum separation for multiple unit installations must be observed.
• The outdoor unit must be out of the prevailing wind, otherwise defrost cycles may be extended or fail completely.
• Trees or shrubs can be used to shield the unit, if none are planned a decorative shield should be constructed or the building design might incorporate a shielded spot for the outdoor unit (see Notes).
• Installers must follow all industry-accepted practices including deep vacuum evacuation. Residual moisture is undesirable in any refrigeration system but improperly dehydrated heat pump systems will experience outdoor TXV blockage in the wintertime leading to potential compressor failure.
• It is best to install a heat pump during the summer months when a proper evacuation can be done.
• The outdoor unit must not be located in any place where water can drip from above or snow accumulation on a roof can slide off and bury the unit.
• As a result of the defrost cycle, ice will build up underneath the outdoor unit. It must be high enough above the ground (12″ minimum) so that a stalagmite of ice cannot “grow” into the cabinet and crush the outdoor coil tubing.
• The homeowner must be given detailed instructions about how to use and maintain the system.
• A premium air filter MERV11 or better should be used with a heat pump air handler
Ian McTeer is a field service representative with extensive experience in residential applications.