Air-source heat pump for heating, cooling, and domestic hot water
We are working on a net zero project here in Dauphin, MB (Western Manitoba) and are interested in reducing our annual electrical usage (thereby minimizing the size of our required PV system to achieve net zero ‘status’). I have been encouraged by our energy modeller to consider a cold climate air source heat pump for space heating and cooling. I am seeking to know if a practical solution exists to use air source heat pump technology for space heating, cooling AND domestic hot water with a single outdoor unit.
The annual usage is estimated as follows:
Space Heating 2100 kWh
Space Cooling 1700 kWh
Domestic Hot Water 3000 kWh
Design Loads:
Space Heating 3.5 kW
Space Cooling 4 kW
All advice is appreciated.
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Replies
Eric,
Yes, it's possible to install an air-to-water heat pump (or heat pumps) that are sized to provide your home's space heating and domestic hot water needs. For more information on equipment available in North America, see Air-to-Water Heat Pumps.
In your case, there are several important reasons to proceed with caution:
1. Residential air-to-water heat pumps are rare, so you are unlikely to have contractors in your area who are familiar with the equipment and able to provide service in an emergency.
2. The climate in western Manitoba is challenging for any type of air-source heat pump, whether you are talking about an air-to-air heat pump or an air-to-water heat pump. Once the temperature drops to -25°C (-13°F), many types of air-source heat pumps are going to struggle. Some brands of equipment will struggle well before it gets that cold. So you'll end up needing electric-resistance backup equipment for use during cold weather.
The sticking point is the requirement that it all has to be packaged as a pre-designed single unit. For N. America that pretty much narrows it down to the Daikin Altherma air-to-water which isn't necessarily ideal for locations with 99% outside design temperatures of -30C or lower. The extended temperature capacity charts for that series only go down to -20C, but it'll keep on chugging at any arbitrarily low temperature with some real (but unspecified) heat output.
What is your outside design temperature in Dauphin? [edit: found it- it's -31C, at the airport's weather station]
Custom designs using other air-water heat pumps can work, but with any custom design comes higher expense and higher risk.
Air-air cold climate heat pumps have more available options, but they don't make domestic hot water directly. A separate heat pump water heater could take heat from indoors and put it into the domestic hot water though, lowering the cooling load, but raising the heat load.
You can review here for use of a Chlltrix for space heating/cooling and domestic hot water:
http://www.chiltrix.com/documents/Chiller-with-solar.pdf
It sounds like you've done a great job of designing the envelope; with those design loads, it's really hard to justify the expense of an air-to-water system, which, in my experience, will run about 2-3x the cost of a single-head mini split heat pump.
Here's what I would install in your situation:
1. A mini split with one outdoor unit and one indoor unit. Whether you want a ducted or a ductless indoor unit is a question to consider; the answer will depend on the layout of the house and your tolerance for a few degrees of temperature variation between rooms. I'd size the mini split so that its temperature-corrected output at its lowest operating temperature (somewhere around -25C) equals the home's heat load at that temperature.
2. Because your outdoor design temperature is below the cutoff temp for the heat pumps I'm familiar with (Mitsubishi and Fujitsu), you will probably also need 3.5kW of resistance heat. This will be very rarely used but will be there for extreme conditions and will also provide a backup if the heat pump requires service.
3. Heat your water with an inexpensive electric resistance tank (my assumption here is that, because the design heat load is small, the house is also small enough that the noise of a heat pump water heater would be problematic).
4. Use a portion of the $5k-$10k in savings versus the air-to-water system to buy additional solar panels.
This is a simple, cost-effective, robust approach that will avoid the custom design and service issues Dana alluded to.
There is a Panasonic Aquare all-in-one air to water heat pump solution rated up to -28C with a claimed 100% capacitiy up to -20C.
http://www.aircon.panasonic.eu/GB_en/product/aquarea-all-in-one-h-generation-t-cap-bi-bloc-single-phase-three-phase-heating-and-cooling/
Though, I was unable to find a USA distributor on the Panasonic website.
Jon: " Because your outdoor design temperature is below the cutoff temp for the heat pumps I'm familiar with (Mitsubishi and Fujitsu), you will probably also need 3.5kW of resistance heat."
Most mini-splits don't have a "cutoff temp", even though they do have a lowest outdoor temperature at which they specify and guarantee the output capacity. Mitsubishi ductless units specify that it won't cut off until it's at least-18F/-28C outside, even though they only specify an output capacity down to -13F/-25C. In practice many continue to operate at -30C, but at some temperature the Mitsibishis are designed to turn off, and will re-start automatically when it warms up to -25C.
Fujitsu & Gree cold-climate mini-splits units do not turn off, and will keep running at arbitrarily low outdoor temperatures. Even though the manufacturer does not specify the output at the lower temperatures, they DO continue to deliver heat, and installing 100% design load backup of 3.5 kw of resistance heat is not warranted.
A Quebecer posting here under the handle "Jin Kazama" a few years ago heats his house with four xxRLS2H series Fujitsu ductless mini-splits with no backup. Even though the exit air at the mini-splits was noticeably tepid, they continued to heat his house, at temperatures in the -30C range. The newer RLS3H series Fujitsus have a specified output at -26C, and with just about any oversize factor at all for the calculated heat load at -26C, they would cover the heat load at -31C.
Thanks everyone for these meaningful responses.
I do get nervous about the lack of qualified service people for an air to water unit and also do like the idea of some redundancy. I will follow up to post what we decide on and will try to follow up with some feedback on how well it works.
Thanks again,
Eric
btw - is there a way to get notified whenever a new answer is posted on a specific GBA Q&A thread?
Eric,
Q. "Is there a way to get notified whenever a new answer is posted on a specific GBA Q&A thread?"
A. Unfortunately, no. This feature is on our "wish list" for web site improvements.
Here's an air-source mini-split heat pump with built-in water-heating capability.
http://www.hotspotenergy.com/air-conditioner-water-heaters/
But it's not very helpful here because it has a 14 F (-10 C) outdoor temperature. So that's not very useful for your location.
Air / water HP systems would in almost every case require a buffer tank . This type of building may also require lower supply water temps than most are used to seeing or even designing for . Any idea of what temps you would require at design ? This information would help quite a bit . So far all that has been stated is opinion , chances are that the building could be heated with sub 90*f water even at design . Some of the A/W HPs mentioned may be able to perform that task even at your -31 temp .
This unit will make 125 * water at 0* f .
http://spacepak.com/products/solstice-heat-pumps.asp