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141 comments on Ground Source Heat Pumps

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AlanfromBigEasy on February 10, 2008 - 2:25pm Permalink | Subthread | Parent | Comments top

I think that I found a flaw. Air source heat pumps output declines significantly at cooler temperatures. More electricity AND less heat as the temp drops. This adjustment is not apparent in your spreadsheet.

In New Orleans, I found that heat pumps sized for a/c cooling load could provide adequate heat down to about +3 C (with interior heat from office building). GREAT for us (rarely below 0C), minimal gas heat supplement.

On an province wide basis, this has strong implications. Minimal demand at 8C (air source works wonderfully), MUCH higher demand (and resort to resistance heat) at, say, -20 C. TOOOO much for grid :-(

If you would like to talk, send me an eMail (click my name and it is in my profile).

Best Hopes,

Alan

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DaveMart on February 10, 2008 - 4:00pm Permalink | Subthread | Parent | Comments top

Check out information in this thread on the new Eco-Cute CO2 air pumps - they are not yet in the States, Japan only, but they are more efficient and good for down to -20C

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HereinHalifax on February 10, 2008 - 6:16pm Permalink | Subthread | Parent | Comments top

Hi Alan,

Thanks for your comments and for your kind offer to assist; both are much appreciated and I might just take you up on that.

Actually, the spreadsheet makes adjustments for both output and power consumption based on outdoor ambient temperature. At 8.3C/47F, the Fujitsu 12RLQ produces 4.68 kW of heat and its power consumption is 1.25 kW (COP = 3.75). At 24C, heat output climbs to 6.18 kW, but so too its power demand -- maximum demand is said to be 2.14 kW. At -15C, we're told heat output falls to 0.9 kW; I don't honestly know the exact numbers, but I'm guessing at this point its COP runs in the range of 1.75 to 2.0 and that periodic defrosting of the outside coils kicks us closer to 1.5 or perhaps 1.8. I wish I had better numbers to work with and as soon as I find them I'll incorporate them into the model.

That said, based on published specs, we would expect heat output to rise an average of 0.094 kW per degree C and for our purposes, I rounded that down to 0.09 kW/C. Likewise, for each degree above 8C, power consumption should increase by no more than 0.059 kW and I rounded that up to 0.06 kW. On that basis, I'm reasonably confident our performance estimates are accurate for temperatures 8C and above. When temperatures fall below 8C, we assume heat output drops by 0.16 kW/C which is in line with published specs. With regards to power demand, one would expect it to fall largely in proportion to heat output, but this would be tempered by defrost demand; in our model, I assumed power demand would only fall by just 0.004 kW/C which is far more pessimistic than need be. For example, at -15C, we know heat output is 0.9 kW, but my calculations have power demand at 1.16 kW, giving us a negative COP when, in fact, we know it would be positive. I figured it would be better if I intentionally underestimated performance rather than the other way around so, if anything, the numbers should be even better than what we show here.

Best regards,
Paul

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AlanfromBigEasy on February 10, 2008 - 8:13pm Permalink | Subthread | Parent | Comments top

at -15 C... but my calculations have power demand at 1.16 kW, giving us a negative COP when, in fact, we know it would be positive.

Actually not. It is entirely possible for a heat pump to generate less heat than electrical resistance heat (COP < 1.0). Given the stress on the equipment, and defrosting, it is better to turn off the heat pump and go to resistance heat (or oil) before this happens (at COP 1.5 or so). 28 F or so depending upon the model.

Alan

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HereinHalifax on February 10, 2008 - 10:03pm Permalink | Subthread | Parent | Comments top

It is entirely possible for a heat pump to generate less heat than electrical resistance heat (COP < 1.0).

Hi Alan,

I'd be shocked if a heat pump's COP would be allowed to fall below 1.0 within its normal operating range; presumably manufacturers would avoid this for all the obvious reasons. As mentioned, I don't have the operating specs on the Fujitsu 12RLQ (HSPF = 10.55), but I do have them for a York BHX024 which is a less efficient unit with an HSPF of 8.0. At -23C and with 21C dry bulb temperature over the evaporator coil at 800 CFM, the BHX024 produces 2.43 kW of heat and has a power draw of 1.25 kW, which includes the blower. So even at -23C, a full 8 degrees below the -15C cut-off of the Fujitsu, the COP for this particular heat pump is a still respectable 1.95. Defrosting will obviously take the final number down somewhat, but I couldn't imagine a scenario short of entombing the outdoor compressor in a thick block of ice where more energy would be expended performing this task than what would be gained through normal operation.

I tend to believe my numbers are, if anything, unfairly conservative but, again, this isn't my area of expertise, so I would encourage you and anyone else to challenge my assumptions and poke holes in my arguments.

Cheers,
Paul

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Saratoga Peak on February 11, 2008 - 9:36am Permalink | Subthread | Parent | Comments top

Actually,
Air Source Heat Pumps are designed to fall below a COP of 1.0 when the outdoor temperature gets near or below freezing. What happens is the outdoor coil begins to Freeze up. Then, to combat this, by design, is the Unit switches to Air Conditioning mode to generate heat on the Outdoor coil, thereby cooling indoors. Then, since it is winter, and you want heat, the resistance coil turns on, thereby taking cooled air and reheating it, and heating it more in order to heat your home. This is painfully inefficient. And is what in fact has given the Heat Pump industry a serious black eye in the majority of North America. And sadly, Ground Source Heat Pumps have had to work hard to rid this stigma, since they are entirely different and not prone to the same problem.

Now I don't know about this fujitsu model. It must use a different refrigerant to make this possible.

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HereinHalifax on February 11, 2008 - 12:31pm Permalink | Subthread | Parent | Comments top

Air Source Heat Pumps are designed to fall below a COP of 1.0 when the outdoor temperature gets near or below freezing

Hi Saratoga Peak,

Frankly, I don't see how this is possible and it certainly doesn't reflect my own experience. Air source heat pumps are rated by their HSPF (Heating Season Performance Factor), which is a ratio between how much heat they produce over the heating season versus the amount of energy they consume, in total; obviously, higher numbers are better. The Fujitsu 12RLQ has a HSPF of 10.55 (Zone 4). To convert this to its "seasonal COP", you divide this number by 3.4, which gives us a COP of 3.1. The York product I mentioned above has a HSPF rating of 8.0, so its seasonal COP is 2.4. Just to be clear, these numbers take into consideration the energy used to defrost the outside coils. I should also add that Halifax, N.S. is located in Zone 4 and our heating degree days number 7,800, which means our winters are as cold as those of Minneapolis, MN (not exactly tropical by any means).

Prior to installing my ductless heat pump, I used, on average, a little over 2,000 litres of heating oil a year for space heating and domestic hot water purposes. The following winter, this number dropped to 827 litres and last year it came in at 830 litres. My records show my DHW related consumption during the summer months averages 1.2 litres/day and about 1.5 litres/day during the winter months when water inlet temperatures are lower, I do more laundry (bulker and heavier clothing) and when longer (and hotter) showers are preferred. On this basis, I can reasonably assume 500 or so litres a year can be attributed to DHW needs, with the balance related to space heating. Thus, with the addition of my heat pump, my space heating consumption has fallen from about 1,500 litres/year to 330 litres, for a net savings of 1,170 litres/year. [My home, btw, is a 2,500 sq. ft., 40-year old Cape Cod that has been extensively upgraded in terms of its thermal efficiency, with a space heating demand that places it somewhere between a conventional new home and a R2000 equivalent.]

My oil-fired boiler has an AFUE of 82%, which means I net about 8.77 kWh of heat from each litre of heating oil. Multiplying 1,170 litres x 8.77 kWh/litre, tells me my heat pump is providing me with an average of 10,260 kWh of heat over the course of the heating season. My electrical consumption averages 17 kWh/day during the summer months and climbs to 40 to 43 kWh/day during the coldest winter months when the poor little guy is working flat out. Taking a look at my most recent power bill, here's how the numbers break out (total consumption / days in billing cycle / kWh/day):

For the period ending:

January 08 -- 2,540 kWh, 59 days, 43 kWh/day
November 07 -- 1,527 kWh, 62 days, 25 kWh/day
September 07 -- 1,136 kWh, 62 days, 18 kWh/day
July 07 -- 1,043 kWh, 62 days, 17 kWh/day
May 07 -- 1,848 kWh, 62 days, 30 kWh/day
March 07 -- 2,397 kWh, 58 days, 41 kWh/day
January 07 -- 2,332 kWh, 58 days, 40 kWh/day

Past Year: 10,491 kWh, 365 days, 29.7 kWh/day

Our heating season basically kicks off October 1st and eventually tapers off towards the latter part of May, so if we look at the consumption for just this period, I used a total of 7,548 kWh over the span of some 210 days, which is an average of 35.9 kWh/day. From that, I can subtract what would be used for other household needs (i.e., lighting and appliances) which, based on my summer usage, seems to be in the range of 17.5 kWh/day. If these numbers are more or less correct, my heat pump consumes a little less than 3,900 kWh/year but provides me with just over 10,000 kWh/year in heat. On this basis, my seasonal COP should be in the range of 2.5. Note too that my heat pump has a HSPF of 7.2 and the Fujitsu has a HSPF of 10.55, so the Fujitsu should be, in theory, 1.5 times more energy efficient than my own.

My apologies for the long post, but I wanted you to understand my own "real world" experience with air source heat pumps. I'd be happy to answer any questions you have and provide you with more information if it would be helpful.

Cheers,
Paul

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docscience on February 12, 2008 - 2:18am Permalink | Subthread | Parent | Comments top

Greetings; I am in Sydney, NS and was wondering if you know anyone else in halifax who is researching peak oil ?
You do not have any email contact info in your personal info section. Can you email me ?

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HereinHalifax on February 12, 2008 - 12:12pm Permalink | Subthread | Parent | Comments top

Hi Gilbert,

I've send an e-mail to your hot mail account.

Cheers,
Paul

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141 comments on Ground Source Heat Pumps

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