Ground Source Heating

By: Graham Doel, March 10th, 2008

In my constant quest to find cheap energy sources that are environmentally friendly and economic I began to have a look at Ground Source Heating. I couldn’t find much that was really helpful in explaining how it works so I asked my friend Andy. He sent me this concise, informative reply:

Graham,

Your Renewable Energy Hotline here! (I’ve asked Andy renewable questions before)

Yes, I’ve looked at Ground Source Heat Pumps quite a bit. You are all right in a way:-

1. They are extremely good for the environment because you get 3-4 kW of heat for every kW of electricity. Looking ahead they are going to play an important part in meeting the commitment the government has signed to meeting 15% of UK’s energy needs from renewable sources (the heat counts as renewable).
2. I wouldn’t describe them as any more complicated than necessary, but they are more complex than a boiler.
3. You are right too, they are expensive, generally expect £5k-£10k on top of normal boiler cost. Grants can offset a small proportion of this: http://www.lowcarbonbuildings.org.uk/how/householders/

How they work:-

1. They extract heat from the ground, which is generally at 5-10 deg C by pumping cold antifreeze or brine around the ground loop at say 0 deg C.
2. The slightly warmed water, now returning at say 5 deg C has heat extracted from it by the heat pump, which takes the heat at 5 deg C and upgrades it to heat at 40 deg C which is then given to the water circulating round the house. This diagram shows it quite well, note how the heat pump takes heat in from the right at low temp and gives out heat into the house on the left at high temp, this is the clever bit you can’t replicate in a DIY manner. http://www.earthwisescotland.co.uk/Tech/sys_diag.htm
3. This is just about warm enough to be useful, but because 40 deg C is not very warm compared to a normal boiler you have to have especially large radiators or underfloor heating.
4. Note this is not hot enough for hot water. Typically a heat pump cannot supply the hot water tank (although one or two can now) and a supplementary boiler is often required for that.
5. A bonus is that in the summer you can use the cooled ground to absorb heat from the house, in this mode the heat pump is not used because the brine is returning at a suitable temperature to cool the house.

Why the Doel It Yourself plan won’t work:
I think you’ve mistaken heat for warmth. The ground isn’t warm, it’s colder than the house, but that doesn’t mean you can’t get heat out of it. In scientific terms heat is a flow of energy, it doesn’t imply warmth. A heat pump uses electrical energy to push heat “uphill” against it’s natural inclination to flow from high temperature to low temperature.

Summary
If you are off the gas grid then although it is expensive it will pay back (in perhaps 10 years) compared to oil or direct electric heating, which have high running cost. If they have gas there is no contest on price both installation and running costs of gas are lower. From a green point of view it knocks the socks of everything else. They are very disruptive to lay, although the ground loop can be in a installed in a 70m deep borehole instead of a 70m long shallow trench. It will also be disruptive inside to lay underfloor heating or up sizing radiators.

And no you can’t rig one up yourself!!

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2 Responses to “Ground Source Heating”

1. Ricky_O Says:
   March 11th, 2008 at 1:04 pm

   No one’s mentioning the effects of pumping that heat in and out of the ground.

   When you move the heat from the building to the ground you are actually ‘putting it into the ground’. That’s a lot of heat. I’ve heard of cases where the ground temp has actually increased significantly, like to the 80’s Fahrenhite. That earth has an ecology, and many of the organisms in it cannot tolerate such changes in temperature. If one upsets that ecology, it is a chain reaction. The waste by-products of one species feeds the next in the change. That simply won’t do.

   Second issue no one is addressing, is the change in the frost line. Buildings are engineered with the location of frost lines considered. Driving heat into the ground can do away with that line. Taking heat out of the ground may cause it to progress downwards. Most building foundations are set to a depth to accomodate where that line is. Moving it may have significant effects. I would check with an independent engineering firm before making any decisions.

2. tony Says:
   March 11th, 2008 at 2:16 pm

   we have done extensive work with ground source heat exchangers. direct exchange systems have more of a propensity to react as ricky notes. however water source and our system - the geocolumn - produce less stress on the soil as the exterior temperatures of the heat exchanger mediums (pipes) are less dramatic. in the case of the geocolumn where it is only located 23 feet below grade the exterior temps are 40F in heating to 70f in cooling. outwardly declining radiation of this heat is also limited to an area approx. 8 feet from column center.

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