Net Zero Air Exchange Webinar

BrightBuilt Home NetZero Heat Pumps Webinar cover photo.

We're just getting warmed up...for the next webinar on our list we'll be joined by Dana Fischer from Mitstubishi, to discuss the wild, magical world of heat pumps.  It's the March event that is sure to be the cooooolest thing aside from a New England spring.  Bring all your hot button questions, like what is a C.O.P? And, can I really just set it and forget it?

Answers to your burning questions!
There were many questions after the webinar and Dana was kind enough to answer all of them, see below.

Recent heat pump improvements for colder climates
Highly efficient inverter driven compressors and fan motors allow for precise control so speed and output of the system and capability to run at very high rates to provide heat even when it is very cold outside. Refrigerants used by new mini-split systems (410a) has a boiling point of -50F allowing for extraction of heat from outdoor air all winter long.

I am interested in solely using a heat pump to heat and cool my home. Is this realistic?
Displacing all of your heating load can be accomplished with only heat pumps, but requires proper sizing and selection of equipment. It is common in homes that are going 100% heat pumps to have more than one outdoor unit to provide redundancy and match seasonal loads throughout the structure.

Would cover two topic of zones and regular maintenance?
Units should be checked after a year of operation to make sure everything is working well and to conduct an initial cleaning of the indoor unit. The level of buildup in the indoor unit will determine whether annual cleaning is necessary or if a 2 or 3 year cycle is satisfactory.Heat pumps come in a variety of heating capabilities, but even if you have a single unit that may be large enough to heat the entire home, you may need multiple indoor units to provide sufficient distribution to have roughly level temperatures throughout.

The cost of operating heat pumps compared to heating with wood in northern New England?
Efficiency Maine has an excellent Cost of Heating Comparison tool where you can adjust the price of fuels including wood and compare seasonal costs of one source relative to another with a variety of common heating systems.
Can an air sourced heat pump provide enough heat in 20 degree weather?
Absolutely, even in -10F weather and colder. The key is sizing. If a unit is selected to meet the need for the home and it is a model that is designed to provide heat at very cold temperatures, it will do just fine.

Substitute two smaller compressor units for one large unit? Can a dual system work (aka heat pump in great room and electric heat in bedrooms) when budget is an issue? We are building a small timber frame house (1,200 sqft) with a partial 2nd floor (master bedroom) and another bedroom on the first floor. Most companies have suggested THREE heat pumps for us which was more than we could fit in the budget for our mortgage. We plan to primarily heat with wood.
There is no harm in walking into heat pumps instead of running. 3 small heat pumps may be ideal to provide full and level heat throughout, but identify what is the most important zone for your needs and select one or two. Down the road if you decide you want the third, you will have clear understanding of how it will affect the dynamic in your home. If can install one in the main living space with the knowledge that there will be some temperature variation across the home, just as would be the case with a wood stove. Cooling does not go up, so you may decide to install the unit in the bedroom as a priority to augment heat, but to provide cooling in your sleeping area, and to some degree down and into the rest of the house.

At what temperature do mini splits become inefficient?
At temperatures above 40F they can provide nearly 4 units of output for 1 unit of energy input making them extremely low cost to operate. At temperatures approaching 0F and colder, they provide closer to 2 units of output for every unit of heat energy input. Even at 0F, heat from heat pumps is cheaper to operate than propane.

Is there a temperature where they just stop working?
There really should be no concern for cut out temperature for true cold climate units. They operate below the most extreme temperatures we have seen in most all of New England anytime in recent memory. The only area this may be of concern would be along the border with Canada in NH and Maine. Mitsubishi Hyper heat units are rated to provide guaranteed heating levels at -13F and continue operating to at least -18F ambient outdoor temperature. Even if conditions require the unit to shut down, it will resume operation as soon as it warms even to -13F.

Is it better to have one central heat pump or use multiple pumps and set up a zoning system?
It depends on the preferences and budget of the homeowner and the layout of the structure as to what the best option may be. I’d recommend having a consultation with a local Diamond
What is cost of electricity per sq ft of home with heat pumps?
It less a question of cost per square foot and how much heat needs to be displaced and the size of the equipment in use and how it is used. Ballpark 1,200 kwh of electricity on a seasonal basis in Northern New England into an appropriately sized heat pump will displace at least the equivalent of 100 gallons of oil.

Do the mini splits have auxiliary heating like older whole hose heat pumps?  What temp does the aux kick in?
No, mini-splits and ductless units do not come with auxiliary electric heat. There are models that have a “base-pan” heater which prevents the drain pan of the outdoor unit from freezing up, but that feature does not impact indoor heat, and is not really that important as long as the outdoor unit is mounted high enough to keep it out of snow drifts and away from where ice will form under the unit. There are full air handler indoor units (furnace style) that can be adapted with an optional electric heat kit to provide backup if desired. Typical control structure only turns the electric heat on if the heat pump is unable to keep, which could be never if the system is sized appropriately.

How many units are needed for 950 sq feet ranch?
Depends on the heating load and the expectations of the homeowner for distribution, but single zone residential systems can easily manage a typical open concept space in a ranch, then perhaps consider either a second single zone in the master bedroom, or a small ducted unit to heat/cool all of the bedrooms. It is easly plausible to duct the entire home and use a single outdoor unit to manage it all.

Energy use chart for low temperature placement and how many per square foot? bathroom heating?
I do not recommend using heat pumps in bathrooms unless it is part of a larger ducted system. Even our smallest indoor units would be overkill for a bathroom. If heat is desired in a bathroom for luxury, consider electric tile or towel racks.

I'm extremely frugal with heating in my current home. Thermostat typically at 58 night, 64 day with all upstairs heating ducts to bedrooms closed 24/7 all year. So my bedroom could get to mid 50s some winter nights, which I'm fine with. My design for a new 950 sf 2 bedroom net zero home has an open space design for kitchen/dining/living room. I'd prefer to get by with just one heat pump head as I'm on a tight budget, probably located in the living room pointing down towards the  length of the hallway to the bedroom.  What's your opinion on whether this 1 head would be sufficient to meet my much lower than average heat needs?
Yes. Given your expectations for some temperature variation and placement where airflow will be directed across the home, it will work fine. It is not uncommon for net zero homes of this smaller size to have a very low load, and so an 18k hyper heat model would likely be ample. If you or someone in a similar home used multiple units, a smaller unit in the main space like a 12k would be good with 6k model(s) in bedrooms or a 12k ducted model for multiple small rooms.

Heat pump for renovation of passive solar?
Passive solar homes are generally well insulated but have lots of south facing glass. Sometimes, if they were not built with ample roof overhang above south facing glass, they can get quite hot during summer months. Heat pumps can work well in these homes due to the relatively tight and well insulated construction, but careful sizing for cooling may be needed depending on how the home generally fares during the summer months.

How far out are you on production if I order this spring?
Global inventory and supply chains have been disrupted this past year due to a number of pandemic related factors. Mitsubishi has more products flowing into the US in coming months than ever to meet demand.

Not sure this is the appropriate forum for this question, but I am concerned about power outages combined with cloudy skies and an all-electric home and car. I know we will have an understanding of the energy we need when the energy model on our home is complete, but I'm wondering what recommendations you have regarding battery backup and related cost?
As with any sizing of battery systems, the anticipated load and the desired duration to be able to maintain the load dictate the sizing of the system. In most residential circumstances, outside of having some number of hours of coverage of regular loads covered by battery backup, having a generator of some nature would provide assurance of heat pump operation regardless of what the sun might be providing to solar panels. While batteries continue to drop in price, they have a limited life and cost that can be as much per delivered kwh as the solar array itself.

The decision to have battery back up and the size of the bank relates to the frequency and duration that you anticipate power outages.
See above, although if you anticipate having frequent and long power outages, a generator may be necessary to maintain temperature with a heat pump or any heating source that requires electricity to operate.

What are the annual maintenance requirements? I would like to hear more about their Hyper Heaters recommendation for maintenance cleaning?
Units should be checked after a year of operation to make sure everything is working well and to conduct an initial cleaning of the indoor unit. The level of buildup in the indoor unit will determine whether annual cleaning is necessary or if a 2 or 3 year cycle is satisfactory.When shoveling the front steps, take a minute to shovel out snow from under and around the unit. When mowing the lawn or raking leaves, take a minute to check and clear off grass clippings, spiderwebs, leaves and any other debris from the back coil part of the outdoor unit.

Is it more efficient to have the inverter fan on high rather than turning up the temperature?
Some spaces receive better distribution with the fan set on auto, and in some circumstances it may be better to leave the fan on medium high or high to push heat to a wider space where fewer indoor units are being employed. It is recommended to play around with temperature and fan settings to find out what provides the most comfort, then not modify the temperature significantly. Heat pumps are most efficient when they are maintaining the temperature of a space, as opposed to ramping up to highest speeds to reach a new temperature setpoint.