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Trying to learn about Heat Pumps

NickD on Sun October 25, 2009 6:33 AM User is offline

Refrigerant Type: 410
Ambient Temp: -30*F
Country of Origin: United States

Found a Goodman that would sit nicely on top of my new furnace, AC sounds great with a SEER factor of 18, but that is not of too much concern for me being buried in the north woods. For the same heat pump, reading COP factors of only 4. Currently paying $1.27 for 100K BTU of natural gas, with electrical rates at 9.5 cents a KWH, would cost $2.79 for the same amount of heat energy. With a COP of only four would be like paying 70 cents for a therm of gas, but really doubt if I could use this thing for ambient outdoor temperatures of less than 40*F due to that now evaporator sitting outside with the moist air, it would free up, maybe 50*F would be the coldest temperature I could use it at.

Gas bills aren't that bad in 50*F weather, and considering the cost and upkeep of a heat pump, would more than likely cost even more. If there is payback, may be years or more than likely, never. Finally found a heating contractor that seems to know his business with even a degree of honesty, air heat pumps are not successful up here. Said he did install geothermal heat pumps, but runs around $25,000.00, would never see a payback on that. That COP factor is just way too low, that I don't understand.

Yet another bad joke are these programmable thermostats, what they don't tell you is that your home is constantly undergoing both thermal and moisture cycling, really great for wrecking your clothes, furniture, and any electronic or electrical appliances, plus nothing better than sitting on a cold damp couch. Really difficult in trying to learn the truth about these so called energy saving devices.

Only three years ago, was paying 67 cents for a therm of gas, and 4.5 cents for a KWH of electricity. These are all domestically produced energy sources, but the price skyrocketed with OPEC prices. OPEC prices came down, the domestic suppliers kept them up there. Obama isn't doing anything about this nor is the democratic congress. Same old stuff, Washington represents the super rich. Both republicans and democrats are crooks.

MikeH on Sun October 25, 2009 9:09 AM User is offline

Nick, I live in Dallas so there are a lot of heat pumps here - a lot!! But, it almost never gets below 20F so they work O/K. One of my friends who does Heating and A/C for aliving has told me repeatedly that heat pumps are worthless below 20F. That is why heat strips always accompany them. I really prefer gas heat as it is way less expensive

NickD on Sun October 25, 2009 10:06 AM User is offline

?
This is the COP versus outdoor temperature on the Goodman DSZ180361A 3 ton unit, but I still wonder about freeze up of the now external evaporator in function with these things.


Outdoor Temp 65 60 55 50 47 45 40 35 30 25 20 17 15 10 5 0 -5

COP 4.54 4.39 4.23 4.04 3.91 3.83 3.64 3.44 2.96 2.81 2.65 2.54 2.48 2.28 2.08 1.87 1.64

This is the first unit I have seen where the evaporator sits on top of their gas fired furnace, can buy an outdoor thermostat for automatic switchover from the pump to gas. Most I have seen use resistant electrical heat for back up, that could really run up your electrical bill.

mk378 on Mon October 26, 2009 2:28 PM User is offline

Somewhat comparing apples to oranges as there are different units. COP is watts heat supplied indoors / watts electric consumed but SEER is btuhr removed from indoors / watts consumed.

One watt of heat is 3.41 btu/hr. So to get COP into the same units as SEER, multiply by 3.41. Or vice versa.

Resistance electric heaters thus have a COP of unity. Even at -5 the heat pump is outperforming resistance(*), but is it cheaper than gas?

To make a cost analysis, you need to know how much money a watt-hour of heat from your furnace ends up costing on the gas bill versus a watt-hour of electricity from the electric bill. Divide the electric rate by the COP of the heat pump at your outdoor temperature, and compare to the cost of heat from gas.

Heat pumps do freeze up, they have a defrost system which runs a cycle of reversing the heat pump with the external fan off to thaw the coil. I would assume the energy lost by this process is included in COP figures. But of course the rate of defrost cycles varies with outdoor humidity.

(*) though in cold weather as COP falls and heat loss from the house rises, the heat pump will be unable to supply enough heat on its own. Thus supplemental heating is necessary -- in a typical installation it's electric resistance. Note that if you put a heat pump coil downstream of a gas furnace, it will be unable to pump heat against the high outlet temperature when the furnace is lit-- you need to run either/or but not both.

Edited: Mon October 26, 2009 at 2:39 PM by mk378

NickD on Tue October 27, 2009 6:14 AM User is offline

Goodman did think about using one or the other with an ambient crossover switch, would never use both. But I suspect being half surrounded by Lakes Superior and Michigan, my unit would be spending more time in defrost than in the heating mode. Yet another factor is snow, would be outside most of time digging the darn thing out, can't have air flow if the outside unit is buried deep in snow.

Sounds good on paper with a maximum of a 25% savings in fuel cost provided we get dry air and no snow, but that is not about to happen, so gave up on the idea.

Another problem I have given great thought is with these PVC flues and air intakes. Need a measley 3 square inches for both air intake and exhaust, but also a perfect spot for insects and even small animals to crawl into especially in the spring time for them to find a spot to build a home where it's warm. Been talking to my neighbors, yet another problem is freeze up particularly in the exhaust that is mostly moisture forming huge icicles in the outlet, plus keeping the air sources well above the snow line.

Seen some very poorly installed flue systems, wonder if these HVAC guys do that intentionally so they can make 85 bucks an hour with service calls. Son had a very poor installation, didn't know about that until I studied it, with his share of service calls that amount to a winter's month heating bill. Not saving money that way either.

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