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Conversion form baseboard heating to heat pump

Discussion in 'Home Power and Microgeneration' started by Alan C37, Aug 7, 2006.

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  1. Alan C37

    Alan C37 Guest

    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
     
  2. 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
     
  3. daestrom

    daestrom Guest

    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
     
  4. Alan C37

    Alan C37 Guest

    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
     
  5. Alan C37

    Alan C37 Guest

    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
     
  6. Guest

    Maybe not.
    Suppose the heat pump cools it to 35 F and the ground warms it to 55.
    The ASHRAE HOF says damp (20% mc) sand has about 18 Btu-in/h-F-ft^2, or
    maybe more, for upward heatflow with evaporation below and condensation
    above, so you might get 10K Btu/h with a 10 F dT from 1000/18 = 55 ft^2
    of pipe, eg 53 ft of black corrugated 4" pipe with holes, so we can add
    water to the soil if needed. This pipe costs about $30/100', and it has
    lots of surface compared to smaller water-filled pipes with the amount
    of surface exposed to drier soil as a limiting factor.
    With a 20 F temp change and no change in moisture content, we need to move
    10K/20/0.24 = 2083 lb/h of air, ie 2083/60/0.0771 = 450 cfm. But if the air
    that goes to the heat pump has 100% RH at 55 F (w = 0.009233) and the air
    that leaves it has 100% at 35 (w = 0.004277), each pound of air can supply
    another 1000dw = 5 Btu, ie a total of 0.24x20+5 = 9.76, so we only need
    10K/9.76 = 1025 lb/h, ie 222 cfm.

    How much fan power is needed to pull that air through 4 4"x100' pipes?
    The friction loss might be about 0.2 "H20. It looks like Grainger's 23W
    3VU71 $64 fan might work up to 176 F. It can move 665 cfm at 0" SP and
    550 at 0.1" and 350 at 0.2".

    Nick
     
  7. Alan C37

    Alan C37 Guest

    Nick,
    Many thanks. Perhaps I should not give the idea up just yet.
    With such a short pipe I need to be sure that the air would actually be in
    the pipe for long enough to warm (or cool) sufficiently in the time it is in
    the pipe each circuit. Come to that will the sandy soil around the pipe
    recover its temperature quickly enough? I should probably set up a simple
    test to see what can be achieved since I find calculating this stuff in
    detail more than a little difficult.
    Thanks for the information about blower capacity. I had assumed i would
    need a much bigger one than the one you mentioned.
    The dealer I am considering for the purchase of the heat pump tells me
    that any such non-standard installation would completely void all warranties
    so I will have to think about that too. Maybe a much cheaper system and no
    warranty coverage would be worth the risk. Such things can be bought in
    Canada for around $1000 or so. The excellent two zone Mitsubishi model we
    have in mind will cost me about $8000 (installed), which is a little high in
    my opinion. Decisions, decisions!!
    Yet another possible variant of the ground source heat pump might be to
    bury a long length of water pipe and circulate water or anti-freeze
    relatively slowly through it. Near the heat pump outside unit would be some
    sort of radiator, perhaps finned pipes, to warm/cool the air which then is
    used by the heat pump.
    Thanks again,
    Alan C
     
  8. Guest

    I don't think we need to worry about that.
    We might find the thermal resistance of a collection of coaxial half-
    cylindrical shells in series, starting with a 24" diameter shell 1" thick
    with 24Pi/2/12 = Pi ft^2 of surface and a thermal resistance of 1/(18Pi)
    = R0.0177 and adding the resistance of a 26" half-cylinder 1" thick, and
    so on. The total comes to about R0.5, out to a 240" diameter. So 4 100' pipes
    might have a thermal conductance of about 1/0.005 = 200 Btu/h-F. With a 20 F
    temp diff, they might supply 4K Btu/h. Then again, if pump groundwater into
    the pipes... 6K/20 = 300 lb/h, ie 0.6 gpm. Maybe we need fewer holes.

    Nick
     
  9. Alan C37

    Alan C37 Guest

    Thank you,
    About the only options I have here (for a reasonable cost) are to use air
    source heat pumps for both heating and cooling. This means, as you point
    out, that I will certainly need auxiliary heat during really cold days.
    This would be supplied by my existing electric base boards. If I can figure
    out how to preheat (or precool) the air going into the heat pump I just
    might be able improve the performance on really low temperature days to the
    extent I would need less direct heating from the baseboards. This heat
    would come from the ground via buried pipes and yet another heat exchanger.
    I presently believe that I should circulate water (or anti freeze) fairly
    slowly through the pipes and a radiator in an enclosed, insulated space
    around the external heat pump unit during very cold or very hot days. I
    might be able to raise (lower) the temperature of this air by about 20 deg F
    if all goes well. There should not be a lot of power needed to circulate
    the water. I intend to start with lots more insulation then a single large
    air sourced heat pump, get some performance data and then play with air
    preheating. I will report back here when I have obtained any useful data,
    probably late next year (if I live that long). Thanks again

    Alan C
     
  10. daestrom

    daestrom Guest

    Trouble with circulating liquid 'fairly slowly', is it's a double-edged
    sword. If it goes slowly, it will pick up more heat from the ground and be
    closer to the ground temperature when it comes into your unit (assuming it's
    well insulated between where it comes out of the ground and the unit). But
    moving slowly, it will not heat up the unit as much, the fluid will just
    cool to ambient early in the coil and then contribute nothing more to the
    unit's heating.

    Well, a lot of insulation around the house is *always* a good place to start
    :)
    Looking forward to your results...

    Good Luck

    daestrom
     
  11. Hi Harry,

    Just to provide you with a little more background ... This homeowner
    currently heats his home with electric baseboard heaters and the heat
    pump is not intended to replace his existing heating system, but
    simply to augment it. The goal is to offset as much resistance heat
    with this less costly heat source, as outside temperatures permit.

    Due to relatively high heating demand (and increasingly higher
    electricity rates), even if this heat pump displaces only half of his
    total requirements, he stands to save a substantial amount of money on
    his annual heating bill. Since he also desires air conditioning and
    his home has no existing ductwork, a ductless heat pump will provide
    him with both heating and cooling at about the same cost as a
    cooling-only unit. So, in effect, he gets the space cooling he wants
    and a cost effective supplemental or secondary heat source at
    basically no additional cost.

    Having had one in my home for a year now, I'm kicking myself for not
    installing one sooner.

    Cheers,
    Paul
     
  12. Alan C37

    Alan C37 Guest

     
  13. Alan C37

    Alan C37 Guest

    I think my computer (or I) is going mad. It just sent a reply which I had
    not yet written. Sorry.
    I do know, of course, that available heat pumps have lower limits of
    temperature and will not give any significant heat gain at air input
    temperatures below, perhaps, freezing point or a bit higher. It is for this
    reason that I am speculating about the possibility of preheating the air
    using heat from the ground. I am no expert but have been doing a bit of
    research about heat flow and find it more than a little difficult to
    accurately determine what I will need to do. If I use water or antifreeze
    and a big radiator in an enclosed and well insulated space around the
    outside part of the heat pump I need to calculate how big the air space
    needs to be; how long and what diameter the buried pipes must be. How fast
    should the water circulate to achieve a more or less constant air
    temperature going into the heat pump. How fast, and how much, will the
    temperature around the buried pipes vary. I'm sure there will be many more
    things to think about. At least it will give me something to play about
    with over the winter when I can't play golf.
    I will install a basic heat pump in the next week or so, with no
    modifications to the inlet air. This will certainly reduce my overall
    heating costs significantly and will probably cost me about $1000 only. I
    am hoping to eventually get my electricity bill down from about $4000 per
    year to $3000 or less. I'll let you all know next spring what happened over
    the winter.
    Thanks for the interest.
    Alan C
     
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