Posted by Alan C37 on August 7, 2006, 8:33 am
About three months ago I got a lot of excellent advice from many of you
good people regarding how I might be able to significantly reduce my heating
and cooling costs. I intend, among other things, to install a good ductless
2 zone heat pump, possibly from Mitsubishi, and thus get a lot of heating at
about half, or better, the cost I am paying right now. I will also get
decent air conditioning. Both are "good things".
These heat pumps are very efficient but use external air as the source (or
sink) for the heat and thus will not usually provide enough heat on the
really cold days. It can sometimes fall to minus 40 degrees here in Ottawa
so a back-up, "heavy lifting" heat source is needed for these times. To
cover this I can simply leave my baseboards in place and hope to use them as
little as possible.
If I place the external part of the heat pump system underground and have
a large metal pipe buried about 4 feet deep running around the lawn the air
from which the heat pump draws, or dumps, heat will try and remain at about
55 deg F, if the surface area of the pipe is large enough. This should, I
believe, make the system significantly more effective than placing the unit
above ground. I suspect I might not even need a back up heat source. I may
need to install and auxiliary blower to move enough air through the system.
I may also need to be able to open the top of the enclosure sometimes to use
the actual ambient air when temperatures dictate so doing.
I have not seen any such scheme elsewhere although it obviously has
something in common with the ground source heat pumps using buried water (or
antifreeze) pipes.
Any suggestions or comments would be much appreciated. In the meantime I
will try and calculate what pipe length and diameter I would need.
Thanks,
Alan C
acombellack@nspmsympatico.ca
--
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Posted by Paul M. Eldridge on August 7, 2006, 9:13 am
Hi Alan,
Glad to hear you will be going ahead with a heat pump; I think it's a
wise decision. This is strictly a personal opinion (and I'm sure
others here will provide you with a proper assessment), but I would be
very leery of the setup you described. The outside compressor
requires a high volume of air for proper operation and it's unlikely
you could preheat/cool that much air to any significant degree.
Secondly, that auxiliary blower is going to consume a fair amount of
electricity, even during milder weather when this type of setup would
be of little practical value (if any); it's also one more potential
failure point. In addition, it would presumably make servicing that
outside unit more difficult.
I would leave the outside compressor above ground. Ideally, it should
be placed in an area with unobstructed air circulation but sheltered
from strong winds.
Good luck!
Cheers,
Paul
On Mon, 7 Aug 2006 08:33:39 -0400, "Alan C37"
> About three months ago I got a lot of excellent advice from many of you
>good people regarding how I might be able to significantly reduce my heating
>and cooling costs. I intend, among other things, to install a good ductless
>2 zone heat pump, possibly from Mitsubishi, and thus get a lot of heating at
>about half, or better, the cost I am paying right now. I will also get
>decent air conditioning. Both are "good things".
> These heat pumps are very efficient but use external air as the source (or
>sink) for the heat and thus will not usually provide enough heat on the
>really cold days. It can sometimes fall to minus 40 degrees here in Ottawa
>so a back-up, "heavy lifting" heat source is needed for these times. To
>cover this I can simply leave my baseboards in place and hope to use them as
>little as possible.
> If I place the external part of the heat pump system underground and have
>a large metal pipe buried about 4 feet deep running around the lawn the air
>from which the heat pump draws, or dumps, heat will try and remain at about
>55 deg F, if the surface area of the pipe is large enough. This should, I
>believe, make the system significantly more effective than placing the unit
>above ground. I suspect I might not even need a back up heat source. I may
>need to install and auxiliary blower to move enough air through the system.
>I may also need to be able to open the top of the enclosure sometimes to use
>the actual ambient air when temperatures dictate so doing.
> I have not seen any such scheme elsewhere although it obviously has
>something in common with the ground source heat pumps using buried water (or
>antifreeze) pipes.
> Any suggestions or comments would be much appreciated. In the meantime I
>will try and calculate what pipe length and diameter I would need.
> Thanks,
>Alan C
>acombellack@nspmsympatico.ca
Posted by daestrom on August 7, 2006, 4:39 pm
> About three months ago I got a lot of excellent advice from many of you
> good people regarding how I might be able to significantly reduce my
> heating and cooling costs. I intend, among other things, to install a
> good ductless 2 zone heat pump, possibly from Mitsubishi, and thus get a
> lot of heating at about half, or better, the cost I am paying right now.
> I will also get decent air conditioning. Both are "good things".
> These heat pumps are very efficient but use external air as the source
> (or sink) for the heat and thus will not usually provide enough heat on
> the really cold days. It can sometimes fall to minus 40 degrees here in
> Ottawa so a back-up, "heavy lifting" heat source is needed for these
> times. To cover this I can simply leave my baseboards in place and hope
> to use them as little as possible.
> If I place the external part of the heat pump system underground and have
> a large metal pipe buried about 4 feet deep running around the lawn the
> air from which the heat pump draws, or dumps, heat will try and remain at
> about 55 deg F, if the surface area of the pipe is large enough. This
> should, I believe, make the system significantly more effective than
> placing the unit above ground. I suspect I might not even need a back up
> heat source. I may need to install and auxiliary blower to move enough
> air through the system. I may also need to be able to open the top of the
> enclosure sometimes to use the actual ambient air when temperatures
> dictate so doing.
> I have not seen any such scheme elsewhere although it obviously has
> something in common with the ground source heat pumps using buried water
> (or antifreeze) pipes.
Your basic idea is sound, but I'm afraid you'll have some trouble
implementing it ("the devil is in the details").
What you're describing is a ground-source heat pump with air as the outside
medium instead of the usual water/anti-freeze.
The trick will be in figuring out just how much 'large metal pipe' buried
around the lawn in order to pick up as much heat in the circulating air as
the amount of heat the heat pump will draw out of that same air.
Most ground-source units use water/anti-freeze for the outside loop as it is
easier to bury than what you propose. (a "slinky" style installation only
needs a narrow trench about 6" wide at most).
And of course, as you draw heat from the ground with your metal duct/pipe,
the ground immediately surrounding the duct/pipe will cool and get colder
and colder. How much colder depends in part on how long the duct/pipe is
and how much heat you're sucking out of the ground on the average day.
This, coupled with how quickly heat flows from surrounding ground into the
cooler ground is a major part of the long-term performance.
You might look into direct ground-source heat pumps instead. The
installation costs more (because of all the trenching/drilling), and the
design has to be done by someone with experience in your area. One poster a
while back had problems with ground-source because the installer put in the
'typical' amount of wells for it, but neglected that the customer's site was
atop a large granite mountain. And granite behaves differently than your
average soil with a water table.
daestrom
Posted by Alan C37 on August 8, 2006, 9:07 am
Thanks to both who replied.
I will look into this a bit further. I have tried before to do some
calculations about heat transfer rates and find it far from easy to be
really accurate. Usually too many unknowns. It should be easy enough to
get a ball-park estimate though so I'll open Excel and see what I can do. I
will let you know what I find out.
My site should be just about ideal. I have about three quarters of an
acre and most of it is deep sand. The water table is about 40 ft deep in
late summer and about 3 ft in spring. Digging is easy with a small back
hoe. At the moment I have no idea how heat flows through sand but I'm sure
I can find out.
There is a very excellent seeming Canadian web site devoted to ground
source heat pumps but I have not had any response to at least two queries to
him. I tend to disregard people who don't answer questions reasonably
quickly. Maybe I am getting a bit cranky.
I am sure that in theory this would be an excellent solution to my problem
but I can't do much of the work myself and I don't know of a convenient
source for the equipment. Hence my interest in air sourced systems.
I do wonder why, if ground sourced systems are potentially so good, major
manufacturers do not offer them. One dealer told me that "they are known to
be very unreliable". I have no idea if this is true. Surely if the hype is
right and I can expect to reduce my heating cost by at least one half there
is a great potential financial incentive to installing and using such a
system. It is certainly very much better than switching to oil or gas or
even wood burning, especially if this involves the installation of air ducts
throughout the house. My total electricity costs over $4000 per year now so
if I can reduce this to less than $3000 I could spend a fair amount on the
new system, whatever it is.
Thanks again
Alan C
> Your basic idea is sound, but I'm afraid you'll have some trouble
> implementing it ("the devil is in the details").
> What you're describing is a ground-source heat pump with air as the
> outside medium instead of the usual water/anti-freeze.
--
Posted via a free Usenet account from http://www.teranews.com
Posted by Alan C37 on August 8, 2006, 10:26 am
Sorry to have wasted your time folks. I should have thought a bit more
about my "idea" before going on line and making an idiot of myself. I
haven't bothered to try and do any detailed calculations about heat flow as
I soon realised the idea, while just about feasible, is definitely not worth
the trouble.
Consider the heating situation. I wanted to use the same air over and
over. Take the heat out and pump it into the house. Then the same air then
goes back into the pipe and is cooled by the ground, ideally to about 55 deg
F. It then goes back through the heat pump again. The house needs, say,
10000Btu/hr. I don't want the air to be raised in temperature by very much
so as to minimised the heating of the ground around the pipe. Say I want
this rise kept to 10 deg F. Specific heat of air is about 0.23 Btu/lb/deg
F. Air density is about 0.075 lb/cubic ft Do the sums and we find that we
need nearly 1000 cubic feet of air per minute and this assumes that heat
flow is virtually instantaneous with no thermal resistances involved. Not
at all a practical proposition methinks. Pity!!
Alan C
> About three months ago I got a lot of excellent advice from many of you
> good people regarding how I might be able to significantly reduce my
> heating
--
Posted via a free Usenet account from http://www.teranews.com
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>good people regarding how I might be able to significantly reduce my heating
>and cooling costs. I intend, among other things, to install a good ductless
>2 zone heat pump, possibly from Mitsubishi, and thus get a lot of heating at
>about half, or better, the cost I am paying right now. I will also get
>decent air conditioning. Both are "good things".
> These heat pumps are very efficient but use external air as the source (or
>sink) for the heat and thus will not usually provide enough heat on the
>really cold days. It can sometimes fall to minus 40 degrees here in Ottawa
>so a back-up, "heavy lifting" heat source is needed for these times. To
>cover this I can simply leave my baseboards in place and hope to use them as
>little as possible.
> If I place the external part of the heat pump system underground and have
>a large metal pipe buried about 4 feet deep running around the lawn the air
>from which the heat pump draws, or dumps, heat will try and remain at about
>55 deg F, if the surface area of the pipe is large enough. This should, I
>believe, make the system significantly more effective than placing the unit
>above ground. I suspect I might not even need a back up heat source. I may
>need to install and auxiliary blower to move enough air through the system.
>I may also need to be able to open the top of the enclosure sometimes to use
>the actual ambient air when temperatures dictate so doing.
> I have not seen any such scheme elsewhere although it obviously has
>something in common with the ground source heat pumps using buried water (or
>antifreeze) pipes.
> Any suggestions or comments would be much appreciated. In the meantime I
>will try and calculate what pipe length and diameter I would need.
> Thanks,
>Alan C
>acombellack@nspmsympatico.ca