Hello:

My wife likes a 'hot' house - 73 degrees F.

In my area, the HDD using 73 F is 11,520. Using 33 F it is 1,936. (I chose 33 F as I have heard that air-to-air heat pumps are not efficient somewhere around that temperature)

If I understand things correctly, this would mean that, except for the days 33 F and below, using a heat pump would cost less to heat my house than baseboard heaters, right?

If all other considerations are equal, is this how you make the decision to look at a heat pump? (Then you do the pay-back calculations.)

Or, assuming that heat-pumps are less efficient as the temperature drops, do you have to plot that fact against more detailed HDD info such as HDD at each temperature from 73 F to 33 F?

Thanks,

Rob.

(I chose 33 F as I have heard that air-to-air heat pumps are not efficient somewhere around that temperature)

The heat pump I grew up with went to resistance heat at about that temperature. The heat pumps available now put out efficient heat down to less than 0F.

That's a mighty big heating season you've got there. As long as your deep lows don't go out of the heat pumps capabilities, you should consider it along with any other options that make sense for your area. Other possibilities are a hybrid system. I am just starting to heat with a Daikin Altherma air to water heat pump for my radiant system. It will also heat coils for air to air and it has been used for some time in Northern Europe in what they call a "bivalent" mode in which the heat pump utilizes an inexpensive gas or some other type of boiler when the temperature drops too low for the heat pump functions.

the Mitsubishi Hyper Heat shut off between -13 & -15o. HO many days do you have where it's below -15o?

Thats -26c and Montreal gets that somewhat regularly but far less than it use to, at least when I used to live there. -30-35 was normal in the middle of the night. Robert, for this you will need some electric baseboards or some other back depending on how you deliver that heat to your house, up but it won't run that many day a year now.

the other option is to install a few "bathroom heaters" - small 12"x16" resistance heaters - in the walls. Keep in mind that the house will "coast" - hold heat, so the temp drops aren't what you may be used to.

the other option is to install a few "bathroom heaters" - small 12"x16" resistance heaters - in the walls. Keep in mind that the house will "coast" - hold heat, so the temp drops aren't what you may be used to.

Hi Bob:

According to www.degreedays.net, in the last 36 months we have had 43 HDD below -15 F; 20 HDD in the last 12 months & 18 average during the past 5 years:



Rob.

The base temperature for heating/cooling purposes has nothing to do with the temperature at which your heat pump operates. The base temperature is intended to be the outdoor temperature at which you usually have a cooling load when it's warmer than that, and a heating load when it's cooler, and it's usually a few degrees cooler than the interior temperature you like to maintain in winter.

In non-superinsulated houses maintained at 20C interior, a base temperature of 18C is appropriate. But the better you insulate the house, the lower that balance point temperature is, and in houses built to 2x code minimum R values with better-grade windows, 15C is not an unreasonable base number.  In your "hot" 23C but better-than code house, using 18C or 65F is about the right number.

Using 1C or 33F as a base number makes sense if you are freeze-protecting a greenhouse or warehouse, but it has no relevance to home heating & cooling (outside of the rare Inuit traditionalist dwellings?)

The 99% outside design temperature is a far more relevant number when looking at air source heat pumps, and for estimating how much boiler/furnace/heat-pump you need to heat the place.

http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf

Despite very cold record low temps, the 99% design temp at at High Falls is probably not much cooler than about 1C below that of Trois Rivieres which has a 99% design temp of -25C/-13F. St. Jovite is about 2C cooler than Trois-Rivieres. That is pretty much at the limits of where a Mitsubishi H2i ductless has a specified output, but it is at least possible to size it correctly for that temp.

http://weatherspark.com/#!dashboard;a=Canada/QC/High_Falls

http://weatherspark.com/#!dashboard;a=Canada/QC/Trois-rivieres

http://weatherspark.com/#!dashboard;a=Canada/QC/St.-jovite

You will definitely need some backup heat for the coldest days, but combined with the heat pump it will cost about half as much to run as with electric baseboards alone.

I visited with a Factory rep from Mitsubishi a couple of weeks ago. I was told that the Hyper heat doesn't actually shut down until -18F.

It starts back up at -13. Even at -13, I was told the COP was about 2.0

At -13F there's no way it's hitting a COP of fully 2.0 at full output, even if it might at some lower speed.

Independent testing on a randomly selected Mitsubishi Hyper Heating 1-ton (FE12NA) isn't quite pulling a COP of 2 even at +4F, let alone at -13F. But it's still running a COP better than 1.5 running full-blast at -10F, which isn't terrible. See figure 9 page 14 (page 22 in .pdf pagination) of this document:

http://www.nrel.gov/docs/fy11osti/52175.pdf

I guess it matters what "...about 2.0..." really means, but "anything over 1.5" is probably good 'nuff for the kind o' girlz he goes with, eh? :-) (From Ecotopes testing it looks more like about 1.8 to me, but what's 10% between friends?)

If not quite two it's two-ish, and I'd expect the seasonal efficiency of the unit itself to be in the low-mid 2s in this location. With some amount of resistance heating backing it up at the temperature extremes the combined efficiency would run about 2.

If it doesn't actually shut down until -28C it may not stop at all some years, which would be great, even if it's not quite keeping up with the full load at that temp.