Is a heat pump neccessary for closed loop geothermal installation?

Discussion in 'Solar and Geothermal Water Heating Forum' started by sprinkler, Oct 23, 2011.

  1. sprinkler

    sprinkler New Member

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    Hi, I was wondering if it would be a good idea to run the DHW pipe under the ground before reaching taps and Boiler), and run this looped
    plastic pipe in a 2 meter deep x 50 meter long trench underground. Then this pipe would deliver water to the oil boiler (and hot taps).
    I would be thinking of just depending on the present gravity (attic tank - about 13-15 meters higher than proposed looped pipe in trench) - to circulate the water. Would this be feasible as a way of pre-heating the water before it enters the boiler? Though I realise that the water tank in the boiler would not increase in temperature until enough "new" underground water runs into it, once the taps are turned on. I am trying to think of some way of
    avoiding the high cost of a heat pump. Thank you.
    Last edited by a moderator: Oct 23, 2011
  2. Dana

    Dana In the trades

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    I'm not sure I fully understand the configuration. If the unheated water storage tank tank is in the attic, is it inside the homes insulation, or outside? (In other words, is the attic insulation at the roof deck or is it at the attic floor?)

    What is the temperature of the ground?

    To pre-heat the water the pipe needs to pass through something that is warmer than the stored-water temperature. In the summer an uninsulated attic will be at temperatures well above that of the ground below, and routing the water underground will cool it off ahead of your boiler, but in the winter it would be the other way around- the attic would be cooler than the ground, and the ground would add some heat.

    Hot water need to be at least 40-45C, and unless you live in an equatorial desert your ground temps are far below that. To raise the temp to useful levels requires either burning fuel in your boiler, or using a heat pump (air-source or ground source) to move heat into the potable water from whatever the heat source is (air or water), or using a solar panel, etc.
  3. sprinkler

    sprinkler New Member

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    ah, I see. That sounds logical. There is a water tank in the attic (around 250 liters), which stores the well water, and as you say, it

    would probably heat the water more in the summer than the underground pipe, as there is no insulation in the roof of the house. So maybe a solar panel would be a better way to pre-heat the water?

    But I was wondering about the typical closed loop geothermal installation (using a special anti-freeze liquid). Does a heat pump use lots of electricity to heat a normal 3 bedroom house? Is the heat pump very expensive to buy? Thanks for the advice.
  4. Dana

    Dana In the trades

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    The amount of electricity a heat pump uses is substantial, both in terms of peak-draw when running, and total kilowatt-hours consumed. But depending on your oil & electricity costs, a heat pump is usually cheaper (sometimes FAR cheaper) than a high-efficiency oil boiler at current (and anticipated) oil prices.

    In climates where it rarely drops below -20C (-4F) it's often possible to heat homes using ductless air-source heat pumps (mini-split heat pumps), at nearly the same efficiency as ground source heat pumps, but at a fraction of the up-front system cost. There are also hydronic (pumped hot water) versions, but they tend to need a lot more radiator area than typical oil-fired systems, in order to be able to run reasonably low water temps for highest efficiency. Some are capable of heating the potable hot water as well. In Europe and the UK there are CO2-refrigerant heat pumps available that can efficiently run somewhat higher temperature heating water than R410A-refrigerant heat pumps, but they're not yet sold in N. America. eg: http://eu.sanyo.com/aircon/Products/CO2-ECO-Heating-System/Introduction/ These are also likely to be pretty expensive to buy & install relative to an oil boiler, but the operating cost can be pretty low.

    To figure out what makes the most sense we'd need to know what does electricity and heating oil costs in your area, your location (for weather data) and how much heating fuel you use in a season (to estimate the minimum heating system size necessary to stay warm.)
  5. sprinkler

    sprinkler New Member

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    yea, kind of as I suspected - heat pumps use quite a lot of electricity. Now I think a solar panel, just to heat the water may be a better option. The house is in

    a fairly sunny location (north Spain), and the current heating system is oil boiler and rads (which can heat the whole house) + a wood stove which gives heat to 4 rooms. So usually it's

    only necessary to use the oil boiler for an hour or even less, until the wood stove takes over, and the stove heats around 65% of the house. So I was thinking more of a way to heat water or at least pre-heat it, before it enters the oil boiler, so saving oil costs.
  6. Dana

    Dana In the trades

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    If the temperatures rarely drop below 0C (an almost never go below -5C) you may be able to use an inexpensive "batch solar heater" as a pre-heat to the boiler. These are fairly simple to build, but there are commercially made versions as well. In much of rural China and rural central/south America batch-solar heaters are often the ONLY hot water heater, and in Israel they are quite common as well.

    http://www.builditsolar.com/Projects/WaterHeating/FSECFS36Batch11349.pdf

    [​IMG]
    Home-built batch heater.

    Many of the commercially made batch heaters utilize evacuated-tube collectors & insulated tanks, and can be installed in colder climates.

    [​IMG]
    Evacuated-tube batch heater

    Batch heaters are very simple and inexpensive- they require no circulation pumps or sensors.
  7. sprinkler

    sprinkler New Member

    Messages:
    72
    yea, the batch water tank sounds like a good idea, but in this case the winter air temperature

    can fall to -15º C, or even -20 C, so the water would freeze. Is there a way to get around this problem

    by using a sealed system with a heat exchanger and some kind of anti freeze fluid? Or is that system only

    for a solar panel? Oh, I just remembered - isn't there a system that works as follows? You have say, a 500 liter

    insulated water tank in the garage, BUT it contains only maybe 300 liters of water,the rest is air. Then this tank

    is connected by a pipe to an exterior solar panel or batch heater, which must be at a higher level than the garage tank.

    So, this solar panel or exterior tank contains water ONLY when the circulation pump is turned on. When switched off, the water

    falls to the garage tank and the air rises to fill the exterior tank /solar panel. But I suppose this would only work in a sealed (heat exchanger)

    system? And the more usual in this case is to use an anti-freeze fluid?
    Last edited by a moderator: Oct 27, 2011
  8. Dana

    Dana In the trades

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    Some of the evacuated-tube versions of batch heaters can work at -20C temperatures. On those the thermal mass and insulation of the tank protect the tank from freezing. The risk is primarily to the pipes going to & from the tank.

    The "drain-back" approach with the pumps works, but it's more expensive and has more maintenance & repair issues to attend to. Flat panels with glycol anti-freeze in the panel loop, with a heat exchanger for the potable water is the most common sort of system in that sort of climate, but it is even more expensive.
  9. Earth Fire Energy

    Earth Fire Energy New Member

    Messages:
    3
    Location:
    Prince George, British Columbia
    Heat Pump Requirements

    In order to preheat the water entering the boiler with a closed loop geothermal system without a heat pump, the ground temperature would need to be significantly warmer than the water your trying to heat. Otherwise the water will be transfering heat into the ground. A heat pump allows you to use the heat obtained from low temperature sources such as the ground.

    Best Regards,
    Bret Hutchinson
    Earth Fire Energy Inc
    www.earthfire-energy.ca
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