Radiant Heating HVAC

Special Feature Article

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Geo-exchange Systems and Radiant
By Geoff McDonell P.Eng. LEED ^(R)AP
Copyright (c) 2006, All Rights Reserved

A match made in heaven? Might be. Considering that a geo-exchange heat pump system can make warm and cool water from a single unit makes it an ideal match to radiant heating and cooling systems, with energy efficiency thrown in to boot!

First things first: a lot of people call these systems “geo-thermal” heat pumps. Strictly speaking, “geo-thermal” systems take heat energy from the ground, usually steam, or hot water from volcanic sources. Iceland, for example supplies a lot of it’s energy for heating and electrical generation from these geo-thermal sources using direct use of the heat energy from the volcanic sources. A “geo-exchange” system on the other hand uses the ground as a heat exchanger for both heating and cooling operation. The traditional use of geo-exchange heat pumps has been for heating applications, using the relatively steady ground temperature as a heat exchanger to generate warm water for heating applications, so the term “geo-thermal” has come into common use. The reality is that these systems are just using the ground as a heat exchanger, thus should rightly be called “geo-exchange” systems.

The most common form of a geo-exchange heat pump is a “water to air” unit where the ground water loop is used to exchange heat from/to the refrigerant circuit which is connected to a refrigerant coil and a blower to make warm or cool air. Basically a similar appearing system to a conventional gas fired heating only furnace, but with a ground loop pump circuit and a small refrigerant compressor system inside the blower unit to provide heating or cooling to the coil that the air is blown across.

A water to water geo-exchange heat pump unit uses the refrigerant system to make warm or cool water. The warm and cool water could be pumped and piped to conventional fan-coil units to create warm or cool air for room temperature control, OR, pipe the warm and cool water to a radiant system for room temperature control, cutting out the conversion of energy exchange using the air system.

A radiant temperature control system must still use a ventilation system, but it’s much smaller than a conventional “air conditioner”, so the cool and warm heat pump water can also be used at small heating and cooling duct coils at an air to air heat recovery ventilator to touch up the ventilation air, and if need be provide some supplemental heating and cooling to the rooms, as needed. In hot humid climates, the ventilation system must also perform de-humidification so radiant cooling can be used effectively without the danger of condensation.

A well designed building envelope will allow radiant heating and cooling systems to operate in the range of 85F to 90F for heating water supply temperatures and 62F to 66F for radiant cooling system temperatures. If you look at the Coefficient of Performance (COP’s) of water source heat pumps, you can see that the temperatures required for radiant heating systems can result in much higher energy efficiency than if the heat pump was used for more conventional water temperatures required for higher temperature heating systems.

The comparison above uses a constant ground water loop temperature of 50F in all cases. Note that if the ground temperature was able to provide 50F-55F during the summer, the heat pump unit could be bypassed and the direct ground exchange could be used to generate the cooling water for a radiant cooling system and the cooling coil in the ventilator, via a plate heat exchanger and mixing valve control. The key point is: if the house is designed such that the heating and cooling loads are minimized, the geo-exchange system can be kept small and cost effective, AND operate at higher COP’s.

Now, another myth is that geo-exchange heat pumps “need” vertical wells for the ground loops. The trick to provide cost-effective geo-exchange heat pump systems is to use the integrated design process to find areas of “free excavation” where you can get ground loops buried at suitable depths in horizontal configurations where you have cut and fill sites, or into areas of additional excavation required to put in structural fill, or use geo-exchange tubing integrated into structural piles. Vertical wells are a solution for a compact site where there isn’t any room for horizontal fields. And again, the lower the building heating and cooling loads are, the smaller the geo-exchange system needs to be.

There are other considerations for geo-exchange systems design that are topics for other discussions, but should be kept in mind when considering the building or house design:

 Local soil and water table conditions.
 The energy balance of the house/building – is it heating dominated or cooling dominated?
 Local expertise for geo-exchange and soil conductivity testing
 Legalities of ground use and aquifer protection

- Geoff McDonell -

REFERENCES:

1. International ground source heat pump association (IGSHPA) http://www.igshpa.okstate.edu/
2. Canadian RETScreen tools for evaluating ground source heat pump systems: http://www.retscreen.net/ang/g_ground.php
3. General ground source (earth energy) heat pump systems descriptive material: http://oee.nrcan.gc.ca/publications/infosource/pub/home/
Heating_and_Cooling_with_a_Heat_Pump_Section4.cfm
4. Earth Energy Society of Canada: http://www.earthenergy.ca/conta.html
5. Canadian Geo-Exchange Coalition: http://www.geo-exchange.ca/en/home/index.htm
6. Geothermal heat pump consortium: http://www.geoexchange.org/