Air to Air Heat Pumps in Zone 7

Zman

Recently a 3rd party conservation group has started lobbying to change our local energy code to either require or heavily incentivise air to air heat pumps. We are at high altitude with heavy snow fall, deep in Zone 7. They are claiming that electricity is easier to offset with renewables even though the grid is currently supplied by ~22% renewable.
My concern with the plan is the real world performance of heat pumps in this climate. Does anyone have actual COP numbers for any of the new heat pumps running at low outdoor temps? The manufactures seem to be very eager to show that the product will run at low temps but provide zero documentation for efficiency at those temps.
Attached are our average temps.
Thoughts?

Jamie Hall

At those temps, even the best low temperature heat pumps will struggle to get a high enough COP to match a mod/con. Many will have to resort to backup resistance at night, although there are a few very pricey units suing carbon dioxide refrigerant which still perform pretty well.

I could add a rant here... but I'll be good and shut up.

jumper

Air to water makes more sense because then you can have thermal storage. Run heat pump during warmer daytime. Presumably more solar is then available as well?

Of course if enviro-whacko s were really serious,they'd advocate atomic energy for ordinary electric heating.

hot_rod

One issue is A2A blow cool air in extreme conditions, not comfortable to be around even if they maintain room temperature.

ccA2WHP make more sense. And you have the ability to mix all sorts of hydronic emitters, cool in the summer with some AHs.

Zman

Jamie Hall said:

At those temps, even the best low temperature heat pumps will struggle to get a high enough COP to match a mod/con. Many will have to resort to backup resistance at night, although there are a few very pricey units suing carbon dioxide refrigerant which still perform pretty well.

I could add a rant here... but I'll be good and shut up.

Oh please rant Jamie,
I need ammo for my rant at the code input meeting.
The way I see this, with only ~30% efficiency from the power plant to the appliance, the break even point for heat pumps is COP of 3. That makes it 90% efficient just like the condensing boiler.
What is driving me nuts is the complete lack of data on this. The manufactures will state their ability to run at lower temps and their derations at lower temps but not their COP's.
Does anyone have the data?

kenjohnson

This link has data that could let people make an informed choice
https://ashp.neep.org/#!/

There are plenty of mini-split ASHPs that have a COP of 2 (or better) at 5 degrees F. These don't use electric resistance backup heat.

There are plenty of conventional heat pumps that do far worse and require backup electric resistance heating at far higher temperatures. It would be good to not confuse the two different types.

Zman

@kenjohnson
That is exactly what I was looking for.
Thank you

jumper

hot_rod said:

One issue is A2A blow cool air in extreme conditions, not comfortable to be around even if they maintain room temperature.

Yes comfort is not emphasized enough. High end all electric homes often have electric furnaces/AC rather than HPs.

kenjohnson

In regards to "A2A blow cool air in extreme conditions", I would again emphasize that one must differentiate between conventional ducted heat pumps by the likes of Carrier, Lennox, York, etc. and mini-split heat pumps. My Fujitsu RLS2H mini-split blew hot air (over 100 degrees F) when it was -5 degrees F outside. It is simply not true to say that they blow cold air or have bad COPs in cold temperatures

I also have a Trane conventional heat pump in a condo - wouldn't get another one of those - they do need electric backup and electric resistance heat augments the heat pump at 32 deg F or so and lower. But that is a different technology from a mini-split.

Having said all of the above, I wouldn't trade my new underfloor radiant hydronic with GSHP HW for a mini-split, but will augment the underfloor hydronic with a mini-split to give summertime cooling and wintertime backup.

RonnieJ

@Zman,

For your reference - I've attached the specs for the Spacepak Solstice Extreme. See the asterisks at the bottom in relation to heating (95F) and cooling (0F) extremes. I pulled this from the latest brochure. These are significantly better numbers for low-ambient than the older version I have - which dropped to a COP of 1.91 at 20 degrees ambient, and the supply water temp dropped to 105 from a max of 115. It's pretty interesting to see these improvements. I use mine for cooling and emergency heating - I prefer the baseboard heat for comfort.

mikeg2015

Do you have actual climate data? Advantage of high altitude is low humidity. But also lower air density so I assume capacity drop a little as it will impact both indoor and outdoor coils.

I didn’t think heating degree days in high altitude was that much different that somewhere like Madison, WI or Minneapolis. Heat pumps work fine there. Not as effective during part of the season, but there are still wide shoulder season same as everywhere.

Their question is how many hours above abut 20F, and how many below.

Next challenge becomes sizing. Since humidity isn’t an issue in cooling season, you can probably get away with oversizing for heating capacity or better yet inverter units.

Zman

@mikeg2015
The altitude deration is around 40%. The average temps are in the original post.
What I am finding is that there is really zero actual test data to support the efficiency claims at low temps...

hot_rod

Zman said:

@mikeg2015
The altitude deration is around 40%. The average temps are in the original post.
What I am finding is that there is really zero actual test data to support the efficiency claims at low temps...

Drop Siggy a line, he has two winters in a rather cold upstate NY climate on his Soltice. I know he is watching and recording. Probably a few articles out there on his experience, not sure if he has anything in writing on the current weather. Google Siegenthaler/ heat pumps.

mikeg2015

The challenge with actual efficiency is it depends largely on relative humidity. Defrost is a massive energy suck because it 1) causes a loss in net output so the system has less capacity and run longer 2) the power needed to melt the ice itself.

As a general rule, we disable the units at around 20F. Below that air gets cooler so it’s a comfort issue and with defrost factored in net COP is getting close to 1.0. But we have no hard data for that temperature. But our customers are pretty happy with that.

On dual fuel with my steam boiler, I use 40F to minimize defrost so it just runs in the shoulder seasons.

mikeg2015

For reference I ran mine in -15 to +15F on a really col day when my boiler was out of service. Indoor air temps were just 55F. It spent I think almost 50% of the time in defrost. Not easy to heat up coils to melt ice when it’s -15F and 10-20mph winds.

hot_rod

Here is one of the articles I was referring to. It shows some performance of mini splits in heat mode.

I think the Spacepak Solstice uses heat from the buffer to defrost instead of resistance strips. I have a video somewhere of a defrost cycle in real time on this installation.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&ved=2ahUKEwiX6MPaiLjiAhVFYKwKHa0SCZoQFjAJegQIABAC&url=http://www.homeperformance.org/file/1441/download?token=mG712xbm&usg=AOvVaw057FU44vHlVJnLB_tpxqtN