How do I know when it's efficient and safe to heat with air-source heat pump?

lkhelp

Hello,

Moved into a house that has a Fujitsu Halcyon mini-split in 2 rooms and have been using it for heat in Massachusetts. I tried googling and searching in the manuals but still don't understand 2 questions:
1) When do they become inefficient (as compared to forced water baseboard heat from propane mod/con boiler)? Is there any rule of thumb, e.g., when it's below 40F outside - they're too expensive?
2) At what low outdoor temperatures is it not even safe to turn them on and they can get damaged? Again, is there some sort of rule of thumb not to turn them on when it's 20F outside? 25F? 15F?

Thank you very much in advance!

Robert_25

The system will shut itself down when it is too cold to operate.

Generally speaking, the efficiency drops off pretty quickly below 32F due to the defrost cycles in the exterior unit.

Jamie Hall

Your operating manual should have information as to what the lowest temperature they can safely be operated at. However, that is not the lowest temperature at which they are economical.

That depends on your cost of fuel and cost of electricity and the characteristics of the heat pump. The heat pump -- any heat pump -- loses efficiency as the outside temperatures drop. Determining the best crossover point is not a simple task. However, at some point the system will start running defrost cycles (somewhere above 32) which lose efficiency, and at some lower point it may start running resistance heating -- which is really costly.

Zman

If you can find a performance curve that shows the COP of your unit, that is where you start. From there you need to find your local cost per KW/hr and gallon of propane. It's easy math after you track down the data.

psb75

Your questions are good. It is good to get the answers you are looking for. I suspect many people have these units and don't know the most effective way to operate them to be cost effective.
They are just sold, installed and run, and often not efficiently in the colder seasons.

lkhelp

Thank you all! Did more digging and found that AOU24RLXFZ can technically heat until outdoors temperature is 5F. Must be super inefficient, but at least I don't need to worry if I forgot to turn it off on a 20F night.
As for COP - so far I only found static number at 47F and it's about 4. Am I understanding this right that for every 1 kW of electricity it uses it generates 4 kW of heat? How do I compare this with efficiency of forced water baseboard heat generated by propane boiler?

EBEBRATT-Ed

I would switch to the boiler at 30 deg outside air. At that point the heat pump will need defrosting. The colder the OA the less efficient they are.

You are correct that the newer heat pumps can run down to about 0

Zman

You are understanding the COP correctly.
By the time you get to 5F your COP is likely <1. Somewhere in the manufactures literature, they should provide a curve that shows the COP at every temp.
In my mind, you make the switch to propane when the cost of propane is cheaper than the cost electricity, compensating for your local pricing.
I would use the attached spreadsheet once you have all the data.

lkhelp

Thank you both!!

TAG

At my place NJ with very expensive electric ... I'm around .19 kWh electric and current propane at $1.69 w/ 95 furnace -- about 3x more expensive running straight electric resistance (cost per 100k BTU: E=$5.57 P=$1.93 )

So anytime I am at COP range of 3 it's about a wash ...

Hunt down those specification --- I think you will find that even at very cold temps you will be there. 3 or better. The Mitsubishi units are and I did a Fujitsu in a barn conversion loft almost 8 years ago and it was .... that's why I picked it. Mine was rated down to -5.

I still run my boilers when cold as it's more comfortable -- but, I bump up areas with the heat pumps. Great in the shoulder season. Modern heat pumps with demand defrost really don't have to all that often and when they do it is quick ... mini-splits don't have back up heat. Even with my Carrier traditional split units they don't have to defrost very often --- they are variable speed as well w/o any resistance backup.

Lots of places have electric in the .11 range ... so it's always going to be cheaper running the HP

GW

You need a simple spreadsheet to quickly compare. It’s not 2 plus 2 math either because the COP drops as it gets colder. Also there are comfort factors that are non tangible (how cold is a “cold” bathroom, for example) 

These days fossil fuel systems are cheaper than electric heat pumps (on paper), unless you’re getting hosed on LP costs or your furnace-boiler is badly inefficient. 

lkhelp

TAG said:

At my place NJ with very expensive electric ... I'm around .19 kWh electric and current propane at $1.69 w/ 95 furnace -- about 3x more expensive running straight electric resistance (cost per 100k BTU: E=$5.57 P=$1.93 )

So anytime I am at COP range of 3 it's about a wash ...

Hunt down those specification --- I think you will find that even at very cold temps you will be there. 3 or better. The Mitsubishi units are and I did a Fujitsu in a barn conversion loft almost 8 years ago and it was .... that's why I picked it. Mine was rated down to -5.

I still run my boilers when cold as it's more comfortable -- but, I bump up areas with the heat pumps. Great in the shoulder season. Modern heat pumps with demand defrost really don't have to all that often and when they do it is quick ... mini-splits don't have back up heat. Even with my Carrier traditional split units they don't have to defrost very often --- they are variable speed as well w/o any resistance backup.

Lots of places have electric in the .11 range ... so it's always going to be cheaper running the HP

19 cents, yikes! I thought MA had most expensive electricity... We're at 11.8 cents now. Conversely, propane is $2.4 a gallon, so that doesn't help.
Definitely need to find those specs at various temperatures...

lkhelp

GW said:

You need a simple spreadsheet to quickly compare. It’s not 2 plus 2 math either because the COP drops as it gets colder. Also there are comfort factors that are non tangible (how cold is a “cold” bathroom, for example) 

These days fossil fuel systems are cheaper than electric heat pumps (on paper), unless you’re getting hosed on LP costs or your furnace-boiler is badly inefficient. 

@GW Thank you, Gary! Definitely need to find those COP numbers at various temperatures. No luck so far.
I just got a new boiler that I'm having trouble understanding - any chance you could take a look at this thread?

Zman

Is your heat pump part of this study?
https://www.nrel.gov/docs/fy11osti/52175.pdf

lkhelp

Zman said:

Is your heat pump part of this study?
https://www.nrel.gov/docs/fy11osti/52175.pdf

Unfortunately, not

TAG

At 11c and those propane numbers .... the heat pump is going to win.

A very tight/ insulated house with open spaces is ideal for the mini type heat pumps. Not so great in a multi room typical leaky 60 year old colonial.

lkhelp

TAG said:

At 11c and those propane numbers .... the heat pump is going to win.

A very tight/ insulated house with open spaces is ideal for the mini type heat pumps. Not so great in a multi room typical leaky 60 year old colonial.

lol, I guess I have both! There are 3 small original bedrooms from 1950s and then the open space addition from 1990s (which has the heat pump). Insulation in the old part was just done - so that should be well insulated at least on the roof, not sure about walls and windows (those are older, but seem to hold up).

GW

I'm sorry i don't work with that boiler line. If you have a general question I'm happy to attempt to answer.

Macro

Caleffi just released an idronics issue all about heat pumps and I've been deep in it myself

TAG

Macro said:

Caleffi just released an idronics issue all about heat pumps and I've been deep in it myself

looks like a good read .. just skimmed it. I wanted to do a air/water heat pump for my new build ... there are not many possibilities and none in my area. The DHW was another question. With an all radiant house it seemed like a great solution -- the ones that work like a package unit especially. Just pump the hot transfer fluid in through insulated pipe w/o having to have refrigerant piping to set up

psb75

You should pursue this option. I very much recommend air-to-water heat pump for your new build. Look at units like Spacepak out of MA. Or Canadian unit called Arctic from Manitoba but, importing is now an issue. Install a big enough buffer tank w/ coil and it will also do DHW. Some kind of back-up heat strategy for the 'heart of winter' may be necessary.

ethicalpaul

Please look at water-to-water while you are looking. Wonderful efficiency year-around

psb75

"year-around" for cooling esp. in air-to-air.
No "effective" cooling in air-to-H2O units.