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Actually, you have the problem of COP<1 only with the "cheap" (that is 5000 €) heat pumps fitted on the balcony, and which try to exchange heat with the atmosphere like an air conditionner. They practically stop working at 0°C because of frost on the exchangers.
"Real" heat pumps take the heat from 1.5 m underground, where it's 10°C almost all year long, with burried pipes (that's where you need a lawn and big work $$$). You may need additional fuel heating a few days/weeks per winter if you are in a really cold area where the ground freezes in the winter (like: there is 20" of snow on you lawn for 2 months every winter...)
There is still a variant to fix this problem: the heat well heat pump, with a few 50-100m wells which get the heat from the waterbed, which never freezes in any  place that actually has grid power (Siberia is a special case...). And you actually need less lawn area to conceal those wells, but they're even more expensive to drill (expect 20-30k€ for a mid sized house - you may have a tax rebate on this in some countries).

Pierre
by Pierre on Mon Jul 10th, 2006 at 11:53:24 AM EST
[ Parent ]
That sounds awfully expensive for just the well...

I have just contracted a company for drilling a 650 feet deep well for a 9kW heat pump. The drilling cost is about €5500 or $7200. The heat pump costs about €7500 ($9600). Add installation and the total is about €15000. I already have radiant floor heating and radiators since I've been burning oil until now.

They don't usually drill multiple wells here unless you need more than ~700 feet depth which means 11kW pump and up.

Year average COP should be about 3.5, time to earn back is about 10 years with todays oil price, much less if it hits $100 :)

by jkirkebo (jkk@scm.no) on Mon Jul 10th, 2006 at 06:34:01 PM EST
[ Parent ]
I was thinking of the price for the complete setup (couple of 50-m wells, heat pump, low temperature radiators, possibly solar-hybrid with hot water). Well-collector heat pumps have much higher COPs these days: Viessmann claims up to 6-7 (but that brand is really pricey).

Pierre
by Pierre on Tue Jul 11th, 2006 at 03:27:57 AM EST
[ Parent ]
Peak COP and year average COP is quite different. You'll never get the higher COP when producing hot tap water in the summer. Also a some brands don't include the power consumed by the circulation pumps when calculating COP :(

I've never heard someone reporting a year average COP of more than 4. Still, when already at 3.5 or so, higher COPs mater less and less. The difference in savings from 2 to 3 is huge, from 4 to 5 is miniscule in comparisation.

by jkirkebo (jkk@scm.no) on Tue Jul 11th, 2006 at 04:05:49 AM EST
[ Parent ]
It's true COP doesn't matter on the power savings side, but it helps in that you can heat a bigger space with a pump of a given size (you save upfront cash on the collector, you save room in you basement, you need less of complementary heating, things like that... and also, I like the best technology, better have it in heat pumps than in sport cars !)

Pierre
by Pierre on Tue Jul 11th, 2006 at 04:21:24 AM EST
[ Parent ]
BTW, why do two 50m wells ? A single 100m is far cheaper and just as efficient. Only when one requires more well depth than the max collector length (usually 400m/1312 feet) can support it makes sense IMHO.
by jkirkebo (jkk@scm.no) on Tue Jul 11th, 2006 at 04:09:01 AM EST
[ Parent ]
Viessmann has a system of variable flow rate pumps, so they save on the flow pumps by switching off a well if it's not needed. And you have less load loss with shorter parallel fluid circuits than with a single long one. For horizontal collectors, they also have between 1-8 separate circuits, depending on the heat pump size, for the same reason.

Pierre
by Pierre on Tue Jul 11th, 2006 at 04:17:06 AM EST
[ Parent ]

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